Abstract:
The present invention relates to a high-rate reactor, (applying a high superficial velocity), capable of treating partially soluble, complex, high-strength wastewater. This is the first single reactor system in a compact and mechanically simple package that is configured in multiple stages and has the ability to retain complex insoluble substrates in a wide range of particle sizes in spatially separate stages for sufficient residence time to enable complete degradation, thereby achieving highly efficient performance for the removal of both solid and dissolved contaminants.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device for treatment of wastewater. More particularly, the present invention relates to a device for the removal of biodegradable contaminants from wastewater using biological processes. The present invention also relates to a system for purifying highly contaminated water which contains a large amount of suspended solid impurities (TSS) and high concentrations of BOD (biochemical oxygen demand) and COD (chemical oxygen demand) as, for example, effluents from food processing plants and toilets. The device enables the efficient removal of biodegradable solid substances from wastewater by a combination of filtration and biochemical reaction. In other words, this invention relates to a bioreactor device that enables both biological degradation aid the filtration of suspended solids.  
         BACKGROUND OF THE INVENTION  
         [0002]    The treatment of wastewater for the removal of substances that cause BOD is best accomplished by biological treatment methods such as aerobic degradation and anerobic degradation. There is a wide range of industrial operations that result in liquid effluents containing substantial amounts PBOD, which has to be removed to conform to national discharge regulations. In addition, several of these high-BOD industrial effluents, have substantial quantities of BOD present as suspended solid material.  
           [0003]    In general, examples of BOD exerting suspended solids include, but are not limited to, partially dissolved and partially macromolecular materials, such as proteins, long-chain fatty acids, fats, vegetable oils, tallow, bacterial and yeast cell-walls, celluloses, hemicelluloses, and starch, the suspended solids being present in emulsified, suspended or colloidal state. Effluents of this nature are discharged for example, from slaughterhouses, dairies, rendering plants oil mills pharmaceutical and organochemical plants, pulp and paper factories.  
           [0004]    The relevant organic compounds constituting biodegradable solid organic matter are generally classified as 1) Polysaccharides common among which are cellulose, hemicellulose, starch and pectin 2) Proteins and amino acids which are present as insoluble matter when coagulated by heat, acids or tannins and 3) Fats and long chain fatty acids. All these compounds can be degraded by anaerobic micro-organisms to form methane. The first step in the degradation process is called solubilization which results in the formation of soluble compounds and is carried out by enzyme action outside of the microbial cell. This is a slow process and requires sufficient microbial cells producing the enzymes and sufficient contacting time between enzyme and solids. In fact, the solubilization step is most often the rate limiting step in the sequence of anaerobic reactions that result in mineralisation of the polluting substances.  
           [0005]    The development of high-rate anaerobic reactors, anaerobic treatment has become the economic option in the pretreatment of high BOD industrial effluents. However, high-rate reactors can be used only or the treatment of industrial effluent with BOD in primarily dissolved form. There are no high-rate reactor devices, in use, for the treatment of complex wastewater, i.e., wastewater containing un-dissolved BOD. A survey of prior art, has revealed no apparatus specifically designed to accomplish the primary object of this invention, i.e., the high-rate anaerobic treatment of wastewater containing suspended solids. Therefore, a survey of related art is given below, wherein, some ideas and concepts related to this invention may be found. This survey is merely in support of the practicality of the concepts used in this invention and does not in any way detract from the absolute novelty of the device.  
         PRIOR ART  
         [0006]    Related Art in Anaerobic Treatment of Wastewater  
           [0007]    High-rate anaerobic reactors enable continuous treatment of industrial effluents at small hydraulic retention times. In other words, reactor sizes are relatively small and the BOD load per unit reactor volume per day is high. The primary principle that permits high BOD loading is the retention of a large population of viable microorganisms (biomass) within the reactor by decoupling the retention time of microorganisms from the hydraulic retention time. In simple language, the microbes stay within reactor longer than the liquid being treated.  
           [0008]    A variety of high-rate anaerobic reactor systems are in commercial use. A good description of anaerobic processes and reactors is given by S. Stronach, T. Rudd and J. Lester, “Anaerobic Digestion Processes in Industrial Wastewater Treatment”, 1986, Springer Verlag, Berlin. The prominent designs can be classified into three families: 1) fixed-film reactors 2) sludge-bed reactors and 3) fluidized bed reactors. However, none of these prior art high-rate anaerobic reactor systems are suitable for the treatment of wastewaters where a substantial quantity of BOD is present as solid matter. This will be clear from closer examination of the operating principles and constructional features of these reactors.  
           [0009]    The fixed-film reactors use a stationary inert packing media within the reactor on which microorganisms are retained as a biofilm. The function of the media is enhanced by the increasing the surface area. But the media should have sufficient open porous channel available for free flow with only minor hydraulic head loss and these channel should remain open even after copious growth of biofilm. Among various media types available, are dumped or random packing comprising rings like units and structured media constructed with corrugated plastic sheets jointed to form blocks with dividing straight channels. Both types of packing media are rapidly choked by the deposition of suspended solids present in wastewater. This is especially true in the case of filly submerged upflow type fixed film reactors. In a downflow configuration, the presence of solids in the wastewater will either lead to choking of media or if media has sufficient porosity and straight channels for free outflow of solids, there will be insufficient removal of BOD.  
           [0010]    The sludge-bed reactors enable the retention by using the settling property of sludge which is denser than wastewater, if free of gas bubble inclusions. Specially designed “gas solids separators” are mounted on top of reactor to enable settling of sludge. While these reactors are affected by choking problems as is the case with fixed film reactor, they are still not suitable for the treatment wastewater containing solids. Solids in wastewater will rapidly attach or adsorb to the sludge, decreasing its specific gravity and impairing the settleablity of sludge. This lead to the phenomenon of sludge washout, a common and recurring cause of failure of several installations of such reactor. In general, sludge bed reactors are not recommended for wastewater with more than 30% of COD present as insoluble matter. The originator of UASB (upflow anaerobic sludge bed) technology, Professor Gatze Lettinga, from the Netherlands writes in his comprehensive review: [“UASB process design for various types of waste-water”; Water Science and Technology, 24, 8, 87-107 (1991)] “Regarding the reactor system to be chosen for treating a partially soluble complex wastewater . . . it will be clear that process that apply a high superficial velocity, such as fluidized bed and expanded granular sludge bed (BOSS) reactors are unsuitable, unless they are combined with an adequate pre- or post-clarifier . . . . According to our present experience, application of granular sludge UASB (or EGSB) reactors becomes doubtful at TSS concentration in the influent exceeding 6 to 8 g/l because at such high TSS concentration, the segregation between granular and flocculent sludge does not proceed sufficiently rapidly . . . . For high strength wastewaters with high insoluble fraction, (i.e., exceeding 15%) generally conventional digesters are in favour over UASB and other high-rate systems.  
           [0011]    Fluidized bed reactor using small biofilm carrier particle in fluidized state is free of clogging, choking or sludge washout problems. But fluidized bed reactors are still not suited for treatment of wastewater with insoluble BOD as solids are not retained in the reactor for sufficient time for the solubilization.  
           [0012]    The only reactor system which is suitable for the anaerobic treatment of solid containing industrial effluents is the stirred tank digester, also known as sludge digester. This reactor system is a low rate reactor system where no attempt is made to delink liquid residence time from suspended solid residence time nor is there any attempt to retain and increase the microbial biomass in the reactor. The stirred tank digester essentially maintains a homogenous mixture of solids and liquid for sufficient long duration to effect degradation.  
           [0013]    There is only limited previous attempt in prior art to develop a high rate reactor specifically for the anaerobic treatment of solids containing effluents. In order to overcome the inadequacies of anaerobic filters and encapsulated bacterial retention systems in treating organic materials with suspended solids content, the prior art has identified the use of the so-called anaerobic activated sludge process which has also been called the anaerobic contact process [S. Stronach, T. Rudd &amp; J. Lester; “Anaerobic Digestion Processes in Industrial Wastewater Treatment”, 1986, Springer, Verlag, pp. 93-120, 136-147]. The anaerobic contact process uses a secondary solids separation stage downstream of the anaerobic reactor and the separated solids are recycled back to the reactor. The secondary separation stages may be gravity settling or mechanically separation such as centrifugation or flotation.  
           [0014]    U.S. Pat. No. 5,015,383 describes the difficulties with the anaerobic activated sludge as follows: “The anaerobic activated sludge, or anaerobic contact process, has not been effectively utilised because the bacteria in anaerobic digestion are not easily separated from the mixed liquor effluent. The difficulty his been that actively fermenting organisms do not settle by gravity because of the buoying effects of attached gas bubbles and the fact that the density of the bacteria closely approximate the density of water and do not floc easily. The use of other common liquid/solids separators also have disadvantages. The use of gravity clarification with the addition of high concentrations of flocculating or coagulating chemicals is expensive and harmful to the bacteria. Rapid temperature and pH changes have also been attempted and found to be harmful to the bacteria. Centrifuging has been found to be expensive and detrimental to the bacteria. Conventional dissolved air flotation as well as froth and foam flotation techniques are detrimental to the anaerobic bacteria since even minute amounts of oxygen or air are sufficient to destroy the bacteria. S. Stronach, T. Rudd &amp; J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 35-38”. The Burke patent teaches a method of separation using gas flotation, in particular the use of biogas as flotation gas, to overcome separation problems. It would be clear, that this process is hampered by the expense and complexity of the gas flotation process involving mechanical solids removal systems and carried out in closed equipment with a flammable gas. The effectiveness of this process is not known and there is no known installation of the process.  
