Since more or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants, a means to quantify the extent of inhibition under operational conditions has been sought. Thus a method for the estimation of the actual maximum autotrophic growth rate μ^A,T was developed. This parameter helps to detect low nitrification capacity, before the effluent ammonia concentration increases. Two case studies show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.

Inhibitionof nitrification at combined municipal

Biological nitrogen removal from wastewater depends on nitrogen incorporation and on denitrification. Both processes strongly depend on the degradation of COD. On the hand of operational data from a biofilter pilot plant, the relation between COD-removal and nitrogen removal is shown. Mass balances for the two parameters COD and N are the base of this consideration, as they are excellent tools for the evaluation of WWTPs (large scale plants as well as pilot plants) in order to estimate the fluxes of these substances and to draw conclusions of general validity. When applying the results from the pilot scale investigation to the whole treatment process (which includes sludge stabilisation), reject water must be taken into account. Therefore, the N removal efficiency was calculated both for the process with and without return liquor.

The influent to the municipal WWTP of Linz consists of typical domestic sewage and of the discharges of two major industries which results in a severe basic inhibition of nitrification. In the course of a 3-year investigation program, intensive studies were carried out. Pilot investigations showed the nitrification inhibiting properties of this wastewater. On the basis of the operational data, the variation in the inhibition of the maximum specific growth rate was evaluated. In addition, testing procedures which revealed the sources of inhibition - firstly within the catchment area and subsequently inside the industries concerned - were developed. With the results of these tests, risk assessment and a priority schedule became feasible. Based on the information gathered this way and on future steps which will be taken by the industries, major decisions concerning the upgrading of the existing activated sludge plant could be made.

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