Microfiltration of municipal wastewater for disinfection and advanced phosphorus removal

Abstract

The objective of Research Project 02 WA 9253/4 on “Advanced Treatment of Municipal Wastewater Microfiltration of municipal wastewater for disinfection and advanced phosphorus removal” which is supported by the BMBF (Federal Ministry for Education, Science, Research and Technology) is to show whether microfiltration (MF) is a technically feasible and economically competitive process for disinfection and phosphorus removal of secondary effluent. For bacteria and phosphorus removal, three different microfiltration systems (systems with flat sheet, rube and hollow-fibre modules) with a pore size of 0.2 μm are tested in small-scale pilot plants to find out whether they are suitable for municipal wastewater treatment. The most suitable system will afterwards be tested in one full-scale unit to obtain operational data. The monitoring program with the small-scale MF plants using the final effluent of the Berlin-Ruhleben wastewater treatment plant started in November 1993 and the results obtained so far can be summarized as follows.Total coliforms, E. coli, faecal streptococci and salmonella are removed to levels below the detection limit, less than 1 cfu/100 ml in the effluent of all three MF plants.Coliphage — as a surrogate organism for enterovirus — are significantly reduced with a 2–3 log removal, which means that the limit value for enterovirus laid down in the EU Bathing Water Directive can be met in the effluent of the MF plants.The average concentrations for total phosphorus (PT) in the effluent are 60 μ/l for the Memcor and the DOW units and 90 μg/l for the Starcosa unit without the use of precipitants.With a low ferric dosage of 0.014 mol/m3 prior to the MF, the average effluent PT concentrations of all three MF units are lower than the target concentration of 50 μg/l (no polymer feed).With a specific energy consumption of about 0.2 kWh/m3 filtrate the dead-end MF (Memcor) requires at least five times less energy than the cross-flow MF. Based on the energy consumption dead-end MF should be preferred if large volumetric flows of wastewater with a low concentration of solids have to be treated.Because of unfavourable energy consumption the tests with the cross-flow MF have been discontinued.When using MF systems in the final effluent of wastewater treatment plants, evidence must be produced in a full-scale MF unit to demonstrate that microfiltration is really suitable for practical application. This as well as a reliable calculation of investment and operating costs are the main objectives of further investigations within the framework of this research project.