APPARATUS AND METHOD FOR BIOLOGICAL TREATMENT OF WASTEWATER

Field of the Invention The present invention relates to an apparatus and to a method for decontaminating wastewater. More specifically, this invention relates to an apparatus and method comprising a biomass attached to an inert carrier, for a complete biological treatment and reclamation of municipal and industrial wastewaters.

Background of the Invention

Biological treatment of raw wastewater is a standard part of the processes for purifying municipal and industrial sewage, and comprises contacting said water to be treated with essentially immobilized or suspended bacteria, utilizing their metabolism for removing various organic and inorganic impurities. The principal task of an apparatus for biological treatment of wastewater is producing and developing the active biomass, keeping its position and maintaining its concentration in the reactor. Ensuring the viability and the decontamination proficiency of said biomass allows an efficient use of the entire working volume in the apparatus, a reduced duration of wastewater treatment, as well as the reduction of inefficient energy consumption associated with air supply, discharge recirculation, treatment, transportation and burial of excessive toxic active sludge.

To maintain a high concentration of active sludge in the reactor, almost all the current methods in the field of biological treatment of wastewater employ sludge recirculation from secondary or tertiary sedimentation tanks into the bioreactor. Some examples can be found in WO 00/049140 and WO 2004/076365. In case of biomass recirculation from secondary and tertiary sediment tanks into the initial treatment compartment of the bioreactor, the high nutrient concentration contained at the beginning of the treatment

process causes a shock to the reflowed bacteria, which recuperate their full metabolic activity only after a significant delay. Due to changes in the concentration of nutrients and oxygen in the bioreactor, the recirculation of a biorαass intercepts the natural trophic chain. In fact, the majority of the recirculated bacteria become encapsulated, and they have no time to pass into the active phase of the metabolism before discharge of water from the bioreactor. Furthermore, because of the encapsulation, poorly digestible organic material, like heteropolysaccharides (heteroglycan) from the cell envelope of encapsulated bacteria are formed in the bioreactor.

In bioreactors using immobilized microorganisms on carrying surfaces, one of the principal problems is carriers clogging, necessitating periodic regeneration of said carriers to ensure correct performance of the treatment system. One of the main reason of clogging is said encapsulation of recirculated bacteria. Since encapsulated bacteria adhere more easily and more strongly to surfaces than active cells, microorganism carriers are progressively clogged with poorly active biomass, causing a gradual decrease in the water treatment efficiency. Thickening of the biofilm on the carrier deteriorates the diffusion of nutrients and oxygen into the deeper layers, limits the overall metabolic processes, and increases the duration of the decontamination treatment.

Another frequent problem is related to the choice of a suitable bacterial carrier. The ratio between the carrier surface area and the reactor volume is a critical parameter to be taken into account when choosing a carrying surface. This ratio for three-dimensional rigid carriers seems quite high, as disclosed, for example, in WO 03/068694. However, available carrying surface is usually lower, due to clogging effects or due to low porosity levels. In the latter case, supply of oxygen to the biomass requires more powerful blowers or compressors to supply oxygen to a thick biofilm. On the other hand, intensive mixing causes the carrier parts to collide, while shearing effects lead to additional losses of the attached biomass. An improvement of

the oxygen distribution system can be found, for example, in US 4,863,644, that relates to a gas diffuser where large air bubbles are broken into smaller ones. Generation of such small bubbles results in higher gas transfer efficiency in the bioactive sludge. Various improvements in the use of microorganism carriers in wastewater treatment are proposed, for example, in JP 09094592.

However, in general, no particular attention is paid to improve microorganisms viability or their metabolism efficiency. In most cases, the problems are mitigated by increasing the size and the complexity of the wastewater plants, and by adding additional systems aimed at, e.g. biomass recirculation, carrier regeneration, or sedimentation. However, such solutions lead to a significant increase in energy consumption, and require qualified personnel for operating and maintaining the equipment. It is therefore an object of this invention to provide a method for the biological treatment of wastewater which overcomes the above-mentioned drawbacks.

