TREATMENT OF SLUDGE

Abstract: Sludges, especially sewage sludges, are treated to stabilise them by biological digestion (21) and/or to remove heavy metals (19, 23) by using a sulphur compound cycle in which sulphate is reduced to hydrogen sulphide which is then re-oxidised (36, 40) to sulphate for return to the process. The sulphate reduction is effected during digestion (21) of the sludge and the sulphate from re-oxidation (usually sulphuric acid) is either cycled to the digestion of more sludge, or used to leach heavy metals (14) and subsquently recovered upon their precipitation as sulphides (23). Instead of sulphate, other sulphur species can be used provided they are more oxidised than hydrogen sulphide.

Description: TREATMENT OF SLUDGE This invention relates to the treatment of sewage sludge and like materials to stabilise, disinfect and/or remove heavy metals therefrom. Hereinafter, the term "sludge" is used to include sewage sludge and like materials including surplus biological sludges from the biological treatment of a range of industrial waste waters; any waste material which will benefit from removal of insoluble metal species and/or the stabilisation of organic material, and metal-contaminated inorganic waste containing waste organic material. Currently, in many countries, a significant fraction of the sewage sludge is disposed of to farmland. This fraction is likely to increase following the cessation of marine disposal of sewage sludge and untreated sewage. Prior to disposal to agricultural land, sewage sludge must first be stabilised. Stabilisation involves the destruction of biologically fermentable solids. Typically, stabilisation processes destroy around 407. of the volatile matter in a sewage sludge, itself forming around 757„ of the total solid matter. Various methods, both aerobic and anaerobic, are known for digesting sewage sludges. Though not a requirement, the continuing public acceptability of disposing of sewage sludge on agricultural land would be increased if the sludge was also treated for the removal of potentially pathogenic organisms bv disinfection, pas eurisation or some other process. Methods are known for pasteurising sewage sludges, for example high temperature treatment in certain thermophilic digestion processes. The rate of application of sewage sludges to agricultural land is also limited by the heavy metals content of such sludges. It is known, for example from DD- A-290408 and US-A-4370233 , to remove heavy metals from sludges by treating the sludge with acid, dewatering the sludge, and then precipitating the heavy metals as sulphides, thus regenerating the acid for re-use. The treated sludge can then be safely disposed of on land. We have now found an improved way of treating sludges either to stabilise them by digestion or to remove heavy metals, or to effect both processes which can be operated in a more efficient way than previously. According to the present invention, there is provided a continuous process for the treatment of a sludge to oxidatively digest it and/or to remove heavy metals therefrom in which process sulphur-containing materials are cycled via sulphate (or another sulphur species which is more oxidised than hydrogen sulphide) and hydrogen sulphide wherein the sulphate is biologically reduced to hydrogen sulphide which is removed from the sludge and re-oxidised to sulphate (or another sulphur species which is more oxidised than hydrogen sulphide) which is then mixed with sludge. The use of a sulphur material cycle for these treatments gives rise to a number of advantages including the ability to effect treatments not hitherto practicable and to make significant cost savings over prior proposals. The invention is hereinafter described with reference to sulphate but it is to be understood that other sulphur species can be used provided they are more oxidised than hydrogen sulphide. In the method of the invention for effecting digestion of the sludge, the sludge solids are oxidised and the sludge is thereby stabilised, whilst simultaneously sulphate (usually sulphuric acid) is reduced to hydrogen sulphide. This digestion process is known in itself but little used. In accordance with the present invention, the hydrogen sulphide so formed is oxidised back to sulphate (or some other sulphur species more oxidised than hydrogen sulphide), which can then be recycled back to the digestion step. In this way, the sulphur compounds are used catalytical ly . The hydrogen sulphide oxidation step can be chemical or physical (eg. by burning the hydrogen sulphide) , but we prefer to use a biological oxidation step. The incorporation of biological oxidation of the resulting hydrogen sulphide is a highly preferred feature of the invention . The oxidation of the hydrogen sulphide resulting from the digestion step serves two main purposes. Firstly, the oxidation to sulphate allows the sulphur to be used in -a cyclic, catalytic fashion and, secondly, if hydrogen sulphide is not removed from the digestion step it will build up until it becomes toxic to the process. Because of this toxicity, in one preferred configuration of this process, air is injected into recirculating off-gas and the resultant mixture contacted with water and micro-organisms (typically growing on the surface of a plastic medium, although the air may be sparged through a suspension of micro-organisms in a water based medium) in a bioscrubber. This will preferably be placed inside a gas recirculation loop placed around the digestion step. This will normally need to operate in an oxygen limited fashion such that excess oxygen