           [0015]    U.S. Pat. No. 4,551,250 teaches a process by which wastewaters containing undissolved solids are treated in a two reactor system in series. A filtering process retains solids in the first reactor of sufficient residence time to be degraded to low molecular weight soluble forms which are passed. While the basic concept of retaining solids for sufficient time by decoupling suspended solids residence time from the hydraulic residence time is the same as hi the present invention, there is no mention in U.S. Pat. No. 4,551,250 pf the certain possibility of clogging of filter media nor about methods by which this prevented. In contrast to my invention, the process disclosed specifically is for a first reactor is operated at a low pH in order to inhibit gas production. In a further point of deviation from my invention, the first reactor in U.S. Pat. No. 4,551,250 is preferably maintained in a quiescent state to avoid turbulence and enhance settling. In a still further deviation, the filtering process is accomplished by biofilm media provided in the first reactor. Now, it would be clear to those in the art, that solids settling and clogging of biofilm media is a major problem in such reactors when treating high strength wastes, an in particular wastes with suspended solids. The declogging of such systems is a an almost impractical task. In any case, U.S. Pat. No. 4,551,250 does not teach any apparatus that would specifically enable the operation of the process while overcoming the obvious problems with respect to filtration of heavily contaminated wastewaters.  
           [0016]    A rather crude apparatus and method for the treatment of complex wastewaters is disclosed in GB 2,167,055 s . This system comprises an anaerobic followed by a downflow or upflow filter. No method of backwashing or declogging of the filter bed is mentioned. It is obvious that this does not support high-rate operation and may be considered merely a modification of a pond treatment system, and may be applied only where a pond is considered appropriate.  
           [0017]    A reactor system which theoretically can function with solids containing effluents are membrane bio-reactor systems wherein membrane modules are used for separation of solids from the liquor. These systems have not been successfully applied for anaerobic processes because of membrane fouling problems, which are more than in aerobic processes. In addition, these reactor systems consume large amounts of energy to drive the membrane systems. It may be noted that membrane systems separate very fine and colloidal matter including individual microorganisms in addition to larger suspended solids present in the effluents of interest. Thus the application of membrane systems for the anaerobic treatment of complex wastewaters is an overkill being inappropriate and uneconomic for the application at hand.  
           [0018]    Reference may be made to U.S. Pat. No. 4,613,434 which teaches a device for treatment of wastewater by means of anaerobic fermentation comprising of a single reactor within which is a lamella and biofilm filter divides the reactor into a mixed lower zone and a an upper zone. Although, the lower zone is claimed for ‘acidification’ and the upper zone is claimed for methanogenesis and there is no mention anywhere in the patent of solids hydrolysis or the filtering of solids. Let us assume that the apparatus is applied for suspended solids containing wastewater. It will be immediately clear that the apparatus has no mechanism for clearing choking of filter as a result of retention of filtered solids. The lamella filter in lower zone is of no use for buoyant solids which include fats that are usually lighter than water and other-solids with occluded gas bubbles which those in the art will recognise immediately.  
           [0019]    Reference may be made to U.S. Pat. No. 5,314,621 where a method for biological purification of wastewater by upflow of wastewater through a buoyant biological filter bed of expanded polystyrene beads with simultaneous injection of air flowing co-currently with through the bed. The filter is backwashed as per the invention by backflushing at a rate of 30 to 80 m/h with treated water stored at an upper part to the reactor. What is non-obvious as per the patent is the provision of brief mini-backflushing operations that expands the filter bed just sufficient to loosen suspended solids within the bed and enable deeper penetration of impurities into the filter bed, thus enabling a longer filter run. It may be noted that the invention is rot for anaerobic treatment processes and, further, provides no specific apparatus.  
           [0020]    A study undertaken by Nuri Azbar, Pepi Ursillo and Richard E. Speece [Water Research 35, 3, 817-829, 2001] of the effect of reactor configuration and substrate complexity on the performance of anaerobic process is relevant to the present invention. Based on their experiments with various types of well known reactor, they write: “There appears to be something profoundly beneficial to phasing and staging of anaerobic treatment process for the substrates studied” which include simple molecules such as volatile fatty acids and complex mixed substrates such as baby formula.  
           [0021]    There is no known teaching in prior of any special apparatus for the anaerobic treatment of solids containing effluents. Prior art only reveals processes and methods of handling such effluents using known apparatus.  
           [0022]    Related Art in Deep-Bed Filtration: A Common Water Treatment Method for Removal of Suspended Impurities  
           [0023]    Buoyant bed filters form a variety of devices in the well known class of granular bed filters prior art. See for example, “Buoyant media filter” [U.S. Pat. No. 4,446,027 issued May 1, 1984] which uses a 12 inch deep filter bed constructed from commercially available hollow glass beads 0.7 mm in diameter to achieve very good suspended solids removal performance. These devices have so far been intended solely for turbidity removal in water treatment Prior art provides schemes and apparatus is provided for cleaning of he clogged filter by back washing or back flushing. None of the prior art devices and backwash methods are suited for have the advantage of removal of high concentration of biodegradable suspended solids m heavily polluted industrial waters, which requires the combination of bioreactor and filter.  
           [0024]    Another related art is found in “bead filter” devices disclosed for use in nitrification and filtration of aquaculture water. Inventor Robert Malone has shown such devices [U.S. Pat. Nos. 5,445,740; 5,126,042; 5,232,586] wherein a floating filter is used as biofilm carrier device and a filter device for accomplishing nitrification of aquaculture wastewater. In particular the device shown in U.S. Pat. No. 5,232,586 employs a tank having an upper filtration chamber and a lower expansion chamber fluidically connected to each other by a constricted passageway. An inlet line supplies water to the tank through the lower chamber, while a floating media pack forms within the upper chamber during filtration, an outlet toe is connected to the tank above the media pack and delivers filtered water back to the aquatic environment. Back-washing is accomplished by the displacement and expansion of the media pack through the constricted passageway using raw water directed to the upper chamber. The turbulence of this expansion causes the filtered matter and sludge to fall toward a drain line located at the bottom of the tank. Since no aeration provision is provided, the apparatus is suitable only for nitrification using dissolved oxygen already present in the wastewater, i.e., only for low concentration ammonia removal. This apparatus is both unintended and unusable for the anaerobic treatment of high suspended solids wastewater, as there is no separate provision for gas collection, and gas bubbles will quickly accumulate in the floating media. It is also clear that the apparatus is intended for filtration of wastewater with suspended solids concentration an order of magnitude lower than that of the invention. U.S. Pat. No. 5,232,586 invention is merely mentioned as a related art aid the simple device therein bears little comparison to our reactor designed to meet the myriad operational requirements tat a anaerobic reactor high strength complex wastewater. It is therefore clear that these apparatus do not in any way impinge on the novelty of our invention not only by way of mechanical arrangement of apparatus, but also by way of concept.  
           [0025]    Some related art in sewage treatment is given herein as our present invention, in addition to its utility in industrial wastewater treatment, also provides a compact and efficient apparatus for treatment of domestic sewage. Sewage is low-strength wastewater and contains BOD in mostly undissolved form. Sewage treatment can take two forms—centralised treatment plants in town served by sewerage systems and decentralised home scale or colony scale units where no fill sewerage systems are not available.8  
           [0026]    The following quote from ‘A review: The anaerobic treatment of sewage in UASB and EGSB reactors; [Lucas Seghezzo et al., Bioresource Technology 65, 175-190 (1998)] reveals the importance of a high-rate anaerobic reactor systems for sewage. According to Jewell (1985), there is little doubt that development of cost-effective and efficient anaerobic sewage treatment alternative would be one of the most significant advances in waste treatment history’. Lettinga et al. (1987) fully agreed with this statement by saying that ‘ . . . a satisfactory application to raw domestic sewage would represent the maximum possible accomplishment for high-rate anaerobic treatment systems’. The term ‘high-rate was once used for the later design of sewage sludge digesters, but it now widely used to refer to anaerobic treatment systems meeting at least the following two conditions: (a) high retention of viable sludge under highloading conditions, and (b) proper contact between incoming wastewater and retained sludge (Lettinga et al.,  1987 )” 
           [0027]    Anaerobic treatment has been applied with moderate success for the centralised treatment of sewage in hot climates. UASB type technology is used for primary treatment of sewage. The sewage COD loading rate for UASB type reactors is low (less than 1.5 kg/m 3 /d) as the primary mechanism of removal of COD and BOD is entrapment of suspended solids in the anaerobic sludge of the UASB reactor; and subsequent degradation. In this manner, suspended solids residence time is decoupled from the hydraulic residence time. But, higher loading rates will lead to sludge washout and failure of the system. Our invention being able to retain solids by active filtration can achieve higher COD and BOD loading without danger of sludge washout. Further it would be clear from the description of the apparatus that floating matter is also effectively retained in our apparatus to complete the degradation process.  
