Fate of surfactants in membrane bioreactors and conventional activated sludge plants

Two membrane bioreactors (MBRs) were operated at high sludge retention time (SRT) (between 30 and 75 d) in parallel to a conventional activated sludge plant (CASP) conducted at SRT = 10 d. The fate of linear alkylbenzene sulfonate (LAS), nonylphenol ethoxylates (NP(n)EO, n = 1-15), nonylphenoxy carboxylates (NP(n)EC, n = 1-2), and nonylphenol (NP) in these systems was investigated. All systems were very efficient in the removal of LAS (around 99%). The analysis of variance showed that the difference in the removal efficiency of LAS in the CASP and the MBR operated at SRT = 65-75 d (respectively 99.0 ± 0.43 and 99.8 ± 0.11) were significant (p < 0.05), confirming the importance of SRT in the removal of LAS. Comparison between the CASP and the MBRs in the removal efficiency of nonylphenolic compounds were conducted considering NP(3-15)EO, the sum of NP(1-15)EO, NP(1-2)EC, and nonylphenol (NP). In all cases MBRs were more efficient than the CASP. In the case of NP the removal was about 76 ± 7.5% for the CASP and 90% ± 12.1 and 82 ± 8.7% for the MBRs. Better performance of MBRs in the removal of nonylphenolic compounds can be attributed to a better degradation. For example, if the sum of NP(1-15)EO and NP(1-2)EC is considered, estimated biodegradation was about 48% for the CASP and 72% for MBRs.