DE2805054C3 - Process for breaking down sludge - Google Patents

Description

25th

The invention relates to a method for breaking down sludge, in which at least part of the sludge passed through at least two covered mining zones, with the formation of stabilized sludge and Methane gas is exposed to aerobic and anaic conditions in succession.

For the degradation of sludge, three methods are currently used on a large scale in practice: namely degradation in oxidation ponds, anaerobic degradation and aerobic degradation.

Oxidation ponds require substantial land areas and do not allow significant degradation of the Amount of pathogens in the mud.

The most widespread is the anaerobic breakdown of sludge, such as is extracted from settling tanks, biological filters and wastewater aeration systems. The sludge is collected in large extraction rooms, where it is fermented anaerobically for 20 to 30 days without heat. The breakdown transforms the solid residues into a sufficiently stable form so that they can be used for landfilling without major nuisance. The volume is mixed by mechanical agitators or circulated compressed decomposition gas in order to increase the sludge stabilization reaction. It is known that elevated temperatures in the mesophilic range of 30 ° C to 4O ⁰ C allow a shortening of the residence time at 12 to 20 days because the activity of the responsible for the degradation of organisms is greatly influenced by the temperature and is between 30 ° C and 40 ⁰ C highly active mesophilic microorganisms represent the dominant microbe strain in the sludge to be degraded. The lowest temperatures for mesophilic degradation between 35 ° C and 38 ⁰ C be> minimum residence times from 12 to 15 days. W ^: ck. Anaerobic digestion produces methane gas, which is commonly used in combustion heaters to compensate for heat loss from the anaerobic digestion system. Seasonal temperature fluctuations and fluctuations in the concentration of suspended solids in the inflowing sludge have a considerable influence on methane gas production and the amount of heat required. Therefore, if elevated temperature conditions are to be maintained in the anaerobic degradation zone throughout the year, an auxiliary heating system is usually provided. A sludge that is as concentrated as possible is expediently fed to the extraction zone in order to minimize heat losses in the outgoing stabilized sludge flow and to maximize methane production in the extraction zone. Even with such precautions, it is difficult to maintain elevated temperatures in the anasrobic mining zone in an economical manner, especially during the winter months.

In the anaerobic digestion process, the treated sludge solids become essentially one at a time exposed to three separate treatment phases, initially to a solubilization period, then a period of intense acid production, and finally a period of intense degradation and stabilization (gasification). Each of these process stages is due to the production of different ones Intermediate and end products labeled Under normal working conditions, all three phases occur at the same time. The gases generated during the gasification phase are mainly methane and carbon dioxide. They usually make up more than 95% of the evolved gas, with 65 to 70% being methane available. Methane production is due to the breaking down of numerous compounds by many of one another dependent biochemical reactions. In particular, the complex organic Sludge components by bacteria known as acidifiers without the formation of methane in volatile acids and alcohols, which are then converted by other bacteria known as methane generators converted into methane gas. The acid-forming bacteria are robust and resistant to process changes highly resilient. Moderate temperature fluctuations change their metabolic activity not worth mentioning. Methane-forming bacteria, on the other hand, require strictly anaerobic conditions; they are against process changes, such as fluctuations in the pH value and the presence of heavy metals, Detergents, ammonia and sulphides, extremely sensitive. Even very small amounts of toxic Metals such as copper prevent the activity of methane generators. The acid generators begin to dominate. The pH value drops, which also affects the activity of the methane-forming agents. Also may there will be no oxygen in the anaerobic degradation zone. Unintentional introduction of air into them Zone interferes with methane fermentation and leads because of the combination of combustible methane gas with oxygen creates a risk of fire or explosion. The methane generators are also particularly sensitive against temperature fluctuations. Even fluctuations of just a few degrees reduce theirs Activity and viability; they result in a relatively excessive growth of acid generators. The acidic degradation intermediates accumulate. In this case, too, the pH value drops in the mining zone. The activity of the methane generator is further reduced. This leads to serious process disturbances. An insufficiently stabilized sludge is obtained which cannot be used without further treatment Land replenishment or suitable for similar purposes.

In order to enliven the described disturbances in the anaerobic degradation zone. often go to this zone

Large amounts of lime introduced, which strengthen the buffer effect and increase the pH value should. However, this corrective action is usually only suitable for short-term fluctuations or Process disturbances. It usually does not offer any advantages in the case of long-term fluctuations or disturbance conditions step out. In addition, the methane generators have a relatively low growth rate, what makes a long residence time necessary even at mesophilic temperatures. Because of the low At the rate of growth there is a risk that the methane-forming organisms will be washed out of the degradation zone if the sludge solids residence time is reduced below the specified minimum value. The lengths Dwell times require a large capacity of the mining zone.

The aerobic degradation of biodegradable sludge can also be accelerated by increased temperatures. When the temperature rises from 35 ⁰ C, the population of the mesophilic microorganisms decreases, while the thermophilic forms increase. In the thermophilic range of 45 ° C to 75 ° C, thermophiles predominate; most of the mesophiles are destroyed. Above this range, the thermophiles decrease. At 9O ⁰ C, the system becomes essentially sterile. Due to the faster oxidation of the sludge, a more complete elimination of biodegradable volatile suspended matter (VSS) is achieved within the same degradation time than at ambient temperature. A more stable residue is obtained which can be disposed of without inconvenience. Thermophilic degradation effectively reduces or eliminates pathogenic bacteria in the sludge, thereby avoiding potential health hazards.

When using diffusion air systems, the heat losses are usually very large. As a result, aerobic degradation with air is typically limited to the exploitation of mesophilic microorganisms. Because air contains only 21% oxygen and only about 5 to 10% of the oxygen is dissolved, a very large amount of air is necessary to meet the oxygen demand. The sensible heat of the stale air and the latent heat required to saturate the stale air with water vapor are significant. At ambient temperature (2O ⁰ C or less), the sludge residence time in the case of an aerobic degradation with air typically 12 to 20 days; Very large containers are required to hold the sludge. As a result of the heat losses, autothermal heat effects are low. An uneconomical large amount of external heat is required to keep the temperatures at usable levels.

If, as is known, instead of air, an oxygen-enriched gas or pure oxygen is used, is the amount of gas that is fed into the aerobic decomposition zone and leaves it, significantly less than air. Heat losses due to the sensible warming up of the gas and the Water evaporation in the gas are reduced to such an extent that the autothermal heat alone increases the temperature Keeps noticeably higher than the outside temperature The degradation zone can therefore be in the thermophilic temperature range work effectively with minimal supply of external heat. Because the thermophilic stabilization Much faster than a mesophilic stabilization takes place, the necessary residence time in the aerobic degradation zone greatly reduced. This allows the use of smaller pools, which reduces heat losses further depressed. A reduction in biodegradable volatile suspended matter from 80 to 90%, for example, can be of the order of magnitude with comparatively short sludge retention times 3 to 10 days can be achieved.

Despite its considerable attractiveness, the thermophilic aerobic degradation is compared to an anaerobic one

ίο dismantling associated with disadvantages. Because the thermophile aerobic degradation process is oxidizing, a bio-oxidation reaction gas is formed, the carbon dioxide and Contains water vapor that can no longer be used. In contrast, anaerobic degradation produces methane gas as a reaction by-product. In addition, the aerobic degradation zone requires a much larger one Energy expenditure for mixing and contacting gas and sludge than is the case with the anaerobic Mining system for mixing the contents of the mining zone is necessary.

It is also known (DE-OS 15 84 958) to be dismantled Sewage sludge in such a way that successively aerobic and anaerobic conditions expose the sludge is introduced into a first digestion tower above, which is aerobic in its upper part and in which the Degradation conditions become increasingly anaerobic downwards, as well as that the sludge at the transition point withdrawn between the aerobic and anaerobic zones and on top for anaerobic degradation a second digester is abandoned. The anaerobically degraded sewage sludge is then in the middle taken from the second digestion tower and in the same way by a third and possibly further ones Digestion towers passed, with the bottom of each digestion tower essentially completely degraded sludge is deducted. In this way, the humic substances remaining in the usual sludge digestion should also be complete be decomposed. The residue should no longer be flammable and predominantly consist of mineral components be composed. In order to maintain the aerobic conditions in the upper part of the first digestion tower, a well-aerated sewage sludge is used. The dismantling then takes place in this digestion tower down through facultative aerobic, facultative anaerobic to purely anaerobic degradation. The removal of the aerobically putrefactive substance from the middle part of the first digestion tower takes place while avoiding a Whirling around and mixing with dead matter. It follows that in the first digestion tower itself no ventilation gas is blown in and may not be blown in, because otherwise in the Digestion tower together with the methane rising from the lower anaerobic part of the digestion tower an explosive gas mixture is formed. The well-aerated sewage sludge loses in the first one Digestion tower quickly changes its aerobic character. A primary sewage sludge, as is the case with the known method is removed upstream of the first digestion tower arranged primary clarifier, namely has a Oxygen uptake rate of about 200 to 300 mg / l / h. If now the one approaching the first digestion tower Sludge has a content of dissolved oxygen of 9 mg / 1, which is the case with conventional fumigation Air corresponds to the saturation value, the oxygen content of the sludge is consumed within a sludge retention time of only a little more than 2 minutes This is extremely short compared to a total dwell time of months for the known

Process is necessary to completely decompose the humic substances. The known method is as a result essentially suffers from the same problems as those described above in connection with the pure anaerobic degradation processes are discussed. In the period that followed, experts actually did combined aerobic and anaerobic sludge stabilization not taken up; rather it was in practice too continued to consider aerobic or anaerobic treatment alone. ι ο

The invention is based on the object of a sludge degradation process with successive To create aerobic and anaerobic sludge treatment in which a significant reduction in treatment time is achieved with complete stabilization with the least possible expenditure of energy.

