WO1999025657A2

WO1999025657A2 - Well-mixed flow bioreactor for aerobic treatment of aqueous wastes at high organic and solids loadings

Info

Publication number: WO1999025657A2
Application number: PCT/CA1998/001076
Authority: WO
Inventors: Denis Pelletier
Original assignee: Dessau Soprin
Priority date: 1997-11-18
Filing date: 1998-11-18
Publication date: 1999-05-27
Also published as: CA2221407A1; WO1999025657A3; AU1221199A

Abstract

The present invention generally relates to an aerobic bioreactor for treating aqueous waste comprising: (a) a reservoir (2) having lower and upper portions, and opposite inflow (14) and outflow ends (16); (b) an inlet (15) located at said inflow end (14) for supplying said aqueous waste into said reservoir and an outlet (17) located at said outflow end (16) for supplying the treated aqueous waste from said reservoir; (c) at least one oxygen-containing medium diffuser (40), positioned in said lower portion of said reservoir; (d) at least one air lift pump (38) located in said lower portion of said reservoir and positioned for sucking from said lower end portion and for transporting upwardly at least in part said oxygen-containing medium and a mixed liquor comprised of said aqueous waste and of a biomass capable of biodegrading said aqueous waste; and (e) a channel system (34) positioned in said upper portion of said reservoir for receiving said mixed liquor from said air lift pump and discharging said mixed liquor in the vicinity of said inflow end (14), whereby said at least one air lift pump and said at least one oxygen-containing medium diffuser (40) enable a substantially homogeneous oxygenation of said mixed liquor within said bioreactor.

Description

TITLE OF THE INVENTION

WELL-MIXED FLOW BIOREACTOR FOR AEROBIC TREATMENT OF AQUEOUS WASTES AT HIGH ORGANIC AND SOLIDS LOADINGS

FIELD OF THE INVENTION

The present invention generally relates to the treatment of aqueous wastes rich in organic content, either soluble or particulate, and is more specifically concerned with aerobic treatment wherein a well-mixed flow bioreactor is capable of handling and treating aqueous wastes at high organic loadings, enabling the production of floes and their subsequent removal in a settling tank, thereby yielding a significant purification performance of the aqueous waste. The present invention also relates to a well-mixed flow bioreactor enabling an efficient and environmentally sound treatment of aqueous wastes at high organic loadings and an environmentally sound disposal of the treated liquid effluent. The bioreactor of the present invention is also designed to accommodate the treatment of aqueous wastes containing high concentrations of suspended solids. In one embodiment, the invention relates to a well-mixed flow aerobic bioreactor, without mechanical part moving therein, for the treatment of aqueous wastes of high organic and solids contents which enables the development of the biodegrading flora, the formation of floes and a substantial reduction of the amount of nitrogen and phosphorus initially present in the aqueous waste.

BACKGROUND OF THE INVENTION

In many countries, aqueous waste management, either domestic, industrial or agricultural, has been and is still carried out without much concern for the environment. However, septic tank sludge, animal waste such as liquid piggery waste, leachate from landfill sites or wastewater from food processing industry have been the target of restrictions and protective measures to reduce the impact of their disposal into the environment. Some solutions have been applied to solve some of the problems associated with these human activities. In some cases, however, the lack of an acceptable solution has hampered or halted the growth of some industries. The best example is the pig industry, which has brought many producing countries into investing significant sums of money into research and development of technologies for treating liquid piggery waste. In conventional fattening piggeries, the raw slurry containing animal urine and feces is usually collected and stored in ponds or large concrete structures where it is allowed to decompose freely until it is used as fertilizer. Odor production and emissions are left uncontrolled, fertilizer quality is highly variable and/or potentially lost to the atmosphere, and the volumes are diluted by precipitations. Furthermore, management of the fertilizer through intensive farming techniques brings a lot of environmental concern with respect to pollution when it is disposed of onto land (soil compaction, excess landspreading dosages, surface water runoff, groundwater contamination). Clearly, there is an important need for an efficient and environmentally sound apparatus and method of treating aqueous wastes rich in organic content, such as liquid piggery waste remains.

Besides measures proposed for pollution source reduction, such as pig-on-litter systems, better nutrient assimilation through enzyme complement to the animal diet, volume reduction through better water management in the piggeries, different strategies for odor control and several types of physical-chemical treatment processes for liquid piggery waste have been studied. These include olygolysis, or electrolytic treatment (Ranalli et al., 1996, J. Env. Sci. Health A., 31:1705-1721), thermal dewatering technologies (Sirven process) or phase separation using membranes, chemical precipitation, centrifuges or other devices. Such technologies either propose only a partial treatment or require major capital investment. Biological processes for treating liquid piggery waste have also been developed. For example, pig slurry treatment has been approached using aerobic or anaerobic technologies or a combination thereof. Potential applications for these technologies have been looked at either as regional facilities in areas where the pig industry is concentrated or as local facilities installed at the production site. Regional facilities using anaerobic technologies exists, for example, in the Netherlands and in Denmark. Such facilities use the Promest and Memon systems (Rulkens and Ten Have, 1994, Wat. Sci. Technol., 30:157-165) or the Ecosun and Lindtrup processes. Because of high energy costs in these countries, regional facilities using anaerobic digestion were thought to be economically feasible since the energy input is very low and the potential recovery of combustible biogas exists. However, energy yields are low and difficult to optimize, raising the question as to whether large scale projects for the sole purpose of treating animal waste should be further developed or be abandoned. Anaerobic processes (Masse and Droste, 1997, Can. Agric. Eng.,

39:35-41 ; Masse et al., 1997, Can. Agric. Eng., 39:25-33; Masse et al., 1996, Can. J. Civ. Eng., 23:1285-1294) or combined anaerobic/aerobic treatment strategies (Le Hy et al., 1989, Wat. Sci. Technol., 21:1861-1864) in local facilities at the production site, are also being studied. Although, these anaerobic processes can be simple in design, produce little residual solids and have low energy requirements, they also have several disadvantages. They are sensitive to toxic constituents of the influent, provide poor clarification, do not assimilate or degrade all nutrients, and produce noxious and potentially explosive gases.

