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

Use of zero-valent iron-modified sand filters for greywater treatment: performance evaluation and modelling using response surface methodology

The conventional sand filter when used alone for on-site treatment of greywater fails to meet different reuse standards, and hence there is a need to improve the potential of sand filters to remove different contaminants from greywater. Performance of zero-valent iron-modified (ZVI) sand filters is investigated in the present study for the treatment of real greywater. The experiments were conducted using three filters: an unmodified filter (SF) and two iron-modified filters, MSF-2 (with 2 kg of ZVI) and MSF-4 (with 4 kg of ZVI). The study evaluated the performance of these filters under different conditions: daily feed volumes of 10 L (72 L/m2/day), 20 L (144 L/m2/day), and 30 L (217 L/m2/day), as well as pause periods of 12, 24, and 36 h. The results showed that the ZVI-modified filters outperformed the unmodified filter significantly. Specifically, MSF-4 showed higher pollutant removal compared to MSF-2. The filter MSF-4 achieved 58% COD removal, 59% BOD removal, 56% NH4-N removal, 82% PO4-P removal, and a significant 1.96 log reduction in fecal coliforms. To optimize the filter operation, three key parameters, amount of ZVI, feed volume, and pause period were considered. The Box-Behnken design (BBD) with response surface methodology was employed to achieve optimization. The results of the optimization study indicated that the optimal conditions for the filters were 2.67 kg of ZVI quantity, a feed volume of 30 L (217 L/m2/day), and a pause period of 32.1 h.

The effect of a first flush rainwater harvesting and subsurface irrigation system on E. coli and pathogen concentrations in irrigation water, soil, and produce

Climate change is stressing irrigation water sources, necessitating the evaluation of alternative waters such as harvested rainwater to determine if they meet water quality and food safety standards. We collected water, soil, and produce samples between June and August 2019 from two vegetable rain garden (VRG) sites in Frederick, Maryland that harvest rainwater using a first flush system, and deliver this water to produce through subsurface irrigation. The raised VRG beds include layers of gravel, sand, and soil that act as filters. We recorded the average surface soil moisture in each bed as well as antecedent precipitation. All water (n = 29), soil (n = 55), and produce (n = 57) samples were tested for generic E. coli using standard membrane filtration, and water samples were also tested for Salmonella spp. and Listeria monocytogenes by selective enrichment. No Salmonella spp. or L. monocytogenes isolates were detected in any water samples throughout the study period. Average E. coli levels from all harvested rainwater samples at both sites (1.2 and 24.4 CFU/100 mL) were well below the Good Agricultural Practices food safety guidelines. Only 7 % (3/44) of produce samples from beds irrigated with harvested rainwater were positive for E. coli. E. coli levels in soil samples were positively associated with average surface soil moisture (r² = 0.13, p = 0.007). Additionally, E. coli presence in water samples was marginally associated with a shorter length of antecedent dry period (fewer days since preceding rainfall) (p = 0.058). Our results suggest that harvested rainwater collected through a first flush system and applied to produce through subsurface irrigation meets current food safety standards. Soil moisture monitoring could further reduce E. coli contamination risks from harvested rainwater-irrigated produce. First flush and subsurface irrigation systems could help mitigate climate change-related water challenges while protecting food safety and security.

Bacterial capture and inactivation in sand filtration systems with addition of zero-valent iron as permeable layer under both slow and fast filtration conditions

To improve bacterial capture performance and inactivate bacteria, zero-valent iron (ZVI) were added into sand columns as permeable filtration media. Both Gram-negative Escherichia coli and Gram-positive Bacillus subtilis (1.25 ×10⁷ cells/mL) could be completely retained in 10 wt% ZVI amended sand columns in different ionic strength solutions (1-100 mM NaCl) at both slow (4 m/day) and fast (90 m/day) flow velocities. The strong adsorption property of ZVI contributed to the improved bacterial capture performance of sand columns. Moreover, ZVI could inactivate nearly all captured bacteria. Clearly, ZVI added as permeable layer not only could significantly enhance bacterial capture but also would inactivate the captured bacteria. ZVI could destroy the structure of extracellular polymeric substance and cell membrane. Intracellular oxidative stress was then increased and ATP content was decreased, causing bacterial death. Furthermore, high bacterial capture efficiencies were achieved with the coexisting of humic acid (0.2-5 mg/L), in actual river water samples, and longtime filtration processes. ZVI could be regenerated and reused as permeable layer to efficiently capture bacteria. Furthermore, sand columns with 10 wt% ZVI amendment could completely capture and inactivate 4.0 × 10⁶ cells/mL algae. Clearly, ZVI amended sand filtration systems have potentials to purify water contaminated by pathogenic bacteria and algae.

