2.2. Experimental procedure

Activated sludge from a nitrifying municipal wastewater treatment plant that received no industrial wastewater (Plomari, Lesvos) was used to seed the reactors. During the first 2 weeks, both systems operated on synthetic wastewater (Table 1), devoid of Cr(VI) for biomass acclimatization. A potassium dichromate solution (K2Cr2O7, Merck) was added to the synthetic wastewater to provide a constant concentration of 0.5mgl-1 Cr(VI) to the influents of the experimental activated sludge system (plant B) from the 15th day and for a period equal to 3 yc (24 days). Cr(VI) was further increased to 1, 3, 5mgl-1 at the 39th, 61st and 85th day, respectively. After completion of the last experimental period, no Cr(VI) was added to plant B for a period of 12 days in order to investigate the recovery capability of the system. Additionally, 5mgl-1 Cr(VI) were added for a 48-h period to the control system (plant A), to investigate Cr(VI) effect on a non-acclimatized activated sludge system.

2.3. Analytical methods

Analyses of influent and effluent COD (dissolved, CODdis and total, CODtot), suspended solids, and mixed liquor suspended solids were performed every 2 days, accordingto Standard Methods [15]. Suspended solids samples were obtained after filtration through 0.45mm pore size glass fiber filters (Whatman GF/C). Ammonium, nitrate and phosphate concentrations were determined on filtered influent and effluent samples (Millipore membrane filters, 0.45mm pore size). Ammonium ions were determined by an acidimetric method, while nitrate and phosphate were determined by ion chromatography. Dissolved oxygen (DO), temperature and pH values were measured daily in both systems. A WTW Oxi 96 portable instrument was used for DO and temperature measurements, while pH was determined using Crison micropH2001. Sludge volume index (SVI) and settling velocity were measured in a 1-l graduated cylinder according to Standard Methods. Activated sludge flocs size distribution was determined using a Mastersizer E instrument (Malvern). This instrument uses light scattering and data are given as frequency by volume [16]. In order to investigate the morphology of activated sludge flocs, the existence of higher microorganisms and the abundance of filamentous microorganisms, a Leica phase contrast microscope was used. For the identification of filamentous microorganisms, activated sludge samples were analyzed every week according to Jenkins et al. [17].