Effects of dissolved organic matter removal and molecular transformation in different water treatment processes on formation of disinfection byproducts

Structure and molecular-level transformation for oxidation of effluent organic matters by manganese oxides

As important organic components in water environments, effluent organic matters (EfOMs) from wastewater treatment plants are widely present in Mn-rich environments or engineered treatment systems. The redox interaction between manganese oxides (MnOx) and EfOMs can lead to their structural changes, which are crucial for ensuring the safety of water environments. Herein, the reactivities of MnOx with EfOMs were evaluated, and it was found that MnOx with high specific surface area, active high-valent manganese content and lattice oxygen content (i.e., amorphous MnO2) possessed stronger oxidizing ability towards EfOMs. Accompanying by EfOMs oxidation, Mn(IV) and Mn(III) were reduced into Mn(II), with Mn(III) as the significant active species. Through molecular-level transformation analysis by ultrahigh mass spectrometry (FT-ICR MS), the highly reactive compounds in EfOMs were clearly determined to be that with more aromatic and unsaturated structures, especially lignin-like compounds (the highest content in EfOMs (over 60 %)). EfOMs were oxidized by amorphous MnO2 into products with lower humification index (0.60 vs. 0.46), smaller apparent molecular weight (386.94 Da vs. 368.68 Da), and higher biodegradability (BOD5/COD: 0.12 vs. 0.78). This finding suggested that redox reactions between MnOx and EfOMs might alter their abiotic and biotic behaviors in receiving water environments.

Untangling the interplay of dissolved organic matters variation with microbial symbiotic network in sludge anaerobic fermentation triggered by various pretreatments

Various pretreatments are commonly adopted to facilitate dissolved organic matter (DOM) release from waste activated sludge (WAS) for high-valued volatile fatty acids (VFAs) promotion, while the interplay impact of DOM dynamics transformation on microbial population and metabolic function traits is poorly understood. This work constructed "DOM-microorganisms-metabolism-VFAs" symbiotic ecologic networks to disclose how DOM dynamics variation intricately interacts with bacterial community networks, assembly processes, and microbial traits during WAS fermentation. The distribution of DOM was altered by different pretreatments, triggering the release of easily biodegradable compounds (O/C ratio > 0.3) and protein-like substance. This alteration greatly improved the substrates biodegradability (higher biological index) and upregulated microbial metabolism capacity (e.g., hydrolysis and fatty acid synthesis). In turn, microbial activity modifications augment substance metabolism level and expedite the conversion of highly reactive compounds (proteins-like DOM) to VFAs, leading to 1.6-4.2 fold rise in VFAs generation. Strong correlations were found between proteins-like DOM and topological properties of DOM-bacteria associations, suggesting that high DOM availability leads to more intricate ecological networks. A change in the way communities assemble, shifting from stronger uniform selection in pH10 and USp reactors to increased randomness in heat reactor, was linked to DOM composition alterations. The ecologic networks further revealed metabolic synergy between hydrolytic-acidogenic bacteria (e.g., Bacteroidota and Firmicutes) and biodegradable DOM (e.g., proteins and amino sugars) leading to higher VFAs generation. This study provides a deeper knowledge of the inherent connections between DOM and microbial traits for efficient VFAs biosynthesis during WAS anaerobic fermentation, offering valuable insights for effective WAS pretreatment strategies.

Suppressing Organic Bromine but Promoting Bromate: Is the Ultraviolet/Ozone Process a Double-Edged Sword for the Toxicity of Wastewater to Mammalian Cells?

Brominated byproducts and toxicity generation are critical issues for ozone application to wastewater containing bromide. This study demonstrated that ultraviolet/ozone (UV/O3, 100 mJ/cm2, 1 mg-O3/mg-DOC) reduced the cytotoxicity of wastewater from 14.2 mg of pentol/L produced by ozonation to 4.3 mg of pentol/L (1 mg/L bromide, pH 7.0). The genotoxicity was also reduced from 1.65 to 0.17 μg-4-NQO/L by UV/O3. Compared with that of O3 alone, adsorbable organic bromine was reduced from 25.8 to 5.3 μg/L by UV/O3, but bromate increased from 32.9 to 71.4 μg/L. The UV/O3 process enhanced the removal of pre-existing precursors (highly unsaturated and phenolic compounds and poly aromatic hydrocarbons), while new precursors were generated, yet the combined effect of UV/O3 on precursors did not result in a significant change in toxicity. Instead, UV radiation inhibited HOBr concentration through both rapid O3 decomposition to reduce HOBr production and decomposition of the formed HOBr, thus suppressing the AOBr formation. However, the hydroxyl radical-dominated pathway in UV/O3 led to a significant increase of bromate. Considering both organic bromine and bromate, the UV/O3 process effectively controlled both cytotoxicity and genotoxicity of wastewater to mammalian cells, even though an emphasis should be also placed on managing elevated bromate. Futhermore, other end points are needed to evaluate the toxicity outcomes of the UV/O3 process.

