title: Effects of chromium (VI) addition on the activated sludge process.
abstract: The effect of hexavalent chromium, Cr(VI), addition on various operating parameters of activated sludge process was evaluated. To accomplish this, two parallel lab-scale continuous-flow activated sludge plants were operated. One was used as a control plant, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5 mgl(-1). Cr(VI) concentrations of 0.5 mgl(-1) caused significant inhibition of the nitrification process (up to 74% decrease in ammonia removal efficiency). On the contrary, the effect of Cr(VI) on organic substrate removal was minor for concentrations up to 5 mgl(-1), indicating that heterotrophic microorganisms are less sensitive to Cr(VI) than nitrifiers. Activated sludge floc size and structure characterization showed that Cr(VI) concentrations higher than 1 mgl(-1) reduced the filaments abundance, causing the appearance of pin-point flocs and free-dispersed bacteria. Additionally, the variability of protozoa and rotifers was reduced. As a result of disperse growth, effluent quality deteriorated, since significant amounts of suspended solids escaped with the effluent. Termination of Cr(VI) addition led to a partial recovery of the nitrification process (up to 57% recovery). Similar recovery signs were not observed for activated sludge floc size and structure. Finally, shock loading to the control plant with 5 mgl(-1) Cr(VI) for 2 days resulted in a significant inhibition of the nitrification process and a reduction in filamentous microorganisms abundance.
authors.affiliation: Water and Air Quality Laboratory, Department of Environmental Studies, University of the Aegean, Theofrastou and Alkaiou Str., Mytilene 81 100, Greece. astas@env.aegean.gr
authors.forename: Athanasios S
authors.initials: AS
authors.lastname: Stasinakis
authors.forename: Nikolaos S
authors.initials: NS
authors.lastname: Thomaidis
authors.forename: Daniel
authors.initials: D
authors.lastname: Mamais
authors.forename: Evangelia C
authors.initials: EC
authors.lastname: Papanikolaou
authors.forename: Angeliki
authors.initials: A
authors.lastname: Tsakon
authors.forename: Themistokles D
authors.initials: TD
authors.lastname: Lekkas
mesh_terms: D001419:Bacteria; D019149:Bioreactors; D002274:Carcinogens, Environmental / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D005427:Flocculation; D009584:Nitrogen / Q000378:metabolism; D011157:Population Dynamics; D012722:Sewage / Q000737:chemistry*; D014865:Waste Disposal, Fluid
chemical_list: D002274:Carcinogens, Environmental; D012722:Sewage; D002857:Chromium; C074702:chromium hexavalent ion; D009584:Nitrogen