On-farm filtration technology for pathogen reduction: re-use of low hygienic quality water for vegetable irrigation

Reusing wastewater for irrigation is an effective way to recirculate plant nutrients and water, particularly in arid and semi-arid regions. However, wastewater reuse in agriculture poses several hazards for human health, because of potential introduction of pathogens into agricultural production systems. Risks are higher in developing countries, where treatment plants face several challenges in adequately treating the wastewater. In order to feasibly address such risks, a new management approach has been posed in which alternative measures act as barriers along the farm-to-fork pathway. The concept is that a cumulative effect of these barriers reduces exposure to pathogens. The overall aim of this study was to evaluate the hygienic quality of produce from agricultural systems using irrigation water contaminated with wastewater and to assess suitability of an on-farm filtering in this system. To achieve this objective, the concentration of bacteriophages, E. coli and helminth eggs was measured in lettuce, water and soil during one cropping season in an agricultural system that uses wastewater for irrigation of vegetables in Cochabamba, Bolivia. Five riverbank wells and the associated river were sampled every two weeks during the cropping season. Soil samples were taken from the five plots that were irrigated with the monitored wells when the lettuce was planted and when harvested. Composite lettuce samples were taken when harvested. In the laboratory, the reduction of bacteriophages (ɸX174 and MS2), E. coli, Enterococcus spp. and Saccharomyces cerevisiae by charcoal filters was investigated in relation to three grain diameter of filtering media. The tested parameters and levels were: two hydraulic loading rates (200 and 400 L m-2 d-1), three grain diameters of biochar (Ø = 1.4, 2.8 and 5 mm), and two inflowing levels of electric conductivities (500 and 1000 µS cm-1). The microbial concentrations found in soil, lettuce and water sources of agricultural system evidenced high probabilities of fecal contamination along the system. Two types of riverbank filtration wells were identified: protected and unprotected. Both types exhibited significant levels (circa 4 log10 E. coli, 2 log10 bacteriophages, 1 log10 protozoa cysts and 70 % helminth eggs) of microbial reduction. Protected wells had significantly higher reduction rates for all microorganisms except virus. Results from biochar filters showed 1 log10 unit removal of all the monitored microorganisms, however, only for the smallest grain diameter (1.4 mm). No difference was found in microbial removal with either tested hydraulic loading rates nor with the tested electric conductivities. Grain diameter and uniformity of filtering media were identified as main factors for microbial removal for the two tested filtration technologies. Full-scale implementation of both is considered extremely context- dependent due to need of specific geological characteristics for riverbank filtration and due to large area requirement for biochar filters.