WO2003020651A2

WO2003020651A2 - Process and installation for the treatment of biological studies

Info

Publication number: WO2003020651A2
Application number: PCT/IT2002/000563
Authority: WO
Inventors: Enrico Calcaterra
Original assignee: Ecodeco Spa
Priority date: 2001-09-03
Filing date: 2002-09-03
Publication date: 2003-03-13
Also published as: ITVI20010184A1; AU2002337618A1; WO2003020651A3; EP1438273A2

Abstract

Method and plant for the continuous treatment of biological sludge for the recycling thereof for agricultural purposes, comprising the steps of a) conveying a densified product from a biological water treatment plant; b) thickening the densified product until a sludge with a predetermined percentage of dry matter is obtained; c) mixing the sludge with a hygienizing treatment product in controlled chemico-physical conditions in order to promote hygienizing treatment thereof; d) maturing the sludge for a predetermined period of time in order to complete hygiening treatment; e) storing the hygienized sludge until it is extracted for use; f) distributing the hygienized end products; characterized in that all said steps are performed in continuous flow conditions and in substantially steady state chemico-physical conditions.

Description

IMPROVED METHOD AND PLANT FOR TREATMENT OF BIOLOGICAL SLUDGE

Field of application

The present invention relates in general to the recycling of biomass and relates in particular to a method for the treatment of domestic, mixed and industrial biological sludge resulting from the biological purification treatment of water.

In particular, the present invention has an application in the hygienizing treatment of biological sludge which may be used as agricultural fertiliser, which represents a natural and efficient system for recycling the nutritious elements, removed by harvesting back, into the soil.

The novel system according to the present invention is particularly suitable for being used downstream of biological processes for the purification of water of urban, agro-industrial, alimentary, opotherapical and pharmaceutical origin.

State of the art

In the present invention, the terms "biological sludge" or "biomass" will be used without distinction to denote the mass of sludge produced by both the aerobic and anaerobic section of biological purification plants.

The term "hygienizing treatment of biomass" is understood as referring to a process which i) eliminates the presence of pathogenic agents or reduces the concentration thereof to levels which are not dangerous for health, using methods similar to those used in pasteurisation, and ii) inhibits in a lasting manner the possibility of recontamination, even in non-sterile environments and the possibility of anaerobic fermentation, this being achieved by means of bacteriostatic substances.

The process of biological purification consists of a growth of a biomass formed by various species of micro-organisms which feed on the pollutants contained in the water and multiply. In the case where the pollutants consist of nutritious elements originally contained in foods and in other biodegradable materials, said biomass, which must be removed as it is produced, may be used as fertiliser. The regulations establish, for this use, upper concentration limits for other possible materials adsorbed, such as heavy metals.

Biological sludge has chemical characteristics which are in line with the current regulations for agricultural recycling, but which may potentially be or are actually contaminated by pathogenic organisms or parasite eggs.

Before being reintroduced into the environment, waste water produced by domestic or mixed settlement containing a high organic content is normally purified down to a residual BOD (Biological Oxygen Demand) value corresponding to the local regulations by means of active-sludge biological purification plant.

In said plants, the active sludge, present or added, metabolises the organic matter present by using atmospheric oxygen to produce biomass and a liquid with an organic content below the maximum values prescribed by local regulations.

At the end of the biological purification process, the biomass is separated by decantation in the form of a sludge containing about 2-4% dry matter and a clarified liquid which is reintroduced into the environment.

Generally, according to the prior art, the sludge obtained in this way is reprocessed in a sludge treatment line. In the simplest cases, for relatively small quantities, said treatment consists in simple further densification.

Ordinarily, the sludge obtained, which contains a significant fraction of material which can be putrefied easily, is stabilised by means of further aerobic or - particularly for large-size plants - anaerobic treatment with the production of biogas, a reduction in organic material and the elimination of the fraction which can be putrefied easily.

At the end of said treatment, the sludge is densified further, for example up to about 10% of dry matter, by means of centrifugal densifiers, or up to a content of about 20-25% of dry matter in press or press-band filters.

Plants for the biological purification of waste water produced by industrial settlements are substantially similar.

