Water Research 36 (2002) 3341–3349 Effect of chromium(VI) on bacterial kinetics of heterotrophic biomass of activated sludge Athanasios S. Stasinakis a, *, Daniel Mamais b , Nikolaos S. Thomaidis a , Themistokles D. Lekkas a a Department of Environmental Studies, University of the Aegean, Theofrastou and Alkaiou Street, Mytilene 81 100, Greece b Department of Water Resources, Faculty of Civil Engineering, National Technical University of Athens, 5 Iroon Polytechniou Street, Zografou, Athens 15773, Greece Received 13 July 2001; accepted 20 December 2001 Abstract The effect of hexavalent chromium, Cr(VI), on the maximum specific growth rate, m m and biomass yield, Y H ; of heterotrophic biomass was studied in batch tests conducted under high (=10) and low (=1.5) substrate-to-biomass ratios ðS 0 =X 0 Þ: The effects of sludge age and biomass acclimatization to Cr(VI) on the bacterial kinetics were also studied. The m m values were determined by measuring oxygen uptake rate (OUR) and volatile suspended solids (VSS) increase. Cr(VI) concentrations equal or greater than 10 mg l 1 inhibited the growth of unacclimatized activated sludge and caused a significant decrease in m m and Y H values. The acclimatization of biomass and the selection of a high operating sludge age reduced the inhibitory effect of Cr(VI). At a sludge age of 20 days, Cr(VI) concentrations of o10 mg l 1 stimulated bacterial growth as evidenced by an increase in both the m m and Y H values. Determining m m values by OUR and VSS methods, revealed that the presence of Cr(VI) in unacclimatized biomass caused an inhibitory effect mostly on substrate oxidation, while in acclimatized biomass, anabolic pathways were inhibited more. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Toxicity; Cr(VI); Activated sludge; Bacterial kinetics; Batch methods; S 0 =X 0 ratio 1. Introduction The increasing trend towards combining industrial and municipal wastes for treatment in sewage plants increases the possibility of contamination of the influent by metal ions. Since microorganisms are key compo- nents for the decomposition of organic matter, the effect of metal toxicity on microorganisms has received special attention in recent years. Chromium is usually encountered in the environment in the oxidation states of (III) and (VI). It is released by effluent discharge from steelworks, chromium electro- plating, leather tanning and chemical manufacturing. Each of the above oxidation states has very different biological and toxicological properties. Cr(III) accumu- lates in the cell membrane, while Cr(VI) is transported into the cells, where it is reduced to three valent form and react with intracellular material [1]. There are numerous methods for evaluating metal toxicity in activated sludge systems. The most widely used are enzymatic and nitrification inhibition [2], inhibition of respiratory activity [3], influence on activation process efficiency [4,5], influence on activated sludge microfauna [6] and influence on activated sludge bacterial kinetics [7]. Various techniques have been developed to determine the kinetic constants of the biomass growth. However, in the literature, the values of kinetic parameters vary considerably for growth and *Corresponding author. Tel.: +30-251-36225; fax: +30-251- 36226. E-mail address: astas@env.aegean.gr (A.S. Stasinakis). 0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0043-1354(02)00018-0