Aerobic treatment of aqueous waste is well known and has been used for many years. Its most notorious application is in municipal and industrial wastewater treatment. Numerous processes and apparatuses based on aerobic treatment of aqueous waste have been described (for example, U.S. Pat. Nos. 2,907,463, issued to Light et al. on Oct. 6, 1959; 4,522,722 issued to Nicholas on June 11 , 1985; 5,798,673 issued to Huntington on Jan. 17, 1989; and Canadian Patent No. 1 ,117, 042 issued to Spector on Jan. 26, 1982). Many variations of such aerobic treatment are used and adapted around the world depending on the quality of the wastewater, the way by which the wastewater is fed into the system, the process efficiency, and the local regulations governing the quality of the effluent. Most of these systems are large in dimensions, command high capital costs and a significant input in energy, and have not been designed for compactness and high organic loadings. Some efforts have been dedicated to adapting various designs that use aerobic or sequential aerobic/anaerobic processes for the treatment of aqueous wastes at high organic loadings. More specifically for the treatment of liquid piggery waste, nitrification/denitrification and aerobic sequential batch reactor processes have been studied and developed (Fernandes and McKyes, 1991 , Trans. ASAE, 34:597- 602; Martinez, 1997, J. Agric. Eng. Res., 66:51-62; Su et al., 1997, J. Env. Sci. Health A., 32:391-405). Treatment at the production site proposed by some promoters has been considered a potential solution to the problem. Technologies such as nitrification/denitrification processes for nitrogen reduction in France (Agroclar,

Denitral, Val-Epure, Technolyse, Ternois), or different composting methods of liquid manure in France (Guernevez, Isateri, and Lisia-post) and in Belgium (Menart) have been used or are currently used in such applications. However, most of the technologies proposed are either expensive to build and/or operate, are complex to operate, or treat the waste material only partially.

There thus remains a need to provide an aerobic treatment of aqueous wastes containing high levels of suspended solids, organic matter and nutrients that can be oxydized and assimilated into more mineralized, stable and/or innocuous forms during the process. There also remains a need for improving the concentration of the fertilizing value in the sludge in a small fraction of the influent volume for improving the odor control, for improving the proper handling of the biological floe for best settling properties, and for improving the quality of the aqueous effluent. There also remains a need to provide a one-step method for the treatment of aqueous wastes at high organic and solids loadings and to provide a significant purification performance thereof while yielding a sludge having added value.

The present invention seeks to meet these and other needs. The present description refers to a number of documents, the content of which is herein incorporated in reference. SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide an aerobic bioreactor wherein efficient biodegradation of aqueous organic wastes at high organic loadings takes place. It is also an object of the invention to provide a bioreactor and method of treatment of aqueous organic wastes at high organic loadings which yield a treated water which is substantially free of phosphorus and nitrogen. Furthermore, it is an object of the present invention to provide a bioreactor and method using same which yield a product sludge having a better fertilizing value than the influent waste. An additional object of the present invention is to provide a bioreactor which can treat the aqueous waste without any prior treatment thereof and which can remove nutrients such as nitrogen and phosphorus from the process effluent.

A further object of the present invention is to provide a well-mixed flow aerobic bioreactor, which is free of mechanical parts moving therein, enabling an efficient treatment of the waste material and allowing the concentration of the fertilizing value in the sludge in a fraction of the influent volume, while controlling odor emissions.

Yet, a further object of the invention is to provide a well-mixed flow aerobic bioreactor of compact size wherein agitation and aeration are supplied by an air lift pump and air diffusers and which is adapted for the treatment of aqueous wastes at high organic loadings and methods of use thereof. In a preferred embodiment, the present invention provides a bioreactor wherein agitation and aeration are supplied by at least one air lift pump, and at least one diffuser, which can maintain homogeneity of inert and biological suspended solids at concentrations of at least 2 to 10 times (1 % to 5 % mixed liquor suspended solids) that of conventional municipal wastewater treatment systems. Indeed, in most activated sludge processes, reactor solids concentration are in the range of 2,000-7,000 mg SS/L. The present invention provides the means to handle reactor solids concentrations of about 24,000 mg SS/L (from 3,000-53,000 mg SS/L). It will be appreciated by the person of ordinary skill that pure oxygen can enable a handling of reactor solids of even higher concentrations. It should also be recognized that the type and concentration of specific gases (nitrogen, ammonia, etc.) introduced into the bioreactor can be adapted, by the person of ordinary skill, to meet specific needs of the substrate to be treated and/or of the level of performance of purification to be achieved.

Another object of the present invention is a well-mixed flow aerobic bioreactor, wherein aqueous waste at high organic loadings (above 0,30 lb total COD/ffAd or above 0,09 lb filtered COD/ft³.d) is biodegraded while high concentrations of volatile suspended solids (in excess of 30 000 mg VSS/L), i.e. the biocatalyst, are present in the bioreactor.

Broadly stated, it is an object of the present invention to provide a bioreactor, wherein raw aqueous wastes containing grains, sand particles, animal hair and other small debris, can be handled without plugging, thereby overcoming or reducing the need for a separation of the suspended solids from the aqueous waste as in necessary in the systems of the prior art.

