Membrane Bioreactor Processes to Meet Todays and Future Municipal Sewage Treatment Requirements

Several municipal wastewater treatment plants (WWTPs) in Stockholm, Sweden, will within the near future face both an increased load due to a growing population as well as more stringent effluent quality requirements. The latter mainly regarding nutrients due to Sweden‘s commitment to the Baltic Sea Action Plan and the implementation of the European water framework directive (WFD). In addition, removal of emerging substances such as pharmaceutical residues, micro plastics and antibiotic resistance are gaining more attention since WWTP effluent is the most or one of the most significant sources of such loads to the environment [1-4]. Pharmaceutical residues and other emerging substances are generally not efficiently removed in conventional WWTPs [5]. The WFD has defined a list of prioritized substances including pesticides, biocides, flameretardants and metals [6], which already today require monitoring and treatment. Several other substances, including some pharmaceuticals, are on the ‘watch list’ of emerging pollutants that may be placed on the WFD priority list. Requirements for additional treatment, in larger WWTPs, for the reduction of some pharmaceutical residues and other micropollutants (MP) could thus be expected, also in other countries than Switzerland, where such a regulation is already in place. The potential negative effects on aquatic organisms, the aquatic food-web and higher organisms, as well as the risk of increased numbers of antibiotic resistant genes in bacteria, all present a threat to our environment, health and society [7-9]. Another increasing concern for wastewater treatment are emissions of greenhouse gases (GHGs). At WWTPs, special attention is given to nitrous oxide (N2O), which is a highly potent GHG (298 times more potent than carbon dioxide (CO2 ) [10]). At incomplete nitrification and denitrification N2O can be emitted, which may cause a significant negative overall environmental impact of the treatment process [11,12]. Even though regulations may earliest come in place in some years from now, many WWTPs actively work on reducing GHG emissions from wastewater treatment processes. Besides the requirements to increase capacity, improve treatment efficiency and reduce GHG emissions, many WWTPs also face the problem that they cannot expand spatially as they are located in densely populated areas or underground.

New solutions for space-efficient, high-capacity and flexible municipal wastewater treatment processes are thus required. Stockholm Water and Waste Company (Stockholm Vatten och Avfall), Sweden’s largest water service organization, is directly facing the above problems of space limitation, increased capacity need and stricter effluent requirements at the Henriksdal WWTP in Stockholm. As a result, the existing conventional activated sludge process (CAS) will be converted to a Membrane Bioreactor (MBR), doubling the capacity by using existing process volumes only. The new process will be the world’s largest MBR facility with a capacity of 1.6 million PE (predicted load year 2040).

MBRs combine the biological activated sludge process with membrane separation, which provide distinct advantages over the CAS. Advantages include a significantly better effluent (permeate) quality regarding particles, disinfection capabilities due to the membrane pore size, higher volumetric loading due to higher sludge concentrations in the biology, reduced footprint and process flexibility towards influent changes. Even the treatment of MP may be more efficient using MBRs compared to traditional treatment systems. This is partly explained by the fact that MP attached to particles can effectively be removed by filtration whereas dissolved MP can be degraded more effectively because of the higher biological activity in a MBR process. In addition, a more efficient polishing treatment compared to CAS can be achieved [11,13-18]. Drawbacks of the process are the high energy use for aeration and the use of cleaning chemicals in the filtration step to curb fouling and scaling on the membrane surface, which reduces the permeability of the membranes.

MBRs have been used for a number of decades but only in the last decade, MBRs gained more attention for the treatment of both municipal and industrial wastewater. This is mainly due to a significant cost reduction of membranes and process development decreasing energy requirements [19-23].

The aim of this research work is to investigate the MBR technology concerning the overall holism and resource efficiency towards some of the most central treatment aspects including nutrient removal, removal of micropollutants and minimizing of GHG emissions. Through actual pilotscale experiments, the paper describes the performance of the studied system under various test periods defined to meet present and future requirements of the growing region of Stockholm, Sweden.