In particular, plants for the biological purification of water produced by the food, agro-industrial, opotherapical and antibiotics manufacturing industries produce sludge which, for the purposes of the present invention, is of the same nature as the sludge produced by plants for the purification of water of domestic or urban origin, although they are regulated in a different way by local regulations.

The filtration cake formed as indicated above must be disposed of, for example in landfill.

The use in agriculture of sludge resulting from biological purification is undoubtedly one of the most positive and feasible disposal solutions, with the simultaneous recycling of resources. In fact, owing to the high organic substance content and the good nitrogen and phosphorus content of the sludge, said practice offers the advantage of making fertilisation more economical and efficient and, at the same time, is an effective alternative to more expensive forms of disposal.

However, this cannot be regarded as the only solution to the problem of disposal, in as much as it is only applicable in the presence of certain financial and agronomic requirements, which are combined with precise guarantees for human and environmental health. In fact, biological sludge may contain potentially toxic substances and pathogenic organisms or micro-organisms. Moreover, even in the case where the sludge has chemico-physical characteristics which are suitable and in line with local regulations, a series of problems linked to unpleasant odours and hygienic risk resulting from the need for storage and the method of distribution restrict considerably its use in the agricultural sector.

Among the various possible treatment systems which are capable of reducing the pathogenic microbe content and the fermentability of the sludge, only some have proven to be effective and compatible with the use of said sludge in agriculture.

From the point of view of hygiene, optimum results are obtained by means of drying with heat or with sterilising methods, but with prohibitive energy costs. Pasteurisation processes, which are less demanding from an energy point of view, do not eliminate the phenomena of recontamination of the material during storage, which is by necessity very long as a result of the seasonal nature of its reuse in agriculture.

On a practical level, it is not necessary to eliminate the bacterial content totally but it is sufficient to reach a steady state which does not increase the quantity thereof (stabilisation of the sludge) and a simultaneous reduction in the pathogenic germs to a level such that their action is unlikely (hygienizing treatment process). The total bacterial content will therefore be practically unchanged but with a different population distribution.

In the remainder of the present invention, the term "hygienizing treatment" is understood as referring to the process which causes the reduction in the reference bacterial content which the pathogenic agents contain inside them.

The fecal coliforms, which are typically present in the initial sludge in quantities in the region of 10⁶-10⁷ N/g of dry substance, are used as the reference bacterial content.

The hygienizing treatment effect caused by an acidic environment with a pH below 4 or a basic environment, for example through the addition of basic solutions such as potassium, hydroxide, ammonia or lime, is known from the scientific literature. The use of acid substances with a pH below 4 results in the need for the use of corrosion-resistant materials, the cost of which is economically incompatible with the process which forms the subject of the present invention.

Treatments with lime, if increased to a pH over 12, have an extremely high hygienizing treatment effect but, in addition to requiring a large quantity of reagent, limit the possibility of using the hygienized sludge in agriculture.

EP-A-143392 describes a hygienizing treatment process by means of the addition of ammonium salts or alkali or alkaline earth metals with a pH between 2 and 10.

According to known methods, sludge produced by biological plants, which is generally densified to a consistency which can be shovelled in order to limit transport costs, is mixed with more diluted sludge, again resulting from biological purification treatment, and is optionally diluted with water to a consistency which can be pumped (about 10-15% dry substance), in stirred tanks into which the basic solution and optionally other solutions containing elements advantageous for agricultural use are added. After a residence time of about 15 minutes, the sludge is transferred into the final storage tank pending use. The hygienizing treatment is completed in said tank.

A drawback of said known methods is the need to transport the sludge, composed essentially of water, from the biological treatment plant to the fertiliser preparation plant. The cost of said transportation is the largest part of the overall cost of preparation of the fertiliser and its disposal.

For transportation, the sludge obtained from biological treatment is generally thickened in belt or belt-press filters, which are expensive and complex to run, to a consistency where it can be shovelled.

Moreover, as a result of the effect of the presence of air and a high bacterial content, during the filtration and transport operations before hygienizing treatment, the sludge ferments further with the formation of malodorous substances which will still be present even in the end product and which may restrict its use, for example resulting in the need for special distribution techniques, such as direct injection under the surface of the soil.

