Reuse of domestic wastewater treated in macrophyte ponds to irrigate tomato and eggplant in semi-arid West-Africa: Benefits and risks
Research highlights
▶ Risks and benefits assessed on crops irrigated with macrophyte treated wastewater TWW. ▶ TWW physico-chemical quality is suitable for irrigation but health risks remain. ▶ TWW provided very variable nutrients supply depending on year and element (N, P, K). ▶ TWW improved (less than fertilizer) eggplant yield but extra nutrients (N, P) are needed. ▶ Remaining health risks call for precaution measures when reusing TWW in West-Africa.
Introduction
In semi-arid zones, the scarcity of freshwater resource is a critical problem and the reuse of marginal quality water in agriculture is increasing regularly (Cissé, 1997). In towns such as Ouagadougou (capital city of Burkina Faso), with a population estimated at more than 1.2 million people (2001) and a mean annual growth rate of 6.5% against 2.4% for the whole country, urban agriculture is rapidly developing. Urban agriculture cannot rely solely on the limiting freshwater resource, and calls for the use of secondary quality waters. According to the National Institute of Water and Sanitation (Office National de l’Eau et de l’Assainissement ONEA, Burkina Faso; Vezina, 2002) the total discharged wastewater represents more than 20,000 m3/year of domestic wastewater and 600,000 m3/year of industrial effluent. Until recently, and because of the absence of any sewerage system, raw wastewater from the central market, the main hotels, the hospitals, the brewery, the tanneries and the abattoir were discharged without any treatment into natural canals and mixed with run-off rainwater (Cissé et al., 2002). The surroundings of these canals were spontaneously invaded by market gardeners who use the water to irrigate their vegetables without any kind of treatment. Cissé (1997) identified 48 sites of market gardening between 1995 and 1996 with a total area of 174.35 ha in Ouagadougou. This extensive use of untreated wastewater for urban agriculture has led to various consequences on vegetable quality, population health and soil quality that have been reported by several studies. In Ouagadougou, soil, crop leaves and fruits (carrot, lettuce, tomato, etc.) were reported to be heavily contaminated by faecal coliforms and helminth eggs (Cissé, 1997, Cissé et al., 2002). The authors reported also frequent pathogen health effects on market gardener family members. Similar vegetable contamination was reported in other areas, such as in Ghana (Amoah et al., 2006), Turkey (Erdogrul and Sener, 2005), Morocco (Amahmid et al., 1999) and Mexico (Blumenthal et al., 2001). Moreover other problems may be caused to soils irrigated with raw wastewater. Suspended matters in untreated wastewater can accumulate and create clogging, reducing soil aeration and hydraulic conductivity (Viviani and Iovino, 2004, Toze, 2006), sodium and nitrate in excess may create salination and nitrate groundwater contamination (Ayers and Westcot, 1988, Oron et al., 1999). To alleviate the health risks of raw wastewater use on crop and soil quality and on consumers many research works, focusing on treatment possibilities through low cost lagoon systems, were performed with encouraging results in Ouagadougou at the pilot center for the semi-arid SSWA (Koné et al., 2002, Koné, 2002, Klutsé, 1995). Reuse of treated wastewater for agricultural irrigation may be beneficial for different reasons: (1) water scarcity can be alleviated by the large amount of wastewater that is available during the whole year, (2) pollution hazard from direct release to environment is diminished, (3) economic benefits attributed primarily to the nutrients content of the wastewater, which reduces fertilizer expenses to farmers (Lubello et al., 2004, Oron et al., 1999). In the semi-arid SSWA, lagoon treated wastewater was claimed to be biologically sound and has interesting nutrients content for vegetable cropping (Cissé, 1997, Koné, 2002, Koné et al., 2002). Because on-site experiments are still lacking to evaluate the remaining health risks, the nature and amount of nutrients supplied by treated wastewater, the aim of this study was to evaluate (1) the physico-chemical and biological risks to irrigated soil, (2) the pathogen health risks on fruit and consumers, (3) the nutrients supply, and (4) the effect on the yield of tomato and eggplant crops of lagoon (macrophytes) treated wastewater irrigation.
Section snippets
Experimental site
The experimental site is located at the Institut International d’Ingénierie de l’Eau et de l’Environnement (2IE) in Ouagadougou (12°20–12°25 N and 1°27–1°35 E), Burkina Faso. The climate is tropical soudano-sahelian characterized by less than 800 mm mean annual rainfall and 25–30 °C mean monthly temperature. The rainy season extends from May–June to October. The evaporation rate is high with an average of more than 6 mm/day (Mermoud et al., 2005). The soil is ferruginous with a of 7.7, low
Physico-chemical quality of irrigation waters
Table 2 shows the average physico-chemical characteristics of the two irrigation waters. Overall the load of most of the analyzed parameters was much higher for TWW than for the FWL. The physical characteristics (SM, EC, pH and turbidity) of both waters were in agreement with the recommendations of FAO (Pescod, 1992). In terms of chemical quality, high values (with high standard deviation SD) were observed in 2002 compared to 2001 and 2002 for TWW. FWL was in agreement with the normal ranges
Discussion
The reuse of treated wastewater in agriculture has become in the whole world and particularly in the semi-arid SSWA, a strategic mean to save and complement first quality water resource which is more and more limiting. We irrigated tomato and eggplant for three years (2001–2003) using TWW (and FWL as control), with the addition of mineral fertilizer or not, in order to evaluate the physico-chemical and biological risks for soil-fruit-consumer, the nutrient supply and crop yield response.
In this
Conclusion and recommendations
During three years of experiments, we used treated wastewater, and fresh water as control, in combination with or without mineral fertilizer application to evaluate the physico-chemical and biological risks on irrigated soils and fruits of macrophyte TWW, the nutrients supply, and the effect on tomato and eggplant production in semi-arid Burkina Faso. We found that: 1) the physico-chemical quality of lagoon-treated wastewater was acceptable whereas bacteriological quality (faecal coliforms) and
Acknowledgements
This research was funded by the Swiss Agency for Development and Cooperation (DDC). The authors thank Mr Mathieu Kaboré for his assistance in field works, Mr Dieudonné Affagnon and Mr Guy Messan Byll-Cataria for their assistance in the physico-chemical and biological laboratory analyses. The valuable contribution by the anonymous referees in the review process is also gratefully acknowledged.
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