Effect of sodium dodecyl benzene sulphonate (SDBS) on nitrogen removal in activated sludge processes using sequencing batch reactors (SBRs) and a pilot plant activated sludge of modified Ludzack-Ettinger (MLE) configuration

Municipal wastewater treatment plants generally utilise biological activated sludge processes to remove organic compounds and nitrogen. The biological nitrogen removal (BNR) occurs in two steps, nitrification (ammonium removal) and denitrification (nitrates removal). This study focused on a medium-sized wastewater treatment plant (WTP) which experienced poor nitrification for years, mainly when the weather temperature drops. Further investigations showed that the wastewater influent had high concentrations of surfactants during the same period, indicating a relationship between the WTP performance and presence of surfactants in the influent. <br><br>The aim of this study was to assess the effect of the anionic surfactant, sodium dodecyl benzene sulphonate (SDBS) on nitrification in activated sludge systems, under batch and continuous flow conditions. SDBS was selected as a model compound because it is the most used surfactant in large range of detergent and cleaning products. Also, many researchers used SDBS to assess the effect of surfactants on activated sludge activities, e.g. oxygen uptake rate. The batch tests were carried out according to the standard method for assessing the inhibition of nitrification of activated sludge micro-organisms by chemicals and wastewaters. Continuous flow conditions was investigated using bench-scale activated sludge sequencing batch reactors (SBRs) fed with synthetic wastewater and a pilot-scale activated sludge system fed with domestic wastewater.<br><br>Using batch test, the results showed that 30 mg/L concentration of SDBS led to a 9% inhibition of ammonium-nitrogen (NH4-N) removal.<br><br>The lab scale SBRs were operated under typical activated sludge process conditions, e.g. sludge retention time (SRT) of 11 days, and hydraulic retention time (HRT) of 16 hours. The influent to the SBR was synthetic wastewater spiked with SDBS at designated concentrations of 10, 20 and 30 mg/L. The results obtained showed SDBS below 30 mg/L had no effect on the performance of the SBRs, whereas at 30 mg/L SDBS, NH4-N and COD removal decreased by 82% and 34%, respectively. <br><br>Improving denitrification by dosing sugar into the anoxic tank as a carbon source was also assessed using the pilot plant. This caused nitrate-nitrogen (NO3-N) levels in effluent to fall from 34 mg/L to around 13 mg/L, which was similar to BioWin software simulated effluent NO3-N of 10.9 mg/L.<br><br>The effect of SDBS on activated sludge using pilot plant at low concentration of 10 mg/L SDBS were in agreement to the SBRs results, indicating no long-term inhibition of nitrification and COD removal. However, a significant impact was observed at 30 mg/L, where NH4-N and COD removal decreased by more than 50% and 20%, respectively during the first two SRT. The results meant that bench-scale reactors can be used to assess the surfactant’s effect on activated sludge process but the influence varied at different scale. In addition, although the pilot plant performance recovered almost after two SRT (indicating acclimatisation to the surfactant), the poor performance during the first 20 days was crucial as NH4-N and TN levels in the plant’s effluent were high and exceeded the license permit. To avoid and minimise inhibition problems, operational changes such as varying SRT or internal recycling can be trialled, in the future, to mitigate the poor performance during this period.