Performance of downflow stationary fixed film reactors depended on factors such as type of film support, film surface-to-volume ratio, height, temperature and fluid flow pattern. The downflow operation ensured the dilution of the waste as soon as it entered the reactor. Loading rates and organic removal efficiencies depended on the total amount of active biomass retained as well as on type of waste. For a wide variety of industrial wastes, including some of the forest industry, loading rates of 5 to 15 kg Chemical Oxygen Demand per m3 per day were readily obtained with removals of Chemical Oxygen Demand of 70-95%, depending on loading rate and type of waste. Severe hydraulic and organic overloadings were tolerated with operation back to normal, usually in 24 hours following cessation of adverse treatment. Reactors were operated with dilute and concentrated wastes (4000-130000 mg COD/1) and changed readily over from one waste to another. Intermittent loadings at high loading rates were also possible. Loading rates decreased linearly with temperatures between 35 and 10°C; at 10°C they were about 20% of those at 35°C. At thermophilic temperatures, loading rates and performance were similar to those at mesophilic temperatures. Results of a detailed study of mixing characteristics showed that mixing patterns, which depended on number of channels, their configuration and recirculation, affected performance noticeably.

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