Quantifying Microbial Risk Factors for the Consumption of Wastewater Irrigated Salad Crops

Author: Prasert Makkaew

Makkaew, Prasert, 2017 Quantifying Microbial Risk Factors for the Consumption of Wastewater Irrigated Salad Crops, Flinders University, School of the Environment

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Wastewater reuse for irrigation in agriculture is increasingly used worldwide due to the freshwater shortage, the growth of population, and the growth of water demand for producing foods. The major public health concern is the risk from the consumption of the crops irrigated with wastewater, particularly salad crops which are eaten uncooked. This research assessed the microbial risk of wastewater irrigated lettuce from the ‘farm to fork’ continuum, to contribute to minimising the health risk from its consumption.

A field based experiment was conducted to determine the degree of Escherichia coli (E. coli) contamination on lettuces following partially treated domestic wastewater-spray and drip irrigation. A higher risk was found for spray irrigated-lettuces compared to the drip-irrigated lettuces. However, the microbial quality of irrigation wastewater, the time of harvest following the last irrigation, and climate conditions such as rainfall and sunlight, were also shown to impact the microbial quality of the lettuce at harvest.

A laboratory scale experiment investigated the volume of wastewater retained on the surface of three different lettuce cultivars, Iceberg, Cos, and Oak leaf, following submersion in wastewater of different microbial qualities as a surrogate method for estimation of contamination of spray-irrigated lettuce. The concentration of E. coli recovered from lettuce using the direct enumeration method, where E. coli were directly enumerated on the leaves after submersion or by the indirect method, where the E. coli concentration was estimated from the volume of wastewater retained by the lettuce and the E.coli concentration of the wastewater, were compared. The results demonstrated the different variety of lettuce has different wastewater retention capabilities (p < 0.01). No statistical differences (p > 0.01) were detected between E. coli counts obtained from different parts of lettuce, nor between the direct and indirect enumeration methods.

The survival of E. coli at postharvest and the decontamination process at home on wastewater-irrigated lettuce were also studied in this research. Wastewater-irrigated Cos lettuces were kept at different temperatures, 4°C and 20°C. At 4°C, there was no significant effect on the survival and growth of E. coli on wastewater submersed lettuces over 48 h; while the populations of E. coli on lettuce decreased by 0.05 log10 E. coli MPN/ 100 g over 48 hours on the lettuce kept at 20°C. Moreover, storage wastewater-irrigated Cos lettuces at 20°C for more than 48 hours causes deterioration in visual quality. Ten different home washing methods were performed to remove E. coli on wastewater-irrigated Cos lettuce leaves. The results showed that these methods could remove E. coli by 1.3 – 3.3 log10 reduction, and the greatest log10 reduction was achieved from a method using 50 ppm sodium dichloroisocyanurate (NaDCC) based chlorine tablets. However, where chemical sanitisers are not available, pre-soaking for 3 min and running under tap water for 20s was the best alternative method to apply.

A quantitative microbial risk assessment (QMRA) was used to estimate the microbial risk from the consumption of wastewater irrigated Cos lettuce, and factors influencing the microbial risk was determined by Yates analysis. Sixteen exposure scenarios were simulated using a 2-level factorial experimental design based on 4 factors: the effect of rainfall on the concentration of E. coli in irrigating wastewater, withholding period, supply chain, and the effect of decontamination process prior consumption at home. The results from QMRA and Yates analysis determined that the decontamination process at home prior to consumption was the most important factor impacting the level of E. coli contamination at the point of consumption, hence, this step should be considered a Critical Control Point (CCP) from a Hazard Analysis Critical Control Point (HACCP) approach for a wastewater irrigated vegetable production chain from a ‘farm to fork’ perspective.

Keywords: lettuce, microbial contamination, microbial risk, salad crops, wastewater irrigation

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of the Environment
Supervisor: Prof Howard Fallowfield