Municipal wastewater treatment by microsieving, microfiltration and forward osmosis : Concepts and potentials

University dissertation from Department of Chemical Engineering, Lund University

Abstract: Conventional wastewater treatment plants are designed for treating manmade wastewater (e.g., from households and industries) and to protect the environment (e.g., receiving water bodies) and humans from adverse effects.The objective of this work was to investigate the feasibility of treating municipal wastewater without a biological treatment step by applying different separation processes, such as microsieving, microfiltration and forward osmosis. The scope of this work was to treat municipal wastewater with a lower area demand while meeting the Swedish wastewater discharge requirements and allowing for the integration of the new separation techniques with existing full-scale wastewater treatment plants. To achieve these goals, pilot-plant and bench scale studies were conducted using raw municipal wastewater on-site at a full-scale wastewater treatment plant.Two different treatment concepts were identified to be feasible for municipal wastewater treatment based on the experimental findings. The first concept comprised coagulation and anionic flocculation before microsieving with subsequent microfiltration. The second concept only included microsieving and forward osmosis. Both concepts were evaluated for their specific electricity, energy and area demands, including sludge treatment, and were compared with five existing conventional wastewater treatment plants.Both concepts complied with the Swedish wastewater discharge demands for only small- and medium-sized wastewater treatment plants because up to only 65% of the nitrogen was retained. Nevertheless, both concepts achieved high retentions, with ≥96% for biochemical oxygen demand, ≥94% for chemical oxygen demand, and ≥99% for total phosphorus. Furthermore, the evaluation of both concepts showed that the specific electricity demand was 30% lower than the average specific electricity demand for 105 traditional Swedish wastewater treatment plants with population sizes of 1 500-10 000. In addition, the specific area demand could be reduced by at least 73% for existing wastewater treatment plants supporting the same population or a population of equivalent magnitude. Moreover, the results indicated that the new method had positive effects on electricity and energy due to the increased biogas potential compared to conventional wastewater treatment.