does not pass into the digestion step. When a bioscrubber is operated under oxygen limited conditions, the end product of the biological oxidation of hydrogen sulphide is usually colloidal sulphur suspended in the water used by the bioscrubber. Excess water from this bioscrubber can be drawn off and passed to a second bioscrubber operating under air rich conditions. This will complete the oxidation of the colloidal sulphur, and will also oxidise hydrogen sulphide from excess gas from the digestion step together with hydrogen sulphide from subsequent steps in the process, to sulphuric acid, typically at a pH value less than 1. The off-gas from the second bioscrubber will still contain traces of hydrogen sulphide which will need destruction to avoid the production of unpleasant odours. This could be carried out in a third bioscrubber operating under air rich conditions at neutral pH .values or by any of a range of standard technologies. The recycled off-gas, after treatment for removal of hydrogen sulphide, can advantageously be sparged back into the sulphate digester contents. This both acts to mix the contents of the digester and to increase the efficiency of stripping hydrogen sulphide out of the process. In one possible configuration of the process, the process for removing the hydrogen sulphide may be in a separate off-gas recycle loop to that used for sparging. This is because it may be desirable to prevent the stripping out of the hydrogen sulphide from being too efficient. A certain level of hydrogen sulphide in the digester (by being toxic to methanogenic micro-organisms) may be useful for preventing the competing production of methane and also for enhancing the destruction of any pathogenic organisms that were not destroyed in the acid digestion step described below. The treatment according to the invention for removal of heavy metals initially involves the use of sulphuric acid, preferably from the sulphur-containing materials cycle. One way in which this can be obtained has been described above with reference to the digestion step and in the following description it is assumed that this source is used, i.e. that the overall process is being effected to carry out both digestion and heavy metal removal. However, it will be understood that sulphur- co-mpound cycles can be used for either treatment alone, independently of the other. In the preferred heavy metal treatment, the sulphuric acid produced in the digestion step is mixed with feed sludge in an acid digestion step, with a retention time ranging from a few hours to 1-2 days. This digestion step serves two purposes. At a pH value of typically 1-1.5, there will be considerable die-off of pathogenic and indicator faecal organisms. At the same time many insoluble forms of heavy metals will be solubilised. Beneficially this acid digestor can be. aerated. This will act to provide useful agitation and mixing and, in addition, strip out any hydrogen sulphide resulting from the action of acid on heavy metal sulphides, thereby further promoting the solubi1isation of these metals. Such an aeration may also increase the die-off of pathogenic organisms. The air from this stage will pass from the acid digestion step to the air-rich bioscrubber (when one is used). The sludge leaving the acid digestion step is then dewatered. This can be performed typically by a centrifuge or a drum filter or any of a range of standard dewatering techniques. The liquor from this step will contain the majority of the solubilised heavy metals. A portion of the hydrogen sulphide rich gas from the sulphate digestion step can then be sparged through this liquor. This will cause the previously solubilised heavy metals to re-precipitate as their sulphides. The insoluble sulphides are removed from the liquor by settlement or filtration, for final disposal or possible recovery, while the now metal free liquor, containing the sulphuric acid, is recombined with the dewatered sludge cake and passed forward into the sulphate digester. The gas leaving the precipitation step will pass forward to the air-rich bioscrubber. It will be noted that sulphur in this process is used catalytically. Of course no catalytic cycle is perfect and some sulphur will need adding to the cycle. Almost any form of sulphur can be added, the most appropriate place in the cycle depending on the chemical form of the sulphur, for example the addition of calcium sulphate to the sulphate digestion step. Suitable calcium sulphate may be obtained as a waste product from a number of other industries. The sludge leaving the sulphate digester may still contain traces of hydrogen sulphide and other odorous compounds. It may be beneficial to aerate this for a short period to remove these odourous compounds. The air from this aeration step would pass on to the air-rich bioscrubber . Insofar as the invention relates to sludge digestion or stabilisation, it combines a biological digestion step in which organic material is oxidised and sulphate is simultaneously reduced to hydrogen sulphide, with the oxidation of that hydrogen sulphide, typically also in a biological process, back to sulphuric acid, which is returned to the process, resulting in a catalytic sulphur cycle. As a metals removal process, the invention uses sulphuric acid to solubilise and leach heavy metals out of sludge, followed by the biological reduction of that sulphuric acid to hydrogen sulphide, its stripping out of solution and use to regenerate sulphuric acid, typically in a biological process, for use in the solubilisation and leaching stage. This is another cyclic process based on