IES20010342A2 - Sludge and slurry destruction plant and process - Google Patents

Abstract

A process for the substantial destruction of sludge is described, comprising diluting the sludge to 1,000 - 5,000 mg/litre COD and reacting the sludge with a bacterial population at oxygen levels greater than 1ppm, under conditions of high intensity mixing. The biological process for the destruction of sludge described herein is particularly useful for the destruction of organic slurries that arise from intensive animal rearing units and sludges resulting from the biological treatment of industrial and domestic wastewater. <Figure 1>

Description

The present invention relates to a process for the destruction of organic waste and slurries from a variety of sources which hitherto have been difficult to dispose of. In particular the invention relates to a biological process for the destruction of organic slurries that arise from intensive animal rearing units and sludges from the biological treatment of industrial and domestic waste water.

Background to the Invention In recent years changes in Environmental Regulations have resulted in stringent legislation coming into force in relation to effluent from waste water treatment plants. However, whilst the quality of effluent being released into the environment from waste water treatment plants may have improved, there is still the problem of disposal of the residual solids which are produced as a by product of the waste water treatment. Sewage treatment plants treat the waste water from domestic and industrial sources. Such treatment results in the formation of residual solids which are termed sludge. The disposal of the sludge can prove problematic and can be very costly and labour intensive.

The liquid or solid organic sludges that arise from intensive animal rearing units and the sludges arising from the treatment of domestic sewage or industrial waste water, are often quite intractable and the routes for the disposal thereof are often difficult and expensive.

In this document, the term slurry is defined as the waste product arising from the treatment of animal waste. Sludge is defined as the \ ^aste end product of the biological OPEN TO PU8UC INSPECTION U^OfrSkSECTION 28 ANO RUifd3 i JNL No. treatment of domestic and industrial waste water. Sewage is defined as the waste water from domestic or industrial sources or a blend of these. Henceforth, and for the purpose of brevity, the term “sludge” in this document shall encompass the term “slurry”.

Such organic waste may be an inevitable consequence of the process itself such as in intensive animal rearing. The composting process may allow the recycling of some sludges to land, but the sheer volume of waste from such intensive processes often makes the disposal an expensive and environmentally sensitive issue.

Other sludges such as those arising from the waste water treatment of industrial effluent, food and drink processing plants, paper manufacturing, pharmaceuticals manufacture or domestic sewage treatment plants will arise as a liquid waste and may be amenable to further treatment such as anaerobic digestion processes to reduce the overall volume. The liquid sludge is contacted with anaerobic bacteria in a tank over a period of about fourteen days. Such secondary sludge processing will produce methane gas as a by-product and provide a source of power to help offset treatment costs. Aerobic digestion is also used by contacting the liquid sludge with blown air sometimes at elevated temperatures and in the presence of thermophilic bacteria, although it does not produce methane.

Currently known processes for treating sludge are in essence sludge reduction processes. There is currently no process of total organic sludge destruction available, other than incineration.

Sludge reduction programmes only produce at best a reduction of forty to sixty percent of the mass of sludge. The sludge from the digestion processes also still have to be disposed of.

German Patent No. 19502856 describes a waste water treatment plant which produces more methane and less sludge than conventional anaerobic digestion processes. The supply to the waste water treatment plant passes to a primary sedimentation tank. The plant comprises at least one transfer device and thickening system for sludge produced. Settled sewage passes to an aerobic activated sludge plant and an anaerobic reactor digests the sludge. The system uses centrifuges to treat the thickened reactor sludge prior to the digester and the waste sludge from the digester. However, some solid waste product is still produced for disposal from the final centrifuge. The system described uses centrifuges which allow high-g forces to act on the biomass and lyse a proportion of the cells thus releasing nutrients for the digestion process and therefore producing more methane.

There are a number of disposal routes commonly used to deal with both raw and digested waste sludges. In Europe, a common route until recently was simply dumping the liquid sludge at sea, sufficiently far from land to ensure that the material did not find its way back to the shore. However the environmental effect of millions of tonnes of sludge, some of which was contaminated with heavy metals or chemical toxins, was deemed to be highly inappropriate, and the practice was banned by the European Union member countries. However this method of sludge disposal is still practised in other areas but it is unlikely to be allowed to continue in view of the environmental implications.

The liquid sludge arising from, for example, a digestion process would contain typically about 5% solids in water. A dewatering process would convert this to a solid sludge containing 25-35% solids and will give a significant reduction in volume. Dewatering techniques used would include for example, filtration, perhaps using high pressure filter presses or belt presses or, alternatively, the use of centrifuges to separate solid from liquid. However, the waste water stream from the dewatering process is usually highly contaminated and it is necessary for it to be returned to the inlet of the treatment works. The solid cake is transportable but still has to be disposed of, thereby increasing costs.

Another route for disposal of sludge is the use of landfill. Landfill is used, often for dewatered sludges that may be contaminated with traces of heavy metals or with toxins. Landfill regulations are however usually very stringently enforced since the site must be efficiently sealed to prevent leaching into the water courses and potentially causing contamination of rivers and other sources of potable water. The landfill option is therefore becoming ever more expensive as suitable land areas become scarce and environmental legislation increases costs.

