A pilot-plant scale evaluation of potential precipitants in the secondary precipitation process

Chemical precipitation of presettled domestic wastewater (secondary precipitation) was studied in three pilot-plants operating in parallel with alum, ferric iron and lime as precipitants. With all precipitants phosphates were effectively removed down to less than 0·1 mg P l⁻¹. Total phosphorus removals were closely connected to the settling properties of the flocs. The factor that seemed to influence settling properties most was pH. In a favourable pH-range it was possible to achieve total phosphorus concentrations in the effluent of less than 0·5 mg P l⁻¹. The COD removal was at an average 70, 60 and 55% with alum, ferric iron and lime as precipitants.