Development of pre-coagulation and bio-filtration process for advanced treatment of sewage

A pre-coagulation and bio-filtration process for advanced treatment of sewage is 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-C-BOD, T-N and T-P in the effluent were stably kept at less than mg/L, 20 mg/L, 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 (HRT) of 3.2 h in the bio-filtration parts. S-CODCr/NOx⁻-N ratio of higher than 12 mg COD/mg N is required to keep the remaining NOx⁻-N concentrations less than 1 mg N/L in the effluent from the denitrification filter. Methanol addition rate of higher than 4.5 mg COD/mg N is necessary to accomplish denitrification with the remaining NOx⁻-N of less than 0.5 mg N/L in the effluent from the polishing filter. Backwashing in each filter as well as methanol addition and aeration in the polishing filter were able to be operated successfully by the automatic control systems. Therefore this process is proved to be applicable to advanced treatment of sewage with easy maintenance.