AIS/CESIO environmental surfactant monitoring programme. Part 1: LAS monitoring study in “de Meern” sewage treatment plant and receiving river “Leidsche Rijn”

This manuscript reports on the outcome of a 7-day pilot monitoring study on the anionic surfactant linear alkyl benzene sulfonate (LAS) at the “de Meem” municipal sewage treatment plant. The receiving surface water, the Leidsche Rijn is a straight river — about 20 m wide and 1.5 m deep — and dilutes the sewage discharge by a factor 3. The monitoring study illustrates an effective removal of LAS of 99.9% during dry weather and normal operating conditions. The LAS concentrations in daily composite raw sewage samples varied between 3.1 and 7.2 mg/L, with corresponding effluent concentrations generally under the analytical detection limit of 8.1 ug/L. During this same period, total LAS concentrations in the river varied between <2.1 ug/L (detection limit) and 2.9 ug/L in samples taken above the sewage outfall and between <2.1 ug/L and 7.1 ug/L for samples taken below the sewage outfall. Malfunctioning of the primary settler during heavy rainfall conditions, resulted in throughput of primary sludge particles to the aeration tank. The removal of LAS, BOD, SS, and TOC decreased significantly during this period, resulting in higher effluent concentrations. In addition, analyses of river samples taken during and after the rainfall indicated the presence of a sewage overflow which was discharged upstream of the sewage treatment plant outfall.