Another drawback of said known method is the ease with which the presence of pathogenic agents increases, which imposes further restrictions on its use in agriculture.

A further drawback is posed by the need for operators who must work in a malodorous environment and in the presence of possible pathogenic agents.

Disclosure of the invention

A main object of the present invention is therefore that of providing a method and plant for the treatment of biological sludge which is simple, compact and economical.

A particular object is that of providing a method and a plant which are capable of eliminating the risk of contamination of the operators employed for these operations and, more in general, of spreading of pathogenic agents in the environment.

A further object of the present invention is that of providing a method and a plant which prevents refermentation of the sludge with the consequent formation and diffusion of malodorous compounds.

These objects, as well as others which will be better understood hereinafter, are achieved by a method for the treatment and recycling of biological sludge for agricultural uses, comprising the steps of a) conveying a thickened sludge outputting from an upstream biological water treatment plant, b) densifying the thickened sludge until a densified sludge with a predetermined percentage of dry matter is obtained, c) mixing said densified sludge with products which allow the hygienizing treatment and protection from subsequent contamination by pathogenic agents, d) maturing said densified sludge for a predetermined time in order to complete the hygienizing treatment, e) storing said completely hygienized densified sludge until it is extracted for use, f) distributing the hygienized end products, characterized in that at least steps (a-e) are performed in conditions of continuous flow and in a substantially closed environment without contact with the outside and/or the operators.

According to a second aspect of the invention, a plant for the treatment and recycling of biological sludge for agricultural uses is envisaged, said plant, according to Claim 22, comprising a densifier of the thickened product outputting from a biological water treatment plant which is capable of producing a densified sludge and a clarified liquid, a mixer for mixing a hygienizing product with the densified sludge, a first tank for maturing the flow of densified sludge for a predetermined time so as to complete the hygienizing treatment, a second tank for storage of the hygienized densified sludge provided with means for loading or supplying to users, in which said thickener and said mixer are of the type which operate continuously and do not contaminate the external environment.

According to the invention, it is possible to provide a compact and closed plant, which operates continuously, without emissions of malodorous substances, and is located in the same place as the biological treatment plant.

As a result of the present invention, handling of the sludge is avoided and, in particular, the expensive densifying operations in press or press-belt filters to a mass which can be shovelled, containing about 70-80% water by weight, and of transporting said mass to the fertiliser preparation unit are eliminated.

Brief description of the drawings Further characteristic features and advantages of the invention will emerge more clearly in the light of the detailed description which follows and is provided with the aid of the attached illustrative plate, which represents a purely exemplificative and non-limiting embodiment thereof, in which: FIG. 1 shows a flowchart of the method for the treatment and recycling of biological sludge for agricultural uses according to the invention;

FIG. 2 shows a functional diagram of an example of a plant for implementation of the method shown in Fig. 1.

Detailed description of a few preferred embodiments

With reference to the abovementioned drawings, a functional diagram of a plant for the continuous treatment of biological sludge for the purpose of its use in agriculture is shown, said plant being denoted in its entirety by reference number 1 .

The plant 1 is fed with biological sludge produced, for example, by a final decanter D of a plant for the treatment of waste water or sludge, which generally contains between 2 and 4%, preferably about 3%, of dry matter.

The plant comprises, in the order of flow, a thickener 2, a mixer 3, a maturing tank 4, a storage tank 5, which is coaxial with and external to the maturing tank 4 and communicates therewith by means of the overflow 6.

The various units are connected together by means of a connecting line 7', 7", 7'" provided with pumping, cut off and control means - not shown in the figure and within the scope of a person skilled in the art - which are sealed in order to prevent bacterial contamination or the diffusion of volatile or gaseous substances.

According to the invention, the thickener 2 and the mixer 3 are of the type which operate continuously. For this purpose, the lines 7', 7" are capable of ensuring continuous operation with the thickener 2 and the mixer 3 in a steady state.

In particular, the sludge is fed continuously by means of the sealed line 7' to the continuous thickener 2, producing, again continuously, a clarified product, which is recycled by means of line 9 to a previous biological digestion stage, and a densified sludge, which is fed, again continuously, by means of the sealed line 7" to the continuous in-line mixer 3, via which the hygienizing treatment solution is added in a predetermined ratio.

