The impact of the Thaba-Nchu sewage treatment plant on the freshwater ecology of the surrounding area

[摘要] English: Thaba-Nchu is located about 62 km south-east of Bloemfontein, in the Free State Province of South Africa. Sepanespruit is a tributary of the Modder River which drains the Thaba- Nchu area. This area has a population of about 70 000 people and is served by one sewage treatment plant whose hydraulic capacity is 6 MI/day. This study was conducted to evaluate the effectiveness and the efficiency of the Thaba-Nchu sewage treatment works and the effects of the sewage effluent into the receiving stream; Sepanespruit. Sources of pollution into the stream were identified as the poorly treated sewage effluent, domestic wastes, urban litter and run-off, raw sewage, livestock and human trampling. The municipal truck tankers that disposed-off sewage at the manhole which connected to the sewage treatment plant polluted an area of about 30m in radius with raw sewage, the origin point being the manhole. A canal of raw sewage originated from that area and became a point source of pollution to Sepanespruit. The sewage effluent caused thermal pollution to the stream, bacterial contamination (measured as total coliform bacteria, faecal coliform bacteria and E. coli), and addition of excessive salts (measured as electrical conductivity (EC) and nutrients (PO4 3- and NH3). Pathogens that occurred in Sepanespruit water were E.coli, Shigella dysentriae, Faecal enterococci and Faecal streptococci; they are all of faecal origin. Endocrine disrupting chemicals such as dyes, herbicides, pharmaceutical and cosmetic compounds were also identified in the sewage effluent; the implication was that the conventional treatment processes did not remove those chemicals from the sewage stream. The sewage effluent was sometimes released in a red-brown coloured state. The colour was attributed to the rich dyes that a meat processing factory dumped at the sewage treatment plant. The most common algal divisions identified in Sepanespruit water were Cyanophyta, Chlorophyta, Bacillariophyta and Euglenophyta. The vegetation along Sepanespruit was classified using JUICE (the computer vegetation classification program) into one community of Paspalum dilatatum - Rumex crispus. Highly enriched water contributed to the observed vegetation structure; the stream banks were covered by monotypic stands of plant species that are adapted to disturbances such as Paspalum distichum and Rumex crispus. The canal water and sewage effluent were the major sources of point pollution to Sepanespruit. Poor sewage quality was a result of poor management, old non-functional unit processes and unskilled operational staff. The section of Sepanespruit that formed part of this study was deemed eutrophic but the dam water was oligotrophic because of the biofiltration process that the reed beds of Typha capensis and Phragmites latifolia undertook before water moved into the dam. Alternative methods of factory dyes disposal and sludge handling have to be sought in order to reduce water and environmental pollution. The waste management department of the water affairs has been advised to monitor disposal of sewage of sewage at the source manhole and rehabilitate the polluted area.

[发布机构] University of the Free State