Spiked urea was completely removed in the moving bed biofilters at all loadings within the 350 min sampling period (Fig. 4).Degradation of urea followed first order kinetics throughout when spiked to concentrations of approximately 2.5 mg urea-N/l in the bulk water (Table 2). First order degradation rate constants of urea were significantly lower than those of ammonia and nitrite(P < 0.001 in both cases). As a result of urea degradation ammonia was generated and peaked shortly after spiking with urea, which in turn was followed by a peak in nitrite (data not shown). Nitrate accumulated in the biofilter systems over time reaching a stable value only after urea removal was complete (Fig. 4).