Other methods of disposal of dewatered sludges include sludge drying as exemplified by the Swiss Combi process which takes a centrifuged cake and passes it through a specially designed dryer. The dried sludge is pelleted, bagged and sold as a fertiliser or may also be used as a low grade fuel. An alternative fate, and one that is being adopted as the method of choice for treatment of sewage sludge in large communities is the use of incinerators. The sludge cake in this case must be autothermic, i.e. its moisture content must be sufficiently low to allow combustion to occur without the addition of extra fuel except at start-up. The dewatering system selected must be appropriate for this. Using this system, the sludge is destroyed but leaves an ash which still has to be disposed of. The ash may be used for brick making or in the case of contaminated industrial sludge, calcined and land filled.

Studies have been carried out on the use of blends of sewage sludge and domestic waste for incineration in local refuse disposal incinerators or as a fuel in cement kilns. It is not known if these routes have ever been proven as commercial propositions. However, blends of sewage sludge and domestic garden and other organic waste have been used in a commercial composting process.

Both the drying and the incineration process require the building of very large and expensive plants which adds significantly to the cost of disposing of the sludge. Additionally, the drying process requires a ready market for the sale of the pelleted dry material, which may or may not be available.

One outlet for the dried sludge, assuming it has no toxic contaminants such as heavy metals, is its use as a fertiliser. Sewage sludge in all its forms has always been used as a fertiliser. At least half the sewage sludge generated is used for agricultural or land reclamation purposes. Some types of liquid sewage sludge because of their content of harmful bacteria have to be injected into the soil. Other types, particularly those which have been subjected to the digestion process which pasteurises the sludge, may be sprayed on to the land. Dewatered cake may also be used by the farmer or forester. The stabilisation of sludge with lime has long been used to assist in the sludge pasteurisation process and assists in its use as a fertiliser product. One development has been to take the waste ash from the production of cement, called cement kiln dust and blend this with dewatered sewage sludge cake. The product is a pasteurised and easily handled product that has the appearance, texture and odour of soil. It is known as the NViro™ process and has been used in a number of countries to assist in the presentation of sewage sludge as an acceptable product to agriculture and horticulture. However beneficial, the product is still the blend of two wastes and doubles the quantity of products to be sold.

U.S. Patent No. 4079003 describes a process for transforming sludge into ecologically acceptable solid material. The process involves the use of lime to stabilise sludge and render it inert. The sludge is converted to a form which is useful as an agricultural fertiliser.

U.S. Patent No. 5,948,261 describes a process for the treatment of biosolids resulting from waste water treatment which results in the destruction of organic contaminants, which results in biosolids which can be disposed of on land or under ground. Likewise, U.S. Patent No. 4277342 discloses a method for the biologicalchemical detoxification of sewage sludge.

U.S. patent number 4341632 relates to a process for the biological purification of waste water and in particular to the problem of undesirable bulking sludge formation caused by the proliferation of certain microorganisms. The document suggests that elements of the bulking sludge can be destroyed by using a centrifuge to place a high mechanical shear upon the bacteria. Normal bacteria found in the activated sludge are not affected. The process does not deal with the destruction of sludge.

Japanese patent specification no. 9066298 describes a process whereby after organic sludge is anaerobically digested, digested sludge is subjected to solid-liquid separation treatment and the solid part of the digested sludge is solubilized by alkali treatment before being returned to an anaerobic digestion process.

Japanese patent no. 9052100 describes a method for reducing the volume of organic sludge by using an aerobic digestion method. The method comprises a first aerobic digestion process aerobically digesting organic sludge, a solid-liquid separation process separating digested sludge into a solid and into a liquid, a solubilizing process heating at least a part of the separated sludge from the solid-liquid separation process or treating the same with alkali.

U.K. patent number 2 298 195B describes a process in which the production of sludge is reduced or even eliminated by the deactivation of a proportion of the bacteria in the biomass. In this way the total number of bacteria in the reaction zone can be maintained constant while maintaining a high rate of bacterial growth and aerobic digestion. It is a feature of activated sludge processes that they depend upon the facility of the bacteria to clump together naturally (flocculation). This ensures good and quick settlement in the separation phase and little carry over in the final effluent. The patent relates to a system in which the settled biomass is returned in full to the reactor and a high shear is applied to the floe, breaking them up and thereby allowing easier predation by rotifer and protozoan microorganisms in the digestion process.

In Europe the banning of sea dumping as an acceptable route for sludge disposal has led to an increase in the building of incinerators to take up the huge volumes to be destroyed. Additionally the increasingly stringent regulations on the water industry in Europe to treat domestic and industrial waste water to high standards before disposal has IF β 6Β8Φ and will continue to lead to an increase in sludge volumes. There is a limit to the amount of sludge that can be used for agriculture, land reclamation, forestry or horticulture.

Increasing global populations will make the safe disposal of sludge, whether industrial or domestic a major problem for all developed and developing countries.

Object of the Invention It is therefore an object of the present invention to provide a process and plant for the substantial destruction of sludge resulting from the treatment of waste water or from agricultural slurries. It is a further object to provide a process for sludge destruction without the requirement of chemical additions.

Summary of the Invention The invention provides a process for the substantial destruction of sludge comprising diluting the sludge to 1,000 -5,000mg/litre COD, reacting the sludge with a bacterial population at oxygen levels greater than lppm, under conditions of high intensity mixing.