After mixing, the sludge is fed by means of the sealed line 7'" into the bottom of the maturing tank 4 in such a way as to cause in said tank a plug flow from the bottom upwards as far as the overflow 6, which connects with the concentric outer storage tank 5. The capacity of the tank 4 is such as to guarantee a residence time of about 15 days in order to complete the bacterial population control action and the capacity of the tank 5 at least four months to compensate for the seasonal agricultural use.

Advantageously, the dry matter content of the densified sludge after thickening is equal to about 12%.

The decanter D may equally be part of an aerobic biological water treatment plant or an anaerobic treatment plant or an aerobic or anaerobic sludge treatment plant. An aerobic or anaerobic treatment of sludge resulting from the treatment of water reduces the total quantity of material to be treated.

The capacity of the storage tank 5 is determined by the local seasonal nature of the agricultural use, i.e. the need to store the sludge even in the months when fertiliser is not distributed, or by the explicit demands of the regulations of the local authority.

In temperate climates, said period of time is about four months. Local regulations may result in the need for the insertion of a further tank of predetermined capacity between the storage tank 5 and the loading point of the distribution means, which usually corresponds to about a day's processing. In said tank, corrections to pH values which are too basic for the land on which the sludge must be distributed may be performed, for example by means of the addition of phosphoric acid, which is available in recovery solutions, compatible with agricultural use and adds nutritious elements.

Conveniently, other elements, in solution or in solid form, may be added into the mixer 3 together with the hygienizing treatment solution, in order to correct or increase the content of elements such as nitrogen, phosphorus and potassium, which are useful for the end agricultural use.

Advantageously, the continuous thickener is of the centrifugal densifier type.

Closed stirred tanks with a suitable residence time may be advantageously used as mixers.

The storage tank 5 is provided with suitable means, which are not shown in the drawing, for loading the sludge onto the transportation means.

In order to prevent subsequent fermentations or the formation of a potentially dangerous atmosphere, a gas which is inert to fermentations, such as biogas resulting from the anaerobic fermentation of sludge - if available - or nitrogen, is present under slight overpressure in the upper part of tanks 4 and 5.

The maturing tank 4 and storage tank 5 may be built separately without departing from the scope of the present invention. In the case of coaxial tanks, the sludge present in the outer storage tank exerts a thermal isolating action with respect to the tank 4, avoiding the need for insulation or heating. The preferred form of said tanks is cylindrical but other forms, for example parallelepipeds, may also be used.

The constructional material may be any material suitable for resisting hydrostatic pressure and the corrosive and abrasive action of the sludge.

The effectiveness of the hygienizing treatment depends on the treatment time, the concentration of the reagent, the residence time and the working temperature.

The compact continuous plant according to the present invention allows the use of the heat possessed by the sludge following the biological treatment. In particular, the sludge leaving anaerobic treatment has a temperature in the region of 40°C, which makes the hygienizing treatment particularly fast and with savings in terms of reagent.

In the plant according to the present invention, the optimum temperatures are between 20 and 40°C, but even at 10°C the reaction is completed effectively during the residence time of the maturing tank. Even in particularly cool environments, the sludge leaving the biological treatment is unlikely to reach lower temperatures.

The process and the plant according to the present invention allows a further recycling of energy to be performed.

In the event that an increased speed is desired, the flow after thickening may be conveniently heated, for example by means of the injection of steam.

Advantageously, in the presence of ammonia, the temperature will be between 30 and 50°C. Higher temperatures shift the balance of the ammonia solution towards the gaseous phase, while lower temperatures slow down maturation - very much so in the case of temperatures below 10°C.

π Higher concentrations further reduce the bacterial content, but to values which are not necessary for agricultural use.

Temperature itself is also an effective hygienizing agent. According to the present invention, the sludge may be effectively hygienized continuously by means of the addition, again continuously in an in-line mixer, of steam, so as to bring the temperature of the sludge into a range of between 50 and 60^CC. Higher temperatures cause a higher destruction of the bacterial content, which is not necessary for the objects according to the present invention and has unacceptable and incompatible costs.

