A Real Time Toxicity Bioassay for Activated Sludge Reactor

Activated sludge reactor is commonly employed for the secondary treatment of industrial wastewater, showing a high purification yield, operating simplicity and cost-effectiveness. Nevertheless, industrial wastewater generally contain numerous organic and inorganic compounds which are not biodegradable; heavy metals are often found in effluents from electroplating and metal-processing industries, manufacturing of paints, plastics, scientific instruments, salts are found in wastewater from production of pesticides, pharmaceutical and food industry, waste-dump percolating water. Primary treatments are often designed to reduce heavy metal and salt contents in wastewater, nevertheless trace concentrations persist and may reach the biological reactor, resulting in biomass inhibition and dramatic loss in purification efficiency. Respirometry through Oxygen Uptake Rate measurement is one of the most rapid and reliable methods to assess the inhibition of activated sludge. In previous studies toxicity tests for specific compounds have been performed following different procedures and attaining heterogeneous results. In this work a simple modification of the configuration is proposed, in order to assess in real time the toxicity and the inhibiting effect of the incoming wastewater before it reaches the reactor. A sample is drawn from the influent and a respirometric test is performed on mixed liquor collected from the reactor itself. If the inhibition exceeds the limit, the influent is sent to storage and to an alternative treatment. Preliminary experiments were carried out with bench-scale CSTR, supplied with synthetic wastewater. Copper sulphate and sodium chloride were chosen as inhibiting compounds, supplied separately to batch samples of activated sludge in shock load at different concentrations. Copper concentration in the mixed liquor was measured by atomic absorption and salt concentration was determined through conductance measures. Inhibition up to 90% was evaluated by OUR test with short response time, demonstrating the suitability of this tool to a real time toxicity bioassay for activated sludge reactor.