The accumulation of non- or slowly biodegradable organic compounds in an activated sludge process within a Kraft Pulp Mill

Adsorption of non- or slowly biodegradable organic compounds such as lignin or its derivatives on biosludge may weaken its dewatering properties and suitability for anaerobic digestion. These compounds accumulate in biosludge of an activated sludge process (ASP) within a kraft pulp mill as the sludge retention time (SRT) increases. The specific adsorption characteristics of influent COD and BOD were studied in laboratory-scale with biosludge and ferric sulphate. The accumulation of non- or slowly biodegradable compounds in biosludge was estimated by dynamic modelling using specific influent COD characteristics and the process data from an ECF kraft pulp mill. Approximately 20-30% of influent COD consisted of adsorbable organic compounds that had no effect on BOD₅. Due to this fraction, the non- or slowly biodegradable organic content of mixed liquor volatile suspended solids (MLVSS) in biosludge was 45-70% with SRT varying between 25 and 60 days.