Infoscience

Thesis

Micropollutant removal from municipal wastewater: from conventional treatments to advanced biological processes

Many micropollutants present in municipal wastewater, such as pharmaceuticals and pesticides, are poorly removed in conventional wastewater treatment plants (WWTPs), and may generate adverse effects on aquatic life. The objective of this thesis was to study and develop various options to improve micropollutant removal from municipal wastewaters. Various technologies were investigated, from conventional biological treatments to advanced physico-chemical and biological processes such as ozonation, activated carbon adsorption, enzymatic bio-oxidation with laccase and biodegradation with white-rot fungi. The potential of ozonation and powdered activated carbon (PAC) adsorption was assessed with two large-scale pilot systems at Lausanne WWTP. Micropollutants were removed on average over 80%, with an average dose of 5.7 mg/l O3 or 10 to 20 mg/l PAC. Both advanced treatments led to a clear reduction in toxicity of the effluents, and appeared to be feasible in large municipal WWTPs. The role of nitrification on micropollutant removal in WWTPs was investigated with aerobic granular sludge reactors, operated with or without nitrification. Out of the 36 micropollutants studied, 5 were significantly better removed in the reactor with nitrification. For the other pollutants, aerobic heterotrophic microorganisms played probably a role much more important than nitrifying bacteria. The potential of laccase, an oxidative enzyme produced by many white-rot fungi and bacteria, was assessed for the removal of 39 micropollutants. 9 were oxidized by laccase alone, and three others were degraded in case of addition of a mediator. Despite the limited range of pollutants degraded, laccase (with or without mediators) was able to oxidize several substances of concern. The influence of pH, temperature, laccase concentration and reaction time on the oxidation kinetics by laccase of four micropollutants was investigated. All four factors have a significant effect on the micropollutant oxidation with the greatest influence shown by pH. Optimal conditions for micropollutant oxidation ranged between pH 4.5 to 6.5 and between 25°C to more than 40°C. The influence of pH, mediator, enzyme and pollutant concentrations on sulfamethoxazole and isoproturon oxidation kinetics with laccase-mediator systems (LMS) was investigated. The mediators were consumed during the reaction (no catalytic cycles observed). Faster oxidation kinetics were observed at lower pH, but also higher mediator/pollutant ratios were required. Transformation product mixtures were always less toxic than untreated pollutants. LMS appears to be a promising option to treat concentrated and potentially toxic effluents, but does not seem adapted for the treatment of low micropollutant concentrations in municipal wastewaters. Finally, a sequential batch fungal filter (SBFF), composed of woodchips as substrate and support for the mycelium of Pleurotus ostreatus, was designed and operated during more than 140 d with unsterile wastewaters. A wide range of micropollutants was removed well by a combination of fungal and microbial degradation and adsorption. The SBFFs were able to compete with ozonation and PAC adsorption regarding the average removal efficiency (up to 82%) of 27 micropollutants in municipal wastewaters. This thesis opened new perspectives regarding biological treatment of micropollutants in wastewater, highlighting also the challenges of applying fungal and oxidative enzyme treatments in WWTPs.

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