           [0028]    In the case of decentralised sewage treatment, septic tanks are effective and commonly used the world over. A septic tank is basically an anaerobic reactor with baffle arrangement to provide sufficient solids detention time to effect degradation. Septic tanks can be used for treatment of composite domestic sewage, or for the treatment of black water from toilets only. The residence time for composite sewage in septic tanks is at least 24 hours, BOD loading rate of the order of 0.3 kg/(m 3  d. Septic tanks are designed to provide sufficient sedimentation time for separation of solids from sewage and sufficient volume to provide solids residence time for degradation. Higher loading rates are possible in UASB systems because of better contacting and mixing than in septic tanks, enabling lowering of residence time to 6 hours. At even lower residence time, sludge washout occurs in UASB systems. The effective retention of solids by filtration by our buoyant filter bioreactor, and better contacting and mixing conditions enable the anaerobic treatment of sewage in a compact apparatus. No such filter reactors are available in prior art although several filters for post treatment removal of solids and pretreatment removal of solids from septic tanks are mentioned. The present invention is conceptually different from any of prior art devices septic tank and filter systems, being essentially different in principle, configuration, and method of working, by its use of buoyant filter and gas driven backflush system. Merely for the sake the completeness, a brief review of some septic tank filter devices is given below.  
           [0029]    Sewage disposal apparatus employing circulating filter media is shown in U.S. Pat. No. 5,308,479. A filter bed using buoyant media is provided for filtration and the media is prevented from clogging by use of a circulating fluid flow generated by either a propeller type agitator or by air sparing. The method in accordance with U.S. Pat. No. 5,308,479 attempts to prevent the clogging of the filter media by moving it continuously in a slow circulation with media particulates being maintained at substantially fixed relative positions during operation. There is no attempt to perform a normal filtration operation—(obviously a more effective filtration operation as compared with a moving bed), and intermittent back flush of media by fluidization which are essential in our invention. The U.S. Pat. No. 5,308,479 invention is suited for totally unpowered gravity flow operation unlike our invention.  
           [0030]    Suspended solids free effluent from septic tanks is desired also to prevent clogging of drain field to which it is discharged. Some invention of septic tank filters have this objective—see for example: simple basket type filters U.S. Pat. Nos. 5,198,113, 6,177,004 or simple pipeline filter U.S. Pat. No. 6,136,190, to a complex and rather impractical layered filter as taught in U.S. Pat. No. 6,024,870. These inventions do not seek to enhance the degradation capacity of the septic tank by improving the loading rate.  
         OBJECTS OF THE INVENTION  
         [0031]    A primary object of the present invention is to provide a reactor system suitable for anaerobic treatment of complex wastewater containing substantial amount of BOD as insoluble matter.  
           [0032]    Another object of the present invention is to provide a device tat enables the solubilisation of biodegradable solids and the further conversion of produced soluble compounds into biogas, the two seeps being carried out sequentially in separate compartments arranged in a compact unit.10  
           [0033]    Another object of the present invention is to provide an apparatus which is capable of effectively removing BOD and sided matter from the water without requiring a long residence time or increasing the size of the processing vessel.  
           [0034]    Still another object of the present invention is to provide an apparatus which enable recovery of biogas.  
           [0035]    Yet another object of the present invention is to provide a filter system which is backwashed and de-clogged automatically without the anion of mechanical or electrical valves or devices with moving parts.  
           [0036]    A further object of the present invention is to provide a filter reactor system which is backwashed and declogged whenever the filter pressure drop exceeds a set value.  
           [0037]    Yet another object of the present invention is to provide a filter reactor system which is backwashed and declogged at a regular interval.  
           [0038]    Still another object of the present invention is to provide a reactor with a filter system that is automatically declogged whenever the filter pressure drop exceeds a predetermined value.  
           [0039]    Still another object of the present invention is to provide a reactor that removes biodegradable, settleable and filterable matter from wastewater  
           [0040]    Still another object of the present invention is provide an anaerobic reactor, wherein anaerobic bacterial sludge is retained within the reactor even when it has a floating tendency as a result of adsorption of low density material such as fats.  
           [0041]    Still another object of the present invention is to provide a high rate anaerobic reactor which does not require external energy input for mixing and agitation.  
           [0042]    Still another object of the invention is to provide a automatically declogged floating media filter reactor which has a minimum of moving parts and is easy to operate and maintain.  
           [0043]    Still another object of the invention is to provide a filter bioreactor that does not require periodic stoppage of flow for backwashing.  
           [0044]    Still another object of the invention is to provide a filter reactor with a gas driven backflushing system, which may, where required, operate only on self produced gas.  
           [0045]    Yet another object of the present invention is to provide a multistage anaerobic reactor which can remove successively finer suspended particles and breakdown products.  
           [0046]    Yet another object of the present invention is to provide multistage anaerobic reactor which can provide a high efficiency for the removal of both suspended and dissolved organic contaminants in a single and compact reactor.  
           [0047]    Yet another object of the present invention is to provide a high rate anaerobic reactor, which can be made tall and slender so as to occupy a small footprint.  
           [0048]    Still another object of the present invention is to provide a septic tank and aerobic treatment system that can be used for the treatment of black water sewage from single or cluster of dwellings.  
           [0049]    Still another object of the present invention is to provide a treatment device that can carry out anaerobic treatment and aerobic treatment of black water sewage without external power sources and without loss of hydraulic head.  
           [0050]    All the above objects are achieved by the invention described herein as the buoyant filter bed reactor with gas-driven backflushing and its various embodiments.  
         SUMMARY OF THE INVENTION  
         [0051]    The present invention is the first high-rate reactor, (applying a high superficial velocity), that is capable of treating partially soluble, complex, high-strength wastewaters. Our invention is the first single reactor system in a compact and mechanically simple package that is configured in multiple stages and has the ability to retain complex insoluble substrates in a wide range of particle sizes in spatially separate stages for sufficient residence time to enable complete degradation. Therefore, a highly efficient performance for the removal of both solid and dissolved contaminants may be expected with the present invention. The present invention may be seen as a the first ever synthesis of a innovative self-cleaning deep-bed granular filter with staged anaerobic reactor to achieve the object of solids liquefaction by decoupling suspended solids retention time from liquid residence time. The present invention specifically provides a system for periodic declogging of the filter bed and retention of solids in a first stage digester for liquefaction. Our invention can also perform as a self-pumping system, which can discharge sewage at a hydraulic energy grade above that of the inlet sewage. This function is applicable in the case of high-strength blackwater sewage with COD exceeding 3000 mg/1. There is absolutely no mention in prior-art of a self-pumping system for any sewage or any other anaerobic reactor system.  
           [0052]    Accordingly the present invention provides a device for the biological treatment of wastewater containing biodegradable solids comprising a vertically oriented elongated vessel partitioned, in vertical progression, by impermeable substantially horizontal walls, into a top chamber, a bottom chamber and, where required, a multiplicity of intermediate chambers one below the other, in between the top chamber and bottom chamber, each chamber having a gas retaining space and a liquid retaining space, wherein for any neighboring pair of chambers, the lower one is termed “lower chamber” and the upper one is termed “upper chamber”, a nozzle establishing fluid communication between the bottom chamber and the outside of the vessel for input of wastewater into the vessel, a nozzle establishing fluid communication between the liquid retaining region of the top chamber with the outside of the vessel for discharge of treated wastewater from the vessel, and arranged so as to retain a level of liquid within the top chamber, a nozzle for discharge of gas from the gas retaining space of the top chamber, and further comprising for every pair of neighboring chambers, at least one filter chamber having at least one inlet communicating fluidly between liquid retaining part in the lower chamber and at least one outlet communicating fluidly with the liquid retaining part of the upper chamber, a filter bed constructed using a particulate bed of inert material with specific gravity less than 1.0, placed within the said filter chambers, and partly filling its internal volume, at least one gas conduit establishing fluid communication between gas retaining space in the lower chamber with the upper chamber, -and a device enabling periodic discharge and stoppage of flow of gas through the said gas conduits.  
           [0053]    In a preferred embodiment of the invention the reactor is divided by a fluid tight horizontal a partition plate into upper and lower chambers. The filter chamber is constructed as modular units, say, using a lent of pipe of suitable diameter, vertically penetrating through the partition plate. The module has perforations below a predetermined level in the cylindrical wall opening out into the lower chamber and an impermeable end cap at the bottom. At the top end, a perforated end cap is fitted, opening out into the upper chamber. The module is filled with the buoyant filter media, particle size chosen to effect required degree of filtration of the suspended solids in wastewater. The perforations in the module are chosen to be less than the diameter of the filter particles thus effectively confining the filter bed inside the module. As the bottom end cap is impermeable, it serves to deflect and prevent the escape of rising bubbles into the filter module. The apparatus operates by collection of gas above the liquid surface in the lower chamber below the horizontal partition in the space formed by the inner walls of the vessel, bottom surface of the horizontal partition and outer walls of the filter chamber, above the level of perforation in the filter chamber. The accumulation of gas drives the liquid in the lower chamber through the filter into the upper chamber. Suspended solids are filtered by the granular bed. When sufficient gas has collected in the lower chamber, it is released to the upper chamber.  
