The degradation was constant (substrate independent) at ammonia concentrations above 2 mg TAN/l in the bulk water. Below 1 mg TAN/l degradation rates declined exponentially with concentrations (Fig. 2). The kinetic rate constants k0aand k1adescribing zero and first order degradation, respectively, are shown in Table 2. During degradation of ammonia a minor temporary peak in bulk water nitrite concentration was measured (data not shown) and nitrate concentrations increased consistently with ammonia degradation