Comparative study of suspended and attached activated sludge processes in the treatment of wastewater containing toxic substances

Although activated sludge communities are considered stable, the presence of toxic substances, such as Cr(VI), in the influent may induce changes in the activity and the performance of a wastewater treatment plant. The main objective of this study was the determination of Cr(VI) effects on the performance and on the protistan community of an activated sludge. Six laboratory scale activated sludge reactors, three conventional and three with PVA gel beads, were supplied with synthetic sewage containing Cr(VI), at three concentrations: 1, 5, and 10 mg/L. The protozoan species were identified, quantified, and correlated to the efficiency of each system through microscopic observations. High removal rates of the organic loading, ammonia– nitrogen, and total phosphorus were observed after the acclimatization period of the activated sludge, even at high Cr(VI) dosages. The reactors with the PVA biofilm carriers exhibited increased removal of Cr(VI) from the synthetic sewage. Differences in the abundance and diversity of the protistans were observed at various Cr(VI) influent concentrations. Sludge Biotic Index (SBI) values decreased at the start of the operation, indicating the effects of Cr(VI) influent on the activated sludge microfauna. However, the acclimatization of the activated sludge microorganisms to Cr(VI) concentrations was indicated by the increase of the SBI index after 45 days of operation. This fact indicates that a healthy activated sludge microfauna, performing adequate biodegradation processes, can be retained after gradual