STABILIZATION OF BIO-ORGANIC SLUDGES

Abstract: The stabilization of bio-organic sludges such as those derived from sewage, meat works or dairy works effluent, against putrefication or microbial decomposition is disclosed. The stabilization is achieved by adding a bio-organic material, such as coir fibre, straw, flax and/or ground bark and/or a source of carbohydrate. Preferably, the final concentration of water in the product is at least 15 % by weight water, with a preferred upper limit of 70 % by weight water.

Description: STABILIZATION OF BIO-ORGANIC SLUDGES This invention relates to a stabilized bio- organic sludge and to a method of stabilizing bio- organic sludges which are produced by effluent treatment processes. The term "bio-organic sludge" includes sludges from sewage works, slaughterhouses, farms, meat and dairy processors, and the like, but excludes sludges produced by oil spills or other chemical (non- biological) processes. These bio-organic sludges are produced by aerobic or anaerobic treatment of the liquid and settled solid fraction of the effluent or as the result of flocculation and airflotation or settlement of the effluent, as discussed, for example, in EP-0048723 and O93/15026. Such sludge is conventionally applied to agricultural land or dumped at sea. Bio-organic sludges from effluent treatment often contain significant amounts of proteinaceous material with relatively high amounts of essential amino acids. Extended trials on such sludge has shown it to be of value as a fertilizer or an animal feed. The sludge in a wet state is unstable and rapidly putrefies unless dried or stored at a low temperature. Typically, the dry content of such sludges is 10-30wt.t so drying requires a large amount of energy and maybe prohibitively costly. Some bio-organic sludges also present health problems because they contain large numbers of pathogenic bacteria. These bacteria have to be destroyed by heat or other disinfection treatments before the sludge can be used as a fertilizer or a food product, however, such disinfected sludges, if left in a wet state, will still putrefy if exposed to atmospheric organisms. HU-209271 discloses the addition of organic material such as straw, flax, reed or hemp to pre- stabilized fermented and/or oxidized biogas-free sludge of 70-80% moisture content. The disclosed process relies on a composting process to achieve stabi ization via a lengthy microbial route involving bacterial and fungal processes. Such processes convert a substantial percentage of the nitrogen present in the feed material to ammonical nitrogen, a large proportion of which is lost to the atmosphere or leached out of the material by rain water. In the Hungarian document, denitrification is cited as being beneficial and reduced pollution. However, if the intended product is to be used for agricultural or horticultural purposes, nitrogen should be retained for use by plants or animals utilizing the product. The current invention solves this problem. We have unexpectedly found an easy and inexpensive method' of stabilizing bio-organic sludges against putrefication or microbial decomposition, which advantageously maintains nitrogen levels in the material, thereby reducing nitrogen loss by leaching or degradation into, for example, ammonical nitrogen. Therefore, according to a first aspect of the invention, there is provided a bio-organic sludge stabilized against putrefication or microbial decomposi ion by the addition of bio-organic material . A second aspect of the invention provides a method of stabilizing wet bio-organic sludge against putrefication or microbial decomposition comprising mixing with organic material. The organic material is derived from plants, animals, or fungi, and excludes synthetic chemical or organic compounds. The material may comprise at least one source of bio-organic material with a water content of less than about 5*v/v. The organic material may provide a source of carbohydrate. In a third aspect of the invention there is provide a bio-organic sludge stabilized against putrefication or microbial decomposition comprising an added source of carbohydrate. A further aspect of the invention provides a method of stabilizing bio- organic sludge against putrefication or microbial decomposition comprising mixing with a source of carbohydrate. This may be added in addition to other bio-organic material deficient in carbohydrate. The nitrogen contained in the bio-organic sludge is preferably bound within the bio-organic sludge so that it is only slowly released into the environment. Preferably, the wet bio-organic sludge is a by¬ product of sewage, meat works, dairy works or other industrial effluent treatment. Preferably the water content of the final product is at least 15% by weight water with a preferred upper concentration of water of 70% by weight water, or more preferably 60% by weight water. An especially preferred concentration of water in the final product is 30-40% by weight water, and more preferably 36% by weight water. The bio-organic material or carbohydrate source may be a dry organic fibrous material and/or a dry vegetable matter of less than 5% moisture, and may be added in about an equal proportion to the sludge. One preferred bio-organic material is coir fibre complete with its pith, from coconut shells, others include straw, flax, feathers, hair, fungal material, and/or ground bark, all of which are relatively inexpensive. The stabilized bio-organic sludge of the invention may be used as an animal feedstock, or as a peat substitute, soil conditioner, or potting media in a growing medium. Unexpectedly, we have found that, by mixing wet organic sludge with bio-organic material, putrefaction is prevented. The stabilized product can then be stored indefinitely without the development of offensive odours or other signs of putrefaction. The stabilized product has the advantage that it is inexpensive to produce and may use dry bio-organic material which would otherwise not be used. The presence of the bio-organic material has the additional advantage that it acts as a soil conditioner. Coir fibre, for example, improves soil structure, retains moisture, and provides humus, which are necessary for good plant growth. In general, the protein in the organic sludge has the advantage over inorganic nitrogen fertilizers in that it allows nitrogen to be released slowly over a longer period of time. This