Drinking water treatment sludge (DWTS) is a waste by-product from water treatment plants where aluminum and iron salts are the most commonly used coagulants. DWTS was reused to investigate the effects of DWTS on the digestion liquid quality and microorganism activity of excess activated sludge (EAS). DWTS with four suspended solid (SS) concentrations (0%, 2%, 5% and 10%) was added to EAS which was sampled during aerobic and anaerobic digestion processes, then batch tests were carried out which followed the coagulation-flocculation process. It was found that DWTS can improve total dissolved nitrogen (TDN) and dissolved phosphorus (DP) removal efficiencies for anaerobic EAS. The highest removal efficiency of TDN (29.97%) as well as DP (55.38%) was observed when DWTS dosage was SS = 10%. The release of dissolved organic matter (DOM) by DWTS could increase dissolved organic carbon (DOC) concentration and lead to the accumulation of non-biodegradable humic acid-like substance in aerobic and anaerobic digestion liquid. The dehydrogenase activity (DHA) values of anaerobic EAS were higher than aerobic EAS. DWTS could reduce DHA for both EAS. These results indicate that potential risk of release of DOM should be considered when reusing DWTS in future research.

This research focuses on characterizing the dissolved organic matter found at water treatment plants with closed systems. Recycled water generated as a by-product of water treatment is added to raw water in those systems. The dissolved organic matter in the raw water was found to be higher in summer than in winter, but the water treatment process was able to produce purified water of the similar quality in both seasons. The recycled water contained mostly low molecular weight and protein-like substances, and this composition was different from that of the raw water, which mainly contained humic-like substances. The recycled water did not influence the concentration of humic-like substances or the molecular weight distribution in the influent water.

Rainwater is a potential source of drinking water, but has various components of dissolved organic matter (DOM). DOM is a reservoir of potential hazards in drinking water. Therefore, a new method is required to purify rainwater as a drinking water source in terms of DOM aspects. A radio-frequency (RF) treatment system is introduced here to purify source water with a small possibility of contamination. RF is generated by applying a frequency of 1.5 MHz through a glass reactor with a diameter of 2 mm which is wrapped by a 2 mm copper wire. The results demonstrate that UV260 value and dissolved organic carbon (DOC) are reduced during RF treatment. DOC was reduced by a lower amount compared to UV260, suggesting the partial transformation of bio-refractory DOM. A fluorescence excitation-emission matrix showed that humic-like substances in rainwater were reduced faster than protein-like ones, indicating that humic-like substances are susceptible to reduction by RF treatment. The results offer information on the use of RF treatment in a rainwater purification process for the production of drinking water.

The characteristics of dissolved organic matter (DOM) that formed during the aerobic and anaerobic incubation of drinking water treatment sludge stored at different temperatures (5 °C, 20 °C, 40 °C) for long periods (7, 14, and 21 days) were investigated. Anaerobic incubation at high temperatures with prolonged storage was found to result in higher organic content than aerobic incubation (3.6–6.8 times at 40 °C). The high temperatures caused changes in the DOM fractions, with humic-like substances mainly formed in aerobic incubation and protein-like substances in anaerobic incubation. Results showed that the fluorescence intensity of humic-like and protein-like substances increased by 45% and 22%, respectively, at the end of the anaerobic incubation period. The UV-absorbing DOM constituents in aerobic incubation had lower molecular weights and were more heterogeneous than those in anaerobic incubation.

The recycling process is applied in many water treatment plants (WTPs), although this process can lead to adverse effects. The effect of the recycling process on the characteristics of dissolved organic matter was evaluated based on a fluorescence excitation-emission matrix using the peak-picking technique and self-organizing map (SOM). In this study, an evaluation of two WTPs, one with and one without a recycling system, was carried out. Both WTPs show moderate efficiency during the coagulation–flocculation process in removing DOC, fulvic acid-like, humic acid-like, and tryptophan-like substances. The recycling process causes increased values of fulvic acid-like, humic acid-like, and tryptophan-like substances and specific ultraviolet absorbance (SUVA) after the filtration process of about 31.0%, 35.7%, 22.2%, and 6%, respectively. Meanwhile, the WTP without recycling showed a reduction in the level of fulvic acid-like, humic acid-like, and tryptophan-like substances and SUVA by 23.3%, 52.9%, 27.8%, and 21.1%, respectively. Moreover, SOM analysis based on the peak-picking technique can determine differences in sample clusters due to the recycling process.