It is also an object of the invention to provide adequate control over the shear stress imposed on the biological catalyst to insure swift and proper separation of the biological solids from the liquid effluent by passive settling. The instant invention provides the means to minimize foam formation and wherein excess foam can be controlled chemically vegetable oil, animal fat and the like and/or mechanically (i.e. commonly known foam breakers). It will be understood that foam problems are linked to the substrate or aqueous waste which is treated. In the case of piggery waste with which foam problems can be encountered, minimizing foam formation is important. It will be recognized that foam problems are generally only encountered at the start up of the bioreactor. Once the bioreactor has been stabilized, foam control is usually not necessary.

Furthermore, it is an object of the present invention to provide a bioreactor wherein air is supplied at low pressure and high flow-rate thus preventing ammonia stripping and allowing assimilation of ammonia nitrogen by the microbial flora. The bioreactor of the present invention provides the means to release the oxygen-containing medium (such as air) into the tank of the bioreactor at a pressure which is slightly superior to the hydrostatic water pressure in the bioreactor at the site where the oxygen-containing medium is delivered (a function of the height of the mixed liquor column). By providing a relatively low air pressure, which minimizes or avoids a substantial migration of ammonia to the surface of the mixed liquor and eventually a degasing thereof of its ammonia (i.e. stripping of the ammonia) the bioreactor of the present invention allows a substantial assimilation of the ammonia by the flora. In a preferred embodiment, the invention provides a bioreactor wherein mixed liquor suspended solids and air are drawn into air lift pumps (such as Venturi tubes) for increased contact times, providing internal recycling of the mixed liquor (from as high as 500-1 ,000 cycles/day), as calculated from the flow rate of the mixed liquor from the channel into the discharge area of the bioreactor and from the hydraulic retention time and the sludge retention time (when applicable), good oxygen transfer rate (i.e. 3% for a hydrostatic pressure of 6 feet, at the point of entry of the oxygen containing medium into the bioreactor) and high oxidation levels (removal of between 80-99% of filtered COD at loadings commonly handled by the bioreactor of the present invention and preferably above 90% of filtered COD). The gas pressure of the oxygen-containing medium, as it enters the bioreactor, can be adapted in relationship to the water pressure at the point of entry of the oxygen-containing medium, provided that it is slightly higher than the water pressure and provided that it is not high enough to be accompanied by ammonia stripping, foaming and high shear forces. For a hydrostatic pressure of 10 feet, the oxygen transfer rate should be preferably in the range of 4-5%, which is satisfactory to provide the advantages of the present invention.

In a preferred embodiment, it is an aim of the invention to provide a bioreactor wherein flow patterns and mass transfer can be controlled in different sections of the bioreactor and wherein high dissolved and high rendered oxygen concentrations can be maintained at the bottom of the bioreactor. It will thus be recognized that the bioreactor of the present invention enables a modulation of the level recycling of the mixed liquor as well as of the level of residual oxygen at different levels or different sections within the bioreactor (i.e. by changing the flow rate within the air lift pump or modulating the flow rate of the air diffuser).

It is also an object of the present invention to provide a bioreactor wherein efficient aerobic treatment is effected at mixed liquor temperatures in the mid- range from about 10°C to about 30°C, and preferably ranging from about 15°C to 25°C. The operation of the bioreactor in the mid-temperature range is a significant advantage as compared to the bioreactors of the prior art which operate between 25°C to about

40°C, and for which energy often has to be supplied by an exogenous source. The mesophyllic temperature range of the bioreactor of the present invention therefore provides a significant advantage over such bioreactors of the prior art.

Yet another object of the invention is to reduce the volume of the waste by concentrating organic carbon and sorbed nutrients into the sludge while producing a large liquid fraction (from 45 to 75 % of the influent volume) that can be easily polished or directly disposed of in a sewer or water course.

The invention also relates to a process for the aerobic biodegradation of aqueous organic wastes at high organic loadings. Further, the invention relates to a process wherein the biological catalyst develops as a free suspension from the facultative flora present in the waste to be treated. If the waste lacks a proper biocatalyst, start-up can be initiated using inoculum such as activated sludge from wastewater treatment plants or such as commercial bacteria products. The invention also relates to a bioreactor which optimizes and favors the growth of a mesophyllic flora (optimal temperature for growth being in the mid-range) .

Oxidation and assimilation are stimulated through the supply of air, which generates off-gases rich in carbon dioxide. The off-gases may also contain other compounds in various amounts if their precursors are present in the influent waste and if the conditions for volatilization prevail in the bioreactor. Some non-limiting examples of volatile compounds include low molecular weight fatty acids, alcohols, nitrogen gas, nitrous oxide, ammonia and reduced sulfur compounds. Volatilization conditions can be adapted to specific volatile compounds by the person of ordinary skill. However, temperature, pH and high oxidative conditions prevailing in the bioreactor favour the escape of oxidized gaseous compounds rather than that of reduced gaseous compounds. Also, the low speed of air admission into the bioreactor, the downward succion of the aerated liquor by the air lift pumps and the high internal recirculation of the liquor in the reactor prevent the stripping of gaseous compounds from the liquor. Thus, the bioreactor and method of the present invention minimize the release of reduced gas contaminants. In any event, gas contaminants can be captured and eliminated by a filter, if required. Of importance, the bioreactor of the present invention virtually eliminates odorous gas effluents.

The invention thus relates to a bioreactor wherein the constituents of the influent material are controllably converted into water, gas, and biological solids. The off-gas can be removed from the bioreactor and the liquid effluent separated from the biological solids (sludge) in a settling tank or basin. In most embodiment, the gas produced by the bioreactor is substantially odor-free.

