Wastewater Irrigated Vegetable Production: Contamination pathway for health risk reduction in Accra, Kumasi and Tamale – Ghana.
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2008-10-21
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Abstract
The study was conducted in three Ghanaian cities (Accra, Kumasi and Tamale) selected from different agro-ecological zones (Coastal Savanna for Accra, Moist Semi Deciduous Forest for Kumasi, and Guinea Savanna for Tamale) in two phases. The main methods used included surveys, crop and analysis, and field trials.
In phase I, questionnaire interviews were used to gather background information from a total of 1058 subjects (farmers, sellers and consumers) from the three cities on wastewater use, distribution and handling of wastewater irrigated vegetables. In addition, direct observation and focus group discussions were carried out. Samples of waste water irrigated lettuce, cabbage and spring onions were also collected from selected markets in Accra, Kumasi and Tamale to determine the current level of exposure of the Ghanaian urban population to hazardous pesticide and faecal bacteria contamination through the consumption of fresh vegetables produced with wastewater. All three vegetables were analyzed for total coliform (TC) and faecal coliform (FC) as well as helminth egg populations an all three vegetables using standard methods. Lettuce samples were also analyzed for pesticide residue.
In view of the qualitative nature of most of the results from phase I, scientific quantitative data was provided to complement qualitative results, identify intervention points and provide the baseline for assessing the effectiveness of interventions. The requisite quantitative scientific data and implications were covered in the second phase of the study.
Lettuce was used as test crop because of the higher level of FC from markets (phase I) compared to cabbage and spring onions. Tamale was also dropped because farmers used similar irrigation water sources as in Accra. At this stage, the microbial and physiochemical quality of irrigation water from different urban sources was assessed. From two vegetable production sites each in Accra and Kumasi, lettuce samples irrigated with water from drain, stream, well and piped water were collected at designated points along the “farm to fork” pathway and analyzed for TC, FC, and helminth egg populations. Attempts were also made to isolate and characterize representative types of faecal coliforms present on farm samples collected during the pathway to study to assess the potential health risk to consumers. Methods used for washing vegetables before consumption at households and street food kitchens were surveyed and modifications to improve on their efficacies in removal of FC and helminth eggs carried out in the laboratory.
Some of the agronomic practices employed by farmers who used wastewater for vegetable production could be sources of both microbiological and chemical contamination and potentially put farmers, sellers and consumers at risk. For example, 52 to 65% of farmers in Accra, Kumasi, and Tamale irrigated their crops on the day of harvesting, affirming the need to develop measures to minimize the risk associated with water use in vegetable production. Majority of the irrigated vegetable sellers in Accra, Kumasi and Tamale condemned the use of polluted surface water for growing vegetables but could not indicate which input poses the highest risk. All respondents interviewed washed their vegetable before consumption. This is an indication that the last stage before consumption could be one of the best entry points where health risks reduction strategies could be put in place.
Market vegetables from all there cities carried FC and helminthes egg populations ranging between 4.0 x 10³ to 9.3 x 10(8) /g for FC and 1.1 to 2.7 for helminth and exceeded ICMSF recommended standards. A number of different types of helminth eggs, including that of Ascaris lumbricoides, Ancylostoma duodenale, Schistosoma heamatobium and Trichuris trichiura, were also identified on lettuce, cabbage, and spring onions from the markets. Most of pesticide residues on lettuce exceeded the maximum Residue Limit (MRL) for consumption.
Faecal coliform and helminth egg productions in irrigation water from different sources exceeded WHO recommended standards for unrestricted irrigation. From all the irrigation water sources in both Kumasi and Accra, Ascaris lumbricoides was the most predominant species recorded; population density ranged between 2 to 4 eggs /l. however, heavy metal concentrations were mostly within limits of international standard. Despite poor sanitary conditions in markets, post-harvest handling, and distribution of lettuce did not significantly increase the farm-gat contamination levels. High FC levels exceeding common guidelines for food quality were also recorded on lettuce irrespective of the irrigation water source. The result also showed that apart from wastewater, contaminated soil and poultry manure also contribute to crop contamination.
Identified bacterial isolates from lettuce from lettuce sampled from the farms belonged to 9 genera (Cedecea, Enterobacter, Erwina, Escherichia, Klebsiella, Kluyvera, Aeromonas, Chryseomonas and Seratia). Presumptive test for E. coli 0154:H7 was positive for about 13% of isolates from Accra and 20% from Kumasi.
The food vendor/consumer surveys revealed that the large majority of the households and street kitchens used various methods to wash vegetables before consumption. Washing vegetables, irrespective of the method used reduced FC and helminth populations in lettuce, but at varying degrees. Attempts made to improve on selected existing washing methods showed that there was promise for vinegar (> 4 long reductions possible at increased concentration and contact time) but with high financial implications for poor households: and this could reduce its adoption potential. The WHO has set health protection level of 10 /6 DALY (Disability Adjusted Life Years) pppy, which is achievable by 6 – 7 log reduction. However, none of the methods tested achieved this level of reduction alone.
The study concluded that wastewater irrigated vegetable production threatens public health from the perspective of microbiological and pesticide contaminations. The need to reduce the potential health risks resulting from FC and helminth contamination of wastewater irrigated vegetables needs a more holistic approach than a simple focus on polluted irrigation water sources. The adoption of multiple barrier approach, where complementary risk reduction strategies are applied at various entry points before the vegetables reach the kitchen, is likely to make more significant impact.
Description
A Thesis submitted to the Department of Theoretical and Applied Biology,
Kwame Nkrumah University of Science and Technology (KNUST)In partial fulfillment of the requirements for the degree Of Doctor of Philosophy, 2008