This object is achieved according to the invention in that

a) in a first covered degradation zone, the sludge and a sludge containing at least 50% by volume of oxygen Aeration gas introduced and while maintaining a content of the mixed liquid dissolved oxygen of at least 2 mg / 1 and a total suspended matter concentration of the Sludge of at least 20,000 mg / l are mixed,

b) the sludge in the first degradation zone during the mixing process at a temperature within the thermophilic Range of 45 ° C to 75 ° C is maintained,

c) mixing in the thermophilic temperature range for a sludge residence time of 4 to 48 hours with a partial reduction in the content of the sludge introduced into the first mining zone biodegradable volatile suspended matter is continued,

d) partially stabilized sludge and oxygen-depleted degradation gas with an oxygen content of at least 21% from the first extraction zone are carried out separately and

e) the partially stabilized sludge from process stage d) in a second covered mining zone at a temperature of 30 ° C to 60 ° C under anaerobic conditions for a solids residence time from about 4 to 12 days and which is sufficient to maintain the content of the sludge biodegradable volatile particulate matter to below approximately 40% of the content of the Process stage a) of biodegradable volatile sludge introduced into the first degradation zone Lower suspended solids further.

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becomes one in comparison to the known combined aerobic and anaerobic sludge degradation process achieved quite unusual stability of the overall process. This surprisingly allows one Considerable reduction of the treatment time with complete stabilization or pasteurization of the Mud. In particular, the thermophilic aerobic degradation stage due to the heat content of the eo aerobic, partially stabilized sludge flow directed to the anaerobic stage contribute more than enough heat, to thermally stabilize the anaerobic stage regardless of climatic conditions. Temperature disturbances in the anaerobic zone can therefore be practically eliminated by using parameters such as Sludge retention time in the aerobic zone, the solids content of that fed into the aerobic zone

35

40

45

50 sludge and the degree of heat exchange due to which the feed sludge is heated prior to entering the aerobic zone. In this way, the anaerobic degradation zone can typically be operated independently of seasonal outside temperature fluctuations at a temperature which does not deviate from the optimal temperature state by more than 1 degree. Furthermore, the tolerance to toxic metals in the supplied sludge is increased. This is probably due to the fact that the metals only have a toxic effect in a soluble ion form, but the metal ions form harmless complex compounds in the aerobic decomposition stage at the intended sludge retention time and the thermophilic temperature for the anaerobic stage. The thermophilic aerobic degradation stage is also able to absorb sporadic disturbances, for example shock loads, without the efficiency of the process suffering. If a conventional anaerobic degradation zone suddenly experiences a high level of solids, solubilization and acidification take place at a faster rate than the methane-producing bacteria can use the acidic intermediates. As a result, acidic components accumulate in the degradation zone. The pH falls. The content of the mining zone threatens to become acidic. In the present process, the upstream thermophilic aerobic stage not only promotes rapid solubilization of biodegradable material in the sludge; rather, this stage also tends to reduce the population of mesophilic acid-producing bacteria. The aerobic degradation stage then allows these organisms to grow again. However, their population is smaller and more in balance with the population of methane-producing organisms. Therefore, when a shock load occurs in the aerobic zone, there is a rapid solubilization in the aerobic degradation zone and a stabilization of the most volatile parts of the sludge, whereby the shock load is smoothed and its influence on the downstream anaerobic zone is greatly reduced. During further treatment, the anaerobic zone absorbs a partially stabilized sludge in which the acid-forming and methane-forming bacteria can grow in mutual equilibrium.

The substances that interfere with anaerobic digestion are preferentially oxidized in the aerobic stage. Because the high temperature in the aerobic stage, at which nitrifying bacteria are not viable, appears the released nitrogen as ammonia, which together with the released CO2 Ammonium bicarbonate forms an excellent buffer medium for those facing changes in pH represents extremely sensitive anaerobic level compared to a conventional, purely anaerobic Mining rich mining zones with much lower, z. B. amounting to only 60% of the value of known systems Capacity, which, however, is approximately 75% of the methane production capacity of the known Have attachments. Much more methane is therefore generated than for heating purposes within the process is required so that relatively large amounts of high methane gas are available for other purposes stand. Eventually it was found that oxygen was not in any appreciable amount from the sludge of the first Breakdown zone is transferred into the gas phase of the second breakdown zone. This is likely to be attributed to it

be that the elevated temperatures in the anaerobic second degradation zone for sufficiently intense anaerobic bacteriological conditions ensure the content of the extracted from the first to the second degradation zone Dissolve oxygen in the sludge thoroughly before dissolving oxygen in significant amounts Scope can pass into the gas phase within the second degradation zone. The surprisingly short ones Residence times of the present process, especially in the anaerobic degradation stage, can be combined with the thermal stability of the process and the surprisingly high methane production capacity of the anaerobic stage the result of chemical or biological acclimatization of the sludge and the Microorganisms in the aerobic degradation zone are responsible for improving the efficiency of the subsequent anaerobic treatment stage.

Preferably, the sludge introduced into the first breakdown zone has a total concentration of suspended matter between 20,000 and 60,000 mg / l. This ensures a high speed of the biological Sludge degradation with good oxygen solubility.

With a view to minimizing the total duration of treatment and the tank capacity requirement, a sludge retention time in the first mining zone was found between 12 and 24 hours is particularly beneficial.

Depending on the process parameters given in the individual case, it can be useful for a maximization the thermal efficiency of the process to the sludge before it is introduced into the first degradation zone heat to maintain the thermophilic temperature in the aerobic degradation zone.

For operation with particularly high efficiency, the temperature in the Anaerobic degradation zone for a mesoophilic degradation kept in the range of 35 ° C to 40 ° C. Because of the excellent operational stability is another embodiment according to a modified embodiment Shortening the treatment time possible by increasing the temperature in the anaerobic degradation zone for one thermophilic degradation is maintained in the range of 45 ° C to 50 ° C.

Preferably, the sludge residence time in the anaerobic degradation zone is selected to be sufficient to Biodegradable volatile suspended matter content of the sludge to less than approximately 20% the biodegradable content of the sludge introduced into the aerobic first degradation zone to further press down volatile suspended matter.

The stabilization speed in the anaerobic second degradation zone can be further increased by that the sludge is in the second abscess zone

is by putting Me-.hangas against the one located there Sludge is circulated.

In order to control the temperature in the aerobic degradation zone, the aeration gas supplied to this zone can be used be heated. The heating energy requirement can also be minimized in that the sludge before introduction into the first mining zone by indirect heat exchange with that from the second Dismantling zone discharged, further stabilized sludge is heated. It is useful if the temperature in the anaerobic degradation zone is maintained in the range of 35 ° C to 40 ° C and the heated sludge prior to introduction into the aerobic first degradation zone by indirect heat exchange with the partially stabilized sludge discharged from the first mining zone is further heated.

In a further embodiment of the invention, the second degradation zone has a partial acidification zone and a Methane fermentation subzone provided, and it is partially stabilized sludge from the first degradation zone in the acidification sub-zone initiated and for acidification of the sludge there for a sludge residence time of Held for 24 to 60 hours while acidified sludge is discharged from the acidification subzone and introduced into the methane fermentation subzone as well

ίο is kept there at a temperature between 35 ⁰ C and 40 ° C for a sludge retention time of 4 to 8 days. This allows a particularly good adaptation to the most favorable process conditions for the acidification of the sludge and the subsequent methane fermentation. The sludge is preferably kept in the methane fermentation subzone at a temperature between 37 ° C and 38 ° C, while advantageously the sludge in the acidification subzone is kept at a temperature between 45 ° C and 75 ⁰ C and the acidified sludge discharged from the acidification subzone before introduction is cooled to a temperature of 35 ° C to 40 ⁰ C in the methane fermentation subzone.