It is another object of the invention to provide a method to ease the accessibility of the nutrients to the microorganisms used for the treatment of contaminated water.

It is a further object of the invention to provide an apparatus using specifically-adapted microorganism carriers and gas diffusion systems, enabling to improve accessibility of the nutrients to the microorganisms and to preserve the active biomass concentration.

Other objects and advantages of the present invention will appear as the description proceeds.

Summary of the Invention

The invention provides an apparatus for biological treatment of wastewater containing organic contaminants, comprising i) reaction space (bioreactor) through which a continuous stream of the treated water is kept flowing by the effect of gravity, the space being characterized by the presence of bacteria decomposing at least one of said organic contaminants; ii) inlet means through which said wastewater enters said bioreactor, and outlet means through which the treated water leaves said bioreactor without passing again through said inlet, whereas said means control the water height in the bioreactor at a predetermined level; iii) knitted or woven inert polymeric carrier characterized by dense clusters of protrusions and loops providing a surface to said bacteria to adhere to, wherein said carrier is held by essentially planar static holders which are placed in said reaction space; and iv) aerators inserting and dispersing air into said bioreactor in the form of , small bubbles near the planes of said planar holders holding said carrier, wherein said small bubbles break on said clusters to provide microbubbles; wherein said bioreactor comprises at least two sections differing by the type of bacteria, by the space arrangement of said carrier, or by both. In a preferred embodiment of the invention, said bioreactor is composed of different treatment sections, each section comprising i) knitted or woven inert carriers made of shaggy polymeric multifilament yarns, providing a surface for bacteria to adhere to; ii) a plurality of essentially planar, parallel, vertical carrier holders, holding in position said inert carriers; and iii) a bubbling system providing air micro-bubbles near to said plane; wherein said micro-bubbles are produced by breaking large bubbles on combs, said large bubbles going out from aerators, and being directed to the surface of said planar holders by deflectors, said deflectors being disposed on a deflector plate. Said aerators may be attached to sloped deflectors arranged at an angle α of between 30° to 45°, depending on height of the reactor section, said deflectors having external edge in the form of a comb with teeth, where coarse air bubbles rising under the action of buoyancy are torn into finer

bubbles. In a preferred apparatus according to the invention, the quantities (and composition) of the contaminants change gradually with time in said reaction space i.e. at a certain position in the space, thereby enabling constant nutrient environment for said bacteria in each part of said bioreactor; the concentration of the immobilized biomass in each section is subjected to very small fluctuations and practically does not change over the time i.e. in each section of said bioreactor is being provided the steady-state process. Thus, the quantities of the-contaminants, and optionally the type of bacteria (strain, species, etc.), and possibly the amount of oxygen supply, vary along said continuous stream, but the parameters do not change with time in said sections, or at certain point, of said reaction space, thereby enabling the constant physicochemical and biochemical parameters of environment for said bacteria in all parts of said bioreactor.

In a preferred embodiment of the invention, an apparatus is provided which has at least two sections which employ different bacterial strains. In a very preferred arrangement, an apparatus according to the invention has at least one section in which simultaneous nitrification and denitrification processes occur. Said holders preferably form an angle β of 0 to 20° with the direction of the wastewater flow. In a preferred embodiment of the invention, said inert polymeric carrier comprises polyamide multifilament shaggy yarns. The carrier may be, for example, arranged in the holder to form a kind of a hairy carpet, preferably with a part of the hairs freely floating in the passing water.

Provided is an efficient apparatus for treating wastewater, which comprises a plurality of carrier units intended for static use in treating wastewater, wherein each of said units consists of an essentially planar and vertically placed holder, and of knitted or woven inert polymeric carriers, said carriers comprising dense clusters of protrusions and loops, providing a surface to bacteria capable to decompose organic contaminants in said wastewater. Said plurality may consist of essentially parallel units, placed equidistantly

with a distance of n between adjacent units. Said carrier units offer low resistance to wastewater flowing through them. A special feature of the instant apparatus is the use of carrier units bearing polymeric, highly structured carriers attached to said holders and having dense clusters of shaggy yarn which freely fly, in flowing wastewater, in the space between two adjacent planar units. Said clusters are shorter than said distance n between adjacent, essentially planar, units. Said clusters thus essentially may fill the space between two adjacent units, or an important part of it.

