Chemical treatment of anaerobic digester supernatant

Abstract

The recycle of anaerobic digester supernatant, a low volume, highly concentrated waste stream, to the headworks of a sewage treatment plant has long been suspected and recognized as being detrimental to the wastewater treatment process. This study was concerned with the removal of phosphorus, nitrogenous, and carbonaceous pollutants from the supernatant by use of chemical precipitants. Emphasis was placed on achieving maximum phosphorus removal. Lime, ferric chloride, and alum were the precipitants used in this study.

All precipitants effectively reduced the total phosphate concentration by 98.5% or better. For lime an end point pH of 12.3 or greater corresponded to the maximum phosphorus removal. No attempt was made to adjust the sample pH in the experiments employing ferric chloride or alum. Significant suspended solids and chemical oxygen demand removals were observed at the precipitant dosages which resulted in maximum phosphorus removal. Ammonia removal was insignificant.

Aeration of a supernatant sample to remove carbon dioxide and raise the initial pH resulted in reducing the amount of lime required to achieve maximum phosphorus removal by approximately 40%.