PILOT-PLANT STUDIES ON FILTRATION OF RAW SEWAGE USING FLOATING FILTER MEDIA AND MULTIPLE FILTER COLUMN INLETS

The characteristics and performance of solid-liquid separation of raw sewage using floating media and multiple inlets were studied using pilot-scale equipment. Particles over 7 mum, representing approximately 80% of SS and 50% of BOD in raw sewage were efficiently removed. Smaller particles and soluble components, comprising a large part of total BOD, were difficult to remove by filtration. The maximum SS removal ratio (SSRR) and SS quantity removed per unit filter surface (SSR) was achieved using a medium size of 5.5 mm and a filtration rate of 150 mum/d. Inlet switching to a lower position significantly extended the filter run lengths and increased the total SS removed by filtration. Using 5.5 mm filter particles and a 150 m/d filtration rate, 80-89% of SS was removed, 38-42 kg of SS was removed per unit filter sectional area, and filter runs were 55-65 hours. The backwash-filtrate ratio was around 2%. The space requirement of this system will be significantly smaller than the ordinary primary treatment system judging from the trial design of a prototype plant.