Biomonitoring for effective management of wastewater discharges and the health of the river environment

Abstract

The management of wastewater effluents in South Africa has to date been based on physico-chemical standards although recently the assimilative capacity of the receiving stream has been included but not widely implemented. Very few assessments of the success of these management practices in protecting the downstream aquatic environment have been done. Bioassessments using benthic macro invertebrates were conducted up and down stream of a number of wastewater works to measure the degree of impact that the works were having on the stream ecosystems. A significant deterioration in the benthic invertebrate fauna downstream of most of the wastewater works was observed. This was particularly noticeable where the receiving stream was of good quality while there was little change in poor quality receiving streams. This was confirmed using ordination techniques. The ordination diagrams displayed how community composition varied with important environmental variables although some of these may be surrogates for other unmeasured variables. It was revealed that river flow and the dilution of the effluent were the most important of the measured variables explaining the variability in the biota. Escherichia coli and soluble phosphorus were also shown to be important with suspended solids also significant. Most of the wastewater works, in the month prior to the biological investigation, had complied with their permit requirements. The exceptions, found on infrequent occasions at different works, were soluble phosphorus, chlorine, E. coli and conductivity. One permit requirement that was consistently ignored was a statement forming part of the effluent standards permit system which states that the effluent should not contain constituents that are poisonous or injurious to animal (including aquatic) life. No systems for monitoring this criterion were in place. Despite the general compliance with regulations governing wastewater treatment a lack of adequate effluent dilution appeared to be the major cause of degradation of the downstream biota. These wastewater works are found in a region that does not have an abundant supply of water for dilution so treatment of the effluent to a condition that is not injurious to aquatic life becomes a necessity. Thus it appears that the physico-chemical standards used to characterise and manage wastewater treatment in South Africa are inadequate to protect the downstream environment in the vicinity below many wastewater works.