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Sustainable Tertiary Wastewater Treatment Is Required for Water Resources Pollution Control in Africa Isaiah Bosire Omosa,†,‡ Hongtao Wang,† Shuiping Cheng,† and Fengting Li†,* †
College of Environmental Sciences and Engineering, UNEP-TONGJI Institute of Environment for Sustainable Development, Tongji University, Shanghai, China,200092 ‡ Department of Civil Engineering, Kenyatta University, P.O Box 43844-00100 Nairobi, Kenya treatment. Most Sub-Saharan countries employ wastewater stabilization ponds (WSP) for secondary wastewater treatment because of cost and the salubrious tropical climate. In Kenya 90% of wastewater treatment is accomplished by WSP. Perhaps the major challenge for these systems is overloading as a result of rapid urbanization. Africa’s urban areas are expanding rapidly, growing at a world annual fastest rate of 3.5%.2 Sadly, the high rate of urbanization has not been matched with improvement in sanitation and for existing WWTPs implies overloading and compromised treatment efficiencies. It is for this reason that tertiary treatment is viewed as necessary in a region where it has largely remained alien. Often the reason is financial, but it is also due to an ignorance of low-cost wastewater treatment processes and of the economic benefits of treated wastewater reuse.3 Constructed wetlands, for example, exhibit characteristic properties like utilization of natural processes, simple construction, simple operation and maintenance and cost effectiveness and are well suited for tropical climates. They can treat wastewater with high concentrations of organics and can cope well with water quality and quantity fluctuations as well as being able to operate intermittently.3 Free Water Surface here is a significant lack of proper wastewater treatment in wetlands can enhance further reduction of fecal coliforms and most African countries, especially in sub-Saharan Africa. removal of the excess algal growth which characterizes pond Untreated sewage effluent is one of the most common types of effluents.4 The effluent from the WSP system is normally pollution found around urban rivers and in groundwater relatively oxidized and nitrified; this is achieved in the sources in many African cities. By 2020, 75−250 million people maturation ponds (Figure 1) when properly operated. The in Africa may be exposed to increased water stress due to effluent also contains substantial carbon because of algal cells. climate change and pollution will only exacerbate the situation.1 This scenario favors horizontal subsurface flow constructed In Kenya, for example, most municipal wastewater treatment wetlands (HF CW) for denitrification. HF CWs are also plants (WWTPs) discharge partially treated or untreated excellent in organics and TSS removal. Their major problem is wastewater containing high levels of organic matter with clogging but this can be eliminated by efficient operation and mean effluent TSS, CODcr, and BOD5 being 246 mg/L, 253 maintenance of the whole system and by selection of coarse mg/L and 98 mg/L, respectively, (as of 2004) against a filtration materials. It is therefore suggested that the existing discharge requirement of 30, 50, and 30 mg/L, respectively. WSP be fitted with HF CW as a tertiary mechanism of effluent This situation replicates itself across most cities in the polishing (Figure 1). developing world. Dar es Salaam, Accra, Khartoum, Harare, Owing to their comparative low cost and simplicity, financing for HF CW development should not be an overly uphill task. Maputo, and Kampala, to name but a few, discharge treated and With adequate sensitization and determination, relevant untreated sewage into their water bodies. In 2004, only about stakeholders especially national and local governments, private 14% of sewage was collected and treated in Dakar, and the rest sector, and even external agencies including international was discharged into the sea without treatment.2The story is the programs such as the Millennium Development Goals same in Latin America, where less than 15% of the wastewaters (MDGs) can make it happen. Private finance is usually rare collected in sewered cities and towns is treated prior to in the wastewater sector because of the low financial viability discharge.3 The inadequacy in treatment stems from a number of reasons that include poor operation and maintenance of Received: May 27, 2012 existing systems or complete breakdown of the same, Accepted: June 5, 2012 Published: June 15, 2012 overloading, and lack of tertiary systems for extended
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© 2012 American Chemical Society
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dx.doi.org/10.1021/es3022254 | Environ. Sci. Technol. 2012, 46, 7065−7066
Environmental Science & Technology
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Figure 1. WSP and HF CW in series. (2) UNEP. Green Hills, Blue Cities: An Ecosystems Approach to Water Resources Management for African Cities. A Rapid Response Assessment;UNEP: Nairobi, 2011. (3) Mara Duncan. Domestic Wastewater Treatment in Developing Countries; Earthscan: London,2003; pp 18−23. (4) Constructed wetlands, Treatmentof Municipal Wastewaters, Manual, EPA/625/R-99/010; U.S. Environmental Protection Agency: Washington, DC, 2000. (5) Wang, H. T.; Omosa, I. B.; Keller, A. A.; Li, F. T. Ecosystem protection, integrated management and infrastructure are vital for improving water quality in Africa. Environ. Sci. Technol. 2012, 46 (9), 4699−4700.
passing the onus of responsibility to public budgets, more so since external aid may not be certain in light of the global financial situation. Appropriate increment of user fees to enhance cost recovery and debt servicing will be necessary. Besides the need for tertiary treatment within existing WWTPs, a significant number of urban centers in this region either have no WWTPs or have large areas not covered by the sewerage systems further complicating the situation. Port Harcourt did not have an off-site WWTP as of 2010 yet the city depends on boreholes for freshwater. In 2003, only 25% of Dakar was connected to the sewerage system and in Burundi just 10% access adequate sanitation. 2 Generally sewer connections cover less than 50% of most urban areas and the uncovered areas are normally inhabited by the urban poor who also depend on unsafe water supply sources. 2In Kampala the raw water supply on which the poor residents rely, are polluted from many sources, including industry and households. In Addis Ababa, areas not connected to WWPTs dispose waste into the stormwater drainage system polluting the surface and groundwater yet the river water is used by downstream residents to grow vegetables which are consumed by city inhabitants. These cases obviously require urgent attention but the solutions are more likely long-term. Lack of proper wastewater treatment has far reaching ramifications. Effluent discharged will not meet the prescribed standards while treated wastewater as an alternative water source is diminished if not eradicated and when attempted becomes a public health risk.5 In the contrary, successful and sustainable wastewater treatment can help support peri-urban agriculture to reduce water consumption, address food security and enhance livelihood opportunities.2 It is, in conclusion, viewed that tertiary treatment of municipal wastewater using constructed wetlands will contribute to attainment of the MDGs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is part of the Africa-China Cooperation Program on Environment which is supported by the Ministry of Science and Technology of China (MOST) and coordinated by the United Nations Environment Programme (UNEP). It was supported in part by the Fundamental Research Funds for the Central Universities.
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REFERENCES
(1) World Water Assessment Programme (WWAP). The United Nations World Water Development Report 3: Water in a Changing World; Earthscan: Paris: UNESCO, and London, 2009. 7066
dx.doi.org/10.1021/es3022254 | Environ. Sci. Technol. 2012, 46, 7065−7066