The invention is directed to a carrier unit for static use in treating wastewater in a bioreactor, the unit consisting of an essentially planar holder and a knitted or woven inert polymeric carriers, said carriers comprising dense clusters of protrusions and loops providing a surface to bacteria capable to decompose organic contaminants in said wastewater. The carrier unit of the invention is suitable for static and dynamic use in water treatment. The unit according to the invention offers a low resistance to wastewater flowing through said planar holder, for example water moving approximately perpendicularly to the plane of said planar holder, wherein said carriers comprise elements freely flying in said flowing wastewater. Said carrier unit preferably comprises a textile which is woven or knitted from double non-stranded fibers. The carrier unit may comprise a shaggy fiber made of a single or double non-stranded yarn of at least 350 to 700 Tex, with s number of multifilament of at least 720, woven or knitted to a density of, for example, about 40-80 yarn per decimeter. Said carrier may be knitted or woven from polyamide fibers. The carrier may, for example, have a warp comprising a double non-stranded yarn of at least 700 Tex, being woven or knitted to a density of 40-80 yarn per decimeter. The carrier may, for example, have a weft comprising a single or a double non-stranded yarn of respectively 350 or 700 Tex, woven or knitted to a density of 50-60 yarn per decimeter. The ratio between the width of a weft yarn strip and the length of a warp yarn in an inert carrier to be employed, may equal to 1:3, 1:4, 1:5, 1:6,

1:7 or 1:8, a lower ratio being preferred when said carrier unit is placed in a section with a high concentration of contaminants. In . one example, the distance between the weft yarn strips of said inert carrier is equal to the length (Ls) of a free yarn. The distance (Lk) between interweaved clusters in said inert carrier may be, for example, about the double of a free yarn length.

The invention provides a method of treating a stream of wastewater contaminated with organic impurities, comprising i) passing said water by gravity through a bioreactor composed of a plurality of sections, said stream flowing along a curve that is not closed; ii) contacting said water with knitted or woven inert polymeric carriers held on static, essentially planar parallel holders, the carriers being characterized by exhibiting dense clusters of protrusions and loops arranged in shaggy structures, and by being attached to said holders, while having dynamic elements that freely fly in said passing water, and providing a surface onto which adhered bacteria are decomposing said organic impurities; iii) admixing predetermined volume of air in the form of bubbles into said stream near to the planes of said planar holders; iv) breaking said bubbles on said protrusions, thereby improving the mass transfer of gases in said water; and v) colonizing said carrier in each section of said plurality by bacteria adjusted to the trophic environment of said section; thereby efficiently decomposing said impurities, with minimal output of activated sludge. In a preferred embodiment of the method according to the invention said stream of wastewater is treated in continuous flow without recirculation or sedimentation, whereas at least one section comprises simultaneous nitrification and denitrification processes.

Brief Description of the Drawings

The above and other characteristics and advantages of the invention, will be more readily apparent through the following examples, and with reference to the appended drawings, wherein:

Fig. l.is a top view of the bioreactor;

Fig. 2. is a side view (cross-section) of a nitrification and denitrification section;

Fig. 3. is a side view (cross-section) of one of the bioreactor sections;

Fig. 4.A and B, are front views of two possible arrangements of the clusters in woven fabric;

Fig. 5. is a front view of a knit or woven fabric of inert carrier (without clusters);

Fig. 6.A and B show a side view and a perspective view, respectively, of the air supply system and its arrangement with the inert carriers; and Fig. 7.A and B are plots showing the course of the BOD (mg/1), COD(mg/l), and TSS (g/1) decontamination and fluctuation of the biomass concentration (g/1) during 8 hours of the complete sewage treatment in the different sections of a bioreactor for domestic wastewater.