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

Improving the microbial quality of agricultural water through filtration can benefit small farms globally. The incorporation of zero-valent iron (ZVI) into sand filters (ZVI–sand) has been effective in reducing E. coli, Listeria spp., and viruses from agricultural water. This study evaluated ZVI–sand filtration in reducing E. coli levels based on influent water type and the percentage of ZVI in sand filters. A ZVI–sand filter (50% ZVI/50% sand) significantly (p < 0.001) reduced E. coli levels in deionized water by more than 1.5 log CFU/mL compared to pond water over six separate trials, indicating that water type impacts E. coli removal. Overall reductions in E. coli in deionized water and pond water were 98.8 ± 1.7% and 63 ± 24.0% (mean ± standard deviation), respectively. Filters constructed from 50% ZVI/50% sand showed slightly more reduction in E. coli in pond water than filters made from a composition of 35% ZVI/65% sand; however, the difference was not statistically significant (p = 0.48). Principal component analysis identified that the turbidity and conductivity of influent water affected E. coli reductions in filtered water in this study. ZVI–sand filtration reduces Escherichia coli levels more effectively in waters that contain low turbidity values.

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

Groundwater depletion is a critical agricultural irrigation issue, which can be mitigated by supplementation with water of higher microbiological risk, including surface and reclaimed waters, to support irrigation needs in the United States. Zero-valent iron (ZVI) filtration may be an affordable and effective treatment for reducing pathogen contamination during crop irrigation. This study was performed to determine the effects of ZVI filtration on the removal and persistence of Escherichia coli, and pepper mild mottle virus (PMMoV) in irrigation water. Water was inoculated with E. coli TVS 353, filtered through a ZVI filtration unit, and used to irrigate cucurbit and cruciferous crops. Water (n = 168), leaf (n = 40), and soil (n = 24) samples were collected, the E. coli were enumerated, and die-off intervals were calculated for bacteria in irrigation water. Variable reduction of PMMoV was observed, however E. coli levels were consistently and significantly (p < 0.05) reduced in the filtered (9.59 lnMPN/mL), compared to unfiltered (13.13 lnMPN/mL) water. The die-off intervals of the remaining bacteria were significantly shorter in the filtered (−1.50 lnMPN/day), as compared to the unfiltered (−0.48 lnMPN/day) water. E. coli transfer to crop leaves and soils was significantly reduced (p < 0.05), as expected. The reduction of E. coli in irrigation water and its transfer to crops, by ZVI filtration is indicative of its potential to reduce pathogens in produce pre-harvest environments.

Pathogen and Surrogate Survival in Relation to Fecal Indicator Bacteria in Freshwater Mesocosms

The microbial quality of agricultural water for fresh produce production is determined by the presence of the fecal indicator bacterium (FIB) Escherichia coli, despite poor correlations with pathogen presence. Additional FIB, such as enterococci, have been utilized for assessing water quality. The study objective was to determine the survival times (first time to detect zero or censored) of FIB (E. coli and enterococci), surrogates (Listeria innocua, Listeria seeligeri, Salmonella enterica serovar Typhimurium, and PRD1), and pathogens (four strains each of pathogenic E. coli and Listeria monocytogenes and five Salmonella serovars) simultaneously inoculated in freshwater mesocosms exposed to diel and seasonal variations. Six separate mesocosm experiments were conducted for ≤28 days each season, with samples (sediment/water) collected each day for the first 7 days and weekly thereafter. Microorganisms survