BE532872A - - Google Patents

Description

       

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   The present invention relates to improvements made to the treatment of raw sewage water by anaerobic digestion carried out in a closed digestion tank, the object of this treatment being to convert the putrescible matter of the sewage into gas or methane. combustible, while producing a harmless residue known as "digester sludge"
This anaerobic digestion, carried out in an uninterrupted manner inside the contents of the tank, is due to the action of anaerobic bacteria naturally present in the fresh sludge, the word anaerobic deriving from the fact that these bacteria feed and develop using the putrescible organic constituents of sewage water, in the absence of dissolved oxygen, while converting this material into a gas, which constitutes the net product,

   together with alkaline reaction products, especially ammonium bicarbonate. The gas leaving the digestion tank can be used as a fuel for heating the contents of said tank and maintaining favorable digestion temperatures therein, or for other uses, while the sludge itself is being disposed of, for example. for example by spreading it on drying layers or by dehydrating it, for example on vacuum filters, so as to obtain a cake which can receive any suitable destination.



   The present invention proposes to improve the efficiency and economy of current digestion processes by reducing the volume required by the digestion tank, accompanied by savings in the cost of construction and in capital invested, maintenance costs, depreciation, area of land required, and amounts of fuel required to maintain required digestion temperatures. The present invention further proposes to improve the general efficiency of the sewage treatment plant. Other objects of the invention will be demonstrated in the course of what follows.



   Raw sewage sludge intended for such anaerobic digestion comes from the clarification stage of raw sewage, normally very dilute, which arrives at a sewage treatment plant in from the sewage collection line.

   In other words, the raw sewage water reaches the plant at rates and concentrations that vary throughout the day and enters what is called the primary clarification or sedimentation tank, in which the major part of the liquid, containing the lightest material, or not settable, is separated and flows from the tank through an overflow in the form of a so-called primary effluent while the solid materials, heavier or settable, and a relatively low liquid form a primary raw sludge which is withdrawn from the bottom of the tank to be subjected to anaerobic digestion of the sludge, with a view to its subsequent disposal.

   According to the conventional method, digestion usually provides a digester slurry, coming from the bottom of the digestion tank, and gas coming from the top of the tank, in addition to providing a lighter liquor which flows from the top and is usually returned to the treatment facility in order to remove the solid content, which may be excessive and variable. The production of a supernatant liquid of adequate quality from conventional digestion plants and the processing of such liquid according to usual practice raises special problems as will be seen later.



   The digester sludge itself is usually subjected to dewatering treatment, unless it is dumped into lagoons or transported and discharged at sea; so that a conventional sewage treatment plant applying the digester sludge dewatering method can be considered to include a clarification section, a sludge digestion section and a sludge dewatering section, as well as means for returning the supernatant liquor layer from the digester to the clarification section.

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   Whenever desirable, the primary effluent containing suspended matter, as well as matter in the dissolved state, can be subjected to additional treatment, such as that called "activated sludge treatment". that is to say, a secondary or biological clarification treatment by which the primary effluent is subjected to the action of aerobic bacteria, which derive their name from the fact that they feed and develop from the matter contained in suspension or in the dissolved state in water, in the presence of oxygen, which converts the solid matter of the primary effluent into a more easily settable form o A mode of treatment of activated sludge, called aerobic treatment in the general classification,

   requires that the overflow or primary effluent be subjected to aeration in the presence of a previously formed activated or biological flocculate, which aeration may be carried out in aeration tanks with the aid of vigorous agitation, and that It is then transferred to a secondary clarification tank. The liquid which pours out of this secondary clarification tank is called secondary or final effluent and is usually characterized by a degree of purity which is considered to be sufficient so that this effluent can be disposed of by discharging it into masses of water. open water.

   Activated or secondary sludge which is removed from the bottom of the secondary clarification tank may be subjected to the digestion treatment together with the primary sludge, although most of this sludge must be returned to the aeration section in order to maintain the biological process. Therefore, when it comes to a raw sludge to be fed to the digester, it should be understood, for the purposes of this invention, that this raw sludge can be composed of either a primary sludge or a mixture of primary mud and secondary mud.



   In the operation of a digester, in order to produce and supply a desired quality of digester slurry, care must be taken that bacterial inactivity, or metabolism, proceeds uninterruptedly in the contents of the digestion tank. Therefore, it is necessary that the anaerobic bacteria are continuously supplied with adequate nutrient material in the form of organic gasifiable elements, to allow said anaerobic bacteria to perpetuate their life cycles, while concurrently maintaining an adequate supply of reaction products. alkalis (ammonium bicarbonate) in this mass of sludge during digestion, in order to ensure the maintenance of a state of equilibrium between the bacterial population and the nutrient continuously available inside the tank.

   This is a question of a digestion installation which, in general, fulfills the function incumbent upon it if it stabilizes digestible solids by transforming them into gaseous and alkaline reaction products, while reducing the volume of sludge supplied for its Subsequent evacuation The essential conditions for the functioning of such an installation for a biologically sound and balanced operation are that the installation provides a digester sludge, a digester gas and a supernatant liquor, all at the same time. quantity and quality required, while maintaining a favorable digestion temperature and pH.



   According to current practice, the digestion slurry is usually heated so that it is maintained at a favorable temperature to stimulate and accelerate the bacterial action and thus maintain favorable bacterial conditions and satisfactory bacterial development, as well as to ensure the continuation of the digestion operation, without the risk of stopping the process, despite the fluctuations that occur in the raw sludge feed. The conditions causing the cessation of satisfactory operation of a digester occur when the amount of digestible food required for the bacteria in the digester is reduced to a low critical value or becomes inaccessible to the bacteria, interrupting the continuity of their cycles. of life and causes their death.

   In such a conventional digester, a shutdown occurs when an insufficient

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 food, with the resulting extinction of bacteria, is followed by a resumption of normal eating. The bacteria which tend to multiply under the influence of this refeeding generate excessive amounts of intermediate conversion products, such as volatile acids which, when they exceed a critical content, cause such a formation inside the digester. of foam that the conduct of the digester ceases to be able to be assured in a suitable way.



   Another factor or cause of failure or complete shutdown for any digester is excessive loss of alkalinity, consisting primarily of ammonium bicarbonate. Such loss of alkanility can result from too dilute sludge feed and too high loadings, these conditions having the effect of displacing out of the digester a volume of sludge which contains an excessive proportion of undigested material, together with substances. alkaline in the dissolved state.



   The digester gas itself, and even an auxiliary fuel, is used to maintain a desired digestion temperature. When the temperature is thus maintained at a desired value, or within the limits of a desirable range, a factor upon which the proper functioning of the digestible bacterial action within the tank depends is the pH of the sludge, which should normally be around 6.8 or higher.



   With regard to the problem of foaming on the surface of the digester slurry, foam breaking mechanisms are sometimes provided.



   The intensity of the bacterial digestive action, and consequently the intensity of the gas formation inside the digester, tend towards a state of equilibrium based on the density of the bacterial population, on the quantity of food available for bacteria and on the other above-mentioned work factors; thus, the digestion operation regulates itself to a state of equilibrium in the bacterial economy.



   It is on such a balanced bacterial economy that certain determinations of the capacity or capacity that the digester must have are based, these determinations being largely empirical and usually expressed by the number of dm3 or liters of capacity of the tank. of digester which is required per inhabitant of the town or agglomeration to which the treatment plant is attached., a volume of 42 to 56 dm3 per inhabitant being currently considered necessary for the digestion of a primary sludge, whereas 113 dm3 are needed for the digestion of a mixed activated sludge (primary and secondary) in the case of a modern two-stage digestion such as that which will be explained below.



   The volume of digester required can also be expressed by the permissible load of the digester (expressed in kilos of the quantity of dry solids contained in the sewage water) to be treated per cubic meter of the digester volume and per month. An average load adopted for current digesters is of the order of 16 to 32 kg per cubic meter and per month, when a conventional two-stage digestion treatment is used, as will be explained later. .



   Anaerobic digestion of gasifiable organic matter is the result of a biochemical decomposition by which the matter is reduced or decomposed in stages into volatile acids or dissolved elements constituting an intermediate digestion product, and by which these intermediate products or acids are subjected to further conversion to combustible gas as a final and desired digestion product.

   Simply put, this digestion of gasifiable matter is due to the action of certain kinds of anaerobic bacteria, hereafter called acid-forming bacteria, which feed on organic matter to generate volatile acids constituting their own digestion product, while other kinds of anaerobic bacteria, called

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 Ci, -after gas-producing bacteria, in turn feed on this intermediate digestion product and transform it into gas.

   From this fagbn, under the average conditions of sewage digestion, it has been established that the decomposition or destruction of 1 kg of volatile organic matter which results from such digestion produces about 1 cubic meter of gas, at as a final digestion product, this gas being mainly composed of methane, which constitutes two thirds, and carbon dioxide which constitutes the remaining third.



   The digester sludge coming from a conventional digestion plant and intended for disposal contains solid elements, both mineral (ash) and organic (residues). In other words, from an average raw sludge whose organic matter content is for example 75%, the remainder consisting of incense, only a portion of the 75% organic elements is easily digested to such a degree that 'it is destroyed as a result of its conversion to gas.

   Referring to average conditions, about half of the total amount of solids in sewage is thus subjected to decomposition or gasification; as a result, about two-thirds of the aforementioned organic portions of the raw sludge undergo actual digestion to the point of gasification, while the remaining one-third, which is organic but hardly decomposable, penetrates together with other constituents which are not susceptible to decomposition. digestion as a largely harmless residue inside digester sludge. For example, out of 100 kg of solids of sewage entering the digester, 75 kg is composed of organic elements and 25 kg of core.

   When 50 kg of organic matter (i.e. two thirds of this matter) is converted into gas, the digester sludge contains 25 kg of organic matter, plus the aforementioned 25 kg of ash. Therefore, for every 100 kg load of dry solids from the sewage entering the digester in such a case, 50 kg of solids containing the remaining 25 g of organic matter, plus the 25 g, are removed. kg of the initial ash.