Detailed Description of the Invention

It was found that a surprisingly efficient biological treatment of wastewater can be performed in a full aerobic continuous-flow system, without zones of sedimentation and recirculation of the treated wastewater, if several requirements are carefully addressed: a) supplying oxygen to the bacteria; b) providing minimal conditions for their survival; c) protecting the bacteria from shocks; d) ensuring sufficient contact area between the treated water and the microbes; e) preventing clogging of the working surfaces; and f) performing simultaneous nitrification and denitrification in the same section.

The bioreactor of the invention represents a direct-flow construction without zones of sedimentation or recirculation of the treated wastewater. The wastewater flows by gravity from one section into another through windows arranged in each treating section. In all the sections except that of nitrification and denitrification, the windows are localized at the top. In the nitrification section, the inlet window is at the top and the outlet window at the bottom. The complete biological wastewater treatment, including

nitrification and denitrification, proceeds in compliance with the generic composition of hydrobionts fitted to the inert carrier. The present invention provides in one embodiment a water treatment system using a continuous plug-flow bioreactor comprising inert carriers that ensure fixation of the hydrobionts responsible of the destruction of the water contaminants.

Referring to Fig. 1, shown is a top view of one embodiment of the bioreactor 1 subdivided into sections. The number of bioreactor sections is determined by technological parameters of the contaminations and requirements for the treated water quality. In each section, inert bacterial carrier units 2 are positioned vertically at an angle β of between 0° and 20° to the wastewater flow direction. Blowers 3 provide air supply to each bioreactor section by pipelines 4 and 5 and a system of individual aerators 7. The nitrification section 6 has its own air pipeline 5 separated from other sections.

A part of the invention is a novel treatment section, in which nitrification and denitrification processes take place gradually and synchronously. This particular section contributes considerably in decreasing the complexity of the wastewater treatment system and is designed as follows: the treated water is supplied through the top overflow window into the nitrification and denitrification section which is divided into two parts. Intense aeration is realized using an air supply and distribution system located at a calculated height above the section bottom. Nitrification is realized by nitrifying bacteria (such as, for example, Nitrosolobus multiformis, Nitrobacter hamburgensis, Nitrococcus mobilis, Nitrobacter vulgaris, Nitrobacter wiiiogradskyi) attached to inert carriers in the upper part of the section, and supplied with a high oxygen concentration. Then the water flows into the lower level of the section, wherein the nitrification products are further used by denitrifying bacteria (such as, for example, Veillonella parvula, Clostridium perfringens, Propionibacterium, Wolinella succinogenes, Pseudomonas aeruginosa, Alcligenes). The lower part of the section (anoxic

zone) is not supplied with air and therefore has a very poor dissolved oxygen concentration and a high concentration of nitrification products, i.e. NO3, enabling denitrifying bacteria to metabolize nitrification products into nitrogen gas. Through a bottom window, the water further flows in a similar half section, where an analogous process takes place. Water is supplied from the nitrification and denitrification section through the top overflow window into the post-treatment sections.

Referring to Fig.2, shown is a side view of the nitrification and denitrification section 6. The contaminated water flows into the section divided by a vertical partition 11 into two parts through the top overflow window. Each half- section is divided horizontally into two unequal parts (for example ratios of 2:1, 3:1, 4:1). In the upper part, dedicated to the nitrification process, inert carrier units 2a with immobilized nitrifying bacteria are arranged vertically and separated by a distance nl (nl is about for example 15 cm in the preferred embodiment). Deflectors 8 coupled to aerators 7 are positioned at a calculated height above the bottom of the section and placed above a separating grid 19. In the lower part, dedicated to the denitrification process, inert carrier units 2b with immobilized denitrifying bacteria are placed at a distance n2 = nl/1.5 one from each other (n2 is about for example 10 cm in the preferred embodiment). The treated liquid flows by gravity from one half- section into another through a bottom window 9 and then, moves bottom-up, and flows out through the top window 10.