           [0054]    In yet another preferred embodiment of the apparatus, the gas conduit can be shaped to form simple hydraulic automatic discharge and stop system. In this system the gas conduit forms the shape of a U inside the lower chamber; one arm of the U opening out into the lower chamber and the other arm extending into the gas space of the upper chamber. The diameter of conduit is chosen such that two phase flow of liquid and gas in the operational ranges of gas discharge is the regime of “liquid plug” pushed by gas and not in the bubble flow regime. This may be readily determined by those knowledgeable about two phase pipe flow. Initially, the short arm of the U is totally immersed in liquid in the lower chamber. As gas accumulates in the lower chamber the short arm of U shaped gas conduit is exposed and a plug of liquid in the short arm of the U is pushed by the hydrostatic pressure into the rising longer arm, till it is entirely displaced from the shorter arm. At this stage, the gas pressure in lower chamber gas space just exceeds the maximum hydrostatic pressure of the liquid plug which is then driven upwards and discharges into gas space of the upper chamber. Whence gas flow from lower chamber to upper chamber takes place, simultaneously with back flow of liquid from upper chamber to lower chamber through the filter bed. As the liquid level in the lower chamber rises and reaches above open end of the short arm of the U, the flow of gas is cut off by the formation of a liquid plug inside the gas conduit. The sequence of step is repeated as gas accumulation proceeds once again, thus enabling13 repeated and period backflushing. Further, in order to prevent the entry of suspended solids or floating matter into the U tube, a perforated hood device may be provided at its lower chamber entrance.  
           [0055]    In a further preferred embodiment of the invention, gas recirculation to the lower chambers using a pump from a gas reservoir or from an upper chamber gas collection space is provided. This can be used to enhance the frequency of backflushing or obtain a longer duration backflush by supplementing gas generation. The gas recirculation also aids in vigorous mixing of the reactor contents to improve mass transfer and enhance the rate of reaction.  
           [0056]    In yet another embodiment, the apparatus of the invention may be operated as a sequencing batch reactor—“fill, reactor and draw” method—in a most natural and automatic manner. In this embodiment, the wastewater to be treated is charged into the apparatus which already contains a volume of reacted liquor and a large population of anaerobic micro-organisms. The reaction commences producing gas in the lower chamber. The accumulation of gas in the lower chamber displaces the lower chamber liquor through the filter bed into the active sludge in the upper chamber, wherein further reaction and gas production occurs. At the end of the reaction, the reacted liquor may be withdrawn from the upper chamber.  
           [0057]    Alternately, an overflow outlet may be provided in the upper chamber, so that treated effluent overflows from the upper chamber when the liquid level rises as a result of accumulation of gas in the lower chamber. Periodically the accumulated gas in the lower chamber is discharged, effecting the backflushing of the filter bed. The gas discharge cycle may be repeated several times if necessary before the reaction is complete. In this mode of operation, the volume of wastewater charged into the reactor during each filing cycle is equal to the volume of gas discharged during one gas discharged cycle and discharged of treated effluent takes place only during the first gas discharged cycle.  
           [0058]    A further advantage of the invention is obtained if a non-return valve device is provided at the liquid inlet to the device, which does not permit backflow of liquid from the reactor. Initially the reactor contents are at a hydraulic level which enable the filling of a charge of wastewater by gravity flow. Upon filling, the non-return valve closes preventing reverse flow. As the gas generation proceeds, liquor is displaced to the upper chamber till it reaches an outlet provided at a hydraulic level which may be above the hydraulic level of the inlet stream. The reactor may be so configured so that the upper chamber is tall and narrow with respect to the lower chamber to enable pumping to higher elevation. In addition, the gas discharge volume may be chosen such that there is only one gas discharge operation per sequence. This enables the displacement of a large quantity of liquor from the lower chamber of reactor to the upper chamber. The advantage of this operation is that wastewater is discharged fairly continuously during the gas generation process, at a higher hydraulic level as compared to the source. Hence the reactor, besides providing anaerobic treatment, also provides the function of an equalisation tank and pump system for downstream secondary treatment. It is conceivable that the pumping effect can be beneficially used for downstream treatment, for example, by a exposed aerobic trickling filter or a solar disinfecting basin, while the wastewater drain and suitable collection sump is underground. Therefore a totally unattended operation of the invention is possible with no external power source for anaerobic and aerobic treatment of highly contaminated wastewater.  
           [0059]    Another embodiment of the invention is the multistage buoyant filter bed reactor, where there are several intermediate stages between the bottom stage and the top stage. The passage of liquid from bottom to top passes through filter bed at each intermediate stage. This enables very high efficiency of suspended solids and dissolved organic matter removal. The filter beds may be constructed so as to retain coarse suspended matter at the lower stage with each succeeding stage retailing progressively finer solids. This is easily accomplished by choice of the filter media and in particular the particle size of filter media. Gas collected in the bottom stage discharges periodically to the second stage, backflushing the filter bed connecting the two stages. As gas collection at the second stage exceeds the predetermined volume, it is discharges to the third stage backflushing the filter bed at the second stage. This sequence is repeated at each stage to set up a cascade till ultimately the gas is discharged from the system and all filters are flushed. Those in the art will acknowledge the extreme difficulty in filtration separation of wastewater containing large sized to fine sized suspended solids organic matter and will readily appreciate advantages of the multistage buoyant filter bed reactor of this invention which is not only able to separate but also to degrade the separated matter, in a single, compact and energy efficient device. 
       
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0060]    [0060]FIG. 1 represents a fully functional laboratory realisation of a two chamber buoyant filter bio-reactor constructed using glass and steel.  
         [0061]    [0061]FIG. 2 represents detail of automatic discharge gas conduit mechanism,  
         [0062]    [0062]FIG. 3 represents detail of filter chamber.  
         [0063]    [0063]FIG. 4 shows detail of a baffle device that permits preferred flow patterns during normal operation and backflushing.  
         [0064]    [0064]FIG. 5 represents an anaerobic reactor provided with a gas release controlled by a electromechanical device.  
         [0065]    [0065]FIG. 6 represents another configuration of an anaerobic reactor provided with a gas release controlled by a electromechanical device.  
         [0066]    [0066]FIG. 7 represents a multistage reactor with three filtration stages.  
         [0067]    [0067]FIG. 8 represents a realisation of the invention, showing a two-stage laboratory scale reactor with a biofilm reactor as a second stage.  
         [0068]    [0068]FIG. 9 represents a realisation of the invention showing a fill sized anaerobic reactor with multiple modular filter chambers.  
         [0069]    [0069]FIG. 10 represents a fully functional laboratory sized two-chamber anaerobic reactor, with a biofilm upper stage and an external filter chamber.  
         [0070]    [0070]FIG. 11- a  represents schematically (constructional features which those in the art can readily fill in are omitted for sake of brevity) a preferred embodiment of the buoyant filter bioreactor invention which can be used as a septic tank device for the treatment of household sewage.  
         [0071]    [0071]FIG. 11- b  is an enlarged view of the filter chamber.  
         [0072]    [0072]FIG. 12 shows an embodiment of the invention suited for sequencing batch i.e., “fill-react-draw” mode operation.  
         [0073]    [0073]FIG. 13 shows a realisation of the invention for unpowered gravity flow operation, suitable for anaerobic-aerobic treatment of black water sewage or high strength wastewater with such components as ground kitchen waste and animal waste. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0074]    The invention is explained with respect to the drawing accompanying this specification:  
         [0075]    In the drawings many details pertaining to fabrication not bearing upon points of novelty are omitted in the interest of descriptive clarity. Functionally equivalent components are given identical reference numbers in the various drawings. These components are explained in detail for the first occurrence only for sake of brevity.  
