Removal of organic matters in sedimentation sludge water under pH adjustment: characteristics and mechanisms

Removing dissolved organic matter (DOM) from sedimentation sludge water (SSW) before recycling is an effective method to reduce the impact on drinking water. The pH of SSW was adjusted from 5 to 9, followed by mixing, sedimentation, and filtration to simulate SSW treatment process. The concentration of dissolved organic carbon (DOC), value of UV₂₅₄ and specific UV₂₅₄ absorbance all increased with the increase of pH. The hydrophilicity fractionation and fluorescence excitation-emission matrix spectra showed that the hydrophilic, fulvic and humic acid-like compounds were the primary DOM in SSW at different pH conditions. These compounds were removed at acidic conditions, while adding alkali caused the release of these compounds. Fourier transform infrared spectroscopy showed that the characteristic peaks were not shifted at different pH conditions, however, the intensity of characteristic peaks related to Al and organic matter, such as Al-OH, Al-O, C = C, C = O, and COO^-, were enhanced at acidic conditions. The contents of Al and C in sludge at acidic condition were also higher than that in the unadjusted sludge sample. These results suggested that the incidental coagulation reaction was enhanced at acidic conditions, promoting the removal of DOM from SSW. Nevertheless, the intensity of characteristic peaks and contents of Al and C in sludge at alkaline conditions showed the opposite trend to that at acidic conditions, illustrating that the sludge flocs in SSW were destroyed, causing the release of DOM. Therefore, maintaining a slightly acidic pH is an effective approach for treating DOM in the recycling of SSW.