Solar-based physicochemical-biological processes for the treatment of toxic and recalcitrant effluents
Abstract: Wastewater treatment is a worldwide necessity that is even more crucial to countries deprived from abundant fresh water sources or unable to afford advanced and costly processes. With this perspective, this thesis was conducted within an Egyptian-Swedish research collaboration that aimed to develop solar-based (as free renewable energy source available locally) process for hazardous wastewater treatment. A local coke factory generating a phenol-laden toxic wastewater was used as model. Solar-based biological (sunlight being used to power algal photosynthesis for oxygen production) or physicochemical (sunlight being used to power photodegradation) treatments were first tested independently before the best options were combined in a sequential process. A p-nitrophenol (PNP)-degrading Arthrobacter sp., a phenol-degrading Alcaligenes sp. and a Chlorella vulgaris were isolated from the coke wastewater plant. Alcaligenes sp. could biodegrade up to 1200 mg phenol l-1 as well as other aromatic compounds such polycyclic aromatic hydrocarbon (PAHs). Its growth on phenol was well described by the Haldane kinetic model. Arthrobacter sp. could degrade 120 mg PNP l-1 but was inhibited at concentrations above. C. vulgaris was capable to grow in the presence of 300 or 20 mg l-1 of phenol or PNP, respectively, but was totally inhibited at PNP concentrations higher than 25 mg l-1. Sunlight was advantageously used to reduce the aeration costs and improve the safety (less risk of hazardous pollutants spreading) by supporting photosynthetic oxygen production by C. vulgaris during the aerobic biodegradation of a simulated industrial wastewater (325 mg phenol l-1 and 500 mg NH4+ l-1) by Alcaligenes sp. Continuous treatment at the 3.6 d HRT (Hydraulic Retention Time) in a single stage photobioreactor supported phenol and NH4+ removals of 58 and 18%, respectively. Algae fertilization with 8 g NaHCO3 l-1 increased these values to 100 and 29%, respectively. However, biological treatment of a mixture of phenol and PNP at 100 and 50 mg l-1 or real coke wastewater under photosynthetic aeration was not possible due to algal inhibition. Hence, it was necessary to pre-treat the effluents by photochemical treatment. Microbial inhibition was also observed during biological treatment (under classical aeration) with acclimated (biofilm consortium) or non acclimated (activated sludge microflora) of a mixture of 4-chlorophenol (CP), 2,4-dichlorophenol (DCP), 2,4,6-trichlorophenol (TCP), and pentachlorophenol (PCP) at an initial concentration of 50 mg l-1 each. UV irradiation was efficient to reduce the toxicity of this mixture and thereby allow the complete biodegradation of the remaining pollutants by both inocula. Evidence was however found that the pollutants were partially photodegraded into toxic and non-biodegradable products. UV irradiation of a similar mixture of chlorophenols (CPs) initially supplied at 100 mg l-1 each even caused the release of toxic photoproducts that contributed to inhibit the subsequent biological step. Interestingly, the photoproducts released from solutions of individually supplied CPs could be subsequently biodegraded, showing that results from studies conducted on single contaminants cannot be extrapolated to mixtures. Photochemical pre-treatments (UV/TiO2/H2O2, UV/H2O2, or UV/TiO2), significantly improved the removal of all pollutants. Photochemical degradation was well described with pseudo first order kinetics and UV/TiO2/H2O2 treatment supported the highest pollutant photodegradation rates. UV/TiO2 and UV/H2O2 were efficient to detoxify both CPs mixtures (50 or 100 mg l-1) and allow the subsequent biodegradation of the remaining 4CP and DCP, yet evidence was found that recalcitrant photoproducts (some of which were chlorinated) were formed during these treatments. By comparison, combined UV/TiO2/H2O2 ? biological treatment of both CPs mixtures allowed complete pollutant removal, detoxification, dechlorination, and nearly complete COD removal. It should therefore be regarded as the safest treatment option. A fully solar-based combined photochemical - biological treatment was finally used to efficiently treat various mixtures of pollutants, as well as synthetic and real coke wastewater. UV photolysis was efficient to sufficiently detoxify a mixture of phenol, TCP and PCP and coke wastewater (1st sampling campaign) and support the subsequent biodegradation of the remaining pollutants under photosynthetic aeration. It was however inefficient to detoxify a mixture of 100 mg phenol l-1 and 50 mg PNP l-1. The combined UV/TiO2-algal based biological treatment of this mixture efficiently removed all the pollutants and completely detoxified the effluent. Based on the treatment efficiencies of different mixtures of pollutants as well as artificial or real coke wastewaters, the combined treatments tested could be ranked in the following order of decreasing efficiency: UV/TiO2/H2O2-biological > UV/TiO2-biological ~ UV/H2O2-biological > UV-biological. However, these pre-treatment steps can also be inversely ranked from the most to the least economical as UV < UV/TiO2 < UV/H2O2 < UV/TiO2/H2O2. UV/TiO2 is an interesting cost-efficient alternative (in cases not recalcitrant photoproducts are formed) but its large scale implementation remains limited by the technical difficulties in immobilizing or recycling the catalyst. More research is therefore needed to determine the most cost-efficient method in regards to the affordable risks. More research is also needed to improve algal-recovery to improve the process (biomass control, biomass reuse) and achieve satisfactory effluent quality.
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