The influent organic waste material is passively introduced or continuously pumped, at an injection point opposite to the reactor overflow, directly into the bioreactor from the source or from an upstream homogenizing tank or basin.

The organic waste then comes into contact with the biological catalyst freely suspended in the bioreactor and is oxidized with the addition of air which also provides agitation. Air is injected through air lift pumps, and, whenever higher aeration is needed, through the addition of an appropriate number of air diffusers. The contact time between the substrate and the catalyst, oxygen transfer and dispersion are maximized by the pneumatic Venturi tubes which continuously recirculate the mixed liquor by discharging it into at least one channel installed above the liquid surface. The channel(s) empty(ies) its(their) content into the bulk of the liquid at a discharge area located at the end opposite to the reactor overflow. The incoming waste is thus instantaneously homogenized with the recirculated mixed liquor. Pneumatic mixing provides low shear conditions that preserve the integrity of the biological floes. Consequently, the floes are more amenable to a downstream swift and passive settling step. After settling, the clarified effluent leaves the settling unit. The biological sludge rich in assimilated carbon and sorbed nutrients along with the inert solids fraction that may have been present in the influent material can be collected at that time. The settling unit can be looped with the bioreactor through devices that recycle biological solids as necessary and return any scum that may form at the surface of the clarifier. In certain embodiments, the flow of the liquids to and from the bioreactor can be carried out by gravity.

The quality of the sludge and aqueous outflows of the settling unit will be dependent on the quality of the influent waste. Final treatment and/or disposal options will depend on local environmental standards (sewer and/or water receiving bodies) as well as on biomass reuse capabilities of the local or regional community and can be adapted by the person of ordinary skill. In a preferred embodiment, the bioreactor of the present invention provides the production of a sludge from the settling unit, such that the sludge is a better ingredient in the preparation of commercial fertilizers than the influent material provided to the bioreactor.

The present invention also provides a method of treating aqueous wastes containing high levels of organic material comprising an aerobic biodegradation inside a well-mixed flow bioreactor, whereby reduction levels of 90 % or above are obtained for (1) total suspended solids; (2) organic material; (3) nitrogen and phosphorus contamination; and whereby reduction levels in excess of 99.⁹ % are obtained for pathogenic bacteria. As used herein, the designation "organic wastes" is meant to cover preferably organic wastes having high organic content. Non-limiting examples of such organic wastes include animal slurries such as liquid piggery waste, septic tank sludge, landfill site leachate and agro-food or other industrial wastewater. Preferably, the organic loadings of the bioreactor will be between about 0.17 to 0.46 lb total COD/ft³.d or between about 0.06 to 0.17 lb filtered COD/f .d. It shall be understood that the organic content of the aqueous wastes which can be efficiently treated in accordance with the present invention is dependent on the concentration of volatile suspended solids (VSS) or biocatalyst in the bioreactor. For example, an increase in the concentration of VSS should allow the treatment of aqueous wastes having higher organic concentration. It follows that the concentration of VSS can be adapted as a function of the organic content of the aqueous waste.

The term "biodegradation" denotes the fact that the treatment or degradation of the organic waste in the bioreactor is enabled by a biological catalyst which is freely suspended and develops from the facultative aerobic microbial flora present in the waste to be treated. Of course, it will be understood that the biological catalyst can be added to the waste to be treated and that the microbial flora to be added can be adapted to the type of pollutant to be removed from the aqueous waste. In addition, inoculation of the bioreactor will often take place naturally by the microbial flora contained in the aqueous waste. It shall also be understood that a particular type of aqueous waste generally contains a microbial flora which is usually best adapted to the degradation of the substrate in which it lives. The bioreactor of the present invention and method of degradation using same, favour and select for the development of such a microbial flora, thereby enabling an efficient and complete biodegradation of the organic content, provided that an adequate sludge retention time is applied. The microbial flora refers generally to bacteria and higher life forms such as protozoa which develop in the mixed liquor. The bioreactor also enables the production of floes. Thus, the present bioreactor and method of using same are adaptable to the treatment of different types of aqueous wastes at high organic content, provided that the microbial flora has the necessary growth conditions inside the bioreactor. The adequate sludge retention time can be determined by the person of ordinary skill as a function of the food mass ratio (F/M) of the F/M inside the bioreactor, of the hydraulic retention time and of the level of purification performance of the bioreactor which is targetted. As used herein the term "floes", well known to a person of ordinary skill to which the present invention pertains, refers to a flocculant mass formed by the aggregation of inert and biologically active particles. The present invention enables the production of floes without a dependance upon added flocculating agents. Under certain conditions, the addition of flocculating agents although less preferred (see below) could also be envisaged.

As used herein, "sludge or biological sludge" is well known in the art and denotes that the sludge is of biological origin and that it provides a biomass. Non-limiting examples of sludge thickening include, centrifugation, drainage on porous bed, filtration on membrane or by press. Non-limiting examples of sludge stabilization include, composting, lime treatment, and aerobic or anaerobic digestion. Sludge disposal usually pertains to disposal in a landfill site unless sludge land farming is possible and/or permitted. Thermal destruction of sludge is yet another method to dispose of the sludge. Since flocculating agents are generally not required in the method of the present invention, a decrease in the value of the sludge by these flocculating agents is avoided. Furthermore, the method of the present invention produces a sludge which has a higher fertilizing value than the influent waste. In some applications of the method of the present invention, the product sludge has a potential as an ingredient in the preparation of animal feed. Therefore, the product sludge has a potential value on the market.