The sludge introduced into the first degradation zone can expediently consist of primary sludge and excess sludge consist of a wastewater activation system.

According to a further development of the invention it is provided that the introduced into the first mining zone Aeration gas contains at least 80% by volume of oxygen, the decomposition gas withdrawn from the first decomposition zone has an oxygen content of at least 40% and at least 35% of the oxygen content of the in the first Decomposition zone made ventilation gas, and this decomposition gas in a covered fumigation zone a wastewater activation system than at least the larger part of that fed to a fumigation zone Aeration gas is introduced, while the excess sludge of the wastewater activated sludge in the first mining zone is fed. By such a process management, the degradation gas from the aerobic first degradation zone for the excess sludge of the activated sludge plant analogous to a known wastewater treatment process (US-PS 39 26 794 and corresponding DE-PS 25 28 800) as an oxidant gas for Sewage ventilation can be used.

If, depending on the respective climatic conditions, an external heat supply is required in order to Maintaining the degradation zones at the intended working temperatures is preferably a part of the second degradation zone discharged methane gas mixed with oxygen-containing gas and with the formation of Heat burned, by means of which the sludge in at least one of the two degradation zones on increased Temperature is maintained. In particular,

moderately by burning methane gas with oxygenated Gas the sludge in the first extraction zone must be kept at the temperature of 45 ° C to 75 ° C.

In the present case, sludge is a mixture of a liquid phase and an at least partially

so understood biodegradable solid phase. The sludge can be characterized by its content of biodegradable volatile suspended matter (VSS). This is understood to mean the reduction in solids that can be maximally achieved by aerating the sludge with O ₂ -containing gas at outside temperature, for example at 20 ^° C. and a content of dissolved oxygen of at least 2 mg / l. It is assumed that a maximum reduction

the solids content is reached after a period of fumigation of 30 days ("Water Pollution Control", W. W. Eckenfelder and D. L. Ford, The Pemberton Press, 1970, Page 152). By determining the VSS values of the fresh sludge and after 30 days of fumigation, the biodegradable fraction of the total volatile suspended matter is calculated as follows will:

VSS (fresh) - VSS (30 days)
VSS (fresh)

Under stabilized sludge becomes a sludge with a reduced content of biodegradable understood volatile suspended matter following a degradation treatment. The sludge retention time denotes the mean time τ (in days or hours) during which the sludge in a There is a mining zone; it will be according to the formula

"ΟΙ

calculated, where

Vd = volume of sludge treated in the degradation zone (m ³ ) and

Qs = volumetric flow rate of the sludge fed to the degradation zone (m ³ / day or m ³ / h).

The invention is explained in more detail below on the basis of preferred exemplary embodiments. In show the drawings

F i g. 1 is a flow diagram of a degradation process according to an embodiment in which heat is removed the outflowing streams of both mining zones is recovered,

F i g. 2 is a flow diagram of a modified embodiment, in which discharged from the first mining zone, oxygen-depleted decomposition gas for the oxygen enrichment post-treatment of BOD-containing Water is exploited,

F i g. 3 is a flow sheet of another embodiment in which sludge is obtained from the primary and the secondary wastewater treatment stage is fed to the sludge removal zones,

F i g. 4 shows a graph of the temperature of the sludge entering the first degradation zone, which is necessary to maintain a working temperature of 50 ^° C. there, plotted as a function of the total suspended matter concentration (MLSS) of the sludge flowing into the first degradation zone and

F i g. 5 is a flow diagram of a further embodiment in which a smaller part of the in the plant incoming sludge is discharged to the second degradation zone.

F i g. Fig. 1 shows a flow sheet of a sludge degradation process that utilizes a first thermophile aerobic degradation stage a mesophilic anaerobic degradation is followed by sludge, for example from a primary settling basin, the clarification basin of a wastewater treatment plant working according to the activated sludge process, a trickle filter or other sludge-producing system occurs over a line 8 and is successively in heat exchangers 22 and 15, for example on a Temperature of 30 ° C to 35 ° C heated before it is introduced into the first covered mining zone 10 to there to keep the temperature at a value in the thermophilic range of 45 ° C to 75 ° C. The outside temperature having sludge in the line 8 is first heated in the heat exchanger 22 by being in indirect heat exchange with the further stabilized sludge passed through in countercurrent is, which leaves a second covered mining zone 20 via a line 24 In this way, heat recovered from the further stabilized sludge. The cooled, stabilized sludge obtained exits the heat exchanger 22 and is passed through a line 25 to final disposal or to another Use discharged. The sludge entering the heat exchanger 22 via line 24 can expediently have a temperature of 35 ° C to 40 ° C, so that the incoming sludge, the heat exchanger leaves via a line 9, is heated to a temperature of 28 ° C to 30 ° C. This mud is im Heat exchanger 15 is further heated to a temperature of 30 ° C to 35 ° C by being in countercurrent indirect heat exchange is brought with the partially stabilized sludge from the first mining zone 10 discharged via a line 14 and from the heat exchanger via a line 16 into the second Mining zone 20 reached.

As an alternative to the explained heat exchange with sludge product streams from the relevant The inflowing sludge can be broken down through indirect decomposition zones before it enters the first decomposition zone Heat exchange with an externally supplied heating medium, for example steam or hot water although heat recovery from the warm zone product streams is preferred, because in this way the heat within the process is used effectively and the heating energy requirement of the Process is minimized. The heating of the incoming sludge before it is introduced into the first The degradation zone can vary depending on the solids content of the inflowing sludge, the sludge retention time in the aerobic degradation zone and other process parameters in practice be desirable in order to reduce the thermal To maximize the efficiency of the process taking place at elevated temperatures.

The further heated sludge that leaves the heat exchanger 15 via a line 11 is in the first Degradation zone 10 is introduced from a line 17 together with aeration gas. The vent gas in the line 17 contains at least 50 and preferably at least 80% by volume of oxygen in order to ensure high mass transfer driving force and oxygen dissolution rate in the sludge at the present highs Scb ^ rnmt ^ rnnpratijrpri in Η & Γ first A.bbHUZOne 7ij care for. Line 17 is connected to a source of ventilation gas (not shown). At this source For example, it can be a low-temperature oxygen system, a storage tank or one with an adiabatic pressure cycle acting adsorption air separation plant. The ventilation gas can also be heated in the line 17 by means of a heating device 19 to the temperature to hold in the mining zone 10 at the desired value.

In the aerobic digestion zone 10, the sludge and the aeration gas are mixed; at the same time becomes one these fluids compared to the other fluid in sufficient quantity and with sufficient speed circulated to bring the dissolved oxygen content of the sludge to at least 2 mg / 1 and the

Total suspended solids concentration (MLSS) des To keep the sludge at at least 20,000 mg / l. A contact device 12 is provided for mixing and circulating provided that in Her practice an immersed turbine and a gas compressor connected to the gas headspace in the breakdown zone and to the gas sparger is connected to circulate the oxygen-containing aeration gas against the sludge. Instead of this, a surface aeration device can also be present, which opposes the sludge the ventilation gas circulates in the gas head space of the decomposition zone 10. The mutual circulation of the fluids in the The aerobic degradation zone is preferably used to maintain a high level of dissolved oxygen in the sludge and to use the oxygen in the ventilation gas to a large extent. In some cases it can be possible for sufficient dissolved oxygen in the sludge and for high utilization of the oxygen in the Provide ventilation gas by passing ventilation gas through the aerobic degradation zone only once will. The dissolved oxygen content of the sludge in the first decomposition zone should be at least 2 mg / 1 to give the thermophilic microorganisms in the sludge sufficient oxygen for rapid To provide decomposition of the biodegradable sludge components. The total suspended solids concentration of the sludge is at least 20,000 mg / 1 µm, based on kinetic considerations, to achieve a high rate of biological sludge degradation in the aerobic degradation zone in order to at the same time, the heat of reaction from the exothermic aerobic decomposition to a sufficient level Worth keeping in order to keep the temperature of the sludge in the first degradation zone within the thermophilic Area remains, and a satisfactory degree of partial sludge stabilization in the mining zone with short To achieve retention times.

