Impact of copper and cadmium on aerobic and anaerobic digestibility of sonicated sludge

The effects of the introduction of a sludge reduction process such as ultrasound on batch aerobic and anaerobic biodegradability after exposition to two metals (copper and cadmium) were investigated. The specific energy of ultrasonic treatment applied to the sludge was 200,000 kJ kg TS(-1). Ultrasonic treatment led to floc size reduction and to organic matter solubilization. Low copper (< 5 mg L(-1)) and cadmium (< 1 mg L(-1)) concentration improved aerobic biodegradability. For high metal concentration the maximal instantaneous biogas production rate q(max) inhibition by copper and cadmium was modeled by a saturation-type relationship under aerobic and anaerobic conditions. Under aerobic conditions, respiration inhibition was not affected by sonication. Cadmium inhibition (74%) was more than copper (58%). The positive effect of sonication on CO2 production was maintained after metal introduction. Under anaerobic conditions, metal introduction cancelled out the positive effect of the treatment. The sonicated sludge was 16% less sensitive to copper inhibition but 10% more sensitive to cadmium inhibition compared to non sonicated sludge.