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Both ozone (O 3 ) and UV/O 3 treatment processes can effectively remove organic matter in the flocculated membrane filtration concentrate from landfill leachate, but the ozonation byproducts (OBPs) generated in the processes remain unknown. Using electrospray ionization-coupled Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS), this study investigated the molecular characteristics of unknown OBPs and their formation mechanisms during the treatment of flocculated nanofiltration concentrate (FNFC) using the O 3 and UV/O 3 processes. The results showed that after being treated by the O 3 and UV/O 3 processes, the average value of the oxygen-to-carbon ratio (O/C avg ) in the FNFC organic matter increased substantially from 0.49 to 0.61-0.64 and 0.63-0.71, respectively, with an O 3 dosage of 13.4-54.4 mg/min. The main OBPs were CHO and CHON compounds, which were mainly produced through oxygenation (+O 2 /+O 3 and -H 2 +O 2 ), oxidative deamination (-NH 3 +O 2 ), decyclopropyl (-C 3 H 4 ), and deisopropyl (-C 3 H 6 ) reactions. The hydroxyl radical ( • OH) can intensify these reactions, resulting in an abundance of OBPs with a high oxidation degree and low molecular weight. OBPs at five m / z values were fragmented and analyzed with tandem mass spectrometry, and abundant hydroxyl groups, carboxyl groups, and carbonyl groups were tentatively identified, presenting a potential toxicity to aquatic organisms. Due to the high molecular diversity of the OBPs in FNFC, their lower Δ G Cox o compared to natural fulvic acid, and potential toxicity, their impact on the water environment should be given more attention.

In practice, mature landfill leachate and incineration (young) leachate are mixed to improve the biodegradability and enhance biological treatment performance. However, the ratio of mature-to-young leachates greatly influences MBR treatment efficiency and microbial community structure. This study investigated the treatment efficiency and microbial community structure of full-scale MBR systems operated under two mix ratios, mature leachate: young leachate = 7:3 (v/v, denoted as LL) and 3:7 (v/v, denoted as IL). LL group showed lower Cl - and COD concentrations but a higher aromatic organic content comparing to IL group, and the COD and UV 254 removals for LL were significantly lower than those for IL by MBR treatment. Microbial community structures were similar in both groups at phylum level, with dominant phyla being Proteobacteria (23.8%-32.3%), Bacteroidetes (15.25%-20.7%), Chloroflexi (10.5%-23.1%), and Patescibacteria (9.9%-13.2%). However, the richness and diversity of LL group were lower, and differences were observed at lower taxonomy levels. Results indicated that salinity mainly changed the structure of microbial community, resulting in greater abundance of salt-tolerant microbials, while refractory organics affected microbial community structure, and also led to decreased diversity and metabolic activity. Therefore, in mixed leachates biological treatment, a higher young leachate ratio is recommended for better organics removal performance. PRACTITIONER POINTS: The trade-off between refractory organics and salinity in mixed leachate treatment should be paid attention. Refractory organics reduced alpha and functional diversities of microorganisms. Mixed leachate with a higher young leachate ratio reached a better organic removal., (© 2021 Water Environment Federation.)