   In these conventional sewage treatment operations, the raw sludge from the clarification section is relatively very dilute and of varying concentration and volume due to the way it is necessary to operate the primary clarifier, this difficulty being partly due to the diurnal variations of the feed and partly due to the fact that the clarifier is in itself incapable of playing the double role of concentrating the sludge and of retaining a maximum of solid elements. The quantity of sewage water admitted daily and the solid content of this water are subject to vary considerably during the day.

   For example, during the very first and the very last hours of the day, the sewage admitted is low, both in volume and in solid content, while in the last hours of the morning the volume of water in the day. sewer agumente, together with the concentration of solid elements. If we want to avoid too great a dilution of the sludge, it is necessary that the solid elements are retained at the bottom of the primary clarifier, which involves the risk of gas formation causing the rise of a large portion of the elements. solids and thereby causing deterioration of the effluent. On the contrary, if it is to avoid deterioration of the effluent, the sludge will have to be removed in an extremely diluted state.

   Therefore, sludge is usually removed from the primary clarifier only periodically, or intermittently, to avoid overdilution.



  If this sludge were removed almost continuously, its concentration would vary between that of a very dilute sludge and that of raw sludge.



  To overcome this difficulty, the digestion tank or tank is given sufficient capacity to enable it to withstand such fluctuations without risk of disturbance. Such a disturbance can result either from a disturbance in the equilibrium of the biological operation under

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 the influence of fluctuations in the supply of nutrients, i.e. the presence of excessive volumes of water which dangerously decreases the alkaline content. In addition, the introduction of excessive volumes of a dilute residual sludge into the digester may cause disturbance or prevent the production of a supernatant layer of the desired quality.

   This is why it was necessary to make use of a digestion tank of very large dimensions, and therefore uneconomical, in order to ensure the stability of the volume of material contained in the tank so that the operation of the digester itself remains stable and balanced despite the fluctuations that may occur in the feed.



   The dimensions or volume of the digester are sometimes defined in multiples of the daily feed of raw sludge that the tank is capable of containing or of storing at any time; the tank or the digester apparatus are for example designated under the term "58-day tank or apparatus" if they are large enough to contain the equivalent of a 50-day supply or pumping of raw sludge. In other words, to put such a digester into action from the empty state, it would take 50 days of pumping raw sludge to fill the digester.



   Due to natural limitations, the sludge leaving the digester still contains a minor proportion of undigested gasifiable material. The proportion of residual gasifiable material contained in the sludge in turn depends mainly on the relative dimensions of the digestion tank. In other words, the proportion of undigested gasifiable residual material still contained in the sludge at its exit from the digester is all the lower as the bottom dimensions are larger compared to the volume of raw sludge introduced per day into the bac.



   Other problems and difficulties of the methods currently applied for the digestion of raw sludge and which require the adoption of compromises in the procedures applied are due to the fact that the digester or digestion plant provides a layer of supernatant liquid which separates. the actual digestion sludge contained in the installation. The volume of this supernatant layer may for example represent in practice a proportion varying from half to two thirds of the water contained in the raw sludge, and said layer has the disadvantage of containing a relatively large quantity of impurities, both in terms of suspension only when dissolved.

   Therefore, the supernatant layer of the digester is ordinarily subjected to further treatment, recirculating it through the treatment apparatus, which places a considerable additional load on the primary and secondary clarification sections of the plant. treatment. The supernatant liquor from the digester is thus regarded as an undesirable product, in that its recirculation leads to an increase in the pumping volume of the sludge in the digestion plant and aggravates the problem of the supernatant layer, as well as due to due to the fact that this layer seems to be related to the disturbances of the biological process which intervene in the installation of secondary biological clarification.



   It is only possible to obtain a supernatant layer of relatively good quality from such a digestion plant if a relative slowing down of digester activity or a reduction in the intensity of the formation of digestion is accepted. gas in the digestion sludge, since a more intense gas formation would cause by its agitation a reduction in the quality of said supernatant layer. ,
As a compromise adopted in previous digestion processes, the digesters were made so large that the gas conversion of the solids in the sewage was slowed down to such an extent that the tanks simultaneously played the dual role. to digest the

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 sludge and separate the solid elements of the digested sludge from the supernatant liquor.

   Such attempts at compromise presented the difficulty that one had to solve two contradictory problems, namely a satisfactory digestion of the sludge on the one hand, and a settling of the solid elements (in order to obtain a relatively clear supernatant layer). on the other hand. It was only possible to increase the efficiency of one of these two functions at the expense of the other. In other words, the digester tanks usually contained at all times a large reserve of undigested gasifiable material, both fresh and aged, the digestion operation being maintained at a level capable of ensuring a sufficiently moderate gas production level. , per unit volume of interior space of the digester, to allow the release of an acceptable proportion of the supernatant layer.

   This production of supernatant layer was obtained at the cost of a decrease in the efficiency of the tank and the efficiency of digestion.



   Given the need to introduce a large volume or charge of water into the digestion stage itself, it is essential in the current conventional process: a) to remove this charge of liquid from the plant in the form of a supernatant layer; b) treating said layer; c) to supply the quantity of fuel or thermal energy necessary to heat the large volumes of raw sludge introduced into the digester and to maintain the desired digestion temperatures by compensating for the losses by radiation from large tanks; d) carrying out and balancing the digestion process itself in a manner suitable for ensuring the preservation and maintenance of the desirable pH in the digestion slurry.



   In the efforts made to solve the problem of the supernatant layer, it is digestion by the so-called two-stage method (previously mentioned) which has generally been adopted and which is currently the most widespread method because it decreases. to a certain extent, without however eliminating them, those of the difficulties which are inherent in the operating compromises, previously mentioned, made necessary by conventional digestion processes. Digestion by the two-stage process, as currently practiced, will provide some background for the presentation of this invention.



   In the two-stage, or multiple-effect, digestion process, two digestion tanks, called primary and secondary, respectively, working in series are provided. The raw sludge is admitted to the primary tank, at rates and concentrations which vary during the day, as previously mentioned, the capacity of the tank being sufficient to absorb these fluctuations, while ensuring a stable digestion process. and jerk resistance. No effort is made to collect the supernatant layer from the primary digestion tank;

   on the contrary, it ensures the maintenance in the primary digestion stage of a digestion temperature and a favorable pH of an insignificant quantity of digestible matter, to be transferred from the first to the second stage, that is to say insufficient to cause deterioration of the supernatant layer therein.



   Digester sludge at the normal degree of dilution is forced out of the primary tank, volume for volume, by the diluted raw sludge entering this tank, while gas is generated and collected under corresponding equilibrium rates. The primary digestion sludge thus displaced must contain a residual proportion of undigested gasifiable matter which depends on the working level at which the balance of the bacterial economy is maintained in the contents of the tank, this level itself being governed by the dimensions given to the tank in relation to the imposed sludge load.

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   In other words, the raw sludge introduced into the primary digestion tank has the effect of transferring from this tank to the secondary digestion tank a volume of digester sludge equal to the volume of raw sludge introduced into the primary tank, the primary sludge necessarily containing a residual proportion of undigested material capable of being gasified, of which an additional digestion will take place in the secondary digestion tank or stage. In this case again, a necessary functional compromise lies in the acceptance of a primary digester sludge, the degree of digestion of which is in relation to the functional stability of the secondary tank, since it is necessary that this sludge be digested to a sufficient degree. pushed so that it can settle and be decanted in the secondary digestion stage.

   It is therefore important that the capacity of the primary digestion tank is large enough to supply the secondary digestion stage with a primary digestion sludge containing a quantity of residual gasifiable material not exceeding that giving rise to the secondary stage. to an evolution of gas sufficiently attenuated to allow a supernatant layer to form and to discharge from the tank in as little disturbance as possible. In addition, the primary digestion stage requires that the volume available is high enough to absorb the volume of variable sludge from the primary settling tank of the ordinary treatment plant, while transferring well digested sludge to the second stage. for liquor separation and reduced gas production.

   Experience has shown that a holding period or sludge stay of about 20-30 days is required, in a well-functioning two-stage digestion plant, for this single first digestion stage. Current practice provides for the secondary digestion tank a capacity approximately equal to that of the primary digestion tank which supports the greater part of the digestion load and of which no supernatant layer is collected, the secondary tank supporting only a relatively low digestion load, allowing the supernatant liquor to separate from the digestion sludge while very slow gasification takes place in this tank.

   For example, such a practice adopted as a compromise results in a primary digestion sludge which is digested at approximately 90% and contains the remaining fraction (i.e. 10%) of the material capable of being gasified, intended to be transferred to the tank. secondary digestion to undergo slower digestion concurrently with the separation and collection of the supernatant layer. An almost completely digested sludge is thus collected from the secondary digestion stage.



   In spite of the relative progress represented by the present practice of two-stage digestion, the problem of making the digestion treatment efficient and economical by a stable and shock-resistant process remains unresolved, as the efforts made in this process remain unresolved. The effect of solving it consisted only of a compromise measure consisting in providing bins whose dimensions are excessively large in relation to the daily volume of raw sludge to be treated, as well as in relation to the residence time of the sludge.

   Thus, the two-stage digestion tanks are set up to provide a tank volume or holding time equivalent to a working capacity of 50-90 days, that is, a tank volume that can in all time to contain under working conditions 50 to 90 times the daily quantity of raw sludge to be treated. In other words, for certain treatment conditions, a 50-day tank constitutes a lower limit, while for other treatment conditions a 90-day tank constitutes an upper limit, tanks of intermediate dimensions both adapted to the conditions. intermediates, the extent to which the dimensions of the tank must be increased depending on the proportion of biological solids to be digested, as well as on the degree of conversion desired.

   On average, the digestion of a mixture of primary and activated sludge by the conventional two-stage process requires about

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 100 to 142 cubic decimeters of tank volume per head living in the agglomeration served by the installation.



   The second digestion stage was primarily a physical separation of the supernatant liquor from the sludge. This second stage received all the mud which had undergone the primary digestion. The sludge was transferred from one stage to the next by pumping and / or gravity.



  When a biological secondary clarification was applied in the process, a sludge volume shift of about 30-60 days was required for the secondary digestion tanks. This is why the current digestion installations must ensure a residence time of the sludge varying from 50 to 90 days. In fact, builders and legislators dealing with health issues have come to the conclusion that on this basis of calculation, it typically takes 100 to 142 cubic decimetres of tank volume per unit of digestion volume to perform digestion. 'a mixture of primary sludge and activated sludge.



