Physical-chemical pretreatment as an option for increased sustainability of municipal wastewater treatment plants

Keywords : municipal wastewater treatment, physical-chemical pretreatment, chemically enhanced primary treatment, organic polymers, environmental sustainability Most of the currently applied municipal wastewater treatment plants in The Netherlands are based on the activated sludge process and include nitrogen and phosphorus removal. Physical-chemical pretreatment might be an option to improve the environmental sustainability of these wastewater treatment facilities. Physical-chemical pretreatment is meant to separate suspended and colloidal particles in the first step of treatment. The organic concentrate of separated particles is considered a product that can be digested to produce biogas. Since a large part of the organic pollutants in wastewater is present as not-dissolved material (65-70% of COD), the pollutant load to pursuing treatment steps is consequently reduced. In the first part of the thesis the concept of physical-chemical pretreatment was evaluated for its potential to design more sustainable treatment systems. Based on a review of unit operations, ten different scenarios for wastewater treatment were designed and evaluated along environmental and financial criteria. The evaluation showed that these scenarios could be designed more energy efficient (savings of 60-75% of energy) and smaller compared to currently applied treatment systems. The total cost of treatment scenarios, which combine pretreatment with secondary biological treatment appeared to be in the same range as that of a reference scenario. The scenario evaluation also revealed two important bottlenecks, namely (i) the use of metal-based flocculating agents results in the production of a considerable amount of extra inorganic sludge and in an increased salinity of the effluent and (ii) the removal of particles is partially accompanied by the removal of biodegradable COD which may affect the denitrification capacity of a biological post treatment step. The second part of the thesis describes experimental work into the identified bottlenecks. As alternatives to metal-based flocculating agents the potential application of organic polymers was investigated. Jar tests and experiments with continuous reactors showed that particle removal efficiencies of 70-90% could be achieved by applying these polymers in combination with eighter settling or dissolved air flotation. Especially cationic, high molecular weight polyelectrolytes showed interesting characteristics for practical application as they appeared effective at relatively low doses. A polymer dosing control strategy based on continuous measurement of the influent-turbidity proved effective to create different levels of particle removal, despite large concentration fluctuations in the influent. The effect of particle removal on the elimination of nitrogen was investigated by Nitrogen Uptake Rate (NUR) tests and experiments with a pilot scale activated sludge plant. It was found that with the removal of particulate COD, the biodegradable COD of the investigated wastewater was reduced by 50%. Calculations based on the values obtained in the experiments however indicated that well-designed plants that receive wastewater with an average COD/N ratio still contain sufficient denitrification capacity to comply with the nitrogen effluent constraint at particle removal efficiencies as high as 80-90%.