In the present application high intensity mixing is used to indicate that the mixing process is sufficiently intense to break up the floes of bacteria formed within the reactor.

In the present application the intensive mixing produces a shearing force within the reactor system generated suitably by an aeration source and directed at the sludge and the bacterial population. This ensures that the sludge and bacterial population are always maintained in a finely divided form and in maximum contact with each other and the dissolved or micro bubble oxygen.

The process is preferably carried out at ambient temperatures.

Preferably in the process according to the invention the sludge is blended and macerated to allow the intimate homogenisation and blending of waste organic sludges in the solid or liquid form with a diluent liquor. Organic material, both dissolved and suspended is thoroughly mixed and blended and is bacterially treated in the reactor. There are a number of naturally occurring genera of bacteria such as Sarcina, Staphylococcus, Bacillus, Nitrosomonas, Vibrio, Pseudomonas or Nitrobacter which flourish in the strongly aerobic and fertile conditions found in the reactor. These bacteria are preferentially selected because of the rich supply of nutrient and ready supply of oxygen all presented at suitable temperatures and acidity to encourage digestion of the organic waste. The reactor may however be seeded with blends of micro-organisms if necessary. The ammonia present formed by the degradation of some of the nitrogenous waste is used by other bacteria and is oxidised to nitrite and nitrate .

Preferably, the incoming sludge is diluted with water or more preferably, raw sewage. The latter can provide a ready source of micro nutrients such as essential vitamins and minerals necessary for biological growth and activity. Such nutrients may be deficient in some sludge.

Preferably the sludge is enriched with nutrients, e.g. nitrates, phosphates, trace elements, vitamins and micro nutrients. Preferably the enrichment process is carried out by diluting the incoming sludge with raw sewage.

Preferably the process comprises mixing the sludge and diluent under high intensity mixing. High intensity mixing is essential to break up any solid sludge and provide a homogeneous mix. This aids bacterial digestion of the sludge.

Preferably, the level of dissolved oxygen in the destruction reactor is maintained at levels greater than lppm. This allows sludge destruction to occur. The presence of a 010342 ready supply of oxygen and food in the form of waste allows the bacteria to respire and multiply resulting in a bacterial population which destroys the sludge.

Preferably the process allows a rapid reduction of Biological Oxygen Demand (BOD) in the reactor with a dissolved oxygen (DO) level of at least 3ppm at all times. This allows the rapid uptake of organic material and optimises the conversion of ammonia to nitrate. The bacterial population is provided with a source of food in the form of sludge and the population of bacteria rises very rapidly to meet the level of food available. Sufficient oxygen must be supplied to encourage this growth. Organic material is destroyed by bacteria oxidation to carbon dioxide and water. The food level is however constant and soon the bacterial population exceeds the food available. This point is known as endogenous respiration since some of the bacteria die off increasing the food level. Maintaining the feed of waste sludge to ensure that endogenous respiration is just maintained while ensuring a healthy biomass is an essential part of the sludge destruction process.

Further preferably, in the process according to the invention the sludge is retained in contact with the bacterial population in the reactor for 1-24 hours.

Preferably, in the process according to the invention the suspended solids in the reactor mixed liquor is less than 10,000mg/litre and is preferably in the range 6000-9000mg/litre.

Preferably the process comprises separating reactor solids from suspending liquor, suitably in a primary clarifier. This clarifies the resultant liquor. All the separated solids are returned to the reactor inlet. Other means of separation may be used such as hydrocyclones, centrifuges or one of a variety of conventional clarifiers.

Further preferably, the process comprises (1) treating the resultant liquor with denitrifying bacteria; (2) separating denitrifying biomass from supernatant treated liquor; (3) clarifying supernatant treated liquor by adsorption means; wherein blended and waste sludge is contacted with waste sludge from the denitrification process.

Preferably the process comprises contacting the resultant clarified liquor with denitrifying bacteria within the denitrifying vessel for between 3 and 15 hours.

Further preferably, the process comprises agitating the clarified liquor. Agitation may be carried out by stirring or other suitable means of agitation.

Preferably the process comprises separating the denitrification vessel solids from the suspending liquor. Preferably separation is carried out in a secondary clarifier.

Preferably the process comprises returning at least a portion of denitrification vessel solids to the denitrification vessel, the balance of solids being returned to the reactor inlet. The return of a portion of denitrification vessel solids to the denitrification vessel ensures a good concentration of viable biomass in the vessel. The balance of solids is returned to the reactor for destruction.

Further preferably, between 5 and 20% of the solids are returned to the denitrification vessel, the balance of solids being returned to the reactor.

Preferably the process comprises treating the supernatant liquor from the secondary clarifier in an adsorption filter. Current technology is used wherein contact between the clarified liquor and iron salts allows dissolved phosphate ions to precipitate.

Preferably the process comprises contacting diluted and macerated feed sludge with returned sludges from the primary and secondary clarification steps for a period of 0.5-2 hours. The diluent may be a waste water stream, a final effluent from a treatment plant or plain water. Other diluents include domestic waste water, industrial waste water, or blends of water and influent and effluent.

Preferably, the process comprises converting nitrate and nitrite adsorbed on the incoming sludge and that produced by ammonia oxidation, to nitrogen using denitrifying bacteria.