sulphur. A further preferred feature of this process is the use of biologically created hydrogen sulphide to re- precipitate heavy metals that have been leached out of sewage sludge by treatment with acid. As will be understood, the invention also covers a combination of these aspects in a single super-cycle. As has been described above, the invention can also effect disinfection of the sludge, especially in the acid digestion step of the heavy metal removal. In order that the invention may be more fully understood, reference is made to the accompanying drawing whάch illustrates schematically one preferred process of the invention combining both the digestion process and the heavy metal removal process. Referring to the drawing, a sludge supply line 10 delivers sludge (to be treated) to a thickener tank 11. Supernatant water can be drawn off on line 12. The sludge passes on line 13 to a sludge/ sulphuric acid reactor 14 into which air is supplied by line 15 and sulphuric acid by line 16. Exiting gases leave by line 17. After leaching of the heavy metals and their solubi 1isation in reactor 14, the sludge passes on line 18 to a dewaterer 19 from which the dewatered sludge passes via line 20 to a sulphate digestion reactor 21. The filtrate from dewaterer 19 passes on line 22 to a unit 23 for heavy metal removal. Here hydrogen sulphide rich gas is delivered by line 24 and sparged into the filtrate to precipitate heavy metal sulphides. These are then filtered off and the metal-free filtrate is returned via line 25 to mix with the sludge for sulphate digestion in reactor 21. Metal sulphide sludge is removed from unit 23 via line 26. The gases exiting unit 23 are passed on line 27 to join with line 17. Sulphate digestion reactor 21 is provided with a supply line 28 for calcium sulphate make up (to replenish the sulphur in the cycle), and a supply line 29 to carry recycled gas at about 17. hydrogen sulphide into the reactor. After biological oxidation in the reactor, the stabilised sludge is passed on line 30 to a dewaterer 31 (from which filtrate is removed by line 32). The dewatered sludge is then passed along line 33 to a disposal unit 34. In reactor 21, sulphate (largely sulphuric acid) is biologically reduced to hydrogen sulphide. The hydrogen sulphide-rich gas (about 47„ H2 ) passes via line 35 to an air-lean bioscrubber 36. This has an air supply line 37 and a water supply line 38. The aqueous solution (or suspension) from bioscrubber 36 passes via line 39 to the top of air-rich bioscrubber 40. Supply line 29 conducts gas from the top of bioscrubber 36 to reactor 21. Air rich bioscrubber 40 receives air in gas line 17 (from reactor 14) at its base. An interconnecting line 41 joins lines 35, 24 and 17 as shown. Gases exiting bioscrubber 40 are passed in line 42 to filter unit 43 to remove odour etc. and then released (44). The liquid exiting the bottom of bioscrubber 40 passes via line 16 to reactor 14 (the liquid contains sulphuric acid). The process of the invention as exemplified operates very satisfactorily both as a whole and as to its two component parts separately.

Claims: C LA I M S : 1. A continuous process for the treatment of a sludge to oxidatively digest it and/or to remove heavy metals therefrom in which process sulphur-containing materials are cycled via sulphate (or another sulphur species more oxidised than hydrogen sulphide) and hydrogen sulphide wherein the sulphate is biologically reduced to hydrogen sulphide which is removed from the sludge and re- oxidised to sulphate (or another sulphur species more oxidised than hydrogen sulphide) which is then mixed with sludge . 2. A process according to claim 1, wherein the hydrogen sulphide is re-oxidised in a biological process. 3. A process according to claim 1 or 2, wherein the sludge is digested by biological oxidation in the presence of sulphate which is simultaneously ' biologically reduced to hydrogen sulphide. 4. A process according to claim 1 or 2, wherein the heavy metals are removed from the sludge by contacting the sludge with sulphuric acid to dissolve the heavy metals, separating the liquor from the solids, passing hydrogen sulphide into the liquor to precipitate heavy metal sulphides and regenerate sulphuric acid, separating the sulphuric acid liquor and biologically reducing it to hydrogen sulphide, and then re-oxidising at least part of the hydrogen sulphide to sulphuric acid for re-use. 5. A process according to claims 4 and 5, wherein a sludge is treated to remove heavy metals therefrom and is then oxidatively digested, wherein the sulphuric acid liquor is mixed with the separated sludge solids for the digestive oxidation of the sludge with simultaneous reduction of the sulphate to hydrogen sulphide. 6. A process according to claims 4 or 5, wherein during contact of the sludge with sulphuric acid to dissolve heavy metals, the mixture is aerated. 7. A process according to claims 4 or 5, wherein the hydrogen sulphide which is passed into the liquor to precipitate heavy metal sulphides is obtained from the biological reduction of sulphate in the process. 8. A process according to any preceding claim wherein the hydrogen sulphide and sulphate are cycled in effect catalytically and, to compensate for minor losses, a source of sulphur is added during the process. 9. A process according to claim 8, wherein a sulphate soure is added at the stage of biological reduction of sulphate to hydrogen sulphide.

Inventors: HOBSON JOHN ANDREW, HOBSON, JOHN, ANDREW
Applicants: HOBSON JOHN ANDREW, WRC PLC
Application year: 1993
Main classification: C02F11
IPC codes: C02F3, C02F11