The recitation "organic loading" can be expressed as a function of the volume of the reactor or as a function of the concentration of biocatalyst in the bioreactor. It is usually expressed as unit mass of COD or BOD per unit volume of reactor per day (lb COD or BOD/frλd) or as a unit mass of COD or BOD per unit mass of biocatalyst per day (lb COD or BOD/lb VSS.d).

Depending on the sludge retention time and on the hydraulic retention time, the bioreactor and method of the present invention enable a reduction of total COD of the influent waste in excess of 90 % after the clarification step. The reduction can be as high as about 98 %. Thus, the bioreactor of the present invention and method of treating aqueous waste of high organic content of the present invention provide a very efficient reduction in organic content of such aqueous wastes.

The terms "COD" and "BOD", as well known in the art, relate to the chemical oxygen demand and biochemical oxygen demand, respectively. The COD is a chemical oxidation method for the measure of all the matter which is chemically oxidizable. It is always higher than the carbonaceous BOD because it includes both bio- and non-biodegradable matters. The carbonaceous BOD represents the quantity of oxygen used by bacteria to oxidize the biodegradable matter present in a sample of aqueous waste in a period of, normally, five days.

In this context, aqueous wastes having a carbonaceous BOD between about 100 to about 100,000 mg BOD₅/L, preferably 3,000 to 30,000 mg BOD₅/L, and defining an example of aqueous wastes having high organic content, are encompassed as being within the scope of the present invention. As a reference, domestic waste waters have organic contents between 100-400mg BOD_s/L and typically about 250mg BOD₅/L. Using the bioreactor of the present invention to treat such a type of aqueous waste would require an increase in the sludge retention time in order to obtain the optimal F/M for the targeted purification performance. As used herein the BOD values are pertinent to indicate the type of aqueous wastes which are within the scope of the present invention. It should not be used to characterize the size of the treatment units, however. In this context, the size of the unit should be based on the volumetric loading to the reactor (i.e. unit mass of COD or BOD per m³ of reactor volume per day).

The recitation "freely suspended microbial flora" refers to the biomass being in a suspended growth process as opposed to the biomass being in an attached rowth process.

The recitation "aqueous organic waste" refers to the fact that the solvent is water as opposed to oil or the like.

The bioreactor and method of the present invention provide the significant and novel advantage of enabling the treatment of influents containing suspended solids in concentrations as high as 6.5%. The bioreactor and method of the present invention maintain thourough mixing conditions and thereby the homogeneity of the suspended solids in the reactor. The biocatalyst, as characterized by the content of volatile suspended solids, is generally present in the aqueous wastes and contributes to the concentration of suspended solids. The bioreactor and method of the present invention allow the treatment of aqueous wastes containing concentrations of VSS in the range of 0 to 50,000 mg/L, preferably in the range of 10,000 to 30,000 mg/L. It shall be understood that regardless of the suspended solids in the influent material, the reactor can develop and maintain concentrations of VSS up to 50,000 mg/L. The VSS range will vary according to the BOD concentration of the influent, the hydraulic retention time and the sludge retention time. Of course, a person of ordinary skill will be able to adapt these parameters in order to obtain a desired performance of the bioreactor and aqueous waste treatment method. The bioreactors and methods of the prior art in contradistinction to those of the present invention can rarely operate at VSS concentrations superior to 10,000 mg/L. In fact, in general, the preferred range of operation thereof is 1 ,500 - 6,000 mg VSS/L, for municipal applications and slightly higher for industrial applications. The bioreactor of the present invention is thus the first to provide the means to operate at VSS concentrations superior to 10,000 mg/L, as high as 50,000 mg/L, and preferably between 10,000-30,000 mg/L.

It is to be understood that the sludge retention time will have to be adapted by the person of ordinary skill as a function of the specific type of treatment and aqueous waste treated. In certain situations, in which no sludge recycling is used, the sludge retention time will be equal to the hydraulic retention time. However, in order to ensure an efficient clarification, there exists an optimal sludge retention time which is dependent on the type of aqueous waste and the hydraulic retention time. As mentioned above, this optimal sludge retention time can be determined by conventional means by the person of ordinary skill to which the present invention pertains.

It shall also be understood, that the bioreactor tank of the present invention can be under or above the ground level or alternately at intermediate levels. The person of ordinary skill, will be able to adapt the system to the correct level.

While the method of treatment of aqueous wastes at high organic and solids content by the well-mixed flow aerobic bioreactor of the present invention is demonstrated with liquid piggery waste, as mentioned, other aqueous wastes at high organic and solids content can be treated in accordance with the present invention. Non-limiting examples thereof include other animal waste slurry, septic tank sludge, landfill site leachate and agro-food or other industrial or domestic wastewaters.

Broadly, the bioreactor of the present invention is the first bioreactor enabling the presence of a substantial level of dissolved or residual oxygen at the bottom of the reactor (at least 1.5 mg /L and preferably 2.0 mg/L). Thus, the bioreactor of the present invention is the first to enable the obtention of a substantially homogenously oxygenated (or aerobic) mixed liquor.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus generally described the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which:

Fig. 1 is a schematic of a typical set-up for aqueous waste treatment in accordance with the present invention showing the bioreactor of the present invention in relationship with peripheral units that can be used in accordance with the invention;

Fig. 2 is a top plan view of the structure and configuration of the bioreactor;

Fig. 3 is a cross-sectional view on line 3-3 of Fig. 2; and

Fig. 4 is a cross-sectional view on line 4-4 of Fig. 2. Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments with reference to the accompanying drawing which is exemplary and should not be interpreted as limiting the scope of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Broadly, the aqueous waste treatment process of the present invention and apparatus therefor consist of a bioreactor 2 and of two standard units (Figure 1). A homogenizing tank 4 can be placed upstream of the bioreactor 2, the subject of the present invention, and a settling tank 6 installed downstream. The homogenizing tank 4 or basin is optional but preferred for waste containing settleable solids. The aqueous waste 8 flows passively or can be pumped into the homogenizing unit 4 either batchwise or continuously by a pump 10. A pump 12 is necessary between the homogenizing tank or basin 4 and the bioreactor 2 if the outflow from the homogenizing unit 4 is located below the inflow of the bioreactor 2.

