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.
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