Referring to Fig. 3, shown is a side view (cross-section) of one of bioreactor sections. The treated wastewater flows by gravity into the section from a top window 10, passes through inert carrier units 2 with immobilized microorganisms and flows out into the next section by a top window 10 placed at the symmetrical opposite of the section. Aerators 7 are installed at the bottom of the section, with deflectors 8 on top of them.

The number of bioreactor sections is determined by technological parameters

of the incoming contaminations and quality specifications of the purified water. The amount of inert carrier in each bioreactor section is determined by the concentration of the incoming contaminations and varies consecutively from the maximum in the first section to the minimum in the last sections of the bioreactor. An important part of the invention is an inert bacterial carrier, specifically created for the system of the invention, enabling to immobilize microorganisms and to form a three-dimensional space filled with hydrobionts. The carrier immobilizing the biomass is located in the bioreactor in a section with the favorable trophic conditions for the immobilized microorganisms. The creation of optimal conditions for biomass growth, development and conservation in the bioreactor is realized by combining two factors: i) One of the factor ensuring optimal conditions for biomass growth, development and conservation in the bioreactor is a rational relationship between the contact surface area of the inert carrier and the volume of each bioreactor section, which allows holding up to 17 g/1 of attached biomass on the carrier in aerobic process, for instance, in case of sanitary sewage treatment. The carrier is specifically designed to ensure accessibility to nutrients and oxygen of all the attached hydrobionts whatever their location into the bioreactor section. ii) The carrier construction simultaneously operates as a multi-fiber disperser of air bubbles (coarse bubbles) ascending under the action of buoyancy caused by air diffusers below.

The invention provides a new inert carrier unit for immobilizing bacteria responsible of the biological decontamination in the treatment of wastewater, comprising an inert carrier and a carrier holder. Said inert carrier enables to increase the contact area between the microorganisms and the treated water, and to prevent clogging. The inert carriers of the present invention are provided in the form of a textile surface of highly entangled shaggy fibers with dense clusters sticking out in many protrusions in a hairy, carpet-like surface. The textile is woven or knit with double non-stranded polyamide

multifilament yarn, with shags, favoring the adsorption of bacteria without being digested by them. The weaving geometry of the shag provides dense clusters interspersed with pores large enough to let the water passing through. The woven material contains warp and weft polyamide shaggy multifilament yarn that form the cells (see Fig. 4A, 4B and 5). Light- stabilized texturized polyamide multifilament yarn is used as raw material, with a linear density is at least 700 Tex (350 X 2), the number of multifilament at least 720, for the warp shaggy yarn. The weft yarn can be used either with the same linear density than the warp yarn or with single polyamide shaggy yarn with a linear density of 350 Tex. The density of the warp shaggy yarns in the band is about 40-80 thread / dm. The number density of the weft yarns in the strip is 50-60 about thread / dm. The ratio between the weft strip width (A) and the warp thread (Ls) equals to 1:3, 1:4 or 1:5 when placed in concentrated sewage conditions, and equals to 1:6, 1:7 or 1:8 when placed in non-concentrated sewage conditions. The number of weft strips depends on particular conditions, such as section height or impurities concentration, but equals to 11 or less. The dimension of the inert carrier unit depends on the dimension of the section corridor in each particular conditions. The fringe of said woven fabric is melted, in order to prevent fabric disbandment and inserted into the carrier holder to form the carrier unit. Such carrier units offer the following advantages: a) larger setting area for bacteria is provided; b) excellent porosity is achieved, i.e. the treated liquid flow passes through the material with a minimum stream resistance; c) the shag provides additional setting surface and prevents the threads from sticking together; d) the clusters function as a multi-fiber dispersers of air bubbles rising under the action of buoyancy from air diffusers located below; and e) the system has a high specific breaking load and is stable against destructions during the operation in aggressive media.