The bioreactor 2 achieves biological oxidation of the influent waste 14 and flocculation of suspended solids to provide a mixed liquor 16 that overflows into the settling tank 6 in which it can be easily separated to generate an aqueous effluent 18 and a biological sludge 20 to be further treated or disposed of. The product sludge 20 can be managed according to the needs of the user and disposed of in different ways known to the person of ordinary skill. The aqueous effluent 18, depending on its quality and on local environmental standards, can be polished in further purification steps or disposed of in a sewer system or in receiving water bodies as commonly known in the art. An oxygen containing medium such as an air supply has to be provided for the oxidation to take place. In a preferred embodiment of the present invention, an oxygen containing medium 21 (air) is supplied to the bioreactor 2 by a positive displacement blower 22 and off-gas 24 can be treated, if required, as well known in the art. The agitation and aeration design of the bioreactor of the present invention minimizes foam formation. Excess foam building at the liquid surface can be controlled by various mechanical devices or by the addition of chemical foam suppressor. In a preferred embodiment of the present invention, excess foam is suppressed by the addition of a foam suppressor 26 (i.e. vegetable oil) with a dosing pump 28.

The bioreactor maintains homogeneity of the material to be treated and reduces the organic content of the influent waste by 90 % to over 98 % (as expressed in COD) depending upon the hydraulic retention time and the sludge retention time. The bioreactor is designed to handle influents containing suspended solids of up to 6.5 % and organic loadings above 0,11 lb filtered COD/ft³.d. Furthermore, the design of the bioreactor enables its operation at high volatile suspended solids concentrations (up to 50,000 mg VSS/L).

The settling tank 6 passively removes most of the suspended solids leaving the biological unit. The amount of suspended solids removed in tank 6 depends on the operating conditions of the biological unit. If scum is present at the surface or the settling unit, this can be returned to the bioreactor (shown as 30 in Fig. 1). Recycling of the sludge (shown as 32 in Fig. 1) is optional and it can be achieved if the hydraulic retention time does not allow the appropriate sludge retention time in the bioreactor. In such a case, the old sludge provides the biological catalyst for the next round of biotreatment of aqueous wastes.

The bioreactor 2 (figures 1-4) is either under or above ground level. At least one channel 34 installed above the operating volume extends over a substantial portion of the length of the bioreactor 2, preferably over 80% of the length and more preferably over 95 % of its length. Fresh influent material 14 can be fed continuously at any point by an influent pipe 15 or into the channel discharge area 36 of bioreactor 2 where it comes into or is injected directly in the bulk of the mixed liquor. The channel 34 has a slope of at least 1 % to insure a flow of the mixed liquor towards the discharge area 36 located at the end opposite to the overflow 16 and overflow pipe 17 of the bioreactor 2. Alternatively, the influent 14 can be injected into channel 34. In a preferred embodiment, for treatment of aqueous waste at high organic and solids loadings, the influent is injected into the channel discharge area 36, and more preferably, below the surface of the mixed liquor, thereby avoiding odor emissions. As best shown in Figs. 2 and 3, aeration and agitation are entirely pneumatic and provided by Venturi tubes 38 connected to the walls of the channel 34 where they empty their contents into the channel 34. Additional aeration and agitation provided by air diffusers 40 between the Venturi tubes at the bottom of the bioreactor 2. These diffusers should be selected according to mass transfer efficiency and oxygen depletion rates at set operating conditions. Furthermore, the air diffusers should be of the non-clogging type. They should also supply medium and coarse air bubbles into the mixed liquor. This bubble size allows a suitable air transfer to the mixed liquor while preventing the breaking of the biological floes. Such air diffusers 40 also minimize foam formation. The general arrangement of the Venturis tubes 38 can be in staggered or parallel rows and the number of Venturis tubes 38 and rows will vary according to the size of the bioreactor and the number of channels. Each Venturi tube 38 is equipped with a suction device 42 sitting on cross supports 44.

As shown in Figs. 2-4, oxygen containing medium such as air is supplied to the Venturi tubes 38 and diffusers 40 by individual calibrated pipes 46 linked to a manifold 48. If more flexibility in the liquid flow pattern of the reactor is required, the air flow through each pipe 46 can be controlled by individual valves 50. The appropriate air source has to be supplied by a positive displacement blower 22 (Fig. 1) which provides the bioreactor 2 with the oxygen containing medium 21 through the manifold 48. The positive displacement blower 22 should be sized as a function of the oxygen demand and of the hydrostatic pressure to provide approximately 2,500 ft³ of air per pound of BOD₅ per day. Heat transfer may also be considered as a selection criteria for the blower. Indeed, as commonly known the gas pressure and the temperature of the gas (or oxygen-containing medium) as it enters the bioreactor are intimately linked. The mixed liquor 16 leaves the bioreactor 2 passively through an overflow pipe 17 towards the settling unit 6. The overflow pipe 17 and its air vent 52 are located at the end of the bioreactor 2 opposite to the channel discharge area 36. Peripheral flows such as for scum recycle 30 or sludge recycle 32 (see Figs. 1 and 2- 3) from the settling unit 6 can be provided by a scum recycle pipe 54 and sludge recycle pipe 56, installed at the upstream end of the channel 34 and in the channel discharge area 36, respectively. Foam suppressor 26 can be added to the mixed liquor at any appropriate point of bioreactor 2 by a foam suppressor injection pipe 60. Off- gas can exit the bioreactor 2 at any appropriate point and as necessary by an off-gas pipe 58 at the top of bioreactor 2. The off gas pipe 58 can be connected to a filter or gas treating device as commonly known in the art.

