Assessing the effect of surfactants on activated sludge processes using sequencing batch reactors

Anionic and non-ionic surfactants have been detected in the influents to and effluents from wastewater treatment plants (WWTPs). The results obtained from batch tests conducted according to ISO standard methods (ISO 8192 and ISO 9509) indicated that both surfactants, Sodium dodecylbenzene sulphonate (SDBS) as anionic surfactant and Nonylphenol ethoxylates (NPEO) as non-ionic surfactant, can have adverse effects on activated sludge OUR and nitrification reactions. The batch tests showed that NPEO inhibition to activated sludge OUR follows a similar trend to that observed for SDBS but was around 8% to 12% less for all concentrations tested and duration of exposure. Inhibition of a mixture of SDBS and NPEO at a total concentration of 60 mg/L showed lower inhibition to OUR compared with those measured for SDBS and NPEO as a single substract inhibitor.

SDBS and NPEO showed varying levels of inhibition to activated sludge capacity for nitrification. The trend observed was in agreement with SDBS and NPEO inhibition to OUR. Inhibition to nitrification was measured both in terms of reduction to oxidation of ammonia to nitrate and reduction in the production of oxidised nitrogen (nitrite + nitrate), compared with that measured for the control.

Further, the study examined the effect of the presence of both SDBS and NPEO in the influent to Sequencing batch reactors (SBRs) on mixed liquor suspended solids (MLSS), sludge volume index (SVI), chemical oxygen demand (COD) and NH4 removal. The presence of surfactants, especially higher than 20mg/L, showed significant effects on the performance of the SBRs measure in terms of those parameters. It was also observed that the effect was more pronounced using air diffusers with fine air bubbles, whereas, there was no signs of inhibition using the coarse bubble, in some cases.