Open access publications in the SLU publication database

Abstract

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.