  The digestion facilities thus established perform excellently for long periods of time without interruption. They are given great working stability by giving the bins very large dimensions in relation to the load. In fact, the disruption of the biological process of these facilities could only result from the addition of highly toxic substances, cooling of the contents or excessive dilution of the raw sludge load, accompanied by a loss of alkaline element content. In fact, it sometimes happens that an interval of a few days elapses between two consecutive additions or loads of sludge, and the temperatures prevailing in the digester have often varied from 24 to 38 C without any serious disturbance of the process. of digestion.

   The two-stage digestion process with heating of the sludge very satisfactorily and regularly achieves the intended purposes, although the secondary installations are sensitive and susceptible to disturbances as regards the quality of the supernatant liquor.



   The present invention proposes to achieve at least all the advantages of a multiple-effect or multistage digestion, while only having recourse to a tank volume which is only a fraction of the usual volume; Objects of the invention are to produce a well digested slurry, without this requiring the conventional secondary digestion stage giving rise to the difficulties relating to the supernatant liquor.



   In accordance with the invention, these objects are achieved by supplying the digestion plant with an extremely concentrated raw sludge, the degree of concentration of which has never yet been obtained or cannot be obtained from clarification operations; in other words, the digestion plant receives a raw sludge the volume of which is reduced to such an extent that it is substantially equal to the volume of the digester sludge which normally results from an adequate two-stage digestion plant . Thus, the digestion installation is relieved of a water load, the volume of which is equivalent to the supernatant layer of the digester of the previous installations.

   Consequently, no supernatant liquor needs to be withdrawn from the digestion plant to be handled or treated. According to the present invention, the concentrated sludge comes from a source or reserve mass allowing the transfer of said sludge. to the digestion plant at a substantially uniform concentration and at such intervals, or at such charging frequencies, as necessary for the purposes of the invention. In other words, the invention contemplates applying charges at a relatively high frequency and suitable for ensuring extremely high solids feed and gas evolution rates.

   It follows that, by establishing and maintaining a high water conversion of gasifiable matter per unit volume, any appreciable accumulation of digestible nutritional elements is avoided; just keep a minimum

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 of gasifiable material in the mixture to ensure a balanced operation producing a well digested sludge of acceptable quality.



   In addition, in the practice of the invention, the contents of the digestion tank are maintained heated as well as in a state of substantially uniform or homogeneous mixing of the gasifiable material with the digested sludge, so that the food is distributed in a substantially homogeneous manner to the bacteria at all points of the contents of the digester. A substantially constant bacterial population density is thus maintained under high conversion rates in a suitably alkaline medium, although a given volume of well-directed sludge displaced by the raw sludge load is removed from the tank.



   Thus, contrary to the prior practice, it suffices in accordance with the invention to maintain at all times a relatively small quantity of gasifiable material in reserve in all parts of the contents of the digester, such quantity being kept substantially constant and evenly distributed in all parts of said content.



   According to the invention, the installation is supplied with a raw sludge of high density from a reserve mass and such a supply is carried out at relatively short intervals, or even continuously, and in any case at sufficiently short intervals. to meet the practical conditions and needs of the invention. In other words, although a truly continuous feed may provide ideal conditions for the purposes of the invention, some latitude is possible in practice. For example, loads introduced at 30 minute to two hour intervals may be sufficient for high digester loads, which is in contrast to the current two-stage digestion practice, in which 8 hour intervals 3 days or more are common.

   Such a conventional operation is accompanied by accumulations of food producing surges of load which must be absorbed without appreciably influencing the functional stability of the digestion installation.



   In this way, a state of biological equilibrium is established and maintained in which the frequency chosen for the introduction of successive charges ensures maximum production of gas for a minimum tank volume. At the same time, by means of moderate regulation and homogeneous mixing, ambient conditions are established and maintained which ensure the maintenance of a maximum proportion of those of the bacteria on which the high conversion rates to gases and alkaline reaction products depend. solid elements of sewage. In this way, the volume requirements of the digester are extremely reduced and nevertheless a digested sludge is obtained, the quality and concentration of which are comparable to those of well-digested sludge from a properly conducted two-stage digestion.

   The quantities of heat required by the digester and the heat losses by radiation are proportionally reduced; eliminates the need to provide a secondary digestion stage of large size and consequently the problem posed by the supernatant liquor, thus eliminating the burden imposed on the treatment which results from the need to recirculate a supernatant liquor conveying solid elements from the digester . This also results in an improvement in the general efficiency of the treatment plant, considered as a whole. A digestion tank, the dimensions and operation of which are in accordance with the invention will be referred to hereinafter as a “high efficiency” digester.



   In addition to eliminating the secondary digestion stage, in the sense given to this day to this section of the device and its role, the installation is given the flexibility of operation in the sense that the degree of net conversion of digestible solids into gas ceases to be linked to the presence and operation of the digestion stage

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 secondary. This flexibility of operation results from modifications made to the dimensions of the containers, although these modifications are included within a relatively narrow scale which, on the other hand, extends well below and outside the scale of dimensions. classic bins with a capacity of 100 to 142 cubic decimetres per inhabitant.

   In other words, the lower limit of the tank volume necessary for a high-yield digestion operation, according to the invention, should not be less than a point of natural failure of the operation of the digester, i.e. say at a point below which the gas production efficiency of the digester falls in a very steep sloping curve; that the upper limit of the volume of the tank, for a high yield digestion operation, is determined by the factor relating to the degree of net conversion of the solids which is desired to be obtained by the digestion operation.



   When the present process is carried out on a large scale, a high throughput digestion plant will operate satisfactorily with a tank volume which is only one-eighth of that required by the current two-stage digestion process, in the case where the agglomeration attached to the installation has 350,000 inhabitants.



  In the case of an industrial-scale test facility, the present high throughput digestion process will give satisfactory results with only 1/16 - or even perhaps less - the tank volume required. two-stage digestion plant commonly in use today.



   Given the importance attached to the rate of gas production per unit volume of tank, in accordance with the invention, as a criterion of the digestion efficiency of a digester tank, as well as because of the 'importance attributed for this same reason to the quantity of gasifiable matter accumulated at any time in the digester tank, for the purposes of the invention the principle of what has been called the "gas potential" of the digester will be explained, c 'that is to say the quantity of gas which can be drawn from the gasifiable material contained at any time in the unit of volume of the contents of the digester tank when the latter is operating in a state of equilibrium.

   Under operating conditions, the present invention strives to keep this "gas potential" to a practical minimum, as well as to minimize the residence time of fresh sludge inside the tank. While ensuring a rate of gas production. high per unit volume and while simultaneously maintaining this minimum "gas potential, the present invention allows the digestion operation to be successfully located close to and somewhat below the point of natural failure. The invention has shown that the "gas potential" can be reduced to a point which corresponds, for example, to only 15 volumes per volume of digestion tank, while the "gas potential" required per unit volume of digestion tank ,

   in current digestion installations by the two-stage process, is 4 volumes per unit volume of tank, for the digestion of a mixture of primary sludge and activated sludge.



   The present invention is based on the discovery that it is possible to achieve and maintain extremely high bacterial conversion (or az production) rates, in continuous operation, within a suitably balanced digester, said gas conversion and production rates being much higher than the apparent gas conversion and production rates which must be satisfied in a two-stage digestion plant.

   The present invention radially departs from the prior practice of two-stage digestion and hence from the practice that operational stability and jerk resistance must be ensured by huge volumes of tanks which commonly contain or store huge volumes of nutrients, while working at a

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 equilibrium which corresponds to a low conversion rate per unit volume and a low gas production rate.

   In other words, the present invention radically abandons the practice of making determinations relating to tank dimensions in a somewhat "inverted" fashion, accepting dimensions which may have been dictated by empirical stability needs. operation and the needs of the secondary digestion stage, under low gas production rates.



   The present invention abandons the principle of determining or measuring the necessary dimensions of the digestion tank expressed only in terms of working stability and substitutes for it a determination of the dimensions of the tank based on the efficiency of digestion or gas production of a digester tank. In this regard, it also introduces the idea of "gas loss" or "lost gas" to represent the amount of gasifiable material which remains in the sludge delivered by the digester.



   As a measure of the efficiency of gas production or digestion, the present invention uses what has been called the digester index "to denote the percentage of" gas potential "which is actually converted to gas per unit. of time that is to say the quantity '
 EMI11.1
 dç gas ffee.irme.tteraehrlr¯ itrdetêp a $ ippor at there quah% i: tinte de.g.â1.? o.ble at all times.



   The actors of the expression representing "the digester index" are as follows: g: quantity of gas lost by the digester, in cubic decimeters per day;
V: volume of the digester in cubic meters;
G: cubic decimeters of gas produced per day by the digester;
Q: cubic decimeters of raw sludge pumped into the digester, per day; g / @ thus represents the quantity of "waste gas" in each cubic decimeter withdrawn from the digester, and Vg represents the "gas potential" of the digester, that is to say the quantity of gas which can be produced at any time at from the digestible material contained in the digester.



   Since the efficiency of the digester, measured in terms of the 'digester index', hereinafter referred to as I, is defined herein as the ratio of the quantity (G) of gas actually produced per day to the quantity of gas represented by the "gas potential" (Vg) the digester dp index can be represented by the following expression Q; (in per day) G = GQ 100
 EMI11.2
 (in per day) gas potential =:! ..: .. iL iL .. V. g x 100
Q It follows that, in order to obtain the maximum I, the digester is made to work in such a way that it produces the maximum amount of gas and that it loses the minimum of digester sludge, while receiving the maximum amount of raw sludge that it is possible to introduce by pumping into the minimum volume of the digester.

   To this end, a raw, uniform high density slurry is supplied from a reserve mass at very short intervals or continuously, while the contents of the pan are maintained in a state of homogeneous mixing, the frequency of the charges being. regulated in correlation with a high rate of bacterial metabolic conversion, resulting in a high factor I.

