Replacement of secondary clarification by membrane separation - results with plate and hollow fibre modules

Abstract

Submerged membrane systems can replace the final clarification step to separate mixed liquor suspended solids (MLSS) from treated wastewater in the activated sludge processes. Mixed liquor suspended solids concentrations as high as 20 g/l can be maintained compared with the typical 3–4 g/l for conventional activated sludge/secondary clarifier systems. This leads to much smaller reactor volumes. In addition, excellent, solid free effluent qualities can be achieved with this process technology. One major challenge, when applying membrane technology to municipal wastewater treatment systems, is handling the flow variations typically encountered by these systems. This paper reports about the parallel investigation of two membrane-systems installed within a 7 m3 and a 9 m3 bioreactor. One membrane system used submerged plate modules (80 m2 membrane surface), the other hollow fibre modules (83.4 m2). The emphasis of this first investigation was the hydraulic performance and elasticity of the membrane systems. At MLSS concentrations of 15 g/l and hydraulic retention times of 6 hours, flux rates of 15 l/m2 h under permanent operation and up to 30 l/m2 h for short periods of time were achieved (standardised to a temperature of 10°C). A stable operation at these rates could be achieved for a full 4 months period without chemical cleaning of the membranes. Future studies will focus on nutrient removal aspects and the energy consumption of the systems.