         [0076]    [0076]FIG. 1 represents a fully functional laboratory realisation of a two chamber buoyant filter bio-reactor constructed using glass and steel. The reactor vessel  101  is partitioned into lower chamber  102 , and upper chamber  104 , by a plate  103 , fixed impermeably between the two chambers. In actual construction of this laboratory realisation of the invention, the plate  103 , is clamped together in leak tight manner between two flanged glass columns each of 100 mm internal diameter. The lower chamber  102 , is provided with a dished bottom on which is provided15 nozzle  106  for input of the feed wastewater by means of a pump (not shown). A gas solids separator  107 , made of steel is mounted on top of the upper chamber. The gas solids separator ha an outer shell  108 , shaped as a inverted frustum of a cone which is jointed in a leak tight manner to the walls of the upper chamber  104 , at its top. An overflow weir  112 , allows liquid to overflow from the reactor vessel into a circumferential collection launder  113 , and thereafter can be taken out of the reactor through an outlet nozzle  114 . A cylindrical glass vessel  110 , with a open bottom and an impermeable top is mounted on top of the gas-sold-separator, by a fixing means  109 , such that its cylindrical wall projects inside and off the sloping side walls of the gas-solid-separator. The vessel  110 , has a inside diameter larger than the inside diameter of the upper chamber  104  and thus forms a liquid sealed gas collection space  123  inside the lid. A gas outlet nozzle  111  connects fluidly the gas collection chamber with a constant pressure gas reservoir (not shown). A filter chamber  115 , constructed as cylinder, is mounted inside the lower chamber  102 , with its cylindrical walls jointed impermeably to the partition wit!  103 . The partition wall  103  is provided with perforations  117  (see FIG. 3) establishing fluid communication between upper chamber  104  and the filter chamber  115 . The filter chamber  115  is also provided with perforations  118  on the cylindrical wall at its lower end and a impermeable end cap  116 . A particulate filter bed  119  made of polystyrene beads is confined inside the filter chamber  115 . The filter bed  119  occupies only part of the volume of the filter chamber. A glass tube open at both ends, called “return tube  200 ”, is provided penetrating partition wall  103  and sealingly fixed to it. Return Tube  200  is terming at top in gas space  123  and at bottom well within the lower chamber  102 . A gas conduit tube  130 , made of flexible tubing, is provided within return rube  200  sealingly penetrates out of the return tube  200  within the lower chamber. The tube  130  is provided with a U bend within the lower chamber  102 . An additional nozzle  105  extending inside to form perforated pipe ring  120  is provided in lower chamber for sparging gas recirculated from the gas space  123 .  
         [0077]    [0077]FIG. 2 represents detail of automatic discharge gas conduit mechanism, hereinafter called U tube device. The  130  tube diameter in case on this vessel was chosen to be 6 mm internal diameter. Larger diameter tubes would be chosen for larger reactors. Any diameter tube may be chosen as long as two phase flow in slug flow regime is obtained during gas discharge operation. The longer limb  133  of the U tube opens inside a larger diameter return tube  200 . The return tube  200 , open at both ends, is fixed sealingly penetrating the partition wall  103  between the upper chamber  104  and lower chamber  102 . Return tube  200  opens at it lower end at level  201  which well below the lower extremity  129  of the U tube. The top level  202  of return tube  200  is above the level  203  of upper extremity of the U tube. Both tubes are terminated in the gas space  12   36  of the upper chamber and above the liquid overflow level  126  in the apparatus. Further, the vertical distance between  203  and  127  is selected to exceed vertical distance between  204  and  129 .  
         [0078]    [0078]FIG. 3 represents detail of filter chamber  115 . The vertical orientation of the filter chamber ensures that rising gas bubbles are deflected away from the openings  118 . The diameter of the openings provided was 1 mm whereas the filter bed particles were 1 to 2 mm size. The filter bed occupies about 50% of the internal volume of the filter chamber.  
         [0079]    The operation of the invention is explained below with respect to the laboratory scale model represented in FIGS.  1  to  3 . The operation of the subsequent manifestations follows similar principles and has been omitted, except where there are significant differences.  
         [0080]    The reactor vessel is filled with deoxygenated water. The particulate bed being of specific ^ gravity lower than water, forms a floating bed filter bed  119  against the top perforated cover  103 . The reactor is provided with sufficient quantity of acclimatised anaerobic microbial sludge, which may be have granular or flocculant settling characteristics. This forms a sludge bed depicted as  121  in the lower chamber  102  and a sludge bed  122  in upper chamber  104 . Inert gas is recirculated through the gas sparging system  120 . The continuous pumping of wastewater (arrow F) containing suspended organic matter is commenced through nozzle  106 . As pumping proceeds an equivalent quantity of liquor in chamber  102  is forced into filter chamber  115  through perforations  118 , permeates through the filter bed  119 , and exits (arrow F 1 ) via perforations  117  into the chamber  104 . Filtration action at the filter bed  119  retains suspended particles in the wastewater passing through the bed. Some part of soluble matter in the wastewater is immediately converted to methane and carbon dioxide by microbial action in lower chamber  102 . The produced gas and the recirculated gases are collected in the space  124  in the lower chamber  102 , lowering the liquid surface  125 , the displaced liquor again exiting into upper chamber  104  through the filter bed  119 . The filtered liquid containing part of the soluble convertible BOD is contacted with the sludge  122  in chamber  104  for further conversion to gas, which is collected in gas space  123  which exits through the nozzle  111  to a constant pressure gas storage tank. Some part of the gas is recirculated back to nozzle  105  by the gas pump (not shown). A continuous overflow over weir  112  at level  126  is also set up. The level difference between  126  and  127  is maintained at constant level because of the constant pressure gas receiver connected to gas outlet port  111 . It may be noted that the gas receiver is usually at slight pressure above atmosphere and therefore, the liquid surface  127  is below the liquid surface  126  at the overflow level. As the operation proceeds, the liquid surface  125  is pushed below the lower level  129  of the gas conduit tube. Initially liquid fully fills the smaller limb17  132  of the gas conduit tube,  130 , and the longer limb  133  contains liquid to a level determined by the pressure difference between the open ends, which is given by the vertical distance between  134  and  135  as in the case of a manometer. As gas pressure in  124  increases the liquid in the shorter limb  132  is pushed into  133 , lowering level  134  and raising the level  135 . The gas pressure in  124  is given by the vertical distance between  135  and  134  plus the constant gas pressure in  123 . Eventually, the liquid level  134  reaches the lowest point of the U, at  129 , whereupon no further increase in gas pressure can be balanced by the hydraulic pressure of the liquid column in the gas conduit tube  130  and gas will flow out of  124  through the tube  130 , into space  123 . The tube diameter is chosen such that gas velocity is the range where slug or plug flow phenomenon is observed. The liquid column is therefore pushed out of the open end of  133  as a slug and a free flow of gas from  124  is obtained.  
         [0081]    It is noted that the gas conduit tube in the shape of a U is difficult to fabricate and assemble inside the return tube as shown in FIG. 2, which is merely a working laboratory model. In actual application the U will be replaced by a functionally equivalent combination of 90 degree elbows and jointed pipe sections.  
         [0082]    The slug of liquid discharged from  135  falls back into the lower chamber  102  via return tube  200 . It may also be noted that as return tube  200  extends above the open end  203  of the U tube, and terminates at level  202 , which is sufficiently above liquid level  127 , to prevent flow of liquid from chamber  102  to  104  via this tube. In fact, it may be noted that the liquid level inside return tube  200  will be higher than the level  127  by an amount equal to the pressure drop across the filter bed  119 . It will be obvious that the tubes  200  and  130  can be so configured as to capture a high velocity slug of liquid ejected from the U tube. Further, obvious variations of this automatic gas discharge system which are functionally equivalent includes  130  arranged substantially outside the tube  200  with only the outlet of  133  configured to discharge inside  200 . It may also be understood that multiple gas conduit tubes may discharge their liquid contents into a single return tube  200 . Yet another functionally equivalent variation which may be made explicit, is the arrangement of a return chamber integrally constructed in the reactor vessel, instead of a return tube.  
         [0083]    The discharge of gas from  124  leads to a liquid backflow from the upper chamber to the lower chamber through the filter chamber  115 . The flow enters through perforations  117  and exit through perforations  118 . The back flow velocity is much higher than the filtration velocity, and the filter bed  119  expands to a fluidized state as a result, flushing out embedded suspended solid back into the lower chamber. This process is hereinafter termed “backflushing”. The bed expansion also breaks bonding of particles because of microbial biofilm growth. As a result of the backflushing flow, the liquid surface  125  in lower chamber  102  rises till it reaches above  204 , the open end of limb  132  of the gas conduit  130 . The liquid then flows into the tube  130  to once again form a hydraulic column which balances the gas pressure difference between  124  and  123 , thus stopping gas flow. This sequence is repeated, the frequency of operation determined by the gas production and recirculation rate. The back flushed solid particles are thus retained in the lower compartment for duration sufficient for solubilization by microbial action.  
         [0084]    An advantage of hiss U tube gas discharge device is that the automatic backflush system requires no electrical power nor does it involve any mechanical moving parts.  
         [0085]    A further advantage of this U tube gas discharge device is that in addition to backflushing at a set time interval, the device is also automatically backflushed whenever there is an abnormal increase pressure drop in the filter bed  119  as a result of clogging. An increase in pressure drop across the filter bed will be seen as an increase in gas pressure in gas collection space  124 . This increase in pressure pushes down liquid level  134  in U tube limb  132 , irrespective of the liquid level  125 . If the gas pressure is sufficiently large, the level  134  reaches level  129 , and gas is released inducing backflushing of filter.  
         [0086]    [0086]FIG. 4 shows detail of a baffle device that permits preferred flow patterns during norm operation and backflushing. The device described in FIGS.  1  to  3 , allows backflow of sludge from the upper chamber  104  to lower chamber  102  during backflushing. This in itself can be used advantageously because it selects the retention of larger sized sludge granules in the upper chamber. The device can also be used without difficulty when the upper chamber is operated as a fixed film reactor as will be described in FIG. 9. But in certain situations, it may be advantageous to prevent backflow of sludge during backflushing. An improved design of filter chamber  115  that is able to prevent backflow of sludge  122  from upper chamber  104  into the filter bed  119  during backflushing operation is shown in FIG. 4. The backflow of sludge during backflushing can be prevented quite easily by extending filter chamber  115  into the upper chamber so that its upper perforations are at a level above the upper level sludge bed  122 . A further improvement in performance can be achieved by enabling contact of filtered outflow liquor (arrow F 1 ) with sludge  122  during normal operation, while only clear fluid above  122  is utilised for backflushing. This objective is realised by the design of a baffle means described herein. A generally cylindrical filter chamber in cross sectional view is represented. FIG. 4 is to be read with other figures, with functionally equivalent features having the same reference numbers as in previous figures. The filter chamber  115  at its upper end is shaped as a cylinder  401  of smaller diameter than the lower portion, both parts being jointed by a frustum of a cone  402 . It is mounted, sealingly penetrating partition wall  103  separating the upper chamber  104   19  from the lower chamber  102 . The filter chamber is filled partly with a participate media filter bed  119  and confined by lid  116  at the lower end and  406  at the upper end. A sludge bed  122  is shown at its position in the upper chamber. The filter chamber is provided with a multiplicity of perforations  117  at its upper end which extends above the sludge bed level in the upper chamber. A cylindrical baffle termed “sleeve  403  ” is provided as circumferential sleeve coveting the smaller diameter portion  401  of the filter so a form a annular space  405  between sleeve  403  and  401  and a ring aperture  404  between sleeve  403  and the frustoconical portion  402 . The sleeve  403  has an outer diameter not exceeding the outer diameter of  115 . During normal operation, rising gas bubbles generated by the action of sludge  122  are confined to the region outside the sleeve  403 . The rising gas bubbles (arrows Gb) set in motion a circulating liquid flow (arrows L) which is downward inside tee sleeve  403 . The circulatory flow is directed toward the sludge through the ring aperture  404 . The filter outflow (arrows FI) is carried along with this flow to contact with the sludge bed  122 . During backflushing operation, the flow from the upper part of chamber  104  (arrows B), relatively free of sludge, is directed towards the perforations  117  m preference to flow from the sludge region  407  because of the relatively higher resistance to flew through the ring aperture  404 . It may be noted that the resistance to flow of the ring aperture is significant only during the high flow condition at backflushing and is insignificant for the gentle circulatory flow condition during normal operation. Thus the device enables contacting of filtered liquor with sludge and avoids sludge entry from the upper chamber  104  into the filter bed during a during backflushing.  
         [0087]    [0087]FIG. 5 represents an anaerobic reactor provided with a gas release controlled by a electromechanical device, such as a solenoid valve. An external gas conduit  130  is show fluidly connecting the gas retaining space  124  of a lower chamber  102  and the gas retaining space  123  of an upper chamber  104 . The conduit is connected to the lower chamber at a nozzle  501  and connected to the upper chamber gas outlet  111  by a tee connection  505 . An automatic operating valve  502  is provided on the gas conduit  130 . A level sensor  508  which can sense liquid surface at levels  507  and  506  is arranged in the lower chamber. The level sensor  508  excites device  504  with appropriate electronic circuitry that signals valve  502  to open at a level  507  and close at level  506 . The nozzle  501  is provided at a level above level  506 . During operation, the build-up of gas in  124 , as in earlier devices, pushes the liquor in  102  through the filter bed, lowering the liquid surface  125 . At the level  507  the level sensor activates the opening of valve  502 , leading to gas discharge and backflushing of filter bed. The liquid level  125  rises and at level  506 , the level sensor activates the closing of valve  502  and the cycle is repeated. An advantage of this arrangement of the invention is that there is no restriction on gas conduit  20  diameter and there is no entry of lower chamber liquor into the gas conduit obviating the need for a re tube. This arrangement is conveniently used in large size industrial anaerobic reactors which are already provided with instrumentation and automation.  
         [0088]    [0088]FIG. 6 represents another configuration of an anaerobic reactor provided with a gas release controlled by a electromechanical device. The upper chamber  104  is provided with a nozzle  601  and a gas sparging ring  602  for agitating sludge  122  contained in the chamber. Gas conduit  130  connects gas space  124  of lower chamber to the  601  nozzle of the upper chamber  104 . Also provided are an automatic valve  502  in conduit  130 , level sensor  508  and electronic circuit device  504  for actuating valve  502  as previously described in FIG. 5. The gas released through conduit  130  is sparged into the liquid in the upper chamber  104  through a sparging device  602 . The sparging device shown is constructed from a perforated pipe in ring configuration but it may be understood that other sparging devices may also be used. The advantage of this system is the mixing of contents of the chamber  104  during the backflushing operation. The mixing chamber contents will promote the rate for conversion of organic matter. It may also be noted that the entrainment of gas and/or sludge into the filter bed  119  daring can be prevented by the use of a cylindrical baffle device previously described in FIG. 4.  
         [0089]    [0089]FIG. 7 represents a multistage reactor with three filtration stages. Components of identical functionality with previous figures have identical reference number and are not described further. This reactor is a vertical progression of units  600 ,  700 ,  701  each functionally identical to assemblage  600  previously described. When a predetermined quantity of gas has accumulated in space  124 , gas release is triggered by level detector  508 , switch  504  and control valve  502  as previously explained in the description for FIG. 5. Gas release backflushes buoyant filter bed in module  115 . The gas is released into upper chamber  104  and accumulates in space  124   b . When a predetermined quantity of gas has accumulated in space  124   b , gas release mechanism is triggered by level detector  508   b , switch  504   b  and control valve  502   b  as previously explained in the description for FIG. 5. Gas release backflushes filter in  115   b  in the second stage marked  700 . The gas released from  124   b  accumulates in space  124   c  and the process is repeated for in stage  701  as described for stage  700 . At each stage, further anaerobic degradation by contained microbial sludge results in removal of BOD. In this manner, the operation of the apparatus enables filtration and reaction in a staged manner and backflushing of each filter in a staged manner. After 3 stages of filtration and reaction, the final treated effluent is discharged through an outlet  715  fitted with a siphon break means. The gas released from  21   124   c  finally exits the reactor through outlet  111 . A gas-solids-separator is unnecessary after 3 filtration stages, as the effluent will be substantially free of solids.  
         [0090]    It would be understood by those in the art that more than three filtration stages can easily be constructed in this manner and each successive filtration stage can be arranged for retention of progressively finer particles by suitable choice of buoyant filter media particulates. In this manner it may be ensured that a very high efficiency of removal of both suspended solids and their soluble degradation products may be obtained. In this context, it is worth recalling the “profound” advantages of staging for the degradation efficiency mentioned in the Water Research 2001 reference, and it may note how this invention is uniquely and excellently suited for such staging for wastewaters with any level of complexity.  
         [0091]    Another point of interest for the construction of the reactor, is that at each stage of the reactor, gas production leads to progressively larger quantities of gas to be released and therefore, higher frequency backflushing may be achieved for progressively finer filter media.  
         [0092]    [0092]FIG. 8 represents a realisation of the invention, showing a two-stage laboratory scale reactor with a biofilm reactor as a second stage. Once again components, functionally identical to those of earlier figures are identified by same reference numbers as in previous figures and are not explained further. The upper chamber  104  is filled with a biofilm support packing material  801 , several of which is commonly known in prior art. The upper chamber  104  is also provided with a effluent discharge nozzle above the packing level and fitted with a common siphon break overflow assembly  802  to maintain a level of liquid  803  in the reactor. Suspended solids are retained substantially in the lower chamber and dissolved contaminants are passed on to the upper chamber wherein reactions catalysed by an attached biofilm on the packing material result in further conversion of BOD to gaseous products. This reactor is also backflushed by the mechanism previously described in FIG. 2. The level of liquid in the reactor upper chamber fluctuates between  804  at end of backflush and  803  at discharge level. Large back-flow velocities obtained during the backflushing operation also help in removing biofilm debris from the packing material. It may clearly be understood that biofilm stages may used as final stage of multiple filter stage reactors very profitably to obtain extremely high BOD and COD removal efficiency for complex wastewaters.  