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   For example, if the digester index I is equal to 50%, this means that a proportion equal to half of the "gas potential" is converted per day. In an operation carried out, by way of comparison with the present invention, in a conventional installation with a mixture of primary sludge and activated sludge, the index 1 was found to be 40% per day. In contrast to this result, by performing the high throughput digestion in accordance with the present invention, an I index of 120% per day was achieved, which means that 120% of the "gas potential" is actually converted. in gas per 24 hour day.

   In fact, "high yield" digestion was performed according to the present invention in a test facility and gave an even higher I index, while maintaining continuous balanced operation.



   The importance of the present invention, expressed as a function of the volume of digester tank required, emerges clearly and graphically from the evaluation in% of the above expression: I = G x Qx 100 by plotting (Figure 3 ) the curve of I, plotting the quantities of gas generated in percent of the total available on the ordinate, as a function of the digester capacity (measured in cubic decimeters per inhabitant ') plotted on the abscissa, which makes it possible to obtain: a) an index of 40% per day for the conventional operation carried out using the aforementioned digester and b) an index of 120% per day for the "high yield" operation carried out in accordance with the invention. (This concerns the digestion of a mixture of primary sludge and activated sludge).

   The two resulting curves will be referred to hereinafter as "conventional digestion efficiency curve" and "high efficiency digestion efficiency curve", respectively.



   The two efficiency curves thus drawn comparatively indicate by their characteristics not only the loads which would cause the natural failure of the digester, but also the respective ranges of loads, measured along the ordinate axis, within which the digester " conventional "and the" high efficiency "digester can or should work.



    The limits of the classical working scale R1 of the classical efficiency curve C1, correspond to 100 to 142 cubic decimeters per capita for this operation, this scale being very distant from its point of natural failure on the graph of figure 3 and approaching the side where the safe region of the curve is; that 1 working scale R2 of the "high efficiency" C2 efficiency curve is much closer to or situated in the vicinity of the point of natural failure and that this curve is spaced, along the ordinate axis, by a marked distance of the classic scale and approximate the vertical axis of the graph's coordinate system.



   The two comparative efficiency curves have basically the same shape, but they are separated from each other by a marked distance. In other words, the curve C, known as the "classic efficiency curve" has an intermediate convex curvature, the lower end of which is connected to a steeply sloping portion of the curve ending at the point of origin. of the coordinate system, while the upper end of the convex portion connects to a nearly flat portion going closer to the horizontal;

   the "high efficiency" efficiency curve C has a similar intermediate convex curvature, albeit with a smaller radius of curvature, this convex portion being located above and to the side of the curvature of the conventional efficiency curve aforementioned, its front end connecting to the portion of the curve extending downward, the slope of which is even steeper than that of the corresponding portion of the conventional efficiency curve, while the upper end of said convex portion of smaller radius of curvature connects to a portion of curve which tends even more towards the horizontal and which is even flatter than the

  <Desc / Clms Page number 13>

 corresponding portion of the above-mentioned classical efficiency curve.



   In each of these two digestion efficiency curves, the respective natural failure points are located in the bent portions located near the point where they connect with the respective steep sloping portions. Thus, it emerges from the respective curves that the point of digestion failure of the "high efficiency" curve is shifted upward and to the side with respect to the point of digestion failure of the conventional curve, i.e. that is, it is offset in a direction away from the point of failure of the classical curve and closer to the main vertical axis of the coordinate system.



   Another feature of the "high yield" digestion operation is that it maintains balanced working conditions under high digestion yield, although it allows digestion to be carried out in a relatively area. close to the point of failure, in marked contrast to the conventional two-stage digestion operation, which must be carried out under low digestion efficiency and, graphically, at a significant distance from its point of failure . A more precise and detailed discussion of the relative shapes and scope of these digestion efficiency curves will be set forth below.



   According to one embodiment of the invention, to supply the digester with the sludge load of the high concentration and at the frequencies (or with the measure of continuity) required by the present invention, it is interposed between the clarification section and the digester. digestion section a special dehydration or concentration stage where the raw water from the clarifiers is treated.



  According to the invention, one way to obtain such a high density slurry at the desired frequencies consists of a specially controlled treatment carried out in a thickener. In other words, the raw slurry is transferred to the thickener in the ordinary dilution state, or even in a more diluted state than usual, and with the usual fluctuations.



  The operation of the thickener is adjusted so as to make it possible to form at the bottom of said thickener a layer or bed of sludge from which it is possible to continuously remove a sludge which is admitted to the digester under the high concentration and with the uniform flow rate which are those that the present invention contemplates applying for the supply of the digester, while liquid is pouring through an overflow of the thickener as effluent. Such thickener effluent can be returned to the clarification section for further treatment, the latter imposing only a moderate or insignificant recirculation load on the clarification section.

   This recirculation represents only an insignificant load, compared with that resulting from the need in the prior process, of having to recycle the solids conveyed by the supernatant layer of the digester; in fact, it is possible that the effluent obtained turns out to be of such a high quality that it does not need to be recycled.



   The presence of a supernatant layer in the digester is avoided, in the practice of this invention, because the excess water is removed by the stage of thickening which, according to the invention, is carried out prior to the preparation. digestion.



   To make it possible to obtain from the thickening stage a bottom slurry or lower stream of the desired density, the thickening is carried out in substance as described in the patent application filed in the United States of America under the number 334,513, February 2, 1953.

  <Desc / Clms Page number 14>

 



   Briefly, in this special thickening station, a raw-primary slurry, or a mixture of primary raw sludge and secondary raw sludge, is subjected to a thickening treatment in the presence of a large controlled volume of a liquid diluent, which may be the secondary effluent from the clarifier, or which may be water from any suitable external source. To obtain a sludge having this particularly high degree of dilution, it is even possible to operate the clarifiers, or the clarification plant itself, so as to remove therefrom an extremely dilute sludge for transfer to the thickening section.

   The clarifier can thus be operated so as to provide a sludge having a solids content much lower than that which is possible and desirable to obtain in accordance with usual practice. Thus, for example, if a primary raw sludge is routinely withdrawn from the primary clarifier at a content of about 4 to 6% solids, such sludge could be removed at a solid content as low as 1/4 to 1. / 2% for example, to meet the dilution needs of the thickening operation.



   The interspersed thickening operation is conducted and controlled by establishing and maintaining in the thickening section a relatively dense slurry layer from which high density sludge can be collected. In other words, the thickening operation is carried out so that the solids are retained in said sludge layer for a time of sufficient duration to allow said solids to become more compact, concurrently with removal of the solids. 'water.

   The sludge layer thus maintained can be subjected to small fluctuations, in depth or in volume, while absorbing the diurnal fluctuations in the sludge feed, but it nevertheless constitutes a reserve mass of high density and appreciably constanto This reserve mass thus constitutes a source of supply of fresh and digestible raw sludge which is available at all times, whatever the diurnal fluctuations, to feed the digesters at substantially constant rates and under the high and substantially constant concentration that contemplates the present invention.



   The layer of sludge which it is possible to obtain by this thickening operation is such that it is possible to determine it physically or to measure its thickness, the supply of the digester with this thickened raw sludge 'being adjusted according to such a measurement.



  The sludge layer represents a reserve of nourishing agent constituting a source from which a substantially continuous and uniform stream of digestible sludge can be taken in order to feed the digester in the manner proposed by the invention.



   According to the invention, enormous savings can be made, from the point of view of the capacity which it is necessary to give to the digestion tanks, by supplying the tank with a raw sludge of such a high density - and this at the same time. above-mentioned frequencies or continuously and on the other hand ensuring that the undigested and digestible (carbonated) sludge present at any time in the contents of the tank is in a state of uniform mixture in all parts of the tank. the mass of sludge contained in the digester, such a mixture of sludge which can be obtained by internal stirring and mixing, or even by the combination of internal circulation and mixing with external mixing, by putting back into circulation a digested sludge intended to be mixed with the fresh sludge introduced into the digester.



   With the digester thus supplied, it is made to operate at a considerably increased rate or rate of digestion (conversion, or gasification), in the sense that only a relatively small amount of undigested sludge is allowed to be present at the digestion. any time in the digester

  <Desc / Clms Page number 15>

 per unit volume of this digester, relative to the volume of digester sludge which is withdrawn from said digester; in this way, digestible matter, or bacterial food, is available in all parts of the mass contained in the digester, whether fresh or unaged;

   that is, together with a reduction in the volume of the digestion tank, it is possible to remove from the tank a much larger volume of digester slurry compared to the total content of the digester, which is accompanied by 'a reduction in the residence time of the fresh sludge in the digester.



   In other words, the relative content of digestible sludge elements is kept very low., While the conversion, or gasification, is allowed to proceed in a much more intense or much faster manner. per unit volume of digestion tank than had been possible to date. In this way, due to the joint effect of the previous removal of the dilution liquid (high sludge content), the introduction of the raw sludge at intervals of which the frequency is specially regulated, or even continuously, and By uniform distribution of the nutrients in all parts of the entire digester contents (with proper pH and temperatures maintained), the present invention ensures a high gasification rate per unit digestion tank volume.

   Thus, by maintaining a sufficient quantity of carbonated, digestible, unaged and fresh nutrients in all parts of the contents of the digester, and an alkaline and safe surrounding environment at high concentration the density of the bacterial population is itself constantly maintained at a level high enough to balance the nutrient load that is present per unit volume of digestion tank for high or high efficiency gas production. In this way, any accumulation of nutrients is avoided beyond the desired amount which corresponds to maintaining a sanitary working balance inside the digester, under a maximum rate of gasification and a maximum rate of withdrawal of digester sludge.



   The maintenance of this state of equilibrium, carried out concurrently with the sludge feed regulated from the sludge concentration section, prevents the accumulation of nutrients, whereas if such an accumulation had the possibility of occurring, it would have the effect of displacing out of the digester (by the introduction of the fresh charge) a wlume of digester sludge containing an unwanted proportion of gasifiable elements greater than that which is compatible with a working equilibrium, thus reducing the efficiency of the digester.



   To make the invention a reality, the operation of a conventional digestion installation was checked by the formula given above for the digester index. To this end, the loss of gas "g" of the digested sludge was determined and measured by passing this sludge through another tank where it was held for a time sufficient to complete the digestion. The volume of gas "G" generated per day in the digester under usual working conditions was also measured. The quantity of sludge transferred daily to the tank was also measured. The volume "V" of the tank being known, the resulting digester index obtained was 40%.



