A 2-year pilot study was conducted by the City of Malmo, Sweden, to determine the maximum capacity of existing trickling filters when converted from carbonaceous duty to nitrification duty. Operating variables examined included a comparison of alternating two-stage to single-stageoperation, flushing intensity, and predator control techniques. Distributor speed control had only a small effecton nitrification efficiency, and motorized distributors are not required in this application. Twostage operation in an alternating mode provided for higher nitrification rates and lower effluent ammonia values than in single-stage operation. A nitrification model was used to analyze reaction rate data. The analysis showed that alternating two-stage operation mitigated the suppressing effect of influent suspended solids, resulting in enhanced nitrification rates. The filter macrofauna were dominated by worms rather than filter flies, so that regular filter flooding did not enhance nitrification rates.Waler Environ. Res.. 67, I11I (1995).

After falling in disfavor in the U.S. in the 1970s, trickling filters are now reappearing in new applications. The Trickling Filter/Solids Contact (TF/SC) process uses physical and biological flocculation features to transform the poor quality of the trickling filter effluent into an effluent equal to the best activated sludge system. Over 50 TF/SC plants have been built since the process was introduced in 1979. The recent development (1988) of the Biofilm-Controlled Nitrifying Trickling Filter (BCNTF) has permitted higher rate operation and rendered the process less costly than the activated sludge system for nitrification. Three BCNTF projects are now underway in the U.S.

A two year pilot plant study has been performed in order to evaluate a nitrifying trickling filter (NTF) process within an upgrading project for increased nutrient removal. The operation of the filters was very stable without upsets due to predators. The filter microfauna was dominated by worms and the presence of filter fly larvae was limited. Suggested predator control methods like flooding or varying the flushing intensity did not affect the identified microfauna. By operating the filters in a two stage alternating series filtration mode, higher nitrification rates and lower effluent ammonia concentrations could be reached simultaneously in comparison to operating the filters in a single stage filtration mode.

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