A continuously fed, sequential anaerobic-aerobic-aerobic reactor system was used to treat wool scouring effluent. The chemical oxygen demand (COD) in the anaerobic reactor was reduced from 30500 mg/ℓ to 3000-5000 mg/ℓ. In the first aerobic reactor this was further reduced to 1200-1800 mg COD/l. The final discharge from the second aerobic reactor had a COD of 500-1000 mg/ℓ at a hydraulic retention time of 2-3 d. Nitrification was erratic but sufficient to reduce the ammonia-N to levels of 20 mg/ℓ and less. Mixed liquor suspended solids and volatile suspended solids could be maintained at sufficiently high levels in completely mixed systems and efficient biomass retention in the aerobic rotating biological contactor resulted in an overall removal of 98.4% COD.

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