  This determination is based on the normally accepted load for which the digester was built and under which it was working.



  To examine the effect of varying the load of the digester on its operation, the particular loads introduced into the digester in an industrial operation were determined for both the first and second stages of digestion, which were combined as follows:
For the activated sludge process, the amount of sludge produced was 4.5 cubic meters of sludge per thousand cubic meters, in the case of a facility treating 151.4 milliqrs of cubic meters per day.



  Therefore, the total daily sludge production was 151.4 x 4.5

  <Desc / Clms Page number 16>

   = 680 cubic meters. As this factory was attached to an agglomeration of 3,500,000 inhabitants, this gave 680 @@ = 1.95 cubic decimeters 350,000 of sludge per capita per day. Under these conditions, if the sludge load is to be controlled by the two-stage process, a total stay of the order of 60 days in the digesters is necessary, according to current practice. The total capacity of the digester tanks required was therefore 60 x680 = 40.8 cubic meters.

   By dividing this number by the number of inhabitants, we get 40.8 = 116 decimeters
 EMI16.1
 cubes per capita. 350,000
To collate the examination of the efficiency of digestion, it will be assumed that the two digestion stages have been combined into a single digester volume.



   A curve is plotted representing "the efficiency of conventional digestion" on the basis of the percentage of gas generated compared to the total available (or also on the basis of the percentage of gas loss), as a function of various digester loads per inhabitant, expressed in cubic decimeters of digester volume per capita. The plot of this curve is established on the basis of numerical results obtained by assuming that 116 cubic decimeters of digester volume per capita constitutes the equivalent of a stay of 60 days under a digester index of 40% per day. 'follows that
GQ / gV x 100 = 20% per day = Digestion index in% per day; per 100 G.Q. = 40 gV, which gives g = 100GQ / 40V = 2.5GQ / V.



   Since the total volume of the digester is V and the volume of sludge pumped per day Q, the residence time of the sludge, in days is V / Q.



   We then have Q / V = 1 / residence time in days
 EMI16.2
   'E3ps * 3.e' eZ'ur * 'en "bear
For 116 cubic decimeters per inhabitant, we obtain, by substituting the residence time (60 days) g = 2.5G = 0.042G
60
The percentage of "gas loss" is the quotient of the gas loss times the total amount of gas produced, which is expressed as: g + G x 100 = gas loss.



   Substituting the value g in the expression ### x 100
 EMI16.3
 g + G gives: 0.042G - 0.42G gives ######### x 100 = 0.42G / 1.042G x 100, which gives a loss of
 EMI16.4
 gas of: 10/2 100 = 4.; / i.e. the quantity of gas generated is 1.042 re quan gas generated 100 - 4 = 96%.



   From the fundamental relation that 116 cubic decimeters per inhabitant are the equivalent of 60 days (by applying the activated sludge process), we deduce, by a simple proportionality, the residence times which are the equivalent of the gradually tank volumes decreases needed per capita. For example, for 100 cubic decimetres per capita, we get: 100 x 60 = 51 days. If we continue the evaluation relating to the decreasing volume requirements of the tank, we obtain a table (T) (given later) allowing the plotting of the curve (C1) representing the efficiency of conventional digestion. The discussion of tables and curves will be given later.

  <Desc / Clms Page number 17>

 



    By plotting from table T1 the curve of tank volume requirements per inhabitant as a function of the respective calculated gas losses or of the percentage of gas generated in relation to the total gas available, we obtain the classical digestion efficiency curve C1 .

   By following this curve from its outer extreme portion towards the interior, we see that it has a horizontal portion which illustrates high tank volume requirements per inhabitant, then a bend or connecting portion, and finally a steeply sloping portion which represents the low needs in tank volume per inhabitant; current practice requires tanks sized at the outer end away from the horizontal portion of this curve (approximately on the scale of 100 to 142 cubic decimetres of digester volume per capita.)
A corresponding curve is plotted to represent the efficiency of gas production obtained when operating under the "high yield" digestion conditions according to the invention.

   Under these conditions, the volume of sludge entering the digester is approximately reduced by half by dehydration, so that the tank volume requirement of 116 cubic decimeters per capita of the conventional process would actually provide a retention time. 120 days mud instead of 60 days.



   Compared with what was previously called "conventional digestion efficiency", the operation of the test plant working in accordance with the invention thus resulted in an enormous increase in the efficiency of gas production. , since this operation has shown that it is thus possible to obtain a digester index of 120% per day (and perhaps even higher) instead of the index of 40% per day. day obtained in the conventional process.

   Based on these results, a C2 curve representing "high throughput digestion efficiency" was established based on the following calculations:
GQ / gV x 100 = 120% per day = digestion index
100GQ = gV.120
Hence, g = 100GQ / 120V = 0.833GQ / V
However, if we express in days the residence time V / Q of the sludge inside the tanks, we have Q
G / V = 1 / residence time in days, which gives; g = 0.333G / residence time in days
For example, with the above value of 116 cubic decimetres per capita, substituting 120 days of residence time, we get g 0.833G / 120 = 0.007G.



  120
Since the "gas loss" in% is equal to the gas loss per day divided by the total volume of gas produced per day we have g / g + G x 100 = gas loss%. = 0.007G / 1.0070 x 100 = 0.007 / 1.007 x 100 = 0.65% (loss of gas).



   The gas production is therefore 100 - 0.065 = 99.35% relative to the total volume of gas available.



   For lower tank volume requirements per inhabitant, correspondingly reduced residence times are obtained o For 58 cubic decimetres per inhabitant, we obtain 58/116 x 120 = 60 days of residence time, and so on . The table (T2) given below was thus established in order to obtain the plot of the curve (C2),

  <Desc / Clms Page number 18>

 called "high-yield digestion efficiency".



   According to the invention, the "high efficiency digestion efficiency" curve (C2) is obtained, which has a substantially horizontal portion located above that of the "conventional efficiency curve" and a sharp bend followed. of a steep sloping portion. In accordance with the invention, it is possible to use and work within a scale of digester tank volumes, relative to the per capita requirements, of between about 3 cubic decimetres and 28 cubic decimetres per capita.



    A comparison of the C1 and C2 digestion efficiency curves and the corresponding working scales shows that the "high efficiency" digestion conditions lie within a scale in which the conventional process would cause excessive gas losses, or even a complete failure of the biological process.



   The following conditions or criteria, relating to the respective operating modes, are derived from these efficiency curves: In the conventional digestion efficiency curve C1, it is necessary that the digestion operation be carried out at a significant distance from the portion steeply sloping the curve, area where a dangerous loss of alkalinity is likely to occur concurrently with the excessive loss of gaseous material. Such a high loss of alkalinity causes the pH to drop below the critical level, so that the organisms on which the perpetuation of the desired digestion process depends are destroyed.



    Therefore, failure of the biological process would be the result of practicing the conventional process in this portion of the curve.



  It is in this range, however, that the "high yield" digestion operation works most effectively.



   Figure 1 is the flow diagram of a sewage treatment plant operating in accordance with the invention and in which a mixture of primary and secondary clarifier sludge is passed through a section of special thickening, the slurry from the thickener being transferred to the digester at intervals the frequency of which is regulated for a "high efficiency" digestion operation.
Figure 2 shows a curve relating to the operation
 EMI18.1
 of the thickening section.



   Figure 3 illustrates the efficiency curves of conventional digestion and "high yield" digestion, respectively.



   FIG. 4 is a semi-schematic vertical section of a digester equipped with special devices suitable for effecting a homogeneous mixture of the entire contents of the tank.



   Figure 5 is a plan view, with partial horizontal section, of the digester of Figure 4.



   Figure 1 is an example of a flow diagram of a sewage treatment plant according to the invention, that is to say in which mixed raw sludge which comes from a primary clarification operation and 'a secondary clarification operation (i.e. an activated sludge treatment) are introduced into a thickening section, the operation of which is carried out in a special way to the effect of constituting inside from the thickening tank a thickened sludge layer of controlled thickness which constitutes a reserve mass from which is taken a sludge of the desired high concentration, intended to be introduced into the digester at intervals the frequency of which is imposed by the requirements relating to the "high yield" digestion of this invention,

   with a view to the digestion efficiency curves which will be discussed in detail below.

  <Desc / Clms Page number 19>

 



   For example, high density raw sludge can be obtained by proceeding as follows: The operation of the thickening section is conducted so as to remove about half of the water, so that the volume of the resulting thick sludge which is admitted to the digester is approximately equal to that of the usual digested sludge.

   The subsequent "high yield" digestion treatment according to the invention results in a well digested slurry, in the absence of a "supernatant" layer in the digester. In order to obtain a raw sludge whose concentrations have the high values which are desirable for the practice of the invention, the operation of the thickening section is carried out in a controlled manner, which will be discussed below: The raw sludge is introduced into the thickening tank at a much higher degree of dilution than that to which it is normally desired to withdraw from the clarification operation.

   In other words, the primary sludge itself is constantly withdrawn from the primary clarifier at an unusually high dilution rate; or else is added to the primary sludge (or to the mixture of primary sludge and secondary sludge) a suitable diluent liquid, or what will be called hereinafter a regulating liquid.



   For example, such diluent regulating liquids may be of the nature of a liquor which has been subjected to an aerobic treatment, this liquor being derived from the secondary clarification operation (activated sludge) or may be fresh water, such as than river or city water.



   Raw sludge with a high degree of dilution is thus subjected to sedimentation accompanied by thickening carried out in the concentration section or tank for the purpose of establishing and maintaining the sludge layer or "reserve mass" from from which the digestion tank will be fed. The phase during thickening can thus be safely regulated so as to prevent septic decomposition in the sludge layer of the thickening tank. In other words, the mode of formation, the physical state and the thickness of the sludge layer formed at the bottom of the thickening tank can be adjusted so that an extremely concentrated thickened raw sludge can be removed from the base of the thickener. the layer of mud, without having to fear a significant degree of biochemical decomposition within this layer.

