Linear alkylbenzenes in sewage sludges and sludge amended soils

Abstract

Linear alkylbenzene sulphonates (LAS), manufactured by the sulphonation of linear alkylbenzenes (LAB), became the major anionic surfactant material used in detergent formulations in the 1960s and they have maintained that status to the present day. Commercially produced LAS may contain 1–3% unreacted LAB which are only biodegraded under aerobic conditions. As a result any LAB removed by adsorption onto sewage solids during primary treatment, and which is subjected to anaerobic digestion, will be present on the sludge at the end of the treatment process. Some of the sewage sludge generated is disposed of to agricultural land. A monitoring exercise to determine the concentration and isomer distribution of linear alkylbenzenes in sewage sludges and sludge amended soils is described. Concentrations of LAB in various sludges ranged from 116 to 388 μg g −1 (dry wt). In fields spread with sludge prior to 1989 the concentration of LAB was generally < 5 ng g −1 soil (dry wt). Comparison of these data with the estimated total cumulative load based on known sludge application history indicates losses of >99% for the majority of sites. Time course experiments indicate rapid removal of LAB from sludge amended soils. Calculated half lives for the various alkyl homologues are similar (12–15 days). Phenyl isomer distribution in soils suggests, however, preferential biodegradation of the external isomers. This is confirmed in time course samples which show half lives of 4.5 days for the 2-phenyl isomers and 20 days for the 6/7-phenyl isomers.