Preferably the process comprises maintaining oxygen levels in the denitrifying step at less than lppm. This enables the denitrifying bacteria to flourish. Denitrification requires a different bacterial species than those used in the sludge destruction step. The levels of oxygen within the sludge destruction step work against selection of denitrifying bacteria.

Preferably the denitrification process is operated in a manner to allow sufficient carbonaceous material to be available. Preferably the carbon content of the feed to the denitrifier is kept sufficiently high by controlling the flow rate through the reactor. Too low a flow will degrade all the carbonaceous material and the denitrification step will cease to work effectively.

Preferably the flow rate through the reactor is selected prior to commencing the process and is dependent upon the sludge type and the form of diluent liquor.

Preferably the level of dissolved oxygen within the denitrification tank is maintained in the range 0.1 to 0.3ppm. Such levels of oxygen encourage the denitrification organisms to use the oxygen from nitrated or nitrite ions rather than oxygen in the surrounding water.

Preferably the level of organic carbon is kept at a level sufficient to allow the bacteria to flourish. The level of carbonaceous material is dependent upon the residual nitrate levels to be converted to nitrogen and can vary substantially.

Preferably the level of carbonaceous material in the process according to the invention is calculated empirically during plant set up by controlling flow within the reactor and therefore residence time in the denitrification vessel.

Preferably the process comprises aerating the liquor prior to allowing the denitrified liquor to settle. This aeration step removes micro-bubbles of nitrogen by established principles of mass transfer and assists clarification.

Preferably in the process according to the invention, resulting liquor from the denitrifying step is allowed to settle. The liquor is allowed to settle in a secondary clarifier which separates any remaining sludge.

Preferably, the process comprises maintaining the population of the denitrifying bacteria in the denitrification step at a viable level by returning a proportion of settled sludge to the denitrifier. This also provides a further source of carbonaceous material to assist the denitrification reactions.

In a preferred embodiment, between 5 and 20% of the settled sludge is returned to the denitrification step.

In general, the discharge of nitrate, nitrite and phosphate from a waste water treatment plant is legally regulated. These nutrients may be treated biologically or they may partially rely on adsorption on the sludge usually wasted. As there is no waste sludge in the process according to the invention, there is no waste sludge available to carry adsorbed nutrients to alternative disposal processes. Therefore it is essential to treat all the nutrients in order to ensure removal from the system.

Preferably, the present invention provides a process having an efficient nitrogen removal regime comprising contacting blended and waste sludge with waste sludge from the denitrification process. The balance of the sludge in the secondary clarification step is returned to a pre-contact step. This allows preliminary denitrification of adsorbed nitrate from the waste sludge to occur. This should commence the fermentation process. Nitrogenous material present on the incoming sludge feed and biomass is converted to ammonia and eventually nitrite and nitrate within the reactor using microorganisms.

Preferably the process comprises operating the pre-contact step for 0.5 to 1 hour under anoxic conditions. The pre-contact step improves flocculation of the reactor biomass.

Conventional biological phosphate removal systems rely on phosphate being adsorbed on the conventionally wasted sludge. In the present invention as there is no wasted sludge, such biological phosphate removal systems are not applicable.

Preferably the process comprises removing phosphate from the effluent. The process provides for removal of dissolved phosphate ions and heavy metals by means of an adsorption filter. The filter allows the final effluent to contact materials with an affinity for phosphate and heavy metals.

In one embodiment the process involves treating and reducing the level of phosphate from reactor effluent using sacrificial iron oxidation at a pH within the range from 6.0 to 7.5. This may be carried out by oxidising iron in the form of coarse steel wool for example, in the water stream by natural processes. Provided that there is little oxygen in the water and the pH is not above 7.5, the phosphate ions contained in the water will react with the ferric iron and precipitate. Alternatively the effluent may be passed through an iron containing mineral such as latterite allowing the phosphate to be reacted in the same way.

Further preferably, the process according to the invention comprises removing heavy metals from the effluent. Such heavy metals may sometimes be found in waste sludge, (particularly sludges arising from the treatment of industrial waste) and are liberated from the sludge into solution by the process. Heavy metals may be removed by passing the effluent through an ion-exchange column. Preferably the phosphate and metal filters comprise two packed beds of media in series. Preferably, the media comprises Latterite, for removal of phosphate, and a suitable ion-exchange medium for removal of heavy metals.

Preferably the process comprises clarifying the final liquor and removing nutrients. The clarification of the final liquor is carried out by passing the liquor through a secondary clarifier, preferably of inclined plate design and of appropriate area for separation of denitrified effluent and the denitrifying sludge. The resulting clarified liquor is passed through a phosphate filter.

Further preferably the process comprises passing the clarified liquor through a heavy metals filter. The resulting effluent is within regulatory limits for discharge to inland or coastal waters. Such limits are set by the appropriate regulatory authorities and may be subject to change. Typical discharges must be at least: BOD (Biological oxygen demand) 20ppm; Suspended Solids 30ppm; Phosphate 2ppm; Nitrate 5ppm and 1.0 ppb (parts per billion) or less for heavy metals.

Advantageously, the process according to the invention allows the waste sludge from a number of sources to be treated biologically in a manner to provide complete destruction of the organic material contained within the sludge. A highly efficient and cost effective process is provided.

