Toxic effects of cupric, chromate and chromic ions on biological oxidation

Abstract

Using a completely mixed, continuously operated, lightly loaded, laboratory activated sludge system, the toxic effects of cupric, chromic and chromate ions under conditions of shock loading were observed. These were determined with the aid of a total carbon analyzer and simple mass balance techniques in terms of conversion of the organic nutrient fed. The distribution of the metal ion between aqueous solution and suspended solids was measured using atomic absorption spectrophotometry. Toxic effects were in the order: Cu 2+ > CrO 4 2− > Cr 3+ while the reductions in conversion were 90, 50 and 20 per cent, respectively, for concentrations of 5 ppm metal ion. Cupric ion toxicity was directly proportional to the weight of copper absorbed per unit mass of suspended matter within the total copper concentration range (0–5.5 ppm) studied. This toxicity decreased markedly with increased suspended solids concentration: an 80 per cent decrease in conversion at 210 ppm suspended solids was reduced to a negligible quantity (3 per cent) by increasing the suspended solids to 4000 ppm. At 210 ppm suspended solids, 34 per cent of the added copper was removed by the sludge in 7 h. The results of this work suggest that the toxic effect of metal ions on a sewage plant activated sludge system could be reduced by rapidly increasing the suspended solids concentration, possibly by the addition of dried sludge. It is also implied that the effect on dilute systems such as lagoons would be much greater because of the low suspended solids. This work supports a mechanism involving rapid adsorption of the cupric ion by both viable and dead sludge followed by a slower rate determining step resulting in the toxic effect. The first order rate constant for substrate utilization was found to be (1.07 ± 0.6) h −1.