Sulfate-rich wastewaters emerge due to increasing seawater intrusion, using seawater as alternative water source in coastal areas and sulfur-laden industrial effluents, posing a major threat to mainstream wastewater treatment arising from unpreventable production of sulfide and associated impact on functional bacteria and reactor corrosion. Given its high resilience against adverse environmental conditions, aerobic granular sludge (AGS) could mitigate the sulfide and sulfate impact compared with the activated sludge. This research investigated the feasibility of using AGS in the treatment of sulfate-laden wastewater with focuses on the impact of sulfate and sulfide on nutrients (Nitrogen-N and Phosphate-P) removal, sludge granulation and revelation of the metabolic relationships in the above aspects. Meanwhile, AGS is superior to activated sludge in terms of resource recovery owing to the enrichment of biomass and various by-products (e.g., extracellular polymeric substances (EPSs)). The feasibility of recovering new resources with higher economic value and market demand from AGS for sulfate-laden wastewater treatment, primary producers as well as metabolic pathways of their bio-synthesis in microbes were secondly studied in this research. Five sequencing batch reactors (SBR) with varying sulfate concentrations and alternating anoxic/aerobic operational condition were applied to study the feasibility of sulfate-containing wastewater treatment by AGS process. Results indicated that sulfate exerted a negligible influence on chemical oxygen demand (COD) removal, while phosphate removal was enhanced from 12 to 87% with an increase in sulfate concentration from 0 to 200 mg/L. However, significant deterioration of phosphate removal was witnessed when sulfate concentration exceeded 200 mg/L due to the intensive consumption of energy by the sulfide detoxification process, leaving insufficient energy for Polyhydroxyalkanoate (PHA) synthesis and phosphate uptake. Granulation was accelerated with increasing sulfate levels from 0 to 1000 mg/L by enhanced production of N-Acyl homoserine lactones (AHLs), a kind of quorum sensing (QS) auto-inducer. This study demonstrated interactions among sulfate metabolism, nutrients removal and granulation, as well as confirmed the feasibility of using AGS process for sulfate-laden wastewaters treatment when sulfate content is below 200 mg/L. Sulfated polysaccharides (SPs) were detected in the EPSs of the above AGS. The SPs, especially those having bio-activities, are a group of valuable and scarce raw materials commonly used in dyeing, spinning, rubber-making, paper-making, food-manufacturing, cosmetic and pharmaceutical industries. Identifying the detailed components, bio-activities and primary producers of SPs in AGS for sulfate-containing wastewater treatment are essential and fundamental for SPs recovery. Sludge samples cultivated in alternative aerobic/anoxic condition without and with sulfate, sludge samples acclimated in intensive sulfate-reducing and sulfide-oxidizing conditions, sulfur-associated phosphorus accumulating organisms (S-PAO) and sludge taken from a local saline wastewater treatment plant were analyzed in this study. SPs contents showed significant variance from 0 to 42% among different sludge samples, with main components including both bioactive species, i.e., fucoidan, carrageenan and heparin, as well as non-bioacitve specie, i.e., alginate. Anti-angiogenesis, anti-coagulant and anti-oxidant bioactivities of all SPs extracts were analyzed. The presence of bioactive SPs in sludge systems for sulfate-containing wastewater treatment was discovered, and the purity of recovered SPs was verified. In addition, cultivation conditions showed significant influence on both total content and component of SPs produced in the sludge system. In order to determine the main SPs producers and how cultivation conditions influenced SPs bio-synthesis, analysis of microbial community structures, contents of monosaccharide substrates and activities of critical enzymes for SPs bio-synthesis of four selectively cultivated sludge samples in previous work, aimed at harvesting different dominant microbes, (i.e., aerobes (mainly glycogen accumulating organisms(GAOs), sulfate-reducing bacteria (SRB) and sulfide-oxidizing bacteria (SOB)), were conducted. Successful cultivation of four sludge samples (ASS sample cultivated under alternating anoxic/aerobic condition with sulfate, AS sample cultivated under alternating anoxic/aerobic condition without sulfate, SR sample grown in intensive sulfate-reducing condition, and SO sample grown in intensive sulfide-oxidizing condition) with different dominant microbes was confirmed by taxonomic analysis. Considerable amounts of monosaccharide substrates were produced in both ASS and AS samples, however, activities of enzymes involved in subsequent SPs synthesis decreased significantly in AS sample owing to the scarce of sulfate. An overall repression on both monosaccharide production and enzyme activities were witnessed in both SR and SO samples, which explained the strong inhibitions on SPs synthesis. This research revealed that a sufficient supply of sulfate is essential for SPs bio-synthesis, and aerobes, other than SRB and SOB, are main producers of SPs in saline wastewater treatment systems due to potent inhibition of enzyme activities related to the majority of steps for SPs production. However, in-depth mechanism of biosynthesis of SPs is still unknown and remains to be investigated in the future work as well as the process development for SPs extraction, purification and separation towards high yields and purity.
| Date of Award | 2019 |
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| Original language | English |
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| Awarding Institution | - The Hong Kong University of Science and Technology
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