Mitigation of irrigation water using zero-valent iron treatment.

Significant problems have occurred in the U.S. with regard to the contamination of produce by pathogenic bacteria. Minimally processed produce lacks the processing and preparation hurdles, such as cooking, to aid in reduction or elimination of the occasional and incidental contamination that can lead to widespread outbreaks and national product recalls. Greater emphasis has been placed on preharvest Good Agricultural Practices and postharvest Good Manufacturing Practices, but the American food production and distribution system is vast, complex and global. Environmental fecal contamination is not uncommon in these foods, and transmission of human pathogens to plants through contaminated irrigation water has been documented under both laboratory and field conditions. This project proposes to develop and evaluate a high-volume treatment for irrigation water utilizing filtration through columns of zero-valent iron (ZVI) and sand. ZVI has been successfully used for over ten years in commercial water treatment operations to remove chemical contaminants. Evidence has described the adherence and inactivation of viruses and Escherichia coli by ZVI used in water treatment. The objective of this project is to optimize removal of E. coli O157:H7 and Salmonella from water treated by passage through ZVI-sand columns under conditions modeling commercial use.

Over the past decade, significant problems have occurred in the U.S. with regard to the contamination of produce by the enteric bacterial pathogens Escherichia coli O157:H7 and Salmonella. Minimally processed produce lacks the processing and preparation hurdles, such as cooking, to aid in reduction or elimination of the occasional and incidental contamination that can lead to widespread outbreaks and national recalls. Consequently, greater emphasis has been placed on preharvest Good Agricultural Practices and postharvest Good Manufacturing Practices to ensure safety, but the American food production and distribution system is vast, complex and global. Environmental fecal contamination is not uncommon in these foods; transmission of human pathogens to plants through contaminated irrigation water has been documented under both laboratory and field conditions. This project proposes to develop and evaluate a high-volume treatment for irrigation water utilizing filtration through columns of mixtures of zero-valent iron (ZVI) and sand. ZVI has been successfully used for over ten years in commercial water treatment operations to remove chemical contaminants. Evidence has described the adherence and inactivation of viruses and E. coli by ZVI during water treatment. ZVI is inexpensive and readily available, has a very high surface area and a long service life. The ZVI process is not based on a chemical oxidant such as chlorine and therefore does not generate disinfectant by-products. Microbial inactivation is not based on physical trapping and therefore does not require small pore or particle size or incur significant pressure fluctuations. The experimental objectives of this proposal are divided into laboratory and field objectives. Laboratory objectives at the University of Delaware include: optimize the effectiveness of ZVI/sand water treatment columns by challenging with inocula from two serotypes of E. coli (O157:H7 and O157:H12) and Salmonella Newport. Experimental variables for examination will include ratios of ZVI to sand in the treatment columns, the optimal age (e.g, level of oxidation) of columns needed to achieve maximum bacterial reduction, the effect of different levels of dissolved organic carbon in water on column effectiveness, and recovering E. coli and Salmonella with regard to temperature and survival from spot inoculation of foliar lettuce surfaces. Field objectives include incorporating ZVI columns into irrigation systems that are currently used in high tunnels and evaluating their effectiveness, and analyze leafy greens for surviving surrogates E. coli O157:H12 after irrigation with ZVI-treated water. E. coli O157:H12 will be carried in water contaminated with various sources of sterile feces (dairy cattle, pig, and poultry).

1. Design and evaluate ZVI columns to remove bacterial pathogens taking various water conditions into consideration. Bacterial cells that survive ZVI treatment will be assessed for survival and attachment to lettuce as if irrigated with ZVI-treated water. 

2. ZVI columns will be scaled-up and built into irrigation systems that are currently used in high tunnels, greenhouses and growth chambers. These systems will be used to water leafy greens and assess for bacterial survival.

These studies indicate that even though sand filtration reduced microbial populations in irrigation water, the lack of an immediate reduction in E. coli and Salmonella populations by sand filtration may make it a less effective mitigation treatment compared sand‐ZVI filtration. These results indicate the sand‐ZVI filter may provide an important, cost‐effective mitigation step in decontamination of irrigation water for small growers of leafy greens. Future work should evaluate the effectiveness of ZVI in treating surface waters used for irrigation. With optimization and further investigation, ZVI‐treatment of irrigation water may prove to be an inexpensive and simple method for maintaining compliance with LGMA standards while simultaneously allowing for more diverse sources of irrigation water to be used when irrigating leafy green commodities. Due to constraints of flow that would be necessary for irrigation of large fields and contact time with ZVI that is likely necessary for efficient removal of microbial pathogens, ZVI may be more useful for smaller farms compared to larger ones; however, this has not yet been assessed scientifically and engineering likely plays an essential role in design.