In the method and the plant according to the present invention, any agent capable of controlling the proliferation of the bacterial content may be used as hygienizing agents.

Advantageously, sulphur dioxide added to the sludge after thickening in any of its forms, for example in gaseous form directly into the flow, or as an aqueous solution or in the form of a compound capable of producing said effect, such as the sulphite used in the wine industry, may be used.

Example 1

A flow of 30 tonnes/hour of sludge resulting from anaerobic treatment, containing 3% dry matter and about 120,000 N/g fecal coliforms, used as indicators of the bacterial content and positive to salmonella, is thickened in a continuous densifier to obtain 22.5 tonnes/hour of clarified product, which is recycled into the sludge treatment, and 7.5 tonnes/hour of densified sludge, which can be pumped and contains about 12% dry matter. 150 kg/hour of 20% ammonia solution is added, by means of a continuous in-line mixer, to the thickened flow, which is positive to salmonella and contains 390,000 N/g fecal coliforms. After maturing for about 15 days in a tank of 3,500³, the fecal coliforms are reduced to 1 ,100 N/g and the sludge is negative to salmonella. After storage in a tank of 19,500m³, the fecal coliforms and the salmonella are absent. Example 2

0.700 tonnes/hour of steam at 4.5 bar is added, by means of a continuous in-line mixer, to a flow of 7.5 tonnes/hour of densified sludge, which can be pumped and was obtained as indicated in the preceding example. After maturing for about 15 days in a tank of 3,000m³, the fecal coliforms are reduced to 10³ N/g and the sludge is negative to salmonella. After storage in a tank of 16,500 m³, the fecal coliforms and the salmonella are absent.

The plant according to the present invention is compact and may be built next to the biological water or sludge treatment plant and directly connected thereto, avoiding expensive densifying and transport operations, as well as the diffusion of malodorous substances and the bacterial contamination of the environment and the operators.

Since the method according to the present invention stabilises the sludge by chemical means, in biological waste water treatment plants where the sludge line only has the function of stabilising said sludge, said line may be eliminated entirely since the sludge flows directly into the plant, without intermediate aerobic or anaerobic stabilisation, with obvious benefits of both a cost-related and operational nature.

Moreover, the sludge obtained, which is ready for agricultural use, is practically devoid of odours and the pathogenic agents which restrict its methods of application and limit its use.

Advantageously, where local regulations permit it, sludge produced by biological plants in the pharmaceutical industry, for example for the production of antibiotics, may be treated in the same way. In some regions, agricultural use is only prohibited for sludge in which substances which are harmful for the environment are present, except for where they are demonstrated not to be toxic. The invention thus conceived therefore achieves the predetermined objects.

Obviously, it may also assume in its practical embodiment forms and configurations which differ from those illustrated above, without thereby departing from the present scope of protection.

Moreover, all the details may be replaced by technically equivalent elements and the dimensions, forms and materials used may be of any type according to the needs.

Claims

1. Method for the treatment and recycling of biological sludge for agricultural uses, comprising the following steps: a) conveying a thickened sludge outputting from an upstream biological water treatment plant; b) densifying the thickened sludge until a densified sludge with a predetermined percentage of dry matter is obtained; c) mixing said densified sludge with products which allow the hygienizing treatment and protection from subsequent contamination by pathogenic agents; d) maturing said densified sludge for a predetermined time in order to complete the hygienizing treatment; e) storing said completely hygienized densified sludge until it is extracted for use; f) distributing the hygienized end products; characterized in that at least steps (a-e) are performed in conditions of continuous flow and in a substantially closed environment without contact with the outside and/or the operators.

2. Method according to Claim 1 , characterized in that said conveying step a) is performed by means of pumping.

3. Method according to Claim 1 , characterized in that said densifying step b) is performed by means of a centrifugal action.

4. Method according to Claim 1 , characterized in that said densifying step b) is performed in order to obtain a densified sludge with a maximum concentration of dry matter such as to allow it to be pumped.

5. Method according to Claim 4, characterized in that said concentration of dry matter is between 6% and 20% by weight and is preferably between 8% and 12%o by weight.