         [0093]    [0093]FIG. 9 represents a realisation of the invention showing a full sized anaerobic reactor with multiple modular filter chambers. Again, all parts which are functionally equivalent to parts in  22  previous figures are identified by same references numbers as in previous figures, and are not explained further. This reactor is a large sized anaerobic reactor with two chambers. The upper chamber  104  is partly filled with biofilm carrier media  801  explained earlier. The upper chamber  104  and lower chamber  102  are separated by an impermeable partition wall  103 . Fitted on the partition wall are multiple filter chambers  115 , each of which has form identical to that explained in FIG. 4. Automatic gas discharge from gas space  124  of lower chamber is accomplished by multiple U tube type gas conduits  130 . The gas conduit tubes are provided external to the reactor, but it may be understood that these tubes can equally well be provided internally within the reactor vessel. The number of gas conduits  130  provided depends on the gas production and circulation rate, keeping in view the necessity of flow in each of the conduits being in the slug flow regime. Each of the gas conduit tubes  130 , discharge into a liquid collection launder  901 , provided at a sufficient height above the overflow discharge level  804  of liquid in the reactor vessel. A single return tube  200  is connected to collection launder  901 . A gas recirculation pump  902  is provided for recirculation of gas from  123  to the lower chamber. The gas sparging system  120  has multiple gas diffusion devices  903  connected to a common gas pressure header pipeline. A large manhole  904 , in the lower chamber  102 , is provided for maintenance access to the filter chambers. The operation of the reactor follows the description given for device shown in FIG. 8.  
         [0094]    [0094]FIG. 10 represents a fully functional laboratory sized two-chamber anaerobic reactor  1000 , with a biofilm upper stage and an eternal filter chamber. The upper chamber  104  is constructed as a packed bed reactor as described in FIG. 8, and is provided with an additional nozzle  1009 . The lower chamber  102  is provided with additional nozzle  1001 , at a level below the lowest liquid level  125 , in chamber  102 . Also provided is an external filter vessel  1002 , which is partly filled with a buoyant particulate media  119 . Filter vessel  1002  is provided with a bottom nozzle  1003  and fluidly connected to the lower chamber at nozzle  1100  and a top nozzle  1004  fluidly connected with to the upper chamber through nozzle  1009 . The filter vessel  1002  is provided with perforated plates  1007  and  1008  at the top and bottom respectively, coring the buoyant particulate bed  119  within the filter vessel  1002 . During normal operation, the filter bed  119  is retained against the top perforated plate  1007 . During backflush operation, the filter bed is fluidised but retained by the lower perforated plate  1008 . Manual shut-off valves  1005  and  1006  are provided on the conduits connecting the external filter vessel with the reactor vessel. Valves  1005  and  1006  may be closed for isolating the external filter vessel  1002  for maintenance of filter. The reactor  1000  is provided with a U tube type gas discharge conduit system as previously described in FIG. 2, which operates between liquid levels  204  (upper)  23  and  125  (lower). The method of functioning is similar to the apparatus described in FIG. 8 and shall not be described further for sake of brevity. The advantage of this configuration is the greater access to the filter chamber for removal, replacement or maintenance of the filter media.  
         [0095]    [0095]FIG. 11- a  represents schematically (constructional features which those in the art can readily fill in are omitted for sake of brevity) a preferred embodiment of the buoyant filter bioreactor invention which can be used as a septic tank device for the treatment of household sewage. The device is shown under ground level marked as  100 . House sewage connection is provided to port  106  and via tee joint  1111  to the lower chamber  102 . A filter access port  1102  from ground level opens into the lower chamber  102 .  1102  is provided with gas tight lid  1103 . The lid opening is at an elevation above the gravity overflow nozzle  114  of the device so that accessing the filter by opening lid  1103  does not result in overflow of liquid contents. The filter chamber  115  is provided with a long handle  1104  to enable removal of the filter chamber as module for maintenance. The filter chamber is provided with a welded or unitary moulded flange  1105  (refer FIG. 11- b  for enlarged view of the filter chamber) and o-ring  1106  to enable leak-tight seating of  115  at the base  1107  of the filter access port  1102 , preventing liquid communication between  102  and  1102  except through the filter chamber  115 . Also provided for gas discharge from gas space  124  of the lower chamber  102  is a U tube type gas conduit mechanism  130 , with return tube  200 , functioning of these having been described previously in FIG. 2. The return tube is extended upwards to a suitable height for exhaust of the gas without odour nuisance. A hood  1115  is provided for the gas exhaust to prevent entry debris or rainwater. Alternately, the gas exhaust can be provided with a gas biofilter (not shown) for odour control. Additionally, a gas release tube  1112  with a manually controlled shutoff valve  1113  is provided from the top of the gas space  124 .  1112  opens out into the return tube  200  via tee joint  1114  at a height above the gravity overflow  114  from the system and valve  1112  is accessible from ground level. The inlet port  106  is shown attached to the return tube  200  which is expanded to sufficiently large diameter tube, called downcomer tube  1118 , in the embodiment shown. This arrangement reduces the number of nozzles that require to be cast into the septic tank chamber. It works just as well to provide a separate nozzle directly into  102  for input of sewage. The upper chamber,  104 , is provided with a biofilm carrier packing,  801 . Chamber  104  is connected to the filter chamber via conduit  1108 . Chamber  104  is provided with a simple non-gas tight lid,  1116  that may be removed for maintenance of the packing.  
         [0096]    In operation, sewage flows into lower chamber  102 , via the downcomer tube,  1110 , by gravity. The inlet downcomer tube  1118  contains sewage up to level  1110 , which is slightly above the outlet level  126 , the difference being the pressure drop across the filter bed. The input sewage contacts preferably with the settled sludge  121  in  102 . An equivalent quantity of substantially reacted liquor from  102  is displaced to  104  through the filter  119 , as a result of difference in level between  1117  and  126 . Settle-able solids are retained in the  102  chamber along with active sludge,  121 , where anaerobic reactions result in gas formation. Further degradation of filtered BOD takes place in  104  which is provided with a biofilm retaining packing material  801 . The gas produced in  102  accumulates in space  124  and is released through the  130  conduit into the atmosphere periodically when a predetermined level is reached. The gas release process also achieves backflushing of the filter and solids entrapped in the filter fall back into  102  to undergo degradation. The gas produced during sewage anaerobic degradation is low because of the low BOD and COD strength of sewage. It is known that gas production of the order of 0.19 m 3 kg-COD-removed, which amounts to a range of 35 to 40 litres of gas per cubic meter of average strength sewage (COD 300 mg/1). The filter backflush volume, being equal to gas production, is about 1 cubic meter per 25 to 28 cubic meters of sewage treated. Gas release tube  1112  is provided for manual release of gas by opening valve  1113  to trigger additional backflushing or for draining of the  104  chamber for maintenance. The filter can also be removed through by opening lid  1102  for maintenance or replacement of filter module if required.  
         [0097]    An advantage of this invention is that the effluent is efficiently filtered and free of solids, it prevents the final disposal absorption field from clogging.  
         [0098]    The major advantage of this invention is that the size of the plant can be made smaller than conventional septic tanks. As sewage BOD is mostly in undissolved form, an efficient filtration system made of small particle size media, enables the device to be reduced in size, permitting construction as prefabricated units made by plastic moulding. Mass production of the device can lower cost and promote sanitation in developing countries. It is also possible to install this device within the building because of its small size. The device is also suitable for mobile applications.  
         [0099]    In community scale operation, the gas produced in the device, can be used as fuel source for such applications water heating boilers.  
         [0100]    [0100]FIG. 12 shows a realisation of the invention which is particularly well suited for sequencing batch i.e., “fill-react-draw” mode operation. Components of same functionality with previous figures have same reference numbers and are not further described for brevity. A two chamber anaerobic reactor with a buoyant filter bed is shown as  1200 . It comprises a lower chamber  25   102  and an upper chamber  104 . The upper chamber is constructed as a packed bed with attached biofilm as previously described in FIG. 8. A long filter chamber  115  extending well into the lower half of  102  is provided. A nozzle  1205  and valve  1206  are provided for filling of liquid into the lower chamber and another nozzle  1207  is provided connecting to gas conduit  130  communicating with the upper chamber. A valve  1209  is provided in the gas conduit for closure of gas conduit during react operation. A branch  1208  is provided in the gas conduit  130 , before valve  1209  for pumping of gas using a pump,  1211 , into the liquor in lower chamber through a sparging device  120 . This enables gas nixing of the contents of the reactor. The apparatus is operated as follows: Initially the reactor is filled with reacted liquor fill level  1201  in chamber  104  and level  1204  in chamber  102 . The liquor contains active and preacclimatised microbial cultures capable of degrading the contaminants in the wastewater to be treated. The level  1204  or indeed any other level in lower chamber  102 , can be maintained constant during the liquid charging operation by closing of valve  1209  and valve  1213 . The wastewater to be treated is charged (arrow F) into the vessel through nozzle  1205 , keeping gas conduit valve  1209  closed and pump  1211  off. Some part of the reacted liquor in the chamber  102  is displaced by fresh effluent through the filter  119  into chamber  104 . The liquid level in chamber  104  rises to level  1202  at the end of the fill operation. At the end of the fill operation, valve  1206  is closed, valve  1209  is maintained in closed position, and pump  1211  is started beginning the react operation, wherein mixing is enabled by the gas sparging operation of the pump  1211 . The reaction proceeds with gas produced accumulating in space  124 , lowering level  1204 . An equivalent volume of liquor is displaced through filter  119  into chamber  102  wherein the liquid level rises to level  1212 . As the liquid level in  102  is lowered to level  1203 , the gas valve  1209  is opened and gas is released (arrow G) to a constant pressure gas reservoir (not shown). This initiates a backflushing of the filter bed  119 . The gas release is stopped by shutting-valve  1209  when the liquor level in  102  reaches  1204 . This process may be automated by using any of the devices previously described and in the figure a level sensing and valve control devices  508 ,  504  previously described is shown. The react process is continued, if necessary, with several gas release and backflushing operations, until gas production rate is substantially reduced, indicating the end of the reaction. At the end of the reaction, the treated liquor is drawn, (arrow E), from chamber  104  by opening valve  1213 . The process of filling, reacting and drawing may be repeated with a fresh batch of wastewater. It may be understood that this method of operation can equally be applied to apparatus with multiple filtration stages.  
