Eutrophication is one of the most serious environmental problems involving water sources in all industrialized countries. It can be defined as the natural process in which biological products increase with the age of water body. Generally it is caused by the growth of phytoplanktons and other aquatic plants, which results in the death of fish, specially shell fish, due to the depletion of dissolved oxygen. There have been worldwide research efforts to solve the problem by reducing the major pollutants of nitrogen and phosphorus. SS (suspended solids) in the effluents that is removed in a sedimentation tank in the conventional wastewater treatment system represents BOD value. To minimize land requirements for the wastewater treatment and to obtain good quality effluents, the performance of a dual biological reactor system and an attached culture system of depth filter perfusion system (DFPS) were investigated.
The dual biological reactor system (DBRS) was a hybrid wastewater treatment system that combines CSTR and PFR. This consisted of a circular tank (14 cm ID, 25 cm height) with a draft (8.8 cm ID, 20 cm height) in the first stage and a packed column (8.8 cm ID, 41 cm height) in the second stage. As a packing material, celites were used for the formation of biomass film in the second stage. In spite of increasing hydraulic retention time, BOD removal was 95-97%. But totally aerobic, the nitrification alone occurred. The pH of effluents was 6.5 - 6.8 and the suspended solid of the effluents was 60-100. The removal of phosphorus was a little attained. Therefore, for denitrification and phosphorus removal, the dual biological reactor system should be combined with other anaerobic or anoxic systems.
Immobilization of sludges to the depth filter took about 1 hr by recirculation. To test the applicability of depth filter perfusion system (DFPS) to the wastewater treatment, one-stage aerobic system which the pore size of depth filter was 70㎛ was operated. This can remove 90% BOD, and 90% $NH_4^+$-N, but conversion of $NH_4^+$-N to $NO_3^-$-N was very low that the complete nitrification was not achieved. The extent of phosphorus removal was 20-30%. In order to achieve further removal of nitrogen and phosphorus, the two-stage DFPS was tested. The configurations of anoxic/aerobic and aerobic/anoxic were applied to the DFPS. The anoxic/aerobic system could remove about 20% phosphorus and 60-70% $NH_4^+$-N. The performance of aerobic/anoxic system was similar that denitrification and phosphorus removal were not successful. But adding the external carbon source just like glucose and sodium acetate, the denitrification and about 60% phosphorus removal was achieved. Accordingly, the advanced wastewater treatment system with nitrification-denitrification and phosphorus removal could be achieved by the hybridization of a first-stage DBRS and a second-stage DFPS.
