Degradation of anionic surfactants during drying of UASBR sludges on sand drying beds

Anionic surfactant (AS) concentrations in wet up-flow anaerobic sludge blanket reactor (UASBR) sludges from five sewage treatment plants (STPs) were found to range from 4480 to 9233 mg kg−1 dry wt. (average 7347 mg kg−1 dry wt.) over a period of 18 months. After drying on sand drying beds (SDBs), AS in dried–stabilized sludges averaged 1452 mg kg−1 dry wt., a reduction of around 80%. The kinetics of drying followed simple first-order reduction of moisture with value of drying constant (kd)=0.051 d−1. Reduction of AS also followed first-order kinetics. AS degradation rate constant (kAS) was found to be 0.034 d−1 and half-life of AS as 20 days. The order of rates of removal observed was kd>kAS>kCOD>kOM (drying >AS degradation>COD reduction>organic matter reduction). For the three applications of dried–stabilized sludges (soil, agricultural soil, grassland), values of risk quotient (RQ) were found to be <1, indicating no risk.