   By providing a sufficiently diluted raw primary sludge, or by adding to the raw primary sludge an adequate amount of diluent in the form of aerobic (secondary) treated sewage, it becomes possible to safely form a layer of sludge from which a very dense sludge can be extracted, provided that the residence time of the solid elements of the sludge layer is regulated in such a way as to prevent the biochemical decomposition of the sludge. For example, by diluting the primary slurry by the addition of adequate amounts of aerobically treated sewage water, it is possible to concentrate the layer with sufficient rapidity to prevent biochemical decomposition.

   By adjusting the degree of dilution or the amount of diluting regulating liquid (such as aerobically treated sewage) relative to the residence time of the thickened solids in the bed, a very dense raw sludge is obtained after sufficiently short residence time to prevent biological decomposition. This sludge layer thus constitutes a reserve mass from which raw sludge of the required concentration can be taken at intervals the frequency of which is dictated by the "high yield" digestion conditions of the present invention.



   Between certain limits there is a relationship between the residence time and the degree of dilution (or the added amount of aerobically treated sewage water), namely that the residence time required to obtain a raw sludge of the desired density is all the shorter as the added quantity of sewage water which has undergone aerobic treatment is greater. A measure of the dilution or amount of liquid

  <Desc / Clms Page number 20>

   Diluent regulator added to the raw slurry is the "liquor ratio" defined herein as the ratio of the volume of secondary sewage water (aerobic treatment) to the volume of primary slurry liquor (volume of primary liquor).



   The residence time of the solid elements of the sewage water in the thickener sludge layer is measured here in terms of "sludge volume ratio", which can be defined as the volume ratio of sludge maintained in the layer (measurable by its thickness) at the volume of thickened sludge removed from the bottom of the thickener per unit time.



   The operation of this section, or thickening tank, can be adjusted by setting the "liquor ratio" and "sludge volume ratio" (thickness of the sludge layer.) One by one. relative to each other such that the sludge derived from the layer for transfer to the digester has the density and uniformity to meet the needs of the "high yield" digestion of the present invention.



   As previously indicated, the sludge layer of the thickening tank constitutes the reserve mass from which the digester is intended to be supplied with sludge of the desired high density, at the necessary intervals. With a sufficiently high "liquor ratio", the thickening process can be accomplished reliably within an appropriate residence timescale, thereby absorbing the fluctuations which occur in the characteristics of the slurry. gross admitted,
The aforementioned thickening characteristics of the sludge are used to establish and maintain the reserve mass intended for the supply of the desired sludge to the digester,

   the latter having to receive a sludge of approximately uniform composition and of high density for the purposes of the present invention. During the development of the present invention, it has been established that when sufficiently large amounts of the diluent regulator liquid (or secondary sewage) are used, the sludge in the thickening tank exhibits characteristics of 'thickening (measured in% concentrations, plotted on the ordinate), e function of the residence times of the solid elements (plotted on the abscissa) which have a direct influence on the ability of the mass or reserve layer of sludge to absorb these fluctuations of food.



  When these thickening characteristics are thus represented in the form of a curve, it is observed that starting from the point of origin of the coordinate system, the concentration of the sludge withdrawn from the bottom of the tank and of the sludge layer s' rises sharply or "steeply" for relatively small increases in residence time, to a sharp bend resulting in a small radius curvature, then rises more slowly, the curve thus drawn being that relating to any liquor or sewage having an appropriate "liquor ratio". In other words, such a mud thickening characteristic curve consists essentially of an initial portion rising steeply, an intermediate bend, and a relatively flat or low-sloping end portion.

   We could thus represent a whole family of curves of the same model which would start from a common point of origin, each of these curves corresponding to its own dilution. In the case of a sufficiently high dilution, the curve is such that the thickening operation can be carried out reliably within a residence time scale (and with a corresponding thickness of the mass of reserve or layer of mud) which is located along the upper flat part of the curve following the sharp bend.

   In this way, the thickening section ensures flexibility of work, having regard to the absorption of the fluctuations which occur in the feed of the raw sludge admitted to the digester, without this advantage being obtained at the expense of a variation. excessive density of the thickened slurry which must be admitted to the digester to meet the "high yield" digestion conditions of the present invention.

  <Desc / Clms Page number 21>

 



   The thickening section can be operated so that not only the high density sludge is obtained, but also a relatively high quality overflow liquor, or effluent, corresponding to a high solids content. of the contents of the thickening tank. In other words, one applies essentially the maximum of dilution compatible with the maximum of rate of discharge which is admissible for the thickener itself, taking into account that this rate of discharge influences the quantity of solid elements conveyed by the effluent.

   Thus, the degree of dilution is chosen or adjusted so that it corresponds to the extent to which it is desirable that the thickening tank intercepts the solid elements on the one hand, and concentrates these elements, on the other hand; a relative maximum solids content, together with a high slurry density, can be obtained by adjusting or correcting the dilution, which minimizes the recirculation load of solids from the effluent returned to the apparatus. processing list for further clarification.



   With regard to the influence of the temperature and the biological characteristics of the sludge (s) to be thickened on the "sludge volume ratio", or residence time of the solid elements, it should be noted that an increase in the temperature of the sludge intensifies biological activity, which causes an increase in the amount of gaseous decomposition products and results in the sludge rising to the surface and a resulting decrease in the quality of the sludge. the effluent.

   However, the "sludge volume ratio" can be limited in accordance with the slurry temperature so as to obtain a thickener effluent of sufficient quality to meet the needs of the plant, even while maintaining a sufficient concentration. of the thickened slurry so as to meet the "high yield" digestion conditions of the invention.



   In order to adjust the thickening operation, a relative measurement of the thickness of the sludge layer can be made, for example using simple means consisting of a sufficient weight or test body. loaded and a device intended to lower this weight or this body into the thickening tank. When the body is lowered into the tank during operation, there is an apparent loss in body weight as it enters the mud layer, this loss being due to the fact that the density of the mud is high in comparison. with that of the body of sewage overlying the layer.



   A relative measure of the thickness of the sludge layer during the lowering of the test body can be obtained by measuring the distance between the point at which the weight loss takes place and the point at which, through suspension means, it is perceived that the test body comes into contact with the bottom of the tank.



   Figure 1 illustrates, as an exemplary embodiment of the invention, a sewage treatment plant which lends itself to different operating modes by which the results of thickening can be influenced or controlled. in the desired direction to adjust the previously defined "liquor ratio".



   According to a procedure for adjusting the "liquor ratio", the solid elements of the sewage subjected to the aerobic treatment at the thickening section, to be thickened together with the solid elements. of the primary sludge, are taken partly from the lower stream of the primary clarifier and partly, although to a lesser degree of concentration, directly from the aerobic treatment section.

   The solid elements which have been subjected to the aerobic treatment, and which will also be called hereinafter "secondary solid elements", can be taken in part (or even entirely) directly from the aerobic treatment section, while a corresponding portion

  <Desc / Clms Page number 22>

 secondary clarifier sludge (in part or in whole) is returned to said section. Thus, the regulating liquor for adjusting the "liquor ratio" can be taken directly from the aeration section or / and from the secondary clarifier slurry.

   In this way, it is possible not only to ensure the adjustment of the "liquor ratio" itself, but also to exert, simultaneously with this adjustment, an influence or a regulating effect on both the chemical and biological characteristics of the liquid. the regulating liquor and the solid elements it contains.



   Another procedure for adjusting the "liquor ratio" according to the diagram of Figure 1 is to divert a fraction of the secondary clarifier effluent for use as an auxiliary liquid to be added to the secondary sludge portion. which should be sent to the thickener; or to use a portion of the sewage water subjected to the aerobic treatment and withdrawn from the aerobic treatment section, together with (or diluted by) a portion of the secondary clarifier effluent, to send it to the section thickening.



   The installation of Fig. 1 comprises a primary clarifier tank 10, an aerobic processing section 11, a secondary clarifier tank 12 and a thickener 13. The primary clarifier tank 10 has been shown schematically as the well-known type working. in a continuous fashion which has a feed well 14, a rotary raking structure 15, an effluent overflow channel 15a and a lower slurry discharge cone 16. The secondary clarifier tank 12 and the thickener tank 13 may similarly include a feed well, a raking structure, an effluent channel and a lower slurry discharge cone.



   Raw sewage water is commonly admitted to the primary clarifier tank 10 through a supply pipe 18 leading to the well 14 of the clarifier. The primary clarifier sludge, the solids concentration of which is approximately 2%, is discharged through a primary slurry pipe 19 fitted with a valve 20. The primary effluent, containing in suspension solids which hitherto cannot be settled , passes through a pipe 21 going directly from the primary effluent channel 15a to the aerobic treatment section 11, which is shown in this example as an aeration tank although it may consist of any other type of biological treatment apparatus, for example in a "biofilter" "for carrying out the desired aerobic treatment of the primary effluent.



   The aerobically treated sewage water passes from the aeration section 11, through a pipe 22, to the supply well of the secondary clarifier tank 12. An overflow discharge pipe 23 transfers the secondary effluent. of the effluent channel of this tank. The lower secondary sludge stream from the secondary clarifier tank 12 goes through a sludge discharge pipe 24 provided with a tap 25 to a junction point 26 from which a branch 27, provided with a control cock 27a, leads to the effluent end of the aeration tank, another connection 28, provided with a control valve 28a, leading to the supply well of the thickener 13.



   The secondary effluent pipe 23 is provided with a branch 29 on which is mounted, 'a control valve 30 and which terminates at a junction 31 connected to the branch pipe .28 in order to supply this pipe an adjustable volume of secondary sewage. A pipe 32 goes from the junction point 31 to another junction point 33 where it connects to an auxiliary pipe 34 starting from the aeration tank 11 and provided with a control valve 35, this pipe 34 serving to supply the section thickening 13 an adjustable volume of sewage subjected to aerobic treatment. In

  <Desc / Clms Page number 23>

 in other words, a pipe 36 runs from junction 33 to a junction 37 to which the primary slurry pipe 19 connects. A slurry mixture of the appropriate "liquor ratio" is then admitted to the thickener 13.



   The "liquor ratio" can thus be adjusted and regulated, in particular by supplying part of the secondary sludge to an additional portion of secondary liquor, and this in the form of a slurry which has been subjected to the aerobic treatment which is derived from the. aeration tank 11 or / and in the form of a relatively clear secondary effluent from the clarifier 12.