The sludge ^{temperature of 45 °} C. to 75 ^° C. in the first degradation zone ensures rapid oxidation of the volatile suspended matter content of the sludge. The thermophilic degradation is continued for a sludge retention time in the first degradation zone of 4 to 48 hours in order to partially reduce the content of biodegradable volatile suspended matter in the sludge introduced into this zone. The sludge retention time in the aerobic degradation stage should be at least 4 hours in order to achieve sufficient partial stabilization there. With shorter residence times, the degree of sludge stabilization required in the subsequent anaerobic treatment stage becomes disproportionately large compared to the degree of stabilization in the aerobic stage; the total system residence time and tank capacity requirement begin to approach the values of conventional anaerobic digestion systems; the unexpected improvement in these process parameters, which are characteristic of operation with dwell times of four hours or more, is increasingly being lost. For similar reasons, the sludge retention time in the aerobic degradation zone should not exceed 48 hours. Above this value, the degree of sludge stabilization in the aerobic degradation zone becomes excessively large compared to the remaining stabilization in the downstream anaerobic stage. The residence times necessary in the last-mentioned stage to maintain a suitably large population of methane generators in the mining zone tend to be considerably longer than is desirable with regard to effective operation. which can be achieved with a residence time range of 4 to 48 hours. A residence time in the range from 12 to 24 hours is preferably used

Partially stabilized sludge is discharged from the aerobic zone via line 14, while oxygen depleted degradation gas with at least 21% oxygen content and preferably at least 40% oxygen content is separately discharged from the aerobic zone via a line 18. The oxygen concentration in line 18 can easily be kept at the appropriate value by adjusting the relative flow rates of the gas supplied via line 17 and of the gas leaving via line 18 can be regulated. For For example, gas flow control valves in the gas inlet and / or gas outlet line can serve this purpose be provided, which are influenced in a known manner by an oxygen concentration analyzer, which sits in the outlet line 18

By keeping the sludge in the aerobic degradation zone under thermophilic process conditions, an essentially complete pasteurization of the sludge can be achieved. Via line 14 outgoing, partially stabilized sludge has a temperature in the thermophilic range between 45 ⁰ C un <J 75 ° C. Because in this embodiment a mesophilic anaerobic digestion is provided in the second covered digestion zone 20, heat is preferably removed from the partially stabilized sludge in the line 14 in order to ensure an effective operation of the anaerobic sludge treatment stage. For this purpose, the sludge passes through the heat exchanger 15. Alternatively, the partially stabilized sludge in the line 14 can also be cooled by means of an externally supplied cooling medium, for example with the clarified drain of a wastewater treatment plant. When operating in winter it may not be necessary to provide a heat exchange stage for cooling the partially stabilized sludge flow, because such a cooling stage can satisfactorily compensate for heat losses to the surroundings from the second decomposition zone and the sludge flow flowing from the first to the second decomposition zone.

The partially stabilized sludge introduced into the degradation zone 20 via line 16 is kept there under anaerobic conditions at temperatures of 30 ^° C. to 45 ^° C. for a sufficiently long sludge retention time to keep the sludge's biodegradable volatile suspended matter content to less than about 40 pressing down% and preferably less than 20% of the biodegradable volatile suspended matter content of the sludge introduced into the first degradation zone; in addition, methane gas is formed.

The temperature of the slurry in the second covered degradation zone is maintained in the range of 30 ° C to 6O ⁰ C. This includes both an operation in the mesophilic range of 3O ⁰ C to 45 ° C and a work in the thermophilic range of from 45 ° C to 6O ⁰ C. For operation with particularly high efficiency the anaerobic zone in the mesophilic operating at a sludge treatment temperature is between 35 ° C and 4O ⁰ C and preferably between 37 ° C and 38 ° C. A preferred working temperature range for anaerobic thermophilic degradation is between

45 ° C and 50 ° C. Operation in the preferred temperature ranges mentioned above ensures a particularly rapid degradation of the biodegradable volatile substances by the affected Microbial strains.

During operation of the anonymous mining zone 20, the content of the mining zone is controlled by means of a stirring device 21 constantly mixed, creating a large zone for the ■ active degradation and the speed of the Stabilization reactions is significantly increased. The retention time of the sludge in the second degradation zone ranges from 4 to 12 days and preferably between 5 and 9 days. Sludge retention times in the second mining zone of less than 4 days is inexpedient; the length of stay is less and less to maintain a large viable population of methane generators in the anaerobic stage; the Overall sludge stabilization behavior of the degradation system is adversely affected Dwell times in the anaerobic degradation stage of more than 12 days mean the dwell time in the second Mining zone unnecessarily long; it becomes increasingly difficult to take advantage of the synergistic benefits of the built-in Realize process with regard to dwell time and tank space requirements within the dwell time range from 4 to 12 days can be achieved.

After the anaerobic sludge treatment in the degradation zone 20 is completed, the will continue Stabilized sludge is discharged through line 24 and with incoming feed sludge in the heat exchanger 22 brought to heat exchange before the sludge finally leaves the system via line 25. This in the degradation zone 20 leaves methane gas formed due to the biochemical reactions carried out there the anaerobic treatment stage via a line 23 in which a flow control valve 26 is located.

F i g. Fig. 2 shows a flow diagram of a further embodiment which shows how that of the US-PS 39 26 794 known procedure mn can advantageously be combined with the present method.

According to FIG. 2 BOD-containing water, for example municipal wastewater, passes through a pipe 101 into a covered gassing zone 102. The gassing zone 102 is also a first gas, the Contains at least 40 vol .-% oxygen, via a line 118 (dashed) and activated return sludge is supplied via a line 108 in which a pump 109 is seated. Supernatant liquid from a sludge thickener 151 also goes via a line 150 into the covered gassing zone 102. In FIG. 2, liquid- and sludge-carrying lines are shown in solid lines, while gas-carrying lines are shown in dashed lines are drawn. Valves are not shown for the sake of simplicity. The currents mentioned are in the Gassing zone 102 thoroughly mixed by means of a mechanical stirring device 103. The stirring device can be motor-driven, seated near the surface of the liquid or immersed in the liquid Have impellers; the oxygen gas can via line 118 above or below the liquid level are fed. Devices of this type are known; they should be chosen so that a large Contact area between the fluids is obtained with minimal expenditure of energy. Will the oxygen gas blown into the liquid or caused to diffuse in, the bubbles should be so small that their The total surface area is large and its buoyancy is low. The dissolving of oxygen is also supported by that the arrangement provided for introducing the gas is immersed so deeply in the liquid that the hydrostatic effect plays a role.

In the gassing zone 102, one medium is constantly circulated in relation to the other media, for example a compressor (not shown) can be connected to the gas space of the gassing zone in order to To circulate ventilation gas to the lower part of the zone and in the form of small bubbles over a ventilation

advises to release conventionally. Alternatively, the mixing device can also be used to circulate the Fluids are used, as is the case with surface gassing impellers The case is fumigation devices are rated according to their normal air transition efficiency, which characterizes the ability of the device to dissolve oxygen from air in tap water does not contain dissolved oxygen, is under atmospheric pressure and has a temperature of 20 ° C Suitable devices have a normal air transition efficiency of at least 0.92 kg CVkWh and preferably an efficiency of at least 1.85 kg CVkWh. For the dimensioning of the device The energy used is the total energy required for both stirring the Liquid is consumed as well as for bringing gas and liquid into contact.

The media are circulated in sufficient quantity and speed for the content of the Maintain mixed liquid dissolved oxygen at at least 0.5 mg / 1. The liquid temperature becomes also preferably kept at at least 15 ° C, so that in cold Wet'.er means may be necessary to a lower temperature in the gas zone 102 to avoid. For example, the incoming wastewater can be heated in line 101. construction and operation of the wastewater gasification zone 102 can be found in US Pat. No. 3,547,811, 3,547,812 or 35 47 815 be chosen in a known manner.

The mixed liquid enriched with oxygen arrives from the gassing zone 102 via a line 104 into a clarifier 105, where they are converted into purified supernatant liquid and into activated sludge is separated. Unused oxygen-containing gas leaves the gassing zone 102 via a line 119 and may for example be vented to the atmosphere will. This gas is discharged from the aeration zone in a flow rate that is regulated in such a way that that its oxygen content does not make up more than 40% of the total oxygen contained in the (hereinafter explained) covered aerobic degradation zone is introduced. From the clarifier 105 is protruding Purified liquid withdrawn via line 106, while activated sludge via line 107 is carried out. The revitalized sludge contains microorganisms in a correspondingly concentrated form a total suspended solids content (MLSS) of about 10,000 to 40,000 mg / l. The greater part of the busy Sludge, for example at least 85%, is via the line 108 and the pump 109 to the Recirculated gassing zone, preferably in such a flow rate based on the BOD-containing wastewater that the volume ratio of return sludge to BOD-containing wastewater between 0.1 and 0.5. The flow rates of the media introduced into the gassing zone 102 are preferred dimensioned so that the total suspended matter concentration there is between 4000 and 12,000 mg / l and the Volatile suspended matter content (MLVSS) 3000 to

The liquid-solid contact time in the gassing zone 102 for absorption and 10 000 mg / 1 is Assimilation of organic matter takes between 30 minutes and 24 hours. It fluctuates in dependence on the BOD content of the wastewater, the type of pollutants, the solids content in the fumigation zone and the temperature.