Further preferably, and to allow for situations where the effluent from the sludge destruction plant may be returned to an existing water treatment plant, the process may be operated to provide a discharge effluent having a higher COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand) and Suspended Solids than those exemplified above.

The invention provides a process wherein the treatment and destruction of the waste organic sludge generated from the biological treatment of organic waste liquors includes the integral treatment and removal to regulatory levels, of nutrient products such as nitrates, phosphates and heavy metals.

The invention provides a process whereby the organic component of waste sludge is totally destroyed using a combination of mechanical and biotechnological processes. Preferably, all of the sludge is destroyed thereby dispensing with the need for disposal routes.

Use of the process according to the present invention allows for complete destruction of sludge. Therefore there is no requirement for pre-treatment or posttreatment storage.

The invention provides a process whereby the final effluent has a BOD of 10200mg/litre.

The invention provides a process whereby the final effluent has a COD (chemical oxygen demand) in the range 40-500mg/litre. The COD provides a quick and convenient method of determining the organic content of a waste water by using a chemical oxidation technique rather than the biological methods required for BOD determination. It is possible with a reasonable degree of accuracy for those skilled in the art to correlate the BOD figures with those obtained from the COD result. Once such a correlation is achieved, it is possible to control plant operating parameters using COD measurements. Therefore remote process management becomes achievable.

The invention provides a process wherein the level of suspended solids in the final effluent is in the range 10-200mg/litre.

The level of nitrates in the final effluent is in the range 2-10mg/litre, the level of phosphates in the final effluent is in the range l-10mg/litre, and the level of heavy metals is in the range 0.1-0.5mg/litre.

The invention provides a process wherein the levels of total unoxidised nitrogen (TUN) as N is in the range 2.0-10mg/lt and the level of ammonia as N is less than Img/litre.

The process according to the invention does not produce an offensive odour. Further preferably, the by-product of the process described does not have an odour.

This feature makes the process more environmentally acceptable, particularly in urban areas.

The invention also provides a plant for the biological destruction of organic sludge using microorganisms comprising; (a) a reactor adapted to receive liquor and retain it in contact with a bacterial population, (b) an aeration means adapted to provide oxygen levels above lppm within the reactor in order to maintain bacterial population growth, and (c) a means for intensive mixing of the reactor contents.

Preferably the reactor is adapted to be seeded with a bacterial population capable of digesting sludge. The bacterial species feed on the dissolved, colloidal and suspended organic matter.

Preferably the aeration means is suitably adapted to maintain oxygen levels within the reactor at a level of greater than lppm.

Preferably the aeration means has an oxygen transfer efficiency of at least 40% and allows vigorous agitation to produce high shear forces.

Preferably the aeration means comprises a jet aerator. The aerator allows for vigorous aeration and maintains an even homogeneous blend of solids suspension with an intimate mixing of biomass and oxygen to allow rapid respiration and bacterial growth.

Preferably the plant comprises a high intensity mixing system, more preferably a high shear macerator/mixer. The high shear mixer provides high tangential fluid forces that act upon clumps of sludge breaking them up into their constituent parts. The high shear macerator uses knives to cut and mince large masses of foreign bodies which could block the plant. The combined high shear macerator/mixer allows for intimate mixing of the liquor and sludge, and the breaking up of floes of bacteria.

Further preferably the plant comprises a blender means adapted to receive liquid macerated sludge or liquefied and macerated solid dewatered cake and combine it with a liquid stream which acts as diluent.

Preferably the blender means is provided with a high shear macerator. The raw sludge is macerated prior to blending with diluent. The high shear ensures the breaking up of any clumps of sludge and assists in the degradation of cellular material so as to form diluent liquor in a form suitable for biological fermentation.

Preferably the plant comprises a pre-contact tank. The pre-contact tank is suitably adapted for further diluting the blended sludge and mixing it with sludge returned from a primary clarifier.

Preferably the plant comprises a denitrifying system having a primary clarifier, a denitrification tank and a secondary clarifier.

Further preferably, the primary clarifier is adapted to receive liquor from the reactor and to allow sludge to settle.

Preferably the plant comprises a denitrification tank adapted to receive liquor and retain it in contact with denitrifying organisms for a period of 3 to 15 hours.

In a preferred embodiment of the invention the denitrifying tank contains an agitator.

Preferably the plant comprises a secondary clarifier, adapted to return at least a portion of the sludge to the reactor and the balance to the denitrifying tank.

Further preferably, the secondary clarifier is adapted to return all the settled sludge to the reactor.

Further preferably the plant comprises an adsorption filter adapted to receive clarified liquor and remove phosphate. The adsorption filter comprises an adsorption media which converts a proportion of the dissolved phosphate to an insoluble form. Preferably, the adsorption filter is a packed bed of material which has an affinity for phosphate ions. This may, for example, be naturally occurring material such as Laterite or other iron containing materials or further preferably alternative artificial phosphate absorbers.

In a further embodiment of the invention the adsorption filter is adapted to remove heavy metal ions.

In a still further embodiment of the invention the filter comprises an additional section with alternative adsorbers, for example ion exchange media, which have an affinity for heavy metals. The filter is designed to be easily removable for adsorber removal or renewal.

