Risk Assessment to the Environment Due to Anionic Surfactants in Treated Sewages and Dried Sludges

Paper presents the outcome of a 21 month monitoring study on the anionic surfactants (AS) at the 70 ML/d UASB-PP (up-flow anaerobic sludge blanket - polishing ponds) based sewage treatment plant (STP). The average removal of AS was only around 30%. Appreciable concentration of AS was being discharged to the watercourse (average 3.60 mg/L; range 1.69–4.94 mg/L). Compare to this >92% removal was found in an 18 ML/d activated sludge process (ASP) based STP with an average 0.13 mg/L of AS is discharged to the surface waters. AS concentrations averaged 5909 and 810 mg/kg dry wt. in wet UASB and dried sludges respectively. Treated sewage from UASB-PP based STP when discharged to aquatic ecosystems are likely to generate substantial risk. Post-treatment using 1 day detention, anaerobic, non-algal polishing ponds was found ineffective. Need of utilizing an aerobic method of post-treatment of UASB effluent in place of an anaerobic one has been emphasized. Natural drying of UASB sludge on sludge drying beds (SDBs) under aerobic conditions results in reduction of adsorbed AS by around 86%. Application of UASB sludges on SDBs was found simple, economical and effective. While disposal of treated UASB-PP effluent may cause risk to aquatic ecosystems, use of dried UASB sludges is not likely to cause risk to terrestrial ecosystems.