3.2. Ammonia oxidation Spiked ammonia was quickly removed in all five moving bed biofilters (Fig. 2) approaching 0 mg/l at all loadings within the 6 h of sampling. 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