Keywords: aquatic toxicity reduction, carbon adsorption, toxic organics, biological treatment, wastewater treatment, environmental pollution, effluent toxicity
Toxicity reduction in industrial and municipal effluents – the state of the art
Developments in treatment technology have traditionally focused on removal of specific contaminants: BOD, solids, ammonia, TOC and lately, specific organics such as phenol. A limit on effluent toxicity, however, does not fit so neatly into a procedural package, since it truly integrates the presence and impacts of contaminants in a sample. Toxicity reduction evaluation procedures, therefore, are more complex and site-specific than investigation procedures for reduction of other parameters. As described, toxicity reduction evaluations are best performed in six steps divided into three sequential phases. These are as follows: Phase I – Data validation; Phase II – Treatment system optimisation; Phase III – A) Toxicity identification, B) Best management practices implementation, C) Source treatment evaluation, D) Treatment system upgrading. Data validation can involve three to six months, or more, of additional testing to confirm and quantify the problem. Numerous testing concerns are identified which must be addressed prior to moving on to treatment considerations. The existing treatment facilities at the site should be fully optimised before considering addition of new facilities. Reconsideration of many accepted operational techniques has resulted in dramatic improvement in effluent toxicity, not evidenced by improvement in conventional pollutants. Some of the changes described include extended sludge age, increased carbon dosages and simply change in the carbon supplier. Toxicity identification testing is necessary if the existing system is not capable of completely resolving the problem. This consists of laboratory separation of samples into fractions to determine the contribution of various components to the toxicity. Toxicity identification results or literature research may provide a direction to a Best Management Practices or housekeeping approach to toxicity reduction. Elimination of or substitution for a toxic product or raw material has been known to resolve some toxicity problems. Where no specific component can be identified for elimination, sometimes a source area or building can be identified as a major factor. In either of these cases, source treatment can be helpful. Many applicable treatment technologies for small volume and/or concentrated streams are described. Upgrading of the treatment system is necessary where no other option is available or affordable. Selection of the technologies to be evaluated is based upon the results of the previous steps in the effort. Toxicity performance data are provided on a number of end-of-pipe treatment process alternatives.