NITRIFICATION PATTERN IN A FLUCTUATING ANAEROBIC-AEROBIC POND ENVIRONMENT

Nitrification does not occur in conventional fish ponds or in oxidation ponds. Yet, intensive nitrification occurs in continually aerated and mixed fish ponds and in activated sludge systems. In a combined fish pond system, water from a conventional pond is pumped through a continually aerated and mixed pond, with a 5 h average hydraulic residence time. Unlike other aerated ponds, nitrification did not occur in this case. The masons for nitrification inhibition were studied in laboratory and field experiments. Inhibited nitrification was not due to any factor associated with water composition and persisted even when the water was replaced. In addition, it was not limited by a lack of inoculum. Nitrification in a laboratory assay started immediately in samples taken from a continually aerated pond, while it started only following a 24-48 h lag period in water sampled from a conventional pond or from the combined pond system. Nitrifying bacteria have to adapt to the partial anoxic conditions in the conventional pond. It is assumed that the lag period is needed by the reverse process, adaptation to oxic conditions. The ecological and practical implications of this phenomenon are discussed. The lag period is needed when nitrifiers are transferred from anoxic to oxic conditions as a protective mechanism to ensure that nitrification will take place only if continuous or long-term oxic conditions prevail.