is beneficial to seedlings and prevents polluting nitrogen run-off into rivers after heavy rain. The mode by which the bio-organic material stabilizes the wet organic sludge may, in part, be due to the absorption of moisture from the sludge by dry organic material. However, since the water content of typically treated proteinaceσus sludge is around 75%, and assuming that the fibre is dry, then mixing in equal proportions by weight would reduce the mean water content to 30-40%. This is much higher than the generally accepted level of 10% moisture required for protein stability. This implies that some mechanism apart from moisture reduction plays a part in increasing the stability of the product . We believe that a more likely mechanism is the formation of a complex between the protein and a component of the organic material. One such mechanism would be the well-known MaiHard Reaction between protein and carbohydrate. Such a reaction involves the condensation of an amino group with a reducing group, for example, aldehyde or ketone groups. (Reference McGraw Hill Encyclopaedia of Science and Technology 6th. Ed. (1987), Vol.10 pp. 349-350.) It is logical to assume that a MaiHard- type reaction occurs between carbohydrate groups present in the coir fibre and its pith and protein in the sludge to yield complexes with increased resistance to bacterial decomposition. This would account for the increased resistance to putrefaction despite still having a relatively high water content. We have been able to show that the essential components for a MaiHard Reaction are present in our system. Such a mechanism has the result that the reaction between the protein and reducing groups in the carbohydrate binds nitrogen so that it is only slowly released. The formation of a complex between the protein and carbohydrate denies access by microbes, such as fungi, to the protein in the sludge. This inhibits the growth of such organisms and prevents, for example, the formation of offensive odours. In one preferred process described in European Patent No. 0048723, the proteinaceous sludge is processed at elevated temperatures under acidic conditions. This is likely to result in increased reactivity of the protein with carbohydrate due to the rupture of peptide linkages giving an increase in the number of free amino and carboxyl groups. Accordingly, the invention provides an acid treated bio-organic sludge neutralized by the addition of calcium oxide, calcium hydroxide, or other suitable base. The base may be added before, during or after the addition of the other organic material . Neutralization returns any residual metals which have been solubilized to their solid phase, preventing leeching, and achieves a pH suitable for, for example, plant growth. Neutralization may additionally produce an exothermic reaction which heats the mixture, improving the MaiHard Reaction in the mixture. EXAMPLE 1 Surplus activated sludge from a municipal sewage treatment works was processed according to the method described in EP-0048723. The resulting material had a dry solids content at 26wt.%. The sludge was thoroughly mixed, at 60°C after discharge from the treatment works, with dry coir fibre, in a ratio at 1 part treated sludge to 0.8 part coir fibre using a mechanical blender. The resulting product appeared to be substantially dry and when stored under ambient temperature did not develop an offensive odour or show other signs of putrefaction after seven months of storage. The final water concentration of the product was 36%. The nitrogen levels in the starting sludge is typically 8-8.5% dry weight. In the final product, the level of nitrogen is typically 3.5-4.5% dry weight. Although it is preferred that the added bio- organic material i.e. having a moisture content of less than 5% for the best results, this is only important with respect to its influence on the final product moisture content, which ideally should be in the range 30-50% moisture. Also, although it is preferred that the organic sludge be treated to destroy pathogenic bacteria, it may be sufficient to treat the organic sludge in a manner which provides unfavourable conditions for continued growth of such bacteria. Accordingly, the organic sludge may be activated sludge treated by the acid/alkali process of EP-0048723 or WO93/15026, or may be digested sludge. The mixing of the sludge and the organic material may be carried out at ambient or an elevated temperature. EXAMPLE 2 Chemical Evidence for a MaiHard Reaction Mechanism In the case of the stabilization of activated sludge by mixing it with coir fibre, the following experiments demonstrate that the essential components for a MaiHard Reaction are present. Amino groups are provided by the activated sludge which contains approximately 50% of protein of which about 7% is lysine which has a free terminal amino group. Coir fibre was shown to contain carbohydrate with reducing groups by the following procedure. Coir fibre (5g) was extracted with 100ml of 0.5N hydrochloric acid at 100°C for one hour. The mixture was filtered and the clear filtrate analyzed for hexose sugars using the anthrone method (Trevelyan N.E. al Harrison, J.S. Biochem Journal Vol. 50, p.298, 1952) using a glucose standard. The mean of two determinations gave a value of 4.7% (as glucose) for the carbohydrate content. That the extracted carbohydrate contained reducing groups was demonstrated by reaction with Fehlings solution, which gave a red precipitate of cuprous oxide when boiled with the coir extract. Furthermore, coir fibre was shown to react directly with protein by the following experiment. Coir fibre was ground to a slurry with water and allowed to stand overnight to allow complete hydration. The mixture was then poured into a glass column to give a fibre bed 20 x 2 cm. A solution of ovalbumin (0.5mg/ml) was passed through the fibre bed at a flow rate of 1 bed volume per hour. The concentration of protein in the column effluent was determined turbidmetrical ly with phosphotungstic acid solution and compared with the initial value. At the above flow rate, approximately 66% of the protein was absorbed by the fibre. Recycling the effluent (2 bed volumes) through the bed or allowing the system to stand for several hours gave almost complete uptake. The fact that the uptake of protein was a slow process favours a covalent condensation reaction rather than an ion exchange mechanism.