Control of chlorination disinfection by-products in drinking water by combined nanofiltration process: A case study with trihalomethanes and haloacetic acids

Disinfection by-products (DBPs) are prevalent contaminants in drinking water and are primarily linked to issues regarding water quality. These contaminants have been associated with various adverse health effects. Among different treatment processes, nanofiltration (NF) has demonstrated superior performance in effectively reducing the levels of DBPs compared to conventional processes and ozone-biological activated carbon (O3-BAC) processes. In this experiment, we systematically investigated the performance of three advanced membrane filtration treatment schemes, namely "sand filter + nanofiltration" (SF + NF), "sand filter + ozone-biological activated carbon + nanofiltration" (SF + O3-BAC + NF), and "ultrafiltration + nanofiltration" (UF + NF), in terms of their ability to control disinfection by-product (DBP) formation in treated water, analyzed the source and fate of DBP precursors during chlorination, and elucidated the role of precursor molecular weight distribution during membrane filtration in relation to DBP formation potential (DBPFP). The results indicated that each treatment process reduced DBPFP, as measured by trihalomethane formation potential (THMFP) and haloacetic acid formation potential (HAAFP), with the SF + O3-BAC + NF process being the most effective (14.27 μg/L and 14.88 μg/L), followed by the SF + NF process (21.04 μg/L and 16.29 μg/L) and the UF + NF process (26.26 μg/L and 21.75 μg/L). Tyrosine, tryptophan, and soluble microbial products were identified as the major DBP precursors during chlorination, with their fluorescence intensity decreasing gradually as water treatment progressed. Additionally, while large molecular weight organics (60-100,000 KDa) played a minor role in DBPFP, small molecular weight organics (0.2-5 KDa) were highlighted as key contributors to DBPFP, and medium molecular weight organics (5-60 KDa) could adhere to the membrane surface and reduce DBPFP. Based on these findings, the combined NF process can be reasonably selected for controlling DBP formation, with potential long-term benefits for human health.

Wastewater from natural gas Cansolv desulfurization process: Comprehensive characterization and effective removal of organic compounds

The wastewater generated by the solvent amine desulfurization process in natural gas purification plants is characterized by its recalcitrant organic compounds and high salinity. Without effective treatment, it has the potential to inflict severe environmental harm. The composition of organic matter, however, exerts a profound influence on the outcomes of oxidation processes. To rectify the limitations associated with indiscriminate oxidation that yields suboptimal results, this investigation meticulously performed a molecular-level analysis of organic matter. Based on the organic matter composition in the influent, this study compared the treatment efficacy of three oxidation processes and determined O3/H2O2-Fenton as the optimal joint approach. After O3/H2O2 oxidation, long-chain unsaturated organic compounds (C > 40,DBE > 20) underwent degradation into short-chain aldehydes and low-molecular-weight fatty acids, with priority given to reactions involving CC, CO, and OH over CH reactions. Subsequent Fenton oxidation effectively removed the refractory organics (CHOS, CHONS) and significantly reduced the diversity of organic matter (from 7730 to 4237). The carboxylation, demethylation, and dehydrogenation reactions further facilitated the removal of recalcitrant organic compounds. In light of these findings, this study substantiates that the conversion of extended-chain unsaturated compounds into abbreviated-chain saturated compounds within the system through O3/H2O2 oxidation significantly enhances the subsequent efficacy of Fenton oxidation in organic matter removal. These insights offer valuable perspectives for the efficient remediation of analogous high-salinity organic wastewater scenarios.

Photochemical Transformation of Dissolved Organic Matter in Surface Water Augmented the Formation of Disinfection Byproducts

Sunlight may lead to changes in disinfection byproducts (DBPs) formation potentials of source water via transforming dissolved organic matter (DOM); however, the underlying mechanisms behind these changes remain unclear. This work systematically investigated the effect of photochemical transformation of DOM from reservoir water (DOMRe) and micropolluted river water (DOMRi) after 36 h of simulated sunlight irradiation (equivalent to one month under natural sunlight) on DBPs formation. Upon irradiation, high molecular weight (MW) and aromatic molecules tended to be mineralized or converted into low-MW and highly oxidized (O/C > 0.5) ones which might react with chlorine to generate high levels of DBPs, resulting in an elevation in the yields (μg DBP/mg C) of almost all the measured DBPs and the quantities of unknown DBPs in both DOM samples after chlorination. Additionally, DOMRi contained more aromatic molecules susceptible to photooxidation than DOMRe. Consequently, irradiated DOMRi exhibited a greater increase in the formation potentials of haloacetonitriles, halonitromethanes, and specific regulated DBPs, with nitrogenous DBPs being responsible for the overall rise in the calculated cytotoxicity following chlorination. This work emphasized the importance of a comprehensive removal of phototransformation products that may serve as DBPs precursors from source waters, especially from micropolluted source waters.