   A feed pipe 38 from the junction 37 supplies the slurry mixture to the feed well of the thickener 13; that is to say that the thickener 13 is supplied with a mixture of primary sludge and secondary slurry, together with a regulated proportion of secondary liquor, also called "liquor or regulating liquid:" According to the operating conditions installation and local factors, as well as depending on the degree of dilution at which the secondary sludge is discharged from the secondary clarifier the "liquor ratio" itself can vary upwards and reach 10:

     1, or even a higher value, provided that the operation is carried out in accordance with the principles of the invention, to obtain the thickened mixture of sludge having the remarkably high content of solid elements and other characteristics already mentioned.



   The junction point 37 of the primary slurry and the secondary slurry can take the form of a pre-mixing tank which can be fitted with a suitable mixing device, such as a mechanical or compressed air mixer.



   A return pipe 39 provided with a control valve 39a and leaving the effluent channel of the thickener 13 opens into the inlet pipe 18 of raw sewage supplying the primary clarifier 10. There is also provided a pipe 39b ,, provided with a control valve 39c, and which is intended to allow the optional discharge and rejection of the effluent from the thickener 13, or its recycling to the primary clarifier or to the apparatus secondary treatment.



   A bypass pipe 40, provided with a control valve 41, leaves from a connection point 43 on the auxiliary pipe 34 in order to make it possible to take a portion of the aerated sewage water of your choice, either directly from a point of the tank 11 itself, or from the effluent from this tank.



   The thickened sludge coming from the thickener 13 passes through a pipe 44 which is provided with a control valve 45, and allows the transfer of thickened raw sludge to a closed digestion tank 46, itself provided with a. lower starting pipe 47 allowing digested sludge to be removed, this pipe being provided with a control valve 48. This digester may be of a conventional type equipped with means suitable for collecting the gas and for heating the sludge.



   The thickened sludge coming from the thickener or sludge layer tank 1.1-is transferred to the digester 46 in a quantity and at set intervals, such an adjustment of the sludge supply to the digester 46 being provided by a sludge transfer pump motor-driven P, combined with a timer K. The latter may include a device for automatically starting the pump at predetermined intervals and for keeping the pump in operation for predetermined periods of time.



    A heavy weight or test body, indicated in B1 and provided with suspension means M, is used to measure the level of the reserve layer or mass of mud, for example between the position marked in solid lines and the position marked in mixed lines of the test body.

  <Desc / Clms Page number 24>

 



   Figures 4 and 5 show on a larger scale, although partially schematically, a "high efficiency" digester comprising a closed digestion tank. 49 provided with a slightly conical bottom 50, a cylindrical wall 51 and a cover 52. The mass of mud being digested in the tank is indicated by a level 53. The feed pipe of raw mud digester is indicated at 54, and a discharge pipe 55 of the digested sludge is indicated at the base of the conical bottom 50.



  A gas outlet pipe is indicated at 56. A device for externally mixing the digested sludge with fresh raw sludge is indicated by a pump 57 serving to recirculate the digested sludge coming from the bottom of the tank, or well again by a bypass duct (which has not been shown specifically) starting from the discharge pipe 55 and connected, via a return pipe 58, to the raw sludge supply 54.

   To obtain a homogeneous mixture inside the mass of digestion sludge, internal mixing devices have been shown here in the form of vertical shafts driven individually, each of these shafts being pôurvu groups of horizontal vanes or fins, or d 'equivalent organs, serving to communicate to the mass of mud movements suitable for agitating and mixing it in transverse or horizontal zones. As shown, there are six main vertical shafts 59,60,61, 62,63 and 64, each of these shafts being provided, as shown, with one or more groups of horizontal pallets 65.

   The paths described by the vanes of adjacent shafts can be made to partially overlap in order to ensure a more efficient mixing action; or they can be staggered vertically and arranged in such a way as to avoid the mutual discomfort which may be caused by adjacent pallets. Each of the vertical shafts is provided, as shown, with a guide bearing 66 at its upper end and with a clamp 66a at its lower end. Each of these shafts is further provided, as shown, with a separate drive motor 67 and a reduction gear, shown herein as a worm gear 68.

   The mounting of the sliders 66a and of the drive motors 67 takes place by means of a platform 69 surmounting the cover 52 of the tank. Various control valves are indicated at 70, 71, 72, 73. Means (not particularly shown) can be provided if necessary to break up and redisperse the floating scum which may form on the surface of the mass of sludge. during digestion.



   The primary sludge derived from the clarifier 10 of FIG. 1 may, for example, have a concentration of the order of 2%, the secondary sludge derived from the secondary clarifier 12 may have a concentration of the order of 0.1%, and the density of the sludge derived from the thickener 13 may be of the order of 5 to 9% for a mixture of primary sludge and activated sludge which can be obtained with a "liquor ratio" of the order of 15 : 1 or above.



   FIG. 2 illustrates the curves representing the thickening operation carried out in the tank 13, in concentration percentages (plotted on the ordinate) as a function of the "residence time" of the solid elements (plotted on the abscissa), the 0-ab curve (marked in solid lines) showing an example based on a certain liquor ratio, the 0-a'-b 'curve (broken lines) relating to a higher liquor ratio, and the 0-a "-b" curve (mixed lines) pertaining to a lower liquor ratio.

   The thickener can work within the limits of the sludge concentration and residence time scales defined by the extreme flat or low rise portions ab, a'-b ', a "-b", respectively, of so as to absorb the above-mentioned fluctuations, while providing thickened slurry sufficiently concentrated to satisfy the desiderata of the "high yield" digestion of the present invention.

  <Desc / Clms Page number 25>

 



    Figure 3 shows the efficiency curves C1 and C2 of the digesters, based on the 'digester index' referred to in connection with the mixture of primary sludge and activated sludge, namely an index of 40% per day. in the case of the curve C1, which represents "the efficiency of classical digestion" y and an index of 120% per day
 EMI25.1
 in the case of curve C2, which represents "Inefficiency of high-yield digestion", these curves having both been drawn in the same coordinate system, with the percentages of the total volume of gas generated available plotted in ordinates and the digester volumes measured in cubic decimeters per inhabitant plotted on the abscissa.



   Curve C1 is based on the "digester index" equation G.Q./g.V. x 100 = digester index in% per day = 40% per day (as explained previously). This curve has been plotted in Figure 3 using the figures taken from this equation and shown in Table T1 below, these figures being based on the assumption that the conventional operation of the installation requires 116 cubic decimeters of volume of digester per capita, equivalent to a residence time of 60 days, under a supply of 1.94 cubic meters of pumped raw sewage water per thousand inhabitants per day. Table T1 relating to the curve C1 gave below:
Table T1 CALCULATIONS RELATING TO THE CLASSIC DIGESTION OF PRIMARY AND ACTIVE SLUDGE.



   G.Q./g.v. x 100 = digester index = 40% per day.
 EMI25.2
 dm3 / inhabitant Duration of 2.5G, = dm3 per day corresponds to the digestion- 7Less of gas stay = loss of gas ### = loss of days of stay in dm3 by ¯¯¯ā¯¯ loss of days in days day gas 126.7 66 2.5G - 0.038G 0 0 G - 317%
 EMI25.3
 
 <tb>
 <tb> 66 <SEP> 1.038G
 <tb> 116 <SEP> 60 <SEP> 2.5G <SEP> 0.042G <SEP> 0.042G <SEP> = <SEP> 4
 <tb> 60 <SEP> 1.042G
 <tb>
 
 EMI25.4
 99 51 2. 5G - 0.049G ± 0 %% = 4, 7 ru
 EMI25.5
 
 <tb>
 <tb> 51 <SEP> 1; 049G
 <tb> 84.9 <SEP> 44 <SEP> 2.5G <SEP> = <SEP> 0.057G <SEP> 0.057G <SEP> = <SEP> 5.4 <SEP>%
 <tb> 44 <SEP> 1, <SEP> 057G <SEP>
 <tb>
 
 EMI25.6
 70;

  1 37 2.5G = 0.068G o, o68G 6, 37 1, o6G 5h $ 6 29 2.5G = 0.086G 0 "o6G - z 29 1.086G ze 22 2 G - 0.113G 0.113G z
 EMI25.7
 
 <tb>
 <tb> 22 <SEP> 1.113G
 <tb> 28.3 <SEP> 14.6 <SEP> 2.5G <SEP> = <SEP> 0.171G <SEP> 0.171G <SEP> = <SEP> 14.6 <SEP>% <SEP>
 <tb> 14.6 <SEP> 1.171G
 <tb>
 
 EMI25.8
 14.2 7.3 2 G - o, 344G 0.3 = 25.6%
 EMI25.9
 
 <tb>
 <tb> 7.3 <SEP> 1.344G
 <tb>
 
 EMI25.10
 7.1 3.65 2G = 0.69G 0.69G = 41%
 EMI25.11
 
 <tb>
 <tb> 3.65 <SEP> 1.69G
 <tb>
   The "conventional digestion efficiency curve" C1, thus established comprises an initial portion P1 rising steeply from the point of origin and the slope of which is designated by g1 a convex portion or intermediate curvature p2,

   which connects to the upper end of

  <Desc / Clms Page number 26>

 the portion p1 and whose degree of convexity is approximately indicated by the radius L1; an extreme portion p3 which connects to the outer end of the elbow p2 and which is almost flat but slightly convex and tends towards the horizontal. In accordance with this curve C1, the conventional operation of the digester is conducted and is maintained within a region or scale R located near and along the outer parts of the slightly convex portion P3 tending towards the horizontal, said parts corresponding to a scale of approximately 99 to 142 cubic decimeters of digester volume required per capita (measured on the abscissa) with a gas loss of approximately 2% to 3% (measured from top to bottom on the ordinate).

   This graphical representation shows that the classical working scale is and should be located a much greater distance from the point of biological failure of the digester, which would be located approximately where the curvature p2 connects to the upper end. of the rising portion p1 of the curve C1.