The activated sludge process produces an overall excess of microorganisms because the mass of the the impurities in the wastewater synthesized new cells is greater than the mass of the cells that self-oxidize during treatment. In addition, the wastewater usually contains biological non-degradable solids that settle and collect with the biomass. Consequently a small proportion of the activated sludge has to be excreted in order to achieve equilibrium to ensure between the microorganism population and the supply of organic substances (BOD) as well as to suppress the accumulation of inert solids within the system. The sludge makes usually less than 3% of the total separated sludge and rarely more than 15% the end.

It is true that the sludge represents a small proportion of the total solids separated in the clarifier; nevertheless, viewed in absolute terms, it often constitutes a large amount of substance. Regardless of the amount, the Removal of this sludge is a significant part of the cost of wastewater treatment; Besides the sludge constitutes a significant ecological problem. The mud is putrid and to a large extent biologically active; it often contains pathogenic bacteria. The mud is used as a fertilizer and / or for Terrain filling potentially suitable. Before such an application, however, it must be well stabilized, to avoid nuisance and health risks. Its high water content (e.g. 96 to 98%) must be reduced.

The sludge is withdrawn from the clarification hedge 105 via the sludge circulation loop in a line 111. It has a Gesamtschwebstoffkonzentra tion 10000-40000 mg / 1, and initially about the same temperature as the waste water in the Begasungszone 102, ie 15 ° C to 25 ⁰ C. The blowdown the sludge thickener 151 is passed. This concentrates the sludge to a total suspended matter concentration between 20,000 and 60,000 mg / l. The thickened sludge withdrawn below is fed to the sludge digestion system via a line 152.

In some cases, such as wastewater treatment when the outside temperature is high, a Thickening of the sludge from the clarifier may not be necessary; the sludge in line 111 can then via line 153 to the sludge thickener

151 are bypassed and then enter the line 152 to enter the sludge removal system to get. The overflow of the sludge thickener is fed to the aeration zone 102 via line 150.

The thickened sludge in the line 152 can prior to being introduced into an aerobic digestion zone UO by means a methane boiler 130 are heated. Instead, the sludge can also be mixed with the stabilized Sludge discharge from an anaerobic degradation zone 120Z) can be brought into heat exchange. Blowdown is in the first covered mining zone 110 from the line

152 either continuously or intermittently

initiated. The aerobic degradation zone 110 is on a Maintained temperature in the thermophilic range of 45 ° C to 75 ° C. If an autothermal operation in the aerobic degradation zone is achieved, the sludge temperature in the degradation zone can expediently in the range of 55 ° C to 65 ° C. It is difficult to autothermally maintain degradation temperatures above 65 ° C. The elevated temperature in the HO degradation zone can also be achieved by using external heat is supplied, for example by circulating a heated fluid in a heat exchanger, not shown, which sits in the breakdown zone because of the tendency of the solids to form coatings and to become clogged cause, should not be complicated or arranged heat exchanger surfaces within the mining zone be arranged at close intervals; advantageously they can be embedded in the wall of the tank or with this be connected.

A second oxygen gas containing at least 80 volume percent oxygen becomes the heated breakdown zone 110 supplied via a line 117 The amount of this Gas is sufficient to provide for part of the first oxygen gas that enters the aeration zone 102 is initiated via the line 118.

Preferably, the elevated temperature in the degradation zone 110 is achieved autothermally without Heat exchangers such as the methane boiler 130 are necessary.

The concentrated sludge as used in the oxygen gassing process according to US Pat 35 47 813 usually accrues, is particularly suitable for an autothermal operation because it has a has a lower water content compared to the content of biodegradable »fuel«. aside from that the size of the digestion tank becomes smaller with high solids concentrations, which also leads to heat conduction losses can be reduced through the walls of the digestion tank. Because of such considerations, also in the embodiment according to FIG. 2 the total suspended matter content of the sludge in the aerobic Degradation zone 110 maintained to at least 20,000 mg / l.

The upper limit values for the solids concentration in the aerobic degradation zone are essentially determined by two factors. In general, the maximum concentration depends on the ability of conventional Settling and thickening devices. to reduce the water content. Flotation devices, Centrifugal separators and gravity thickeners often provide total suspended solids concentrations of 60,000 mg / l. The solids content can also be increased by using fresh sludge or more concentrated Waste from a source other than wastewater is mixed in. The second factor that the Limited solids concentrations, the increasing difficulty is to dissolve and oxygen in the decomposition zone to mix the solids. A preferred upper limit is 60,000 mg / l, which ensures that the dissolved oxygen is sufficiently uniformly distributed in the sludge. In addition, the most cases temperatures close to the maximum aerobic, thermophilic degradation rates correspond to, achieve autothermal, if the solids content is not higher than 60,000 mg / l. At a a further increase in the solids concentration would lead to more CO2 entering the gas space of the extraction zone; the Oxygen partial pressure of the ventilation gas would be reduced unnecessarily.

The construction of the digestion tank also affects the autothermal temperature values. Concrete walls are preferable to metal walls because of the lower heat conduction losses of concrete. Of the Heat loss can be suppressed further by having the tank embedded underground and earth against all exposed vertical tank walls is raised. If necessary, thermal insulation, for example a low density concrete or foam, applied over a metallic cover will.

Aerobic and anaerobic digestion tanks which have a surface / volume ratio of less than 2.62 m ² / m ³ are preferably used for the present process. The term "surface" includes the entire wall surface of the covered mining tank including the bottom, cover and side walls. Surface / volume ratios of more than 2.62 m ² / m ³ lead to large heat conduction losses through the walls, based on the amount of heat required in the digestion tank. Such heat loss usually requires thermal insulation on the walls exposed to the outside atmosphere. In addition, larger surface / volume ratios generally require digestion tanks with smaller dimensions, in which gassing and / or uniform mixing is difficult

The retention time of the sludge in the aerobic degradation zone also influences the autothermal one Temperature values that can be sustained. The relationship between length of stay and temperature is determined by numerous factors, including the degradability and the strength (the solids content) of the mud.

The degradation zone 110 is equipped with a mechanical agitator 112, which can be of the same type as the agitator 103 in the aeration zone 102. Furthermore ^s: nd means are provided to constantly circulate in the degradation zone, the second gas or the activated sludge over other fluids.

The second gas containing at least 80% oxygen is the covered aerobic degradation zone 110 in Sufficient quantity and speed supplied to the dissolved oxygen content of the sludge to be kept at at least 2 mg / 1.

An oxygen-depleted decomposition gas with an oxygen concentration of at least 40% is turned off withdrawn from the degradation zone 110 via line 118 in such a flow rate that its oxygen content represents at least 35% of the oxygen content of the oxygen feed gas passing through the line 117 flows in. The gas in line 118 arrives the covered gassing zone 102, wherein at least a larger part of the mentioned first gas den Provides oxygen, which is necessary for the biochemical oxygen enrichment of the wastewater. If If necessary, additional oxygen-containing gas can be supplied from an external source to the to enlarge oxygen-containing gas stream in line 118.

Partially stabilized sludge that is essentially completely pasteurized is removed from the breakdown zone 110 via a line 114 to the anaerobic treatment part fed. The second anaerobic degradation zone includes an acidification subzone 120a and the degradation zone 120Zj, which denotes the methane fermentation subzone shall be. The partially stabilized sludge arriving in line 114 is fed into the acidification subzone 120a and held there for a sludge retention time of 24 to 60 hours in order for the to ensure the necessary acidification of the sludge. The content of the subzone 120a is controlled by an agitator 121a constantly mixed to lower carbohydrates, fats and proteins at a uniform rate Break down fatty acids. The acidified sludge is then extracted from sub-zone 120a via line 126

ίο carried out. Since the temperature of the zone 120a leaving sludge is still in or near the thermophilic temperature range of 45 ° C to 75 ° C and thus above the optimal methane formation values, the sludge temperature is generally lowered in order to ensure satisfactory operation of the methane-forming degradation zone 120f> to ensure. The sludge is therefore passed through a heat exchanger 115 passed in countercurrent to a coolant flow, which flows through the heat exchanger via a line 160 The one leaving the heat exchanger 115 Sludge then flows via line 127 to the methane fermentation subzone. The heat exchange medium in line 160 can appropriately be a flow of cooling water, for example part of the Outflow that leaves the secondary clarifier via line 106, or that flows into the degradation system Feed sludge stream.