Claims: CLAIMS 1. A bio-organic sludge stabilized against putrefication or microbial decomposition by the addition of other bio-organic material. 2. A stabilized bio-organic sludge according to claim 1 , comprising added dry bio-organic material with a water content of less than about 5%v/v. 3. A stabilized bio-organic sludge according to claims 1 or 2 , wherein the added bio-organic material includes a source of carbohydrate. 4. A bio-organic sludge stabilized against putrefication or microbial decomposition comprising an added source of carbohydrate. 5. A stabilized bio-organic sludge according to any previous claim, wherein the nitrogen contained in the bio-organic sludge is bound within the stabilized bio-organic sludge so that it is only slowly re eased. 6. A stabilized bio-organic sludge according to any one of claims 1 to 5, containing a final water content of at least 15% by weight water. 7. A stabilized bio-organic sludge according to any one of claims 1 to 6, containing a final water content of less than 70% by weight water. 8. A stabilized bio-organic sludge according to claims 6 and 7, containing a final water content of 30-40% by weight water. 9. A stabilized bio-organic sludge according to any one of claims 1 to 8, comprising organic sludge from sewage, meat works, dairy processing, farm, or other industrial effluent treatment. 10. A stabilized bio-organic sludge according to claims 1 to 9 , wherein the bio-organic sludge is an acid treated sludge neutralized by the addition of calcium oxide, calcium hydroxide, or other suitable base. 11. A stabilized bio-organic sludge according to any one of claim 1 to 10, comprising added bio- organic fibrous material and/or other vegetable or animal matter. 12. A stabilized bio-organic sludge according to claim 11, wherein the added material comprises coir fibre, straw, flax, and/or ground bark. 13. A stabilized bio-organic sludge according to any one of claims 1 to 12, comprising bio-organic sludge and added dry bio-organic material in approximate y equal proportions. 14. A stabilized bio-organic sludge according to any previous claim, for use as an animal feed stock, fertilizer, or peat substitute. 15. A method of stabilizing wet bio-organic sludge material against putrefication or microbial decomposi ion by mixing with at least one other source of bio-organic material. 16. A method of stabilizing wet bio-organic sludge according to claim 15, by mixing with at least one other source of dry bio-organic material with a water content of less than about 5%v/v. 17. A method of stabilizing wet bio-organic sludge according to claim 15 or 16, wherein the bio- organic material is a source of carbohydrate. 18. A method of stabilizing wet bio-organic sludge against putrefication or microbial decomposition by mixing with at least one source of carbohydrate. 19. A method according to claim 18, additionally comprising the step of adding other bio- organic material deficient in carbohydrate. 20. A method of stabilizing wet bio-organic sludge material according to any one of claims 15 to 19, wherein the wet bio-organic sludge material is a by-product of sewage, dairy processing, industrial or farm effluent treatment. 21. A method of stabilizing wet bio-organic sludge material according to claims 15 to 20, wherein the sludge material is pretreated to destroy or render substantially inert any pathogenic bacteria. 22. A method of stabilizing wet bio-organic sludge material according to any one of claims 15 to 21 , comprising mixing wet bio-organic sludge material with a bio-organic fibrous material, and/or other vegetable or animal matter. 23. A method of stabilizing wet bio-organic sludge material according to any one of claims 15 to 22, comprising mixing wet bio-organic sludge with coir fibre, straw, flax, feathers, hair, and/or ground bark. 24. A method of stabilizing wet bio-organic sludge material according to any one of claims 15 to 23, wherein the wet organic sludge material is mixed in approximately equal proportions with the dry bio- organic material. 25. A method of stabilizing wet bio-organic sludge material according to claims 15 to 24, wherein an acid treated bio-organic sludge is neutralized by the addition of calcium oxide, calcium hydroxide or other suitable base before, during or after the addition or the other bio-organic material. 26. A stabilized bio-organic sludge against putrefication or microbial decomposition substantially as hereinbefore described with reference to the accompanying description. 27. A method of stabilizing wet bio-organic sludge material against putrefication or microbial decomposition substantially as hereinbefore described with reference to the accompanying description.

Inventors: DEAR ARTHUR STEPHEN, GRANT ROY ARTHUR, DEAR, ARTHUR, STEPHEN, GRANT, ROY, ARTHUR
Applicants: CHEMRING GROUP PLC, DEAR ARTHUR STEPHEN, GRANT ROY ARTHUR, DEAR, ARTHUR, STEPHEN, GRANT, ROY, ARTHUR
Application year: 1996
Main classification: C02F3
IPC codes: C02F3, C02F11, C05F1, C05F7, C05F15