Bioloop filter (braided cord made of vinylidene chloride thread) was used for the biological treatment of sewage. In this study, the aeration tank (1.8 m³) was divided into 4 portions across the flow direction and bioloop filter (30 or 45 m·m^-3) was submerged in each tank. Amount of biomass attached to the filter was determined after 60 to 100 days operational periods and were regarded as mixed liquor suspended solids (MLSS).
The amount of biomass attached on the filter was ranged from 0.4 to 0.6 kg·m^-3 (bioloop 30 m·m^-3) and from 0.4 to 0.8 kg·m^-3 (bioloop 45 m·m^-3). Acoordingly, BOD loadings in this study were kept in the range from 0.15 to 0.35 kg-BOD·kg-MLSS^-1·d^-1. Effluent BOD was stabilized with the decrease of BOD loading and effluent containing BOD<5 mg·l^-1 was obtained even in winter season at a loading under 0.24 kg-BOD·MLSS^-1·d^-1. Though dissolved oxygen concentration in bulk liquid was maintained at suficiently high level, biomass contained sulfide ion upto 0.4 mg-S^2-·g-biomass^-1. As a result of anaerobic degradation inside the biomass, the excess sludge production ratio of this process was less than that of conventional activated sludge.

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An equation of specific substrate utilization rate for immobilized microorganism was derived in consideration of external and internal diffusional resistances and microbial substrate utilization. It was expressed by the Michaelis-Menten equation modified replacing the maximum specific substrate utilization rate to η_pν_m and the Michaelis constant to (K_m/K+δ_p) in the case of C_bδ_p/(C_b+K_m/K+δ_p)²«1.
The experimental results of immobilized ammonium and ferrous iron oxidizing bacteria showed that δ_p should be considered because ratios of δ_p to K_m were 0.34 and 1.14, respectively. On the other hand, when microorganisms were dispersed in a culture medium, δ_c could be ignored even in a moderate agitation condition.

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Effects of θ_e, needed for Fe (II) oxidation with air, initial molar ratio Fe²⁺/M²⁺ (M : Zn, Cd and Mn) and reaction temperature on ferritization of heavy metal ions were studied by stability of heavy metal, obtained by leaching test of ferrite suspension at pH 3-3.5, and yield of Fe₃O₄ (3·Fe²⁺/Total-Fe).
θ_e≥1.5 h, θ_e is controled by the conditions of aeration and mixing speed, the stability and the yield of Fe₃O₄ were not effected by θ_e, but they were increased with Fe²⁺/M²⁺ and reaction temperature.
Sedimentation of these ferrites was also studied for the separation of sludge from the treated water. Settling velocity of ferrite became faster, when the ratio of spinnel formation is over 0.5-0.6.

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Based on microcosm experiments, a model for self-purification process in eutrophicated water is developed. The characteristics and applicability of the model are summarized as follows :
(1) The self-decomposition process in eutrophicated water is expressed by a model with 7 variables of bacteria, phytoplankton, zooplankton and 4 types of nonbiotic organic matter.
(2) The behaviors of nitrogen and phosphorus are also simulated by this model under the assumption of constant N and P contents in each variable.
(3) The model is verified by the good coincidence of simulated values with observed values with high correlation coefficient values more than 0.9.
(4) The role of each organism group on self-purification process is evaluated with its respiration rate calculated by this model. The contribution proportions of bacteria and phytoplankton are estimated 40-70% and 10-40%, respectively, while that of zooplankton varies widely from 0 to 40% according to sample properties.

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