In an alternative embodiment the plant comprises a reactor comprising (a) two compartments of about equal volume, each provided with an aeration means and (b) a sump. The sump may collect inorganic matter.

Preferably said aeration means comprises a jet aerator.

Preferably, the plant for carrying out the process according to the invention is sized to fit into a standard transport container (freight container) attachable to a means of transport, preferably a container lorry. The plant may be suitable for treatment of sludge at intensive animal breeding farms, industrial factories or domestic and industrial waste treatment plants.

In one embodiment the plant is constructed in concrete or steel as a permanent fixture. The choice of material used in the construction is dependent on the volumes of sludge to be treated.

In an alternative embodiment the plant is a prefabricated steel structure to be placed on a flat concrete foundation.

In a still further embodiment the plant is constructed as a package designed to fit into a standard transport container attachable to a means of transport.

One such arrangement comprises the packaging of the plant to fit a standard transport container attachable to a means of transport. The term standard container herein means standard sizes of transport container such as 20 foot and 40 foot containers. Thus the plant is suitable for the treatment of sludge waste arising from populations of between 1,000 and 20,000 people or the equivalent volume of industrial sludge or animal waste. Larger volumes of sludge could be treated by multiple such container units or by a prefabricated package plant or a concrete or steel structure according to the invention.

Preferably the plant for carrying out the process according to the invention is a self contained modular package for drop-in situ installation.

Further preferably, the plant is adapted for multi point connection for quick and easy commissioning. The plant is suitably adapted for connection to services such as water and electricity.

IE 0 1 a 3 4 2 Preferably the process and plant according to the invention has the capacity to treat organic waste sludges from a variety of sources such as domestic and industrial waste water, including domestic sewage treatment systems, mixed domestic and industrial sewage treatment systems, chemical and petrochemical, pharmaceutical, brewery, paper, sugar, oil and gas, food processing and manufacture and agricultural slurries from the intensive breeding of animals and intensive growing of vegetables.

Preferably the sludge destruction plant allows for a self contained sludge destruction plant to be placed on the site where that sludge is produced and become a part of the existing waste treatment process. The treatment process allows for a substantial reduction in space, typically half the space required for sludge storage and one third the space required where sludge reduction processes such as dewatering or anaerobic digestion are carried out.

The sludge destruction plant may conveniently be sited close to the sludge generating treatment facility. The present invention allows for the sludge destruction plant and process to become a part of an existing biological treatment facility. This eliminates the cost associated with conventional sludge disposal routes, including handling and transport costs.

Alternatively, the sludge destruction plant and process may be a stand alone facility and be suitable for the treatment of a number of different organic waste sludges in solid or liquid form and from a number of different biological waste treatment facilities.

Brief Description of the Drawings The invention will be further described by way of example only, and with reference to the accompanying drawings in which: Fig 1 is a schematic representation of the plant for carrying out the sludge destruction process according to the invention.

Fig. 2 is a schematic representation of an alternative embodiment of the plant for carrying out the sludge destruction process according to the invention.

Detailed Description of the Invention In the process according to the invention, the waste sludge to be treated is pretreated according to its origin. Solid sludges such as those coming from domestic or industrial waste treatment facilities and which have been mechanically dewatered with centrifuges or belt or filter presses are reslurried in a blending tank (a) which contains a high shear mixing head . Such mixers are commercially available and are selected according to efficiency. The sludge is reslurried in plain water, final effluent from a water treatment facility or screened and degritted influent from a water treatment facility. The selection of the diluent is for convenience and cost.

The mixing and blending of the waste sludge is an important step in the process. The mixer must provide sufficient shear to break up any solid sludge and provide a homogeneous mix. The shear effect will also assist in the breaking of cellular material and assist materially in its digestion within the reactor system.

It has been demonstrated however that solid sludge such as that produced from waste water treatment processes after dewatering with centrifuge, belt press or filter press may be fed in bulk to the reactor. The high shear aerator system proved effective in reslurrying the sludge in the reactor so that destruction could occur. While this method is not necessarily the preferred route, a satisfactory rate of sludge destruction was measured as measured by the suspended solids levels in the reactor.

EXAMPLE A waste activated sludge containing 18% solids was fed in bulk to the reactor system of volume 12M . The diluent was a domestic sewage stream flowing continuously 2M3/hr.

The solids were fed twice per day with a six hour gap and were started at a low rate up to a maximum of 500kg/day over a five day period. The method of feeding can be regarded as shock loading. The results shown in the table below indicate the flexibility of the process.

Day Total solid fed (kg. at 18% ds) Reactor solids (mg/lt) Effluentsolids(mg/lt) 0 0 4340 25 1 28 4588 2 56 4472 3 100 4715 4 168 5 280 5680 6 400 6230 42 7 560 8738 152 8 0 6845 38 Incoming liquid sludges such as those from agricultural, industrial or sewage sources will also be subject to the high shear blending process, to ensure a fully homogeneous mix and a degree of shear sufficient to produce cell breakdown where possible.