The present invention is illustrated in further detail by the following non-limiting examples.

EXAMPLE 1

The liquid piggery waste produced by 160 pigs was treated at a fattening piggery with the bioreactor of present invention using a hydraulic retention time of 12 days. The system also included a homogenizing tank as well as a settling tank (without sludge recycle). Characterization of the influent material and of the supernatant leaving the settling tank before polishing yielded the following results:

Conditions were as follows: Influent flow-rate of 1 ,120 L/d; food to mass ratio (f/M) of 0.29 lb total COD/lb VSS.d or 0.09 lb filtered COD/lb VSS.d working at 0.30 lb total COD/f .d or 0.09 lb filtered COD/ft³.d; no sludge recycle. Hydrostatic pressure at the site of the air diffusers was 6 feet of water; oxygen transfer rate was between 2.7% to 3.2%; residual or excess oxygen in the mixed liquor was on average of 2 mg/L. Mass balance around the reactor and the settling tank indicated that each unit mass of liquid piggery waste was converted into 45 % of supernatant and 55 % of sludge containing 3.5 % suspended solids, by weight.

Thus, the bioreactor and method exemplified enable a removal efficiency of the different tested parameters which is almost maximum. During the two months trial run described in example 1 , almost 20 % of the nitrogen and 90 % of the phosphorus were captured in the sludge. The biological sludge contained, on a dry basis, 3.6 % total nitrogen, 6.3 % plant available phosphoric acid (P₂O₅), 2.6 % soluble potassium (K₂O), 14.8 % humic acid and 15.4 % amino acids. Thus, the sludge produced had an increased commercial value as compared to the influent waste of the bioreactor.

EXAMPLE 2

The liquid piggery waste produced by 300 pigs was treated at a fattening piggery with the present invention using a hydraulic retention time of 6.5 days. The system had an homogenizing tank as well as a settling tank, without sludge recycle. Characterization of the influent material and of the supernatant leaving the settling tank before polishing yielded the following results:

Conditions were as follows: Influent flow-rate of 2,140 L/d; food to mass ratio (f/m) of 0.35 lb total COD/lb VSS.d or 0.24 lb filtered COD/lb VSS.d; working at 0,22 lb total COD/ft³.d or 0,14 lb filtered COD/ft³.d; no sludge recycle. Hydrostatic pressure at the site of the air diffusers was 6 feet of water; oxygen transfer rate was between 2.7% to 3.2%; residual or excess oxygen in the mixed liquor was on average of 2 mg/L. No mass balance can be calculated around the bioreactor and the settling tank for this example because results were obtained under non-permanent regime conditions. Interestingly, the bioreactor was able to treat aqueous waste at a very high efficiency even though the non-insulated bioreactor (and above ground) was submitted to ambient temperature levels that reached below zero °C and in which the influent oxygen-containing medium temperature was as low as -20°C.

Conclusion

There has thus been described a compact bioreactor for treating aqueous organic wastes at high organic loadings. The system concentrates into the biological sludge over 85 % of the nitrogen and over 92 % of the phosphorus contained into the bioreactor outflow and it can allow discharge of the aqueous effluent to a water course depending on local regulations. The aqueous effluent is odor-free and the output solids may be processed through a stabilization step if a stable and odor-free product is needed for further utilization. Off-gases can be treated if necessary through a filter.

The system provides very high reductions in total suspended solids

(above 93 %), organic material (BOD or COD, above 90 %), nitrogen (above 93 %) and phosphorus (above 86 %) contaminations that are otherwise discharged into the environment, treated only partially, or treated through more expensive technologies based upon biological, physicochemical or thermochemical processes.

Although the present invention has been described herein by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.

Claims

WE CLAIM:

1. An aerobic bioreactor for treating aqueous waste comprising: a) a reservoir having lower and upper portions, and opposite inflow and outflow ends; b) an inlet located at said inflow end for supplying said aqueous waste into said reservoir and an outlet located at said outflow end for supplying the treated aqueous waste from said reservoir; c) at least one oxygen-containing medium diffuser, positioned in said lower portion of said reservoir; d) at least one air lift pump located in said lower portion of said reservoir and positioned for sucking from said lower end portion and for transporting upwardly at least in part said oxygen-containing medium and a mixed liquor comprised of said aqueous waste and of a biomass capable of biodegrading said aqueous waste; and e) a channel system positioned in said upper portion of said reservoir for receiving said mixed liquor from said air lift pump and discharging said mixed liquor in the vicinity of said inflow end, whereby said at least one air lift pump and said at least one oxygen-containing medium diffuser enable a substantially homogeneous oxygenation of said mixed liquor within said bioreactor.

2. The bioreactor of claim 1 , wherein said oxygen-containing medium is an air diffuser.

3. The bioreactor of claim 1 or 2, wherein said air lift pump is a Venturi pump.

4. The bioreactor of claim 1 , 2, 3 or 4, wherein said Venturi pump enable a modulation of said upward transport so as to enable a modulation of the level of oxygen inside said reservoir.

5. The biorector of claim 1 , 2, 3, 4 or 5, wherein said aqueous waste contains high organic loadings.

6. The bioreactor of claim 1 , 2, 3, 4 or 5 wherein said aqueous waste contains high solids loading.

7. The bioreactor of claim 1 , 2, 3, 4, 5 or 6, wherein said aqueous waste is piggery waste.

8. The bioreactor of claim 1 , 2, 3, 4, 5, 6 or 7 wherein said a residual oxygen concentration of at least about 1.5 mg/L is present in the lower portion of said reservoir.