         [0101]    It may also be noted that short circuiting of feed through the filter bed is avoided by the positioning of nozzle  1205  at an elevation well above the level of the inlet perforations  118  of filter chamber  119 . In any case, even if part of the feed circuits through the filter bed into  104  during the feed operation, this does not affect the efficiency of the process materially, as reactions can proceed in the  104  chamber also. Further, during backflushing operations which may take place several times during the react phase depending on the strength of the wastewater, liquid in chamber  104  gets returned to chamber  102 .  
         [0102]    [0102]FIG. 13 shows a realisation of the invention for unpowered gravity flow operation, suitable for anaerobic-aerobic treatment of black water sewage or high strength wastewater with such components as ground kitchen waste and animal waste. The system is designed to provide both anaerobic and aerobic treatment by self-pumping of anaerobic treated effluent to an aboveground-air contact aerobic treatment stage. It works with complex wastewater whose gas production potential is larger than its liquid volume. Such a situation is possible whet the waste contains degradable COD in excess of 2500 mg/1. In the figure,  1301  is a collection tank provided with a nozzle  1302  connected to sewage line from toilets.  1301  is provided with a outlet conduit  1306  connecting to the treatment unit. A one-way valve  1308  is provided on the line  1307  connecting  1301  with the lower chamber of the treatment unit  102 . The basic treatment unit is a two chamber buoyant filter bed reactor as described previously in FIG. 11. A removable filter and filter chamber as previously described is provided for ease of maintenance. At least part of upper chamber  104  is below the lowest level  1319  of the collection tank. The upper chamber  104 , unlike in the apparatus of FIG. 11, is provided with a gas tight lid,  1321 . The apparatus shown is not provided with a gas storage and utilisation system. Needless to add, gas collection system may be installed, where required, without difficulty. Liquid knockout and return tube  200  is continued vertically to terminate in a gas exhaust hood  1115  at a suitable elevation to eliminate odour nuisance as previously described. An additional gas release tube  1112  with an automatic shut off valve  1316  for release of gas from gas space  124  is provided. The tube  1112  opens out into the liquid knockout and return tube  200  at a suitable elevation,  1320 , above the highest liquid level in the system, i.e.,  1317 . The upper chamber  104  is provided with liquid outlet  1309  at its top. The liquid outlet rises vertically and discharges through a perforated distributor  1312  into a air contact tank  1313  filled with packing media  1315  for the growth of aerobic micro-organisms. The tank  1313  is provided with a outlet  1314  at its bottom for discharge of aerobic treated water safely.  
         [0103]    In operation, wastes such as black water sewage from toilets is discharged into the collection tank  1301 . The level of liquid in  104  is kept sufficiently lower than the bottom level,  1319 , of tank  13   01 . At  27  this condition one-way-valve  1308  opens and permits the flow of waste into  102 , by gravity. Tank  1302  is constructed with its floor being above the full or overflow level of chamber  104  (shown as  1318 ), so that it can be entirely emptied by gravity. When  1301  empties, and the conduit  1307  is empty, one-way valve  1308  is arranged to close. A swing type valve is appropriate as one-way valve  1308 , as it is in normally closed condition when the conduit is empty. In tank  102 , an active population of anaerobic bacteria, start the degradation reactions leading to gas production. Gas is collected in space  124  of chamber  102  and liquor is displaced into  104  through filter  119 . The gas discharge mechanism is positioned so that before each gas discharge takes place, sufficient liquid, at least equal to the capacity of the collection tank,  1301 , is displaced into upper chamber  104 , and liquid is forced out into the rising outflow conduit  1300 , where level  1322  rises above bottom level,  1319 , of tank  1301 , ultimately flowing out through  1312  into the packed bed  1312 . Here aerobic microorganisms continue the process of degradation through secondary treatment, leading to a high degree of removal of BOD. Liquid back flow from lower chamber  102  into the collection tank  1301  is prevented by the one way valve  1308 . When sufficient quantity of gas has accumulated, it is discharge through the operation of the U tube mechanism previously described. At this stage, filter  119  is backflushed and the liquid level  1322  is lowered and gravity flow of liquid through one way valve  1308  is possible if liquid is present in  1302 . If there is no liquid in  1302 , and the gas production rate in  102  is still continuing at an effective rate, a secondary cycle is repeated with gas accumulation in  124 , liquid displacement through filter into  104 , till final gas discharge and back wash of filter. But during this secondary cycle, there will not be any overflow of liquid through  1312 . Now, it is possible that gas accumulation has ceased with a certain amount of gas accumulated in space  124  and level  1322  is above the level  1319 , while influent sewage has accumulated in  1302 , to a predetermined level  1305 . At this stage valve, level operated mechanism  1303 ,  1304 , is activated and valve,  1316 , in additional gas discharged tube  1112 , is opened automatically, and gas is released from the system. The liquid level  1318 , is thus brought to its initial state below  1319 , immediately allowing input of sewage from  1302 , and emptying tank  1301 .  
         [0104]    It is required to have a COD in excess of 2500 mg/1 if sufficient gas production volume is to be generated to enable displacement pumping against gravity. This mode of operation is therefore suitable for strong wastes such as blackwater and ground food waste discharges. However, it is possible that there are occasional unavoidable discharges of lower strength wastes into the system, and sufficient gas production is, therefore, not available to enable displacement pumping of liquid through  1312 . In this case, hydraulic levels  1322  will equal level of liquid,  1305 , inside the collection tank  1301 . There will be no emptying of tank  1301 , even when the valve  1316  is opened. In such instances, a gravity flow option, through a conduit  1324  connected to the conduit  1300  via a tee joint  1309  is provided to enable the operation of this device in a manner similar to device described in FIG. 11. An outflow valve  1311  is  28  provided in this conduit, which when open, enables the discharge of the upper chamber liquor by gravity. Outflow valve  1311  is activated by a timer mechanism  1323 , connected to the level sensing device  1303 . The timer activates the opening of valve  1311  if tank  1301  is not emptied within a predetermined short time interval after opening of valve  1316 , i.e., if the level sensor  1303  continues to detect the presence of liquid level  1305 , even after a set time interval after opening of valve  1316 . When valve  1311  is open, for low strength wastes, the device does not provide pumping into for above-ground-air-contact treatment, but merely discharges anaerobic treated effluent from the upper chamber,  104 . Once valve  1311  is open, collection chamber  1301  is emptied, and the one-way valve  1308  as well as valve  1311  closes. In case, sufficient gas production takes places with the fresh input of waste, the cycle of operation with liquid discharge through outlet  1312  as previously described takes place. Otherwise, wastewater flowing into collection chamber,  1301 , simply flows by gravity into the lower chamber,  102 , displacing an equal quantity through the filer  119  into the upper chamber  104 . As valve  1311  is in closed position, liquid accumulates in  104  and level  1322  rises in the conduit  1300  along with corresponding level,  1117 , in the downcomer tube  1118 . When  1117  rises above at the level of conduit  1306 , no further flow of wastewater from the collection tank  1301  is possible, and an accumulation of wastes takes place in  1301  until it reaches the trigger level of sensor  1303 . At all times during this process, a slow gas accumulation takes place in  124 , as a result of anaerobic degradation. The quantity of gas may not be sufficient to trigger self-release through the  130  U tube mechanism. However, whatever gas accumulated in  124 , is released by the operation of automatic valve  1316 . During each such gas release, backflushing of the filter takes place and simultaneously further wastes flow into the down-comer  1118 , till level  1117  rises to equal level of liquid in  1301 . As there is no outflow from the system, wastes will accumulate in  1301 . The triggering of valve  1316  initiates gas release and backflushing of filter, but liquid flow into the reactor is no longer possible. Under this condition, the timer device activates opening of valve  1311  for a predetermined duration and outflow by gravity takes place from  104  emptying collection tank  1301 . The valve  1311  closes after tank  1301  is emptied. This cycle is repeated as long as gas production is insufficient to create outflow through the  1312  outlet. As soon as gas production at sufficient rate commences, pumped discharge of liquid through the  1312  outlet takes place and no outflow through the  1324  outlet takes place. Depending on gas production potential of the wastes, the system may as operate with discharge through the  1312  outlet and discharge through the  1324  outlet alternately.  
         [0105]    It may also be noted that any gas accumulation in space  123  of upper chamber  104  is discharged through either liquid outlet  1312  or  1324 , whichever is open.