Phosphorus removal in completely mixed systems

Research was carried out in 2 pilot activated sludge plants with completely mixed systems. Both plants were fed with settled domestic wastewater at a sludge loading of 400 and 250 g COD per kg sludge.day. Tabulated results showed sludge loading, anaerobic:anoxic:aerobic ratio, percentage phosphorus removal, percentage nitrogen removal and percentage COD removal. There was no difference between results at the two different loadings, but there was a significant difference between the anaerobic:anoxic:aerobic ratio (1:1:2,7) combination with the internal recirculation flow (anoxic-anaerobic) of 160 per cent and the anaerobic:anoxic:aerobic ratio (1:1:4,1) with a recirculation flow of 30 per cent. At an internal recirculation flow of 30 per cent, phosphorus removal was disturbed, whereas increasing the internal recirculation flow to 160 per cent had a positive effect on settling properties and phosphorus removal.