Large scale tertiary filtration – results and experiences from the discfilter plant at the Rya WWTP in Sweden

This paper summarizes the results and experiences from the full-scale discfilter plant at the Rya WWTP. In 2010 the WWTP was extended with post-denitrification and a discfilter plant for tertiary filtration of the effluent in order to meet the new discharge limits of 10 mg total nitrogen and 0.3 mg/l of total phosphorus. The disc filters receive effluent from both the moving bed biofilm reactor (MBBR) and the secondary settlers. The disc filters are equipped with filter cloths with 15 µm pore openings. The concentration of suspended solids was generally kept below 5 mg/l and total phosphorus was <0.2 mg/l for approximately 60% of the days during the experimental period, with generally lower concentration of particles in the effluent from the disc filters during the summer. The mass load of suspended solids from the MBBR was higher compared to the load from the secondary settlers but this did not influence the concentration of suspended solids in the effluent from the disc filters. The particles in the MBBR effluent are larger and easier to filter compared to the secondary settler effluent which contain a larger number of small particles. During passage through the disc filters, some particles break-up leaving a larger number of particles (1–5 µm) in the effluent. Due to their small mass, this does not affect the effluent suspended solids concentration significantly. The removal of indicator and pathogenic microorganisms was only marginal. Since the discfilter plant has been placed in operation, the operational strategies have improved (e.g. more frequent cleaning of the filter cloths) which has increased the treatment capacity. The study demonstrates successful operation of a large discfilter plant with large variation in flow and particle loading.