For optimal operation, the sludge in the methane fermentation sub-zone is kept at a temperature between 35 ° C and 40 ° C and preferably between 37 ° C and 38 ° C. The contents of the 120Λ zone b is constantly mixed by a mixer 121, thereby providing a large active separation zone and the velocity of the running there Stabiiisierungsreaktionen is substantially increased. The sludge retention time in the methane fermentation sub-zone is preferably between 4 days and 8 days. Methane gas, which is generated by the biochemical reactions taking place in the degradation zone 1206, leaves this zone via a line 128 in which a flow control valve 129 is located. A portion of this discharged methane gas can be fed to the methane boiler 130 via a line 132, while the remainder is withdrawn from the process in a line 131 in order to undergo further treatment and / or to be fed to other end-use stages. The further stabilized sludge is discharged via a line 133.

F i g. 3 shows a flow diagram of a further embodiment in which sludge from sewage feed and post-treatment stages are fed to the sludge removal system. Doing this is a thermophilic aerobic first Degradation stage integrated with a thermophilic anaerobic second degradation stage. Previously was a thermophile Anaerobic degradation little more than a laboratory curiosity. The ones discussed above, with a mesophile anaerobic operation associated problems of known systems with respect to thermal instability and extreme Sensitivity to changes in process conditions are anaerobic in a thermophilic Degradation even more pronounced. Therefore, this sludge treatment process has heretofore been in commercial use Sludge removal applications received little attention. The problems of operational instability and excessive Sensitivity to process fluctuations becomes aerobic / anaerobic with the thermophilic Embodiment overcome in the same way as in connection with the embodiments

is described, which provide an anaerobic mesophilic second degradation stage. In the arrangement according to FIG. 3, raw sewage, which consists for example of municipal sewage, industrial sewage or rainwater, flows via a line 240 into a primary settling zone 241 , which can consist of a gravity clarifier of the conv entioneller type. There the wastewater is separated into a primary drain with a reduced BOD content, which reaches a gassing zone 202 via a line 201 , and a settled sludge flow, which is discharged from the zone 241 via a line 242. Aeration gas containing oxygen is also fed to the aeration zone 202 via a line 218 , sludge thickening liquid which protrudes via a line 250 and activated return sludge via a line 208. A mixing and circulation device 203 is located in the gassing zone 202 in order to mix the various fluids fed to the gassing zone and to circulate the mixed liquid or the oxygen-containing aeration gas with respect to the other fluids. After a gassing time of, for example, 2 to 6 hours, a mixed liquid depleted in BOD and an oxygen depleted ventilation gas with at least 21% by volume of oxygen are discharged from the gassing zone 202 via lines 204 and 219, respectively.

The oxygen-enriched mixed liquid in line 204 passes to the secondary settling zone 205, where activated sludge is separated from the purified liquid; the latter leaves the process via a line 206. The settled sludge is discharged via a line 207. A larger part of this sludge is returned as return sludge to the aeration zone 202 via line 208 , in which a circulating pump 209 is located. The remaining part, which can make up 3% to 10% of the sludge in the line 207 , reaches a sludge thickener 251 via a line 252.

The sludge thickener 251 concentrates the sludge to a solids content between 2% and 6%. so that the total suspended matter concentration is between 20,000 and 60,000 mg / l. The thickened sludge underflow is discharged via a line 245 and combined with primary sludge in the line 242 from the primary settling zone 241 , so that a combined sludge flow in a line 211 is produced. The supernatant liquid from the sludge thickener 251 is fed to the aeration zone 202 via a line 250.

The sludge flow in the line 211 can be partially heated by indirect heat exchange with the sludge from a second Ahhaii7onp 220; it then reaches a first degradation zone 210 via a line 248. Before this, the sludge can be further heated in the line 248 by means of a heating device 231 in which methane gas is burned, which flows in via a line 227

If outside temperature conditions make it unnecessary to heat the sludge fed to the mining system, the sludge can bypass heat exchanger 244 and heater 231 via bypass lines 261 and 263 , respectively.

In the first covered decomposition zone 210 , the thermophilic aerobic decomposition of the inflowing sludge is carried out in the manner explained.Aeration gas, which contains at least 50% by volume of oxygen and preferably at least 80% by volume of oxygen, arrives at the decomposition zone via a line 217 210, in which a mechanical agitator 212 sits.

Degradation gas leaves the degradation zone via line 218. Partially stabilized sludge is introduced from the aerobic degradation zone 210 via a line 216 into the second covered degradation zone 220 . In the reduction zone 220 is a thermophilic anaerobic digestion zone in which the sludge at a temperature between 40 ⁰ C and 60 ⁰ C and is preferably maintained between 45 ° C and 50 ⁰ C. Because thermophilic operation takes place in both degradation zones, the partially stabilized sludge can be fed directly from the first degradation zone to the second degradation zone. Instead, in some cases it may be desirable to work in the second degradation zone at higher or lower temperatures than in the first aerobic degradation zone and, for this purpose, to heat or cool the partially stabilized sludge between the zones. The heating can be carried out with a heating device similar to device 231 . For cooling, the partially stabilized sludge can be heat-exchanged with the inflowing sludge, as described in connection with FIGS. 1 and 2 as it is even more critical with thermophilic anaerobic degradation than with mesophilic anaerobic degradation to avoid temperature fluctuations in the degradation zone, a well-insulated tank can be provided, which offers security against strong climatic fluctuations.
In the degradation zone 220 , the sludge is continuously mixed by means of a mechanical agitator 221. Methane gas is discharged via a line 223. A portion of the methane gas is fed to the heating device 231 via line 227 and is burned in order to keep the sludge in the first degradation zone at a temperature of 45 ° C. to 75 ° C. The remaining part is discharged via a line 228. The further stabilized sludge is discharged from the second degradation zone via a line 225 and is passed via the heat exchanger 244 . It leaves the system via a line 243.

Due to the differences in the sludge sources, the biological activity gives way to the aerobic one Mining zone according to FIG. 3 strongly depends on the biological activity in the aerobic zone of the embodiment according to FIG. 2 from. In the embodiment according to FIG. 2 is the sludge fed to the mining system as feed sludge Sludge alone activated sludge from the secondary sewage treatment system, while in the case of the Embodiment according to Figure 3, the inflowing sludge and secondary sludge from the activated sludge treatment stage as well as raw sludge from the raw sewage settling stage. Because the organic Material of the secondary sludge consists primarily of viable microorganisms, includes the aerobic degradation of this sludge the different biochemical reaction stages of the cell lysis, the Assimilation of lysis products for the synthesis of new viable material, and respiration. Raw sludge, on the other hand, consists primarily of non-viable organic material, which the im Microorganisms present in the sludge can use them as nutrients. Accordingly, during the aerobic degradation of a raw sludge the microbial population of the sludge a considerable cell synthesis phase in addition to cell lysis, assimilation of lysis products and respiration. Hence the aerobic degradation of the raw sludge with a greater degree of both cell synthesis and cell respiration

than in the case of aerobic degradation of secondary sludge. In addition, the aerobic degradation of raw sludge leads to a smaller overall reduction in biodegradable volatile particulate matter than is the case with a comparable sludge retention time is the case with aerobic decomposition of secondary sludge. the resulting reduction in biodegradable volatile suspended matter in the sludge during the Degradation represents a difference in terms of the competing degradation processes of cell synthesis and cell respiration represent.

The cellular respiration in the sludge breakdown process is exothermic; for the reasons discussed above Raw sludge has a higher heat generation capacity per unit weight of biodegradable volatile Suspended solids that are removed during degradation than is the case with secondary sludge. Accordingly is a lower resultant reduction in primary sludge compared to secondary sludge of the volatile suspended solids required to reach a certain temperature level in the aerobic decomposition stage to achieve and maintain. So the embodiment of Figure 3 at a given Temperature with a lower degree of reduction in volatile matter in the aerobic Degradation zone are operated as the aerobic zone according to F i g. 2. A lesser reduction in degradable Volatile suspended matter in the aerobic zone of the degradation system in turn requires that the sludge residence time in the anaerobic degradation zone is extended accordingly by a given total elimination of volatile suspended matter. Because an increased proportion of the total elimination of volatile If suspended matter occurs in the anaerobic degradation zone in such a case, the primary sludge can be processed System in the anaerobic degradation zone generate more methane than the degradation system in which only Secondary sludge is processed. The embodiment according to Figure 3 is therefore inherently able to larger quantities of methane than the system according to FIG. 2 to deliver; but this is at the expense of increased Sludge retention time in the aerobic degradation zone.

