Zero-Valent Iron-Sand Filtration Reduces Escherichia coli in Surface Water and Leafy Green Growing Environments

Abstract

Surface water is used for irrigation by farmers. However, surface waters may be a source of bacterial foodborne pathogens which contaminate fresh produce intended for human consumption. Proposed but not finalized standards for microbial quality of irrigation water through the Produce Safety Rule (PSR) of the Food Safety Modernization Act (FSMA) in the US emphasize the need for effective reduction of levels of pathogens in surface water intended to be used on fruit and vegetable crops. This study evaluated a zero-valent iron (ZVI)-sand filtration system to reduce E. coli populations in pond water and those transferred to growing spinach plants in a field trial. Six filtration events were conducted with the same ZVI-sand or sand (S) laboratory filtration systems. Filtration systems were constructed by connecting 4 PVC pipes (1.25 L) together. ZVI-sand filters contained 50% ZVI/50% sand (0.43-0.60 mm particle size), while sand filters contained 100% sand (0.45-55 mm particle size). In each event, autoclaved pond water (PW) inoculated with E. coli (ca. 4 log CFU/ml)−8 L-was pumped (1 L/min) through each filter followed by uninoculated autoclaved PW (15 L) with samples taken throughout the filtering process for enumeration of E. coli. Data were fit to a linear model to determine reductions of E. coli levels. ZVI-sand filtration removed significantly (p < 0.05) more E. coli (1.1 log CFU) compared to sand filtration. For ZVI-sand-filtered water, there was a statistically significant (p < 0.05) difference of E. coli removal from early trials (trials 1-3, average removal 96%) than in later trials (trial 4-6, average removal 44%), suggesting that age of the ZVI-sand filters influences E. coli inactivation. Overall, ZVI-sand and sand filtration reduced E. coli populations by 70 and −10%, respectively, indicating that ZVI filtration lowered or inactivated E. coli populations while sand filters accumulated E. coli. Field trials showed that soil and spinach plants irrigated with ZVI-sand-filtered water had significantly lower E. coli levels than soils/plants irrigated with sand-filtered water or unfiltered control water. Overall, ZVI-sand filtration significantly reduced E. coli populations in water compared to sand filtration.

Escherichia coli Reduction in Water by Zero-Valent Iron–Sand Filtration Is Based on Water Quality Parameters

“…The decreased ability of 50% ZVI-sand filters to reduce E. coli in pond water after the fourth trial may be due to higher turbidity levels in pond water compared to deionized water (Table 3), due to the organic load accumulating within the filter over time effectively reducing the removal and inactivation efficiencies. The results of the linear regression model are in agreement with Kim et al (2020), where E. coli reduction by ZVI-sand filtration was reported to occur immediately after filtration [29].…”

“…In addition, we hypothesize that organic matter (measured as turbidity) blocked iron surfaces from contacting E. coli cells, resulting in decreased E. coli reductions in trials conducted with pond water. Previous studies have also shown that reduction in E. coli in pond water decreased with repeated ZVI-sand filtration through the same system over time [29]. Many studies have shown that the initial pH values played a critical role in the removal of metals or inactivated microbes [42][43][44].…”

“…Each filter system was made using 10 cm (diameter) by 30 cm (length) pieces of Charlotte polyvinyl chloride (PVC) with a 2.36 L interior volume. Columns were packed following the same procedures that were used previously [29]. To construct 50% ZVI-sand filters, each filter was filled with 1.18 L of ZVI particles (0.43-0.60 mm size, Peerless Metals, Detroit MI) and 1.18 L of sand (0.45-55 mm, Northern Filter Media, Muscatine, IA) using graduated cylinders.…”

“…Marik et al (2019) reported reductions in 2 log CFU Escherichia coli in surface water in over twenty filtration events using ZVI-sand filters [28]. Kim et al (2020) found that ZVI-sand filtration reduced Escherichia coli levels by a significantly greater extent than sand filtration in inoculated surface water [29]. Sand filters containing iron-oxide-coated sand reduced E. coli levels by 1 log CFU compared to sand filters without iron oxide [30].…”

Zero-Valent Iron Filtration Reduces Microbial Contaminants in Irrigation Water and Transfer to Raw Agricultural Commodities

“…The final column of sand was implemented to prevent any potentially dislodged iron particles from leaving the filtration unit. Sand (Northern Filter Media, Muscatine, IA, USA) and ZVI (Peerless Metals, Detroit, MI, USA) particles were prepared as previously described [22], and the filtration unit had a total pore volume of 10 L. Water was collected in an additional sterile container (4) prior to irrigation and sample collection.…”