In the process according to the invention, the blended sludge is prepared as a slurry at 2-5% solids, is screened and passed to a pre-contact tank (b) where it is further diluted to bring the COD to a range of 1000-7000mg/litre. It is intimately mixed with Ε υ ) U α 4 2 the returned sludge from the primary clarifier (e). The residence time within the precontact tank is in the range 0.25-1.0 hours. The liquor overflows to the reactor (c) whose volume is determined by the nature of the waste sludge being treated. The volume typically would be in a range to provide 6-24hrs retention. Once in the reactor (c) the liquor comes into contact with the bacterial species which feed on the dissolved, colloidal and suspended organic matter therein. The aerator (d) within the reactor (c) introduces air into the reactor and also functions as an excellent mixing system which allows the bacterial species to have a ready supply of oxygen, thus allowing rapid. respiration and bacterial growth. Such growth rates are dependent upon nutrient levels, oxygen availability temperature and pH. The latter three criteria are controlled by plant operation and it is therefore the availability of nutrient that will control the growth patterns of the biomass. Thus where there is a nutrient rich stream feeding a low biomass population in highly aerated conditions, an exponential growth of bacteria will occur. As the bacterial population exceeds the level of nutrient able to sustain it, it ceases to grow and allows the level of predatory higher organisms such as protozoa and rotifers to increase. The bacteria are therefore exposed to two constraints, first a lack of food to sustain the population and second a level of predation that is increasing. The bacteria therefore enter a zone of endogenous respiration. In this phase bacteria begin to die, the cells break up thus providing some additional nutrient, which in turn will encourage further growth.

The aeration system provides for intensive aeration. There is always sufficient oxygen to encourage bacterial growth when sufficient nutrient is available. Thus food availability is a limiting step and at the point of endogenous respiration with bacteria dying, the population is vulnerable. If a proportion of the bacterial population are deactivated, for example by the breaking up of the naturally formed clumps or floes by shear forces, then the population pressures on the remaining bacteria are removed and the growth phase will resume. Such shear forces may be provided by the aeration system, the floes are broken up into their constituent bacteria, weakening them and rendering them vulnerable to predation. The continuous removal of bacterial population and encouragement to predation by higher organisms, quickly establishes a balance of growth and digestion within the reactor.

The rate of growth of the biological system can be described by a simple equation: dX/dt = uX - keX - kdX where X is the concentration of microorganisms U is the specific growth rate. This is a measure of the rate of cell division. ke in the above is the specific endogenous respiration rate. This is a measure of utilisation of stored cell materials ending in cell lysis (cell death with membrane breakdown and release of nutrients.) kd is the rate of predation. This is a measure of the feeding rate of the other organisms that rely upon the bacterial population as food (e.g. protozoa or rotifiers).

When the system is in the growth phase, that is when the food supply is plentiful, specific growths are as high as 5 per day. By contrast, in the endogenous phase, when the bacteria are using up their own resources and dying, the specific growth rate is below 0.1 per day.

Negative growth rates are conventionally achieved by holding the system at a low food to micro-organism ration (F/M), either by a slow rate of feed or maintaining the population of biomass for a long period (tens of days). An example of this is the extended aeration process. In such a system the rate of decrease of biomass is slow because it is governed by the endogenous respiration of the population. The death of the micro-organisms is therefore by natural causes. The transfer of oxygen is a critical factor in process efficiency. In the process described herein a jet aerator is used as it offers efficiencies greater than 40% in the transfer of oxygen to the waste water. The jet aerator provides a very efficient mixing action and ensures that the biomass and oxygen are at all times in intimate contact. The ready availability of oxygen and the fluid motion across the bacterial cell surface allows substrate (food) uptake to increase. The substrate in the water now becomes limiting so that additional energy is now required for the digestion of bacteria by other bacteria. The factor ke in the above equation therefore increases.

By maintaining the system in the endogenous respiration phase, conditions are created to allow the favourable growth of certain protozoa and rotifiers, both higher forms of life than bacteria whose source of food is the bacterial population. Bacterial cells naturally floe together to form clumps which act as a defence to predation. In the process described herein the high shear mixing system confers strong tangential fluid forces to the clumps, breaking them down into smaller units and also releasing loose bacterial cells. These cells are captured and digested by the protozoa and rotifiers. This results in an increased size of population of these predatory organisms. Thus instead of the cells dying naturally they are consumed by the predators. The factor kd is now increased.

In the process described herein, the combination of shearing and high oxygen transfer with the consequent increased substrate uptake means that the biomass in the reactor is in a continuously highly active state, with a rapid uptake of substrate, high turnover of bacteria and high predation rates. In such conditions which mimic the situation of the explosive growth phase in conventional systems a high substrate feed rate can be achieved. In the present process waste organic sludge is the substrate and it is rapidly oxidised by the bacterial species.

In the process according to the invention, the influent liquor contains a high proportion of organic nutrient wholly or partly coming from the blended sludge. The bacterial species within the reactor will digest the organic material and convert it to carbon dioxide and water. Some of the nitrogenous material will be converted to ammonia and then oxidised to nitrates. A proportion of the nitrates present will also come from the incoming sludges, the proportions being dependent on the source. In common with many biological oxidation systems, a small proportion of the nitrogenous material will prove resistant to biological conversion and will pass through the plant as unoxidised nitrogen. Levels of nitrate will therefore be high and it is essential to treat this and reduce as much as possible to nitrogen. In addition, the liquor will contain phosphate released from the sludge treatment and any adsorbed phosphate from the incoming sludge. In some industrial sludges there may be adsorbed heavy metals which will remain in solution and pass unchanged through the plant.

