Trickling filters

Trickling filters, utilising plastic media in columns 4.5-6.0 m high, have been used in the treatment of high-strength fruit and vegetable wastewaters (3000-4000 mg/l BOD5). High liquid recirculation rates and forced air circulation are used to achieve BOD5 removals up to 90%. However, these processes tend to fail with the high organic load rates that are typical of some food processing wastewaters as, for example, in the cheese industry (Rusten et al, 1996).

Wastewater is distributed by rotating arms at constant rates on to the top surface of packed columns of plastic or other inert material media. A biological film is formed on the media and cellular material periodically sloughs off when the thickness becomes sufficiently great that oxygen transfer cannot occur throughout its depth and anaerobic conditions develop. Underdrains located beneath the column transport the effluent to settling tanks where the dense sludge is separated from the liquid effluent. A portion of the effluent is recycled to seed the process with biological cells and to promote consistent sloughing. When operating with loading rates up to 5 kg BOD5/m3 per day, filter media achieve BOD5 reductions of 40-50% (Carawan et al., 1979).

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