The increasing trend towards combining industrial and municipal wastewater for treatment in sewage plants increases the possibility of contamination of the influent by metal ions. Although the mechanisms by which heavy metals affect the biological treatment processes are not well defined, it is well documented that relatively low concentrations of various heavy metals may stimulate the biological systems, while increased concentrations may partially reduce system performance [1–3]. Chromium is usually encountered in the environment at oxidation states of (III) and (VI). It is released by effluent discharge from steelworks, chromium electroplating, leather tanning and chemical manufacturing. Each of these oxidation states has very different biological and toxicological properties. Cr(III) accumulates in the cell membrane and is considered to be less toxic. On the contrary, Cr(VI) is transported into the cells, where it is reduced to the trivalent form and reacts with intracellular material [4].

The effect of Cr(VI) on substrate removal, respiration activity and bacterial growth in activated sludge systems has been studied previously, but the results were controversial, in most cases. In particular, early works by Barth et al. [5] and Moore et al. [6] on substrate removal supported that aerobic biological treatment processes could tolerate, without significant loss in treatment efficiency, Cr(VI) concentrations in the range of 10–50mgl-1. Moreover, Moore et al. [6] showed that at a concentration of 5mgl-1 Cr(VI), the unit performed better than the control reactor. However, Lamb and Tollefson [7] reported that activated sludge shock loading of 5mgl-1 CrO4 2- reduced organic substrate removal by 50%. Vankova et al. [8] studied the effect of Cr(VI) on biomass respiration activity and reported that the 1-h EC50 value was in the range of 40–90mgl-1, whereas Madoni et al. [9] reported that 1-h exposure of activated sludge to a concentration of 83mgl-1 dissolved Cr(VI) reduced the specific oxygen uptake rate (SOUR) only by 21.5%. Finally, Gokcay and Yetis [10] and Yetis et al. [11] showed that activated sludge was stimulated in the presence of Cr(VI). They observed an approximately two times increase in maximum specific growth rate, μm values and stimulatory effects on biomass yield in the presence of 25mgl-1 Cr(VI). On the contrary, Mazierski [12] and Stasinakis et al. [3] observed a significant inhibition of heterotrophic growth in the presence of 10mgl-1 Cr(VI). Though it is reported that nitrifying organisms may be much more sensitive to heavy metals than heterotrophic organisms [5,13], only the effect of Cr(III) on the nitrification process has been investigated in continuousflow reactors [14]. Moreover, Cr(VI) effects on the size and morphology of activated sludge flocs, the settling capacity and the presence of higher microorganisms have not been investigated at all. From the above-mentioned literature review, it is evident that Cr(VI) toxicity on the processes and microbiology of activated sludge remains to be clarified. Thus, the purpose of this study was to investigate the effect of Cr(VI) continuous and shock loading on the removal of organic loading and on the nitrification process. Moreover, the Cr(VI) effect on various secondary operating parameters of activated sludge process, such as the size distribution of activated sludge flocs, the settling capacity, the abundance of filamentous microorganisms and the presence of protozoa and rotifers, was investigated.

2. Materials and methods

2.1. Activated sludge pilot plants

Two parallel continuous-flow activated sludge plants were operated during this study. One system was used as

a control, receiving only synthetic wastewater (plant A), whereas the other (plant B) received Cr(VI) in order to investigate Cr(VI) toxicity. The aerobic reactors of both systems were cylindrical, continuously fed, plastic tanks with a liquid volume of 6l. Aeration and efficient mixing were provided using porous ceramic diffusers. The temperature within the activated sludge units was kept at 20711C and the dissolved oxygen (DO) was maintained above 4.0mgl-1. To achieve a sludge age (yc) of 8 days, the appropriate amount of mixed liquor suspended solids was wasted directly from the aerobic reactors, on a daily basis.