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A moving‐bed biofilm reactor has been developed, where the biofilm grows on small, free‐floating plastic elements with a large surface area and a density slightly less than 1.0 g/cm3. Nitrification of preprecipitated wastewater in a once‐through system and raw wastewater in a recycled system with predenitrification has been studied in a pilot plant with an active, specific biofilm surface area of approximately 310 m2/m3. Maximum nitrification rates under comparable operating conditions were 20% to 25% higher with preprecipitated wastewater in the once‐through system than with raw wastewater in the recycled system. A temperature coefficient of θ = 1.09 was found. However, under oxygen‐limited conditions, the apparent temperature effect was insignificant in the temperature range from 7 to 18°C because of increased dissolved oxygen levels at low temperatures. At 10°C volumetric nitrification rates from 300 to 400 g NH4‐N/m3·d can be expected.

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A moving-bed biofilm reactor has been developed, where the biofilm grows on small, free-floating plastic elements with a large surface area and a density slightly less than 1.0 gjcm l . Nitrogen removal. based on nitrification and denitrification, was studied in a pilot plant with an active, specific biofilm surface area of approximately 310 m 2 / m l . Temperatures ranged from 7 to 18°C, and untreated wastewater had concentrations typically less than 100 mg soluble chemical oxygen demand (COD)/L and 25 mg total N/L. Both predenitrifieation, using untreated wastewater as carbon source, and post-denitrification of preprecipitated wastewater, using acetate as an external carbon source, were examined. The predenitrification process was carbon limited. and only 50% to 70% total N removal was obtained, at a recirculation ratio of approximately 2.0 and a total empty bed hydraulic residence time of approximately 6 hours in the biofilm reactors. With post-denitrification and an external carbon source. 80% to 90% total N removal could easily be reached at total empty bed hydraulic residence times less than 3 hours. Waler Environ. Res" 67, 0000 (1995).The background and strategy for the nitrogen removal research carried out in Norway has been described in a companion paper (Rusten el al.. 1995).

Benzotriazole is a commonly used additive in aircraft de-icing fluids. As a result of extensive de-icing activities the compound is detected in the groundwater below de-icing platforms at several international airports. The compound is toxic, and not biodegradable. Laboratory tests have been performed to study if UV irradiation can degrade the compound and reduce the aquatic toxicity. Benzotriazole can be degraded by UV irradiation at pH values below 7. Approximately 65% reduction in the benzotriazole concentration was achieved at a dose of 320 mWs/cm2, and almost 90% reduction was achieved at 1070 mWs/cm2, with an apparent first order relation between the logarithm to the UV dose and the reduction. Benzotriazole is not significantly mineralised by UV irradiation, but transformed into other compounds, of which aniline and phenazine were identified. The metabolites show toxic effects, but they are not as toxic as benzotriazole, resulting in a general decrease in toxicity as a result of UV irradiation.