IWA 1-ST WORLD WATER CONGRESS
/PARIS 3 - 7 JULY 2000/

A NOVEL COMBINED REACTOR (BIOFILTER UP AND AERATION/SEDIMENTATION CHAMBERS DOWN) FOR TREATMENT OF SEWAGE AND AGRICULTURAL WASTE WATER

An innovative combined reactor (biofilter up and aeration/sedimentation chambers down) was developed to decrease an operative energy expenses (in 2-3 times) and labor cost (not less than on 30%). A closed reactor design allowed to improve a quality of air coming out and to keep a temperature needed in the cold climate conditions. Several full-scale reactors treating sewage and industrial wastewaters were operated successfully for a number of years. A mathematical model based on the material balance equations for biofilter and aeration/sedimentation chambers was developed.

Combined reactor, biofilter, aeration/sedimentation chambers, biomass composition, nitrogen removal, mathematical model.

The biological treatment system can be divided into suspended-growth and attach-growth (Metcalf and Eddy, 1991). In suspended-growth systems microorganisms are maintained in suspension in the wastewater. In attached-growth (fixed-film) systems microbial film are attached to an inert material. The activated sludge system (aeration and sedimentation tanks) is the main representative of suspended-growth aerobic system. Attached-growth aerobic biological treatment systems include trickling filter, rotating biological contactor, fixed-bed nitrification reactors, etc. The activated sludge as well as biofilter processes, in its varied forms, has found wide application in the treatment of domestic and industrial wastes. In this paper the original concepts in the combined reactor (biofilter up and aeration/sedimentation chambers down: Applications 1996, 1998) firstly developed by Kolesnikov et al. (1990) are described and the parameters of existing treatment plants with concentrated sewage and agricultural wastes are presented.

According the scheme of reactor (Fig.l), after being mechanically processed, the wastewater is discharged to mixing chamber (1) where it is mixed with the sludge coming from aeration/sedimentation chambers (2). From chamber (1) the mixture of wastewater and sludge is driven by pump (3) to sprinkling system (4). The falling liquid come to biofilter (5) with the immobilized microflora designed to sorb and oxidize 50-70% of the organic material. Oxidation of the remaining organic material of wastewater as well as an excess biofilm breaking away in biofilter is achieved with the help of the aeration/sedimentation chambers (2).

To saturate the biomass with additional oxygen and to stir the aeration chamber mixture, water jet into which air is sucked (0.6 - 0.9 m³ per 1 m³ of liquid) as result of vortex funnels is applied. The aeration columns distribute (6) the air over the aeration chamber. About 30-35% of the required oxygen come to aeration chamber with waste water and sludge mixture, and 70-65% of oxygen come due to the air bubbles lifting during the process of mass-transfer in the columns. To produce a general revolving effect in the aeration tank, a number of columns are provided with the tangential directed taps.

Displaced from the aeration zone, the sludge mixture is discharged through the slot to the settling zone where some part of it is compacted and driven back to the aeration zone. The effluent water is discharged through the collecting chutes. The combined reactors have been applied for treatment of the sewage from small and medium size settlements and for treatment of the agricultural waste characterized by a high influent BOD concentration above 1000 mg/l. The existing combined reactors use as the biofllter medium corrugated asbestos boards, which are hard and rough enough to hold microflora on their surface. The plastic medium is used also now. A closed reactor design allows improving a quality of air coming out and to keep a temperature needed in the cold climate conditions.

Table 1 summarizes the results obtained in some treatment plants. It is easy to see that a deep carbonaceous removal from sewage and agricultural wastes up to 98% is achieved. There are two different zones (aerobic and anoxic) in a biofilm depth of the biofllter. Oxidation of organic material and nitrification are observed in an aerobic zone, but denitriflcation takes place in an anoxic zone. A possibility to realize a circulation of sludge mixture containing an organic carbon material and nitrates achieves denitriflcation.

The Nitrosomonas, Bacillus subtilus and' Bacillus micoides were isolated from the samples of the bottom part of biofllter as well as from the. sludge of aeration/sedimentation chambers. In anoxic zone of biofllter Micrococcus denitrificans were detected. Under low organic loading (less than 0.2 g BOD/g of sludge) an effective carbonaceous removal and nitrification are taking place in aeration/sedimentation chambers.

The function of the combined reactor is 'expanded now to include nitrification, denitriflcation and phosphorus removal. To improve treatment efficiency from nitrogen a modification of the reactor with a separate chamber for denitriflcation has been suggested. A mathematical model taking into account the processes happened in biofllter and aeration/sedimentation chambers for an optimal design between the different chambers of reactor was developed earlier (Vavilin et al., 1993). Six main variables were involved into the model: concentrations of organic carbon, ammonium nitrogen, nitrites and nitrates, inert material, biomass of heterotrophic and autotrophic microorganisms. The material balance equations were written for biofllter, aeration/sedimentation and denitriflcation chambers. A steady-state solution was realized using IBM computer. Some equations in the model as well as the values of some parameters were taken from the well-known IAWQ model (Activated sludge model, 1986).

Activated sludge model N1 (1986). IAWPRC Task Group Mathematical Modelling for Design and Operation of Biological Waste Water Treatment. M.Henze-Chairman, Denmark. Application PCT/RU 96/00202.1 PC 6: C02F 3/00-3/02 International Search Report-Category A. Application PCT/RU 98/00126.1 PC 6: C02F 3/12 C02F3/06 International Search Report-Category A. Kolesnikov, V.P., Koljtsova, T.N., Nikolaenko, N.N., Rudnev, G.F. and Stasenko, E.V. (1990).

      Combined reactors for wastewater. Vodosnabzhenije i Santechnika (Water Supply and Sanitary Technique), N 12,18-19 (in Russian). Metcalf & Eddy, Inc. (1991). Wastewater Engineering: Treatment, Disposal and Reuse. 3rd ed.

      McGraw-Hill, New York, N.Y. / Vavilin, V.A., Vasiliev, V.B. and Rytov, S.V. (1993) Modelling of Organic Matter Destruction by Microorganism Community. Nauka Publisher. Moscow (in Russian).

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