   Curve C2 is derived from the G.Q./g.v digester index equation. x 100 = digester index in% per day = 120% per day, as mentioned previously. This curve has been plotted in FIG. 3 with the aid of the figures, taken from table T2 which will be found later, representing this equation; and it is based on a digester volume (measured in dm3) required per inhabitant equal to 58 (that is to say 116 dm3) per inhabitant, after elimination of 50% of water by operation 2 of thickening . Table T2 is given below:
Table T2 CALCULATIONS RELATED TO THE HIGH EFFICIENCY DIGESTION OF A THICK MIXTURE OF PRIMARY AND ACTIVE SLUDGE.



   G.Q./g.v. x 100 = digester index = 120% per day.
 EMI26.1
 
 <tb>
 <tb>



  Duration <SEP> 0.883G <SEP> = <SEP> dm3 <SEP> by <SEP> day
 <tb> dm3 / inhabitant <SEP> of <SEP> the <SEP> Time <SEP> of <SEP> = <SEP> loss <SEP> of <SEP> gas <SEP> g <SEP> = <SEP> loss <SEP> of
 <tb> diges- <SEP> stay <SEP> in <SEP> dm3 <SEP> by <SEP> g <SEP> + <SEP> G <SEP> gas <SEP>% <SEP>
 <tb> matches <SEP> to <SEP> tion <SEP> in <SEP> in <SEP> days <SEP> Day
 <tb> days
 <tb> 116 <SEP> 120 <SEP> 0.883G / 120 <SEP> = <SEP> 0.007G <SEP> 0.65 <SEP>%
 <tb> 59 <SEP> = <SEP> 60 <SEP> 0.883G / 60 <SEP> = <SEP> 0.0135G <SEP> 1.3 <SEP>%
 <tb> 42.4 <SEP> = <SEP> 44 <SEP> 0.883G / 44 <SEP> = <SEP> 0.019G <SEP> 1.9 <SEP>%
 <tb> 28.3 <SEP> = <SEP> 29 <SEP> 0.883G / 29 <SEP> = <SEP> 0.029G <SEP> 2.8 <SEP>%
 <tb> 21.75 <SEP> = <SEP> 22 <SEP> 0.883G / 22 <SEP> = <SEP> 0.037G <SEP> 3.6 <SEP>%
 <tb> 14.2 <SEP> = <SEP> 14.6 <SEP> 0.883G / 14.6 <SEP> = <SEP> 0.057G <SEP> 5.4 <SEP>%
 <tb> 7,

  1 <SEP> = <SEP> 7.3 <SEP> 0.883G / 7.3 <SEP> = <SEP> 0.114G <SEP> 10.3 <SEP>%
 <tb> 0.362 <SEP> = <SEP> 3.65 <SEP> 0.883G / 3.65 <SEP> = <SEP> 0.228G <SEP> 18.6 <SEP>%
 <tb>
 

  <Desc / Clms Page number 27>

   The "high efficiency" digestion curve C2 thus plotted, although similar to the curve C1 and emanating from the same point of origin of the coordinate system, has much more pronounced characteristics, in that it has an initial rising portion. q1 whose slope is much steeper than that of curve C, as indicated by angle g2, and which rises higher than the portion of curve C1;

   an intermediate bend or curvature q2, which is sharper or has a smaller radius of curvature, as indicated approximately by a radius L2 which is significantly smaller than the radius L1 of the corresponding portion of the curve C1; and an extreme portion q3 which is located slightly above the corresponding extreme portion p3 of the curve C1 and which is still a little flatter or closer to the straight line than the latter.



  Based on this curve C, the "high yield" digestion operation is conducted and maintained at: Inside a region or R2 scale located an appreciable distance (measured horizontally) from the R1 scale of the curve C1. In other words, the R scale, which roughly corresponds to a scale of 4.24 to 28.3 cubic decimetres of digester volume per capita (on the abscissa) is placed much closer and in reality in the vicinity of the bend q2 , that is to say in the vicinity, although a little below, of the point of biological failure of the digester which would be located approximately at the place where the upper end of the rising portion q, connects to the elbow intermediate 22 of curve C2.



  While the lower limit of the R scale must necessarily be situated below the point of biological failure of the digester, the upper limit depends on the "loss of gas" which one considers to be admissible or acceptable in the residual gasifiable material of the digester. digested mud. Preferably, this point will be located at an appreciable distance from the lower limit of the scale R1.



   Note, in the graph of Figure 3, that the extreme left portion of the scale or work area R2 of the "high efficiency" curve is located above and beyond a portion of. the classic efficiency curve where, in fact, the failure of the classic operation has already occurred.



   In view of considerations relating to the efficiency of digestion and on which the present invention is based, the meaning of the term "loss of gas" (g) will also be considered in relation to the practice of conventional digestion, that is, that is, the gas gate so considered relates to the portion of the gasifiable material contained in the "high yield" digested slurry which would have been converted to gas in the conventional two stage digestion previously described; in other words, the loss of gas can be considered as being the quantity of gas generated by the conventional digester apparatus, reduced by the quantity of gas generated by the "high yield" digestion according to the invention.



   The invention is not limited to the example of a sewage treatment plant which has been shown in Figure 1, as regards the way in which the dilute sludge is derived, that is to say say that the dilute sludges intended to be subjected to the 'high-yield' digestion according to the invention, following their specially controlled thickening treatment, can come from any treatment (physical, chemical or biological) of raw sewage water or any suitable combination of these various treatments.

   For example, the diluted sludge can come only from a primary sedimentation treatment; can come from such treatment combined with chemical treatment; can be derived from a mixture of primary sedimentation sludge and sludge from sewer filters and their modifications; it can also be derived from the biological treatment of raw sewage in the case where no primary sedimentation is applied but the raw water

  <Desc / Clms Page number 28>

 is subjected to an aeration treatment followed by clarification and where the biological sludge leaving the clarifier constitutes the sludge which must be treated in accordance with the invention.



   It should be understood that the scale of the per capita digestici tank volume requirements, in the present "high yield" process (see the R scale of Figure 3 relating to the example of the digestion treatment of a mixture of primary sludge and secondary sludge) should be regarded not as having absolute limits, but as giving reasonable approximate values. In addition, the limits of the R2 scale may vary depending on the type of raw sludge treated. For example, the relative lower limits of the working scale R of figure 3 could be given an even lower value than that given for the example of figure 3 of the treatment of a sludge mixture, in the case of the treatment of d 'a primary mud alone.



   Summary.

** ATTENTION ** end of DESC field can contain start of CLMS **.


    

Claims (1)

I- Sewage treatment process, characterized by the following points, separately or in combinations: 1-0 - It consists in subjecting the sewage water to clarification in a clarification tank out of which a clarified effluent is discharges through an overflow, while the sludge is withdrawn from the lower portion of the tank, to subject this sludge to a concentration markedly increased beyond that which can be obtained by clarification, and to sub- putting the sludge thus concentrated for anaerobic digestion in a digester tank; 2 - The concentration of the clarifier sludge is carried out by a regulated thickening inside the thickening tank; 3 - The concentrated sludge is introduced into the digester tank in an essentially continuous manner or at relatively short intervals; 4 - The contents of the digester tank are maintained in an essentially continuous state of agitation and mixing; 5 - The concentration of the sludge admitted to the digestion tank is high enough to essentially eliminate the dehydration treatment of the digestion sludge; 6 - It consists in subjecting the diluted raw sludge to a controlled dehydration treatment having the effect of raising its concentration to a remarkably high degree and of constituting a reserve mass of raw sludge of high density, intended to absorb the fluctuations in l 'raw sludge feed and from which a high density sludge is intended to be fed at the digestion stage, at the desired frequencies or continuously, to subject this high density sludge to anaerobic digestion in a digestion section containing a mass of sludge during digestion, this treatment consisting in maintaining said mass of sludge during digestion in a state of substantially homogeneous mixture which ensures the establishment and maintenance, in all parts of said mass, of a substantially uniform distribution of digestible material as bacterial food, in transferring said high density sludge from the reserve mass to the digestion mass, at sufficiently high frequencies, relative to the solid element load imposed per unit volume of the digestion mass, to ensure the nutritive supply of bacteria and consequently a high bacterial population and free from inadmissible fluctuations, the relations between the degree of concentration of the feed sludge and the frequency of feed, on the one hand, and the high load of solid elements, on the other hand, being such that they ensure a high alkalinity content of the digestion mass by the effect of said degree of concentration; power frequency <Desc / Clms Page number 29> tation, the degree of homogeneity of the mixture, the degree of concentration of the feed slurry being factors which are all coordinated in such a way that it is possible to obtain a stable digester operation even if the feedstocks 'solid elements are notably high, the scale of these high loads in solid elements, expressing the efficiency of the digester, being able to be defined by a curve indicating the relative quantities of gas generated (in ordinates) as a function of the loads of the digester (in abscissa), said curve comprising an initial portion rising steeply, an intermediate transition bend or curvature comprising what represents the state of failure of the digester at the location connecting to the portion rising steeply, and a flat and relatively horizontal portion extending from said curvature, said scale of solid elements loadings and digester efficiency comprising a portion of said curve which extends relatively near the point of failure, so so that a relatively maximum volume of gas can be obtained in a minimum volumetric digestion space, while digested sludge is removed from the digestion mass, and so that, as a measure, of the efficiency of digestion, a relatively maximum value of the expression GQ digestion index in%, per day = GQ / gV x 100 g. v. in which g = quantity of gas likely to be generated in the digester itself, although lost by release from the digester (in cubic decimeters per day), V = volume of the digester in cubic decimeters, G = quantity of gas produced by the digester in cubic decimeters, per day, Q = volume of raw sludge introduced into the digester by pumping, in cubic decimeters, per day, and in which, in addition, g represents the quantity of gas liable to be generated in the digester, although lost, for each cubic decimetre of the sludge removed from said mass, and Vg represents the "gas potential" or quantity of gas which can be produced. Q picked from a gasifiable material accumulated at all times in said mass; 7 - The digestion carried out inside said mass is carried out in such a way that the digestion index is notably greater than 40% per day, and preferably in such a way that said index is of the order of 120% per day; 8 - The digestion is carried out inside said mass so that the load of the digester is notably less than 99 cubic decimetres per inhabitant, when the material subjected to digestion is composed of a mixture of primary sludge and of activated sludge; 9 - The digestion is carried out inside said mass so that the load of the digester is of the order of a few cubic decimeters per inhabitant. In appendix: 3 drawings.