Advanced treatment of sewage by pre-coagulation and biological filtration process

A pre-coagulation and bio-filtration process for advanced treatment of sewage was developed and experimentally discussed with a pilot plant. The bio-filtration unit consists of a denitrification filter, a nitrification filter with side stream to the denitrification filter, and a polishing filter with anoxic and aerobic parts. Concentrations of SS, T-CODCr, T-carbonaceous BOD, T-N and T-P in the effluent were stably kept at less than 3, 20, 5 mg/L, 2 mg N/L and 0.2 mg P/L, respectively, and transparency at higher than 100 cm, under total hydraulic retention time of 3.2 h in the bio-filtration parts (filter-bed). ORP in an anoxic tank before a nitrification tank should be at a low level of less than -120 mV to keep remaining NOx-–N less than 1 mg N/L, but must be maintained at a level higher than -150 mV. The maximum nitrogen-loading rate under a water temperature of 18°C should be less than 0.25 kg N/(m^3-filter-bed·d). Concentrations of microorganisms kept in the reactors were as high as 4000–5000 mg COD/L-filter-bed. Denitrification activity of 0.4 or 0.7 kg N/(m^3-filter-bed·d), and nitrification activity of 0.3 kg N/(m^3-filter-bed·d) were obtained, respectively, under a water temperature of about 18°C. Backwashing in each tank as well as methanol addition and aeration in the polishing filter were operated successfully by the automatic control systems. These results proved that this process is applicable to advanced treatment of sewage with easy maintenance.