Each of the exemplary embodiments described offers the possibility of heating the inflowing sludge before it is introduced into the aerobic degradation zone. Whether or not such heating is necessary depends on various factors, for example the sludge solids content, the outside temperature, the sludge residence time in the aerobic digestion zone and the type of sludge to be treated. F i g. 4 shows the temperature of the sludge flowing into the first breakdown zone, which is required to maintain a working temperature of 50 ° C. there with a sludge retention time of 24 hours, as a function of the total suspended matter concentration of the sludge flowing into the first breakdown zone. Curve A shows a combined primary and secondary sludge flow in which the ratio of volatile suspended solids to total suspended solids (WS / TSS) is 0.75 and the biological heat content is 47 χ 10 ⁶ ] / kg of volatile suspended solids removed (VSS) Curve B applies to a non-combined secondary sludge with a VSS / TTS ratio of 0.79 and a heat content of 33 χ 10 ⁶ ] / kg of removed volatile suspended matter.

As can be seen from the curves, for a given total suspended matter concentration, the combined sludge of curve A requires a higher inlet temperature for the aerobic degradation zone than the secondary sludge according to curve B. As a result, heating the sludge before it is introduced into the first degradation zone can be particularly beneficial if the Sludge contains a significant proportion of primary sludge from a wastewater treatment plant.

Figure 4 also shows that thermophilic operation can be achieved without the need to heat the sludge prior to introduction into the aerobic degradation zone when the sludge is sufficiently thickened. For example, if a combined sludge (curve A) with a total solids concentration of 4% is to be degraded, the temperature of the sludge introduced into the thermophilic aerobic zone need only be about 15 ° C.

The embodiments described provide a completely pasteurized product sludge, since in all cases the sludge flowing into the digestion system is passed through the thermophilic aerobic zone, the high temperature of which ensures complete sludge pasteurization. However, there may be applications in which a fully pasteurized sludge is not required or the sludge itself does not require pasteurization because there are no significant concentrations of pathogens. F i g. 5 shows a flow diagram of a further embodiment in which a smaller part of the inflowing sludge is diverted to the second degradation zone and which is suitable for such applications. A larger part of the sludge entering via a line 311 is fed to a first covered degradation zone 310 via a line 331. The sludge in line 331 can be heated by means of a methane- heated kettle 330, if desired.

Aeration gas containing oxygen is fed to the aerobic decomposition zone 310 , which is provided with a stirring device 312 , via a line 317. Degradation gas depleted in oxygen is discharged from the degradation zone 310 via a line 318 . Fully pasteurized sludge depleted of biodegradable suspended matter arrives via a line 316 to a second covered degradation zone 320, which operates in the mesophilic temperature range. Because the temperature of the sludge leaving the first degradation zone is between 45 ° C. and 75 ° C., its temperature must be reduced before it is introduced into the degradation zone 320. A smaller part of the inflowing sludge bypasses a methane-heated boiler 330 and the degradation zone 310 in a line 329; it is immediately mixed with the warm sludge in line 316. The flow rate of the diverted sludge is adjusted so that the temperature of the combined sludge stream entering the anaerobic digestion zone 320 is sufficient to maintain an operating temperature between 35 ° C and 40 ° C in the zone 320 .

Methane gas leaving the reduction zone 320 through a line 323. A side stream of this gas is discharged into a conduit loop 340 in which a compressor 326 sits Compressed methane gas is in the degradation zone 320, for example, added to the mud through a non-shown blowing device in order for mixing and circulating the To worry about mud. A part of the methane gas can go from the line 323 via a line 327 to the methane-heated boiler 330 ; the remaining part is discharged via a line 328. The further stabilized sludge is discharged via a line 325 . The advantages of the

explained procedure can be made clear with the help of the following examples:

Example I.

In this example, the operating behavior of the embodiment according to FIG. 2 is compared with a conventional anaerobic high-performance system. The further explanation is based on the treatment of the sludge in a wastewater treatment plant with a capacity of 3.8 10 ⁷ l / day based on the flow diagram according to FIG. 2 off.

A mixture of 50% primary sludge and 50% secondary sludge, which initially has a temperature of 1R ° r ^ \ i / irH fihiar Hit »I aittincT 111 inHac Δ kkailcwctArn

• ir - I - ■ ■ U b | π IA ^ η ^ f γ ρ ^ g ^ λ j ^ f * \ j 1 ΐ 1 T j 1 lÄ Λ Λ Λ ϊ (* Λ γ Ll * J 'K Γ Γ f * ^^ * ι * ■ Γ J f fc ^ 11 I according to Fig. 2. The sludge, which has a total suspended matter concentration of 39 400 mg / 1 and a ratio of volatile suspended matter to total suspended matter of 72%, is in a flow rate of 3.4 χ 10 ⁵ l / day introduced. to keep the sludge in the aerobic digestion zone 110 at a 24-hour residence time at a working temperature of 5O ⁰ C, the inflowing sludge by means of the methane boiler 130 to about 23 ° C is heated. Starting from an efficiency of 50% for the reaction of The calorific value of methane gas in heat is required to feed the boiler 130 approximately 710 mVday of the methane gas generated in the anaerobic digestion zone.

In the aerobic degradation zone, a reduction in the content of volatile suspended solids of approximately 8% achieved (16% biodegradable volatile suspended matter; the biodegradable volatile suspended solids make up about 50% of the total volatile suspended solids content), see above that via the line 114 of the acid formation subzone 120a a partially degraded sludge with a content of volatile suspended solids of 26 100 mg / 1 is fed. This sub-zone is at a 24 hour Residence time operated at thermophilic temperature. At this stage there is a 10% reduction in the Proportion of the inflowing volatile suspended matter. A sludge with a content of volatile suspended matter of 23,500 mg / l goes via line 126 to the degradation zone 1206, which is a methane fermentation sub-zone represents. Sufficient heat is extracted from the discharged sludge in heat exchanger 115, in order to ensure a working temperature of 38 ° C in the extraction zone 1206.

The methane fermentation part zone is operated at a Stägigen Schlammvenveüdauer, which leads for the integrated systsm to an overall reduction in volatile suspended solids of 40% (reduction of the biologically degradable volatile suspended solids to 80%) of the methane fermentation part zone produces approximately 2070 m ³ methane gas per day, giving a total calorific value of After deduction of the methane gas for the operation of the methane boiler 130, 1360 m ^{3 of} methane gas per day, corresponding to a total calorific value of 8500 kWh / day, are available for delivery from the sludge removal system.

If the combined sludge in the amount of 3.4 10 ⁵ l / day is instead fed to a conventional high-performance anaerobic digestion tank, approximately 13 days of sludge retention time is necessary to achieve the same volatility reduction. Although the conventional high-performance degradation tank ^{generates 3625 m 3 of} methane gas per day, corresponding to around 22 600 kWh / day, with a 50% conversion of the calorific value into heat, around 17 600 kWh / day of heating energy are required to achieve the optimal working temperature conditions in the Maintain heavy duty tank. The conventional system therefore requires, in comparison to the embodiment according to FIG. 2, based on the necessary dwell time, 86% more tank space; In addition, under normal working conditions, about 40% less methane is available for delivery.

Rpic nipl II
ι · ■ ^c: ■■

This example relates to the embodiment according to FIG. 5. The incoming feed sludge consists of 50% primary and 50% secondary sludge from a wastewater treatment plant in an amount of 2.27 χ 10 ⁵ l / day. The slurry stream in line 311 has a temperature of 2O ⁰ C and a Gesamtschwebstoffkonzentration 4% (VSS / TSS = 0.75); it is divided into a partial flow of 1.74 10 ⁵ l / day, which goes to the thermophilic aerobic degradation zone 310 via line 331, and a partial flow of 5.3 χ ΙΟ ⁴ l / day, which is the bypass flow in line 329 forms. As shown in FIG. 4, it is not necessary in this case to heat the sludge before it is introduced into the degradation zone 310. The residence time in the mining zone 310 is approximately 24 hours; thermophilic temperatures are reached autothermally. A pasteurized sludge is discharged from the breakdown zone 310 in line 316 at a temperature of 50 ° C. and mixed with the cool bypass flow from line 329. This combined sludge stream then flows to the anaerobic digestion zone 320 where the sludge is held in the absence of oxygen for approximately 8 days, resulting in an overall volatile particulate matter reduction of approximately 40% (80% biodegradable volatile particulate matter reduction). The mining zone 320 generates methane gas in an amount of approximately 2040 m ³ / day, which corresponds to approximately 11,700 kWh / day. This methane can be completely released from the process system.