Referring to Fig. 4, A and B, shown is front views of two embodiments of clusters arrangement on woven fabric. Fig. 4A schematically shows a interweaved shaggy cluster of single threads 12i (350 Tex), and Fig. 4B

shows a interweaved shaggy cluster of double threads 122 (350 x 2 Tex). The distance Lk between clusters interweaved with weft fibers 13 interlaced with warp threads 17 is equal to a doubled length (2 x Ls) of a free thread of cluster in order to prevent any contacts between the free ends of the cluster in a turbulizing flow of the treated liquid. The length Ls of the free end of each cluster is less than the distance n between the carrier units 2 in the bioreactor section.

Referring to Fig. 5, shown is a front view of an inert carrier (without clusters) in knit or woven fabric of double non-stranded shaggy polyamide yarn. In a preferred embodiment of the invention, the linear density of warp threads 17 is at least (350 x 2) Tex, and the number of piles per lcm is from 10 to 15. The distance between the weft strips 20 preferably equals the length of a free thread of a cluster Ls.

On the carrier, the ratio between the weft strips 20 width and the warp thread 17 length is preferably 1:3, 1:4 or 1:5 under concentrated sewage conditions, and preferably 1:6, 1:7 or 1:8 under non-concentrated sewage conditions. In said inert carrier, weft strips 20 amount depends on particular conditions like the section height or the impurities concentration, but is preferably at most 11.

The present invention also provides a special construction of air supply and distribution. Diffusers are coupled to deflectors located at a calculated height above them. Deflectors are oriented with an angle α of between 35° and 45°, gathering air bubbles rising from the diffusers and providing them directly to the inert carriers surface. In a preferred embodiment, external edges of the deflector have a comb shape with teeth. Coarse air bubbles rising from the diffusers are therefore turned by the deflectors combs into fine bubbles. The latter are further dispersed and broken into micro bubbles by the polyamide fiber clusters on the inert carrier. The use of such air dispersion system

significantly improves oxygen dissolution and distribution in the bioreactor, and contributes to an effective mixing of the treated water. Moreover, as micro bubbles are created by the combined action of deflectors combs and fiber clusters, use of micro diffusers and compressors is obviated, avoiding diffusers pores clogging, and reducing the need of energy.

Referring to Fig. 6A and 6B, shown are a side view and a perspective view of the air supply system, and the arrangement of the inert carrier units 2. Deflector plate 18 is composed of several deflectors 8 with combs 16 separated by a distance n, being also the distance that separate two consecutive inert carriers units 2. Deflectors 8 are designed so as to direct coarse air bubbles 14 towards the plane of inert carrier units 2 and are disposed above the aerators 7, with a slope angle α. The external edges of the deflector 8 are made in the form of a comb 16 with semi-circular teeth that are breaking coarse air bubble 14 into small air bubbles 15, which are further reduced and dispersed by the fibers clusters 12. Fiber clusters 12 are made flying and essentially filling the space between carrier units 2 by the stream of water. Additionally, deflector plates 8 are arranged at an angle β which corresponds to the fastening angle of the carriers units 2 in bioreactor section. The angle β is the angle between cross-section plane of the reactor section, and perpendicular plane to the real water flow direction between two top windows in the reactor section. Therefore, the value of the angle β depends on corridor length of the reactor section in each specific case. In the next section, the direction of the water flow is in opposite direction and the deflector plate 18 is turned by 180° (degrees) around a longitudinal axis to be adapted to the fastening angle - β of the inert carriers.

The invention will be further described and illustrated in the following example.

Example

An example of a biological wastewater treatment, using an eight sections system according to the present invention, is presented below (data reported in table 1 and Fig. 7A; 7B). In this particular case, the working volume in each section equals to 1/8 of the whole integrated bioreactor and the treatment time of a working volume in each section is about Ih. Typical domestic sewage having starting concentrations of biological oxygen demand

