IMPACT OF LOW-RATE SUBSTRATE REMOVAL ON THE PERFORMANCE OF DENITRIFYING SYSTEMS

Strict effluent N criteria may also require the use of slowly biodegradable substrate as carbon source for denitrification. The paper draws attention to the fact that oxygen penetration through the surface of uncovered denitrification basins may significantly deteriorate the efficiency when substrate removal rate is low, whereas at high consumption rates this impact may prove to be negligible. Results of comparative lab-scale experiments carried out in both batch and continuous-flow operation revealed that the reason why denitrification rates are much more severely affected at low substrate consumption rates is the increased dissolved oxygen concentration that occurs due to the decreased ability of its removal. In the comparative batch experiments a zero-head-space reactor and a reactor with an open surface were applied using different samples of preclarified wastewater deriving from an existing treatment plant. In the continuous-flow experiment two differently arranged activated sludge systems were operated simultaneously, fed by a model wastewater containing peptone as carbon source. The total reactor volume serving predenitrification was identical in both arrangements; however, in one of the systems it was compartmentalized into three reactors. None of the reactors were covered in this experiment. The dissolved oxygen concentration raised significantly in both of the batch and continuous-flow experiments when the readily biodegradable substrate had been depleted. The results supported that in cases when readily denitrifiable carbon source is not in a pronounced excess, staging of anoxic reactors may significantly improve the efficiency of denitrification through maintaining relatively high substrate removal rate and thereby low oxygen concentration in the first basins