Temperature-dependency of nitrification and required anoxic volume for denitrification in the biological treatment of veal calf manure

Since 1976 veal calf manure has been treated aerobically in central treatment plants in the Netherlands. Reduction of BOD and removal of nitrogen by nitrification and denitrification are the main objectives of this treatment. Until 1991 the manure was treated discontinuously in 24-h aeration cycles. One of the conditions for denitrification is the availability of an electron donor (degrable organic matter). Because of the low BOD 5/N ratio in the manure, denitrification is sometimes incomplete in the existing plants, resulting in accumulation of nitrite and nitrate. Introduction of a continuous treatment with an anoxic zone might lead to a more efficient use of the available organic matter for denitrification. The nitrogen removal rates at 5, 10, 15 and 20°C as well as the optimum ratio between anoxic and aerated volumes at 10°C using manure with different BOD 5/N ratios were investigated in a laboratory-scale, contiuous-treatment plant with separate anoxic and aerated reactors. Measured nitrogen removal rates at 5, 10, 15 and 20°C were 32, 66, 92 and 154 mg NH 4 +-nitrogen g −1 volatile suspended solids day −1, respectively. The temperature-dependency of nitrification could be described by a linear equation ( R 2 = 0·93). The effect of BOD 5/NH 4N ratios of 2·4, 2·5, 2·7 and 3·4 on denitrification was investigated in an anoxic/ aerated volume ratio of 0·15. As accumulation of NO 2 − occurred when the BOD 5/NH 4N ratios was under 3 and as these low BOD 5/NH 4N ratios will have to be dealt with in treatment plants, the anoxic/aerated volume ratio will have to be higher than 0·15. A ratio of 0·25 is recommended beacuse no NO 2 − accumulation was found at 10°C when a BOD 5/NH 4N ratio of 2·4 was used.