The efficacy of zero valent iron filtration on the reduction of Escherichia coli and Listeria monocytogenes in surface water for use in irrigation

Authors: MARIK, CLAIRE - University Of Delaware; ANDERSON, BRIENNA - University Of Delaware; GARTLEY, SAMANTHA - University Of Delaware; CRAIGHEAD, SHANI - University Of Delaware; BRADSHAW, RHODEL - University Of Maryland; KULKARNI, PRACHI - University Of Maryland; Sharma, Manan; KNIEL, KALMIA - University Of Delaware

Submitted to: Environmental Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2019
Publication Date: 2/19/2019
Citation: Marik, C.M., Anderson, B., Gartley, S., Craighead, S., Bradshaw, R., Kulkarni, P., Sharma, M., Kniel, K.E. 2019. The efficacy of zero valent iron filtration on the reduction of Escherichia coli and Listeria monocytogenes in surface water for use in irrigation. Environmental Research. 173:33-39. https://doi.org/10.1016/j.envres.2019.02.028.
DOI: https://doi.org/10.1016/j.envres.2019.02.028

Interpretive Summary: The quality and availability of Irrigation water is increasingly critical in the production of fruits and vegetables. The Food and Drug Administration (FDA) Food Safety Modernization Act’s Produce Safety Rule (PSR) states that water used to irrigate fruits and vegetables will be required to meet specific microbial standards. On-farm, cost effective solutions for small, diversified farms which rely on non-traditional irrigation water sources (creeks, rivers, lakes) which can minimize the risk of contamination through irrigation water are needed to help achieve compliance with these PSR standards. Zero-valent iron (ZVI) sand filters were evaluated for their ability to reduction the levels of fecal indicator bacteria (Escherichia coli) and foodborne pathogens (Listeria monocytogenes) in pond water. ZVI is iron used from scrap metal that has been previously shown to reduce viral contaminants in irrigation water. The results presented here show that ZVI-sand filters maintained their ability to reduce a consistent amount of E. coli and L. monocytogenes over 6-month period (13 trials), which would encompass a single growing season. L. monocytogenes populations on Romaine lettuce plants irrigated with ZVI-sand filtered were lower than those irrigated with sand-filtered water. ZVI-sand filtration may provide a mitigation strategy to reduce L. monocytogenes contamination on leafy greens introduced through irrigation water. This research finding will benefit fruit and vegetable growers who have concerns about the microbial quality of their surface (non-traditional) irrigation water.

Technical Abstract: The use of surface and recycled water sources for irrigation can reduce demand on critical groundwater resources. Treatment or mitigation may be necessary for the use of these alternative water sources in order to reduce risk associated with microbial pathogens present in the water. In this study, the efficacy of a zero-valent iron (ZVI) sand filter was assessed for the reduction of Listeria monocytogenes and Escherichia coli in surface water. Water recovered from an agricultural pond was inoculated with E. coli TVS353 and an environmental L. monocytogenes isolate at 7 log CFU/mL and horizontally filtered over a six-month period through a PVC pipe filter, filled with 35%:65% (volume:volume) ZVI:sand or sand alone. Filtered water was used to irrigate lettuce and bacterial persistence on lettuce leaves was determined for 7 days post-irrigation. No significant (P > 0.05) differences in populations (log CFU/ml) reductions by sand filtration of E. coli and L. monocytogenes populations were observed. Whereas, populations of L. monocytogenes were significantly (P < 0.05) reduced using ZVI – sand filters compared to E. coli. However, both ZVI-filtered water and sand-filtered water contained significantly (p < 0.005) lower levels of both pathogens compared to unfiltered control water. Similarly, populations of E. coli on leaves of lettuce plants irrigated with ZVI-treated water were not significantly lower than populations on plants irrigated with sand-filtered irrigation water over the 7-day period. However, populations of L. monocytogenes on lettuce leaves irrigated with ZVI-treated water were significantly lower than counts on plants irrigated with sand-filtered irrigation water on days 3 and 4 post irrigation (p = 0.052 and p = 0.042 for days 3 and 4, respectively. The differences observed in reductions of L. monocytogenes and E. coli by ZVI filtration is due to the differing effect that ZVI disruption has on gram-positive and gram-negative cell walls and membranes. ZVI sand filters show promising results as an inexpensive on-farm technology for the mitigation of enteric foodborne bacterial populations in pond water over a six-month period.