DIFFUSIONAL TRANSPORT MECHANISMS AND BIOFILM NITRIFICATION CHARACTERISTICS INFLUENCING NITRITE LEVELS IN NITRIFYING TRICKLING FILTER EFFLUENTS

The widely reported accumulation of nitrite during nitrification is studied in recirculating fish culture systems. Nitrite oxidation capacity was homogeneously distributed over the trickling filter, as opposed to the ammonia oxidation capacity, which decreased at an increasing filter depth. Under all circumstances tested, nitrite accumulation in the system was restricted to an elevated but stable nitrite concentration, indicating complete oxidation of the produced nitrite. For a specific trickling filter, a fixed ratio between nitrite and ammonia concentrations in the recirculating water was found. Nitrifying biofilms maintaining relatively low nitrite concentrations were characterized by a relatively high nitrite oxidizing capacity compared to ammonia oxidizing capacity, whereas biofilms with a relative low nitrite oxidation capacity induced high nitrite concentrations. The occurrence of high nitrite concentrations in trickling filler effluents can entirely be explained by diffusional transport mechanisms in combination with the characteristics of the biofilm. There is no argument that high levels of nitrite nitrite are caused by insufficient activity or inhibition of the nitrite oxidation process.