(BOD), chemical oxygen demand (COD), total suspended solids (TSS), and ammonium (NH4) of respectively BOD = 420 mg/1, COD = 720 mg/1, TSS = 180 mg/1, NH4 = 80 mg/1 is treated and enters into the bioreactor. i) In the section 1, the biomass concentration is about 20 g/1: 17-18 g/1 of cells are immobilized to inert carriers (Im), and 2-3 g/1 are formed by suspended cells (S). In this section, BOD is reduced down to 285 mg/1, COD down to 430 mg/1 and NH4 down to 58 mg/1. ii) By means of gravity, the water flows into the second section through a top inlet aperture, said second section having a biomass concentration of about 15 g/1 (13-14 g/1 Im, 1-2 S), and contaminants are further reduced down to 190 mg/1 for BOD, 280 mg/1 for COD and 40 for NH4. iii) In the third section, the biomass concentration is about 10 g/1 (9 g/1 Im, 1 g/1 S) and BOD is reduced to 120 mg/1, COD to 180 mg/1 and NH4 to

25 mg/1. iv) In the fourth section, which is a specific nitrification/denitrification section described previously, occurs more intensive nitrification, due to an increase of the concentration of nitrification bacteria, which prefer relatively purified and well aerated water, with a dissolved oxygen content not less than 2-5 mg/1 The biomass concentration is about 5 g/1 (5 g/1 Im, 0.5 g/1 S), BOD is being reduced to 60 mg/1, COD to 120 mg/1; and NH4 to 12 mg/1. v) In the fifth section the concentration of the suspended microorganisms is about 0.2 g/1, attached microorganisms about 2 g/1, BOD is being reduced to 30 mg/1, COD to 70 mg/1 and NH4 to 8 mg/1.

vi) From section six to eight, occurs the deep refining of wastewater, BOD,

COD and NH4 respectively being reduced to 15 mg/1; 45 mg/1, and 1 mg/1 with a biomass concentration of about lg/1 in section 6; 8 mg/1; 30 mg/1, and 0.8 mg/1 with a biomass concentration of about 0.5g/l in section 7; and 5 mg/1; 15 mg/1, and 0.5 mg/1 with a biomass concentration of about O.lg/1 in section 8. In the eighth section the carrier is covered with a very thin bio-film layer and this circumstance hampers the determination of its quantity (suspended microorganisms practically are absent).

The overall treatment was performed during 8 hours. The quality of purified sanitary wastewater at the bioreactor output is in compliance with very rigid standards for the purified water quality: BOD - 5 ppm; TSS - 5 ppm; COD - 15 ppm; NH3 < 0.5 ppm. The amount of excess sludge at the bioreactor ₍ output is reduced 150-200-fold (depending on BOD, TSS, COD concentrations at the input), which allows the elimination of excessive sludge treatment systems from the purification process. It should be noted that the concentration of immobilized biomass in each section is subjected to very small fluctuations and practically does not change over the time. The motion of effluent in the bioreactor is maximally approximates to "ideal displacement" (plug-flow reactor) Furthermore, in order to maintain the equilibrium necessary of microorganism's vital activity, each section of the bioreactor is provided by a dedicated air supply system and a specific concentration of dissolved oxygen. Therefore, the combination of a regulated oxygen serving and of a rate of sewage contamination which inflow into reactor section is the only limiting factor that preserves the biomass concentration in the bioreactor.

Table 1 Waste water purification in an apparatus according to the invention; the course of water purity along an 8-section device is characterized.

Biomass (g/l) BOD (mg/l) COD (mg/l) NH4 (mg/l) NO3 (mg/l)

Section Im SS Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

Inlet 0,18 420 720 70 0

1 17 3 420 285 720 430 70 58 0 20

2 13 1.5 285 190 430 280 58 40 20 40

3 9 1 190 120 280 180 40 25 40 50

4 5 0.5 120 60 180 120 25 12 50 45

5 2 0.2 60 30 120 70 12 6 45 22

6 1 0.1 30 15 70 45 6 1 22 8

7 0.5 0.02 15 8 45 30 1 0.8 8 0

8 0.1 0.005 8 5 30 15 0.8 0.5 0 0

Outlet C .005 5 15 0.5 0

While this invention has been described in terms of a specific example, many modifications and variations are possible. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.