9. The bioreactor of claim 1 , 2, 3, 4, 5, 6, 7 or 8, wherein said a residual oxygen concentration is about 1.5 mg/L is present in the lower portion of said reservoir.

10. An aqueous waste treatment apparatus comprising: a) an aerobic bioreactor for treating aqueous waste comprising: a reservoir having lower and upper portions, and opposite inflow and outflow ends; an inlet located at said inflow end for supplying said aqueous waste into said reservoir and an outlet located at said outflow end for supplying the treated aqueous waste from said reservoir; at least one oxygen-containing medium diffuser, positioned in said lower portion of said reservoir; at least one air lift pump located in said lower portion of said reservoir and positioned for sucking from said lower end portion and for transporting upwardly at least in part said oxygen-containing medium and a mixed liquor comprised of said aqueous waste and of a biomass capable of biodegrading said aqueous waste; and a channel system positioned in said upper portion of said reservoir for receiving said mixed liquor from said air lift pump and discharging said mixed liquor in the vicinity of said inflow end, whereby said at least one air lift pump and said at least one oxygen-containing medium diffuser enable a substantially homogeneous oxygenation of said mixed liquor within said bioreactor; b) a homogenizing tank connected to said inlet, and enabling a supplying of a homogenous aqueous waste to said inlet; and c) a settling reservoir connected to said outlet, and enabling a separation of a liquid phase and a sludge phase.

11. The aqueous waste treatment apparatus of claim 10, wherein said oxygen-containing medium is an air diffuser.

12. The aqueous waste treatment apparatus of claim 10 or 11 , wherein said air lift pump is a Venturi pump.

13. The aqueous waste treatment apparatus of claim 10, 11 or 12, wherein said Venturi pump enable a modulation of said upward transport so as to enable a modulation of the level of oxygen inside said reservoir.

14. The aqueous waste treatment apparatus of claim 10, 11 , 12 or 13, wherein said aqueous waste contains high organic loadings.

15. The aqueous waste treatment apparatus of claim 10, 11 , 12, 13 or

14, wherein said aqueous waste contains high solids loading.

16. The aqueous waste treatment apparatus of claim 10, 11, 12, 13, 14 or 15, wherein said aqueous waste is piggery waste.

17. The aqueous waste treatment apparatus of claim 10, 11 , 12, 13, 14, 15 or 16 wherein said a residual oxygen concentration of at least about 1.5 mg/L is present in the lower portion of said reservoir.

18. The aqueous waste treatment apparatus of claim 10, 11 , 12, 13, 14,

15, 16 or 17, wherein said a residual oxygen concentration is about 1.5 mg/L is present in the lower portion of said reservoir.

19. The aqueous waste treatment apparatus of claim 10, 11 , 12, or 13, wherein said sludge phase can be recycled into said reservoir, thereby increasing the organic loadings of said mixed liquor.

20. A method of treatment of aqueous waste comprising: a) an incubation of said aqueous waste into an aerobic bioreactor for treating aqueous waste comprising: a reservoir having lower and upper portions, and opposite inflow and outflow ends; an inlet located at said inflow end for supplying said aqueous waste into said reservoir and an outlet located at said outflow end for supplying the treated aqueous waste from said reservoir; at least one oxygen-containing medium diffuser, positioned in said lower portion of said reservoir; at least one air lift pump located in said lower portion of said reservoir and positioned for sucking from said lower end portion and for transporting upwardly at least in part said oxygen-containing medium and a mixed liquor comprised of said aqueous waste and of a biomass capable of biodegrading said aqueous waste; and a channel system positioned in said upper portion of said reservoir for receiving said mixed liquor from said air lift pump and discharging said mixed liquor in the vicinity of said inflow end, whereby said at least one air lift pump and said at least one oxygen-containing medium diffuser enable a substantially homogeneous oxygenation of said mixed liquor within said bioreactor; and b) an obtention of said treated aqueous waste at said outflow end.

21. The method of claim 20, wherein said oxygen-containing medium is an air diffuser.

22. The method of claim 20 or 21 , wherein said air lift pump is a Venturi pump.

23. The method of claim 20, 21 or 22, wherein said Venturi pump enable a modulation of said upward transport so as to enable a modulation of the level of oxygen inside said reservoir.

24. The method of claim 20, 21 , 22, or 23, wherein said aqueous waste contains high organic loadings.

25. The method of claim 20, 21 , 22, 23 or 24 wherein said aqueous waste contains high solids loading.

26. The method of claim 20, 21 , 22, 23, 24, or 2, wherein said aqueous waste is piggery waste.

27. The method of claim 20, 21 , 22, 23, 24, 25, or 26, wherein said a residual oxygen concentration of at least about 1.5 mg/L is present in the lower portion of said reservoir.

28. The method of claim 20, 21 , 22, 23, 24, 25, 26, or 27, wherein said a residual oxygen concentration is about 1.5 mg/L is present in the lower portion of said reservoir.

29. The method of claim 20, 21 , 22, 23, 24, 25, 26, 27 or 28, further comprising a homogenizing step of said waste water prior to its entry into said bioreactor.

30. The method of claim 20, 21 , 22, 23, 24, 25, 26, 27, 28 or 29, further comprising a settling of said treated waste water in a settling tank.

31. The method of claim 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29 or 30 further comprising a polishing step of said treated aqueous waste, following the exit of said treated aqueous waste from said bioreactor.