The liquor from the reactor, after the appropriate residence time, overflows into the primary clarifier (e). This may be of the inclined plate design and allows the sludge from within the reactor to settle. The settled sludge from the primary clarifier (e) is returned to the reactor (c). The supernatant liquor, containing any unoxidised BOD plus the nitrate, phosphate and any heavy metals in solution overflows into the denitrification tank (f). This is of a volume sufficient to allow efficient denitrification to occur. Typically the liquor will reside in contact with the denitrifying bacteria for between 3 and 15 hours.

The conversion of nitrate to nitrogen within the denitrification tank (f) requires organic carbon to be present. It is convenient therefore to provide the carbon from the reactor by ensuring that there is sufficient fermentable carbonaceous material present in the influent to the denitrifier. If this is not possible, external sources of carbon such as the addition of methanol may be required.

The liquor from the denitrification tank (f) overflows to the secondary clarifier (g) which may also be of the inclined plate design. This separates the denitrifying biomass from the supernatant treated liquor still containing phosphate and perhaps heavy metals. A proportion (between 5 and 15%) of the settled biomass is returned to the denitrifying tank. The balance is returned to the pre-contact tank (b). This assists in the preliminary removal of adsorbed nitrate from the incoming sludge. It may be necessary to aerate the liquor between the denitrification tank (f) and the secondary clarifier(g). This aeration step will remove micro-bubbles of nitrogen by established principles of mass transfer and assist clarification.

EQ10342 The clarified and treated liquor overflows from the secondary clarifier (g) into a phosphate removal filter (h). This is a sacrificial filter packed with a material such as steel wool which at the appropriate pH will oxidise and simultaneously remove phosphate ions by precipitation as iron phosphate. Alternatively, other phosphate adsorption materials such as laterite may be used allowing the effluent to contact the adsorbent either in the upflow or downflow modes and remove the bulk of dissolved phosphates. The adsorption material must be renewed or reactivated from time to time.

The phosphate filter may, if required, be combined with a heavy-metal ion filter. This may be a filter through an appropriate ion exchange medium, or an adsorption system combined with the phosphate removal filter (h).

In the process according to the invention, the final effluent is of a very high standard, containing low levels of suspended solids and of oxidisable carbonaceous material. The organic content of the influent sludge is completely destroyed and nitrate and phosphate levels are reduced to those specified by the regulatory authorities for discharge to inland or coastal waters. Any heavy metal ions that may have been adsorbed or contained within the destroyed sludge are easily and conveniently removed from the effluent liquor. The process is such that simple remote monitoring will allow for fully automatic operation of the plant once the required operational parameters have been set.

In some cases it is not necessary to remove all the oxidisable carbon to have a high quality effluent, it being necessary, simply to destroy the bulk of the incoming sludge. For example, if the effluent is to be returned to the inlet of a treatment works, then the residence time within the reactor may be reduced to provide an effluent of lower quality.

In an alternative embodiment the plant for carrying out the process according to the invention may be adapted so as to concentrate the inorganic non-degradable materials that are often found in waste sludges. Some of the inorganic material will be colloidal or very fine and will therefore be carried over in the waste effluent. However larger particles will build up as inorganic waste which will have to be removed periodically.

In an alternative embodiment, the plant for carrying out the process described herein comprises a reactor having two compartments of about equal volume. The alternative embodiment showing compartments Cl and C2 is illustrated in Figure 2.

The first section Cl acts as the inlet reactor and is fitted with a jet aerator (d). After an appropriate residence time in the reactor compartment Cl, the liquor is passed into the second compartment C2 which is also provided with a jet aerator (d). Liquor from C2 is passed into the primary clarifier (e) as described above. However the returned biomass from the clarifier is returned to Cl and C2 in different proportions. For example between 15 and 30% is returned to Cl and between 85 and 70% is returned to C2. The proportions will vary depending on the feedstock and the rate of destruction. Over time the proportions of dense inorganic material in C2 will increase and collect in a sump (j) which is fitted to the bottom of the reactor. Accordingly the inorganic material may be removed and disposed of at appropriate times.

The words comprises/comprising and the words having/including when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims (5)

Claims

1. A process for the substantial destruction of sludge comprising diluting the sludge to 1,000 -5,000 mg/litre COD, reacting the sludge with a bacterial population at oxygen levels greater than lppm, under conditions of high intensity mixing.

2. A process according to claim 1 comprising (1) treating the resultant liquor with denitrifying bacteria; (2) separating denitrifying biomass from supernatant treated liquor; (3) clarifying supernatant treated liquor by adsorption means; wherein blended and waste sludge is contacted with waste sludge from the denitrification process.

3. A plant for the biological destruction of organic sludge using microorganisms comprising; (a) a reactor adapted to receive liquor and retain it in contact with a bacterial population, (b) an aeration means adapted to provide oxygen levels above lppm within the reactor sufficient to maintain bacterial population growth, and (c) a means for intensive mixing of the reactor contents.

4. A plant as claimed in claim 3 further comprising a sump for the collection of inorganic matter.

5. A process or plant for the biological destruction of sludge using microorganisms, substantially as hereinbefore described and/or with reference to the accompanying drawings.