TOXICITY REDUCTION OF WASTEWATERS BY 
THE ACIDOGENIC PROCESS

An investigation has been carried out to evaluate the feasibility of reducing the toxicity of wastewaters containing potentially inhibitory organic substances. The toxicity of wastewaters was evaluated by its oxygen consumption rate, measured using a DO (Dissolved Oxygen) meter based on the International Standards Organization's "Activated Sludge Oxygen Consumption Inhibition Test" (International Standards Organization ISO TC 147/Sc 5/W61 No. 88, 1986). In this method, the decrease in DO concentration is recorded as a function of time. An inhibitory concentration of the test substrate decreases the DO uptake by the micro-organisms. The EC50 value represents the concentration of the substance which would cause a 50% reduction in oxygen uptake by the micro-organisms. ISO Method B, which usually represents the conditions in biological wastewater treatment plants, was adopted. The experimental set-up for the toxicity test is shown in Figure 1. The equipment used in this test were a DO meter, a standard BOD bottle, a magnetic stirrer, a DO recorder, a stopwatch and an air diffuser stone.

The investigations were carried out in four phases. In Phase I, the toxicity of a synthetic wastewater sample containing 2-nitroaniline, 3-nitroaniline and 4-nitroaniline was investigated in laboratory-scale acidogenic bioreactors using glucose as a co-substrate and carbon source. In Phase II, the toxicity of wastewater containing nitrobenzene was studied in an acidogenic bioreactor using sucrose as co-substrate. In Phase III, two separate bioreactors were used - one with synthetic wastewater containing 2-4-6 nitrophenol with sucrose as co-substrate, and the other with wastewater containing nitrobenzene and glucose as co-substrate. The toxicity of pharmaceutical wastewater was studied in Phase IV. The activated sludges used in all four phases of investigation were collected from local wastewater treatment plants and a lab-scale wastewater treatment system.

The biomass (activated sludge) from Jurong Sewage Treatment Works which has long been exposed to industrial wastewaters containing potentially inhibitory organic substances was found to be more resistant to the potentially inhibitory effects of nitrobenzene, nitroanilines and chlorophenols. On the other hand, the biomass (activated sludge) from the Ulu Pandan Sewage Treatment Works which dealt primarily with domestic sewage was found least resistant. A typical example of variation of inhibition with increasing nitrobenzene concentration is shown in Figure 2. The acidogenic biotreatment process was found to be most effective in reducing toxicity of wastewaters thereby making them amenable to aerobic biotreatment by removing more than 95% of the potentially inhibitory organic substances present. In designing new industrial wastewater treatment plants, this process could be incorporated as a pretreatment unit to detoxify the wastewaters before they are treated in aerobic processes.

The research project has been funded by University Research Grant No: 950680 which is gratefully acknowledge. This project was carried out in collaboration with Professor W J Ng.

Figure 1: Experimental set-up of toxicity test.

Figure 2: Inhibition of nitrobenzene on UPSTW activated sludge.