pH adjustment improves the removal of disinfection byproduct precursors from sedimentation sludge water

Recycling sedimentation sludge water (SSW) supernatant to the head of drinking water treatment plants also recycles a large fraction of dissovled disinfection byproduct (DBP) precursors, contributing to the overall DBP load of the finished water. pH adjustment of SSW before release to settling ponds may result in incidental enhanced coagulation due to a significant amount of residual coagulant present in SSW, potentially removing DBP precursors. We adjusted the pH of SSW to 5, 6, 7, 8, and 9, and simulated recycling via mixing, sedimen-tation, and filtration. Samples settled at pH 5 or 6 formed the least DBPs, dependent on species, illustrating that buffering pH at slightly acidic conditions is an effective DBP precursor treatment strategy during SSW recycling. Carbonaceous DBP formation increased with increasing pH. An overall decrease in dissolved organic carbon at acidic conditions was observed, likely due to some enhanced coagulation and sedimentation of dissolved com-pounds. For nitrogenous DBPs (N-DBPs), samples at pH 5, 8, and 9 had greater formation than at pH 6 and 7. Fourier transform ion cyclotron resonance mass spectrometry revealed that adjusting pH to 8 and 9 increased the presence of highly unsaturated molecules, while the unsaturated nitrogen-containing formulae were removed at pH 6 and 7. N-DBP formation potential and the intensity (i.e., concentration) of CHON formulae with double bond equivalent > 10 were well correlated, demonstrating that alkaline pH SSW should be avoided to minimize formation/release of nitrogenous compounds and subsequent N-DBP formation.