Recirculation in a small sewage pond system

Recirculation in a small scale waste stabilization pond system operated over a period of 12 months. Effluent from the third pond in a series of four ponds was added to the primary pond at a ratio of 1:1 (recirculated effluent:settled sewage effluent). Recirculation was stopped during winter for 35 days and during summer for 21 days to ascertain the seasonal benefits of recirculation. Dissolved oxygen concentration in pond 1 increased by up to 234% during recirculation in summer, reducing the potential of algal photosynthesis inhibition by high concentrations of ammonia-nitrogen. Algal cell numbers did not increase significantly in pond 1 during recirculation. Recirculation was unable to reduce Total Suspended Solids (TSS) and Chemical Oxygen Demand (COD) concentrations of the final effluent to meet the Government effluent standards of 25 and 75 mg 1-1, respectively. An increased recirculated effluent:settled sewage ratio is recommended