If the combined inflowing feed sludge of 2.27 10 ⁵ l / day is instead fed to a conventional anaerobic high-performance digestion tank, a residence time of approximately 15 days is necessary in order to achieve the same reduction in the content of volatile suspended matter. Although the conventional anaerobic digestion tank generates 2550 m ^{3 of} methane gas per day, corresponding to around 14 700 kWh / day, with a 50% conversion of the calorific value into heat, around 13 200 kWh / day are necessary to maintain the optimal anaerobic working temperature in the high-performance tank. In this case, a conventional anaerobic digestion system therefore requires an approximately 65% longer sludge retention time, while only resulting gas energy corresponding to 1500 kWh / day compared to 11 700 kWh / day in the combined system is generated. After using the internally generated methane gas as a heat source, the conventional system has significantly less methane gas available for delivery.

Example! III

This example compares the performance of the embodiment of FIG. 1 with a conventional high performance anaerobic system.

A secondary sludge from a wastewater treatment system used for oxygen enrichment, which initially has a temperature of 15 ° C, is first heated in the heat exchanger 22 by the outflow of the anaerobic degradation zone 20, whereupon further heating takes place through the outflow of the thermophilic aerobic degradation zone 10 in the heat exchanger 15. In the heat exchanger 22, the temperature of the inflowing sludge is increased from 15 ° C. to approximately 25 ° C., while the temperature of the stabilized sludge from the degradation zone 20 is reduced from approximately 35 ° C. to 25 ° C. In the heat exchanger 15, the temperature of the sludge flowing in is increased to approximately 30 ° C., while the temperature of the sludge leaving the degradation zone 10 is lowered from approximately 50 ° C. to 45 ° C. The incoming sludge has a total suspended solids concentration of 34,400 mg / l and a ratio of volatile suspended solids to total suspended solids of 78%. It is introduced into the degradation zone 10 in an amount of 2.27 χ 10 ⁵ l / day. In the mining zone 10, a working temperature of 50 ° C is maintained during a sludge retention time of 24 hours.

A reduction of about 16% of the volatile Suspended solids (reduction of 32% of biodegradable volatile suspended solids) is in the aerobic stage achieved, so that a partially stabilized sludge with a content of volatile suspended matter of 22,500 mg / l is fed into the degradation zone 20 via line 16.

The anaerobic degradation zone 20 operates with a residence time of 8 days, which leads to an overall reduction in volatile suspended matter of 42% (reduction in biodegradable volatile suspended matter of 84%) for the integrated system. The mining zone 20 generates approximately 1470 m ^{3 of} methane gas per day, corresponding to a total calorific value of approximately 8200 kWh / day. All the methane gas is available for delivery.

If, on the other hand, the ^{sludge flowing in at 2.27 χ 5} l / day is fed to a conventional ariaerobic high-performance degradation tank, a sludge retention time of at least 14 days is necessary in order to achieve the same reduction in the content of volatile suspended matter. Although a conventional high-performance system of this type ^{generates 2400 m 3 of} methane gas per day, corresponding to approximately 13,800 kWh / day, with a 50% conversion of the calorific value into heat, approximately 13,200 kWh per day are required in order to maintain optimal working temperature conditions in the high-performance tank. The conventional anaerobic system therefore requires approximately 55% more tank space; it can emit methane gas energy of approximately 7600 kWh / day less than the plant according to FIG. 1.

For this purpose 3 sheets of drawings

Claims (18)

Patent claims: 1. A method for breaking down sludge, in which at least part of the sludge is passed through at least two covered mining zones passed and thereby 5 with the formation of stabilized sludge and Methane gas is successively exposed to aerobic and anaerobic conditions, characterized in that that ^{a) Introduced the! 0} sludge and an aeration gas containing at least 50% by volume of oxygen into a first covered degradation zone and maintaining a dissolved oxygen content of at least 2 mg / 1 in the mixed liquid and a total suspended solids concentration of at least 20,000 mg / 1 are mixed, b) the sludge in the first degradation zone during the mixing process at a temperature im thermophilic range of 45 ° C to 75 ° C is maintained, c) mixing in the thermophilic temperature range for a sludge residence time of 4 to 48 hours with a partial reduction in the content of that introduced into the first mining zone Sludge of biodegradable volatile suspended matter is carried away, d) partially stabilized sludge and oxygen-depleted degradation gas with an oxygen content at least 21% are discharged separately from the first mining zone and e) the partially stabilized sludge from process step d) is kept in a second covered degradation zone at a temperature of 30 ^° C. to 60 ^° C. under anaerobic conditions for a solids retention time of about 4 to 12 days and which is sufficient to maintain the content of the sludge further lower biodegradable volatile suspended matter to below approximately 40% of the biodegradable volatile suspended matter content of the sludge introduced into the first degradation zone in process step a). 2. The method according to claim 1, characterized in that the introduced into the first mining zone Sludge has a total suspended solids concentration between 20,000 and 60,000 mg / l. 3. The method according to claim 1 or 2, characterized in that the sludge residence time in the first degradation zone is between 12 and 24 hours. 4. The method according to any one of the preceding claims, characterized in that the Sludge is heated before being introduced into the first degradation zone in order to maintain the temperature in process step b). 5. The method according to any one of the preceding claims, characterized in that the temperature in process stage e) for a f> o mesophilic degradation in the second degradation zone in Range of 35 ° C to 40 ° C is maintained. 6. The method according to any one of claims 1 to 4, characterized in that the temperature in process step e) is kept in the range of 45 ⁰ C to 50 ° C for thermophilic degradation in the second degradation zone. 7. Method according to one of the preceding Claims, characterized in that the sludge retention time in process stage e) is chosen sufficiently to the content of the sludge of biodegradable volatile Suspended solids to less than approximately 20% of the content of process step a) in the first Degradation zone introduced sludge to further press down biodegradable volatile suspended matter. 8. The method according to any one of the preceding claims, characterized in that the Sludge in the second mining zone is mixed by adding methane gas against the one located there Sludge is circulated. 9. The method according to any one of the preceding claims, characterized in that the ventilation gas before being introduced into the first mining zone in order to maintain the temperature in the Process stage b) is heated. 10. The method according to any one of the preceding claims, characterized in that the Sludge before being introduced into the first extraction zone by indirect heat exchange with that from the The further stabilized sludge discharged from the second mining zone is heated. 11. The method according to claim 10, characterized in that the temperature Ln of process step e) is kept in the range of 35 ° C to 40 ⁰ C and that the heated sludge prior to introduction into the first degradation zone by indirect heat exchange with that from the first The partially stabilized sludge discharged from the degradation zone is further heated. 12. The method according to any one of the preceding claims, characterized in that the second degradation zone is provided with an acidification subzone and a methane fermentation subzone, partially stabilized sludge from the first degradation zone is introduced into the acidification subzone and held there for a sludge retention time of 24 to 60 hours to acidify the sludge is acidified sludge is discharged from the Ansäuerungrteilzone and introduced into the methane fermentation part zone and is held there at a temperature between 35 ° C and 40 ⁰ C for a slurry residence time of 4-8 days. 13. The method according to claim 12, characterized in that the sludge is kept at a temperature between 37 ⁰ C and 38 ° C in the methane fermentation subzone. 14. The method according to claim 12 or 13, characterized in that the sludge in the acidification subzone is kept at a temperature between 45 ⁰ C and 75 ° C and the acidified sludge discharged from the acidification subzone is at a temperature of 35 ^{0 before being introduced into the methane fermentation subzone} C to 40 ⁰ C is cooled. 15. The method according to any one of the preceding claims, characterized in that in the First extraction zone introduced sludge from primary sludge and excess sludge from a wastewater activation system consists. 16. The method according to any one of the preceding claims, characterized in that the into the Aeration gas introduced into the first decomposition zone contains at least 80Vol .-% oxygen from the degradation gas withdrawn from the first degradation zone has an oxygen content of at least 40% and at least 35% of the oxygen content of the aeration gas introduced into the first degradation zone makes up, and this degradation gas in a covered fumigation zone of a wastewater activation system as at least the greater part of the aeration zone supplied aeration gas is introduced, while the excess sludge of the wastewater activated sludge is fed into the first mining zone. 17 Method according to one of the preceding claims, characterized in that a part of the methane gas discharged from the second extraction zone is mixed with oxygen-containing gas and is burned with the formation of heat, by means of which the sludge in at least one of the both degradation zones is kept at an elevated temperature. 18. The method according to claim 17, characterized characterized in that by burning methane gas with oxygen-containing gas in aer sludge of the first degradation zone is kept at the temperature of 45 ° C to 75 ° C.