6. Method according to Claim 1 , characterized in that the clarified liquid phase resulting from step b) is recycled to the purification plant upstream.

7. Method according to Claim 1, characterized in that said mixing step c) takes place in a substantially hermetically closed environment.

8. Method according to Claim 7, characterized in that the residence time of the densified sludge during said mixing step c) is preferably close to 2 minutes.

9. Method according to Claim 1 , characterized in that said mixing step c) takes place by means of the continuous addition of said hygienizing products into the flow of densified sludge in a predetermined ratio.

10. Method according to Claim 1 , characterized in that the dosage of said hygienizing products in said step c) is such as to complete the hygienizing treatment action in a maturation time of less than 10 days.

11. Method according to Claim 1 , characterized in that in said step c) fertilising elements in solution form are supplied in a metered amount in order to obtain a predetermined value of the concentration of the main fertilising elements.

12. Method according to Claim 1 , characterized in that the thickened sludge is hygienized and treated with bacteriostatic substances immediately after the step of conveying the sludge from the biological treatment plant upstream.

13. Method according to Claim 1 , characterized in that said hygienizing products comprise ammonia and/or potassium hydroxide in solution form.

14. Method according to Claim 1 , characterized in that said hygienizing products comprise sulphur dioxide added in its gaseous phase, in the form of an aqueous solution or as a metabisulphite salt available in the solid phase or in the form of an aqueous solution.

15. Method according to Claim 1 , characterized in that said hygienizing products comprise steam, which is added in said mixing step and is such as to obtain a temperature of the thickened sludge flow of between 50°C and 100°C and preferably close to 65°C.

16. Method according to Claim 1 , characterized in that said hygienizing products comprise basic solutions in a predetermined ratio.

17. Method according to Claim 13, characterized in that said predetermined ratio for ammonia in solution form is such that the concentration of ammonia present after mixing is between 0.2% and 1% by weight and is preferably close to 0.4%) by weight.

18. Method according to Claim 13 or 16, characterized in that said predetermined ratio for basic solutions is such that the pH after mixing is between 8.5 and 12 and is preferably close to 9.5.

19. Method according to Claim 1 , characterized in that the temperature of the sludge after mixing is between 10°C and 60°C and is preferably close to 30°C.

20. Method according to Claim 1 , characterized in that said predetermined time for the maturing step d) is between 1 day and 20 days and is preferably close to 10 days.

21. Method according to Claim 1 , characterized in that said maturing step d) is performed in the presence of a gas which is inert to fermentation and is at a pressure which is slightly higher than atmospheric pressure.

22. Plant for the treatment and recycling of biological sludge for agricultural uses comprising:

- a densifier (2) for the thickened product outputting from a biological water treatment plant, suitable for obtaining a densified sludge and a clarified liquid; - a mixer (3) for mixing a hygienizing product with the densified sludge;

- a first tank (4) for maturing of the flow of densified sludge for a predetermined time so as to complete the hygienizing treatment;

- a second tank (5) for storage of the hygienized densified sludge, provided with means for loading or supplying to users; in which said thickener (2) and said mixer (3) are of the type which operate continuously and with no external contamination.

23. Plant according to Claim 22, characterized in that the movement of said thickened sludge and said densified sludge is performed by means of pumps.

24. Plant according to Claim 22, characterized in that said continuous mixer (3) is closed and has a capacity such as to allow a residence time which is preferably close to 2 minutes.

25. Plant according to Claim 22, characterized in that said thickener (2), said mixer (3) and said tanks (4) (5) are connected in series in a sealed manner against odours, emissions and bacterial contamination by means of pipes 7', 7", 7'" provided with suitable cut off, circulation and control means.

26. Plant according to Claim 22, characterized in that said flow of densified sludge is fed into the bottom of said first tank (4) in such a way as to obtain therein a plug flow from the bottom upwards.

27. Plant according to Claim 22, characterized in that said first tank (4) and said second tank (5) are concentrated in a single container.

28. Plant according to Claim 27, characterized in that said first tank (4) and said second tank (5) are substantially coaxial and are separated by an overflow wall.