Effect of repeated addition of nitrite to semi-continuous activated sludgereactors

To a semi-continuous lab-scale activated sludge system (SCAS), nitrite was dosed discontinuously, i.e. together with the feed. The nitrite was added a t a concentration (50 mg N/1) which had been found earlier not to cause acu te inhibition of the general running of the reactors. Repeated nitrite addi tion, however, exerted an inhibitory effect which was reflected in decrease d removal efficiencies and was also expressed in decreases in total respira tory and nitrifying activity of 20% and 40%, respectively. The inhibition b y nitrite was reversible as removal efficiencies improved after ceasing nit rite addition. The two consecutive test runs revealed that addition of nitr ite gave rise to high amounts of dispersed cells and free-swimming protozoa . Moreover, DGGE patterns confirmed a shift in microbial community structur e upon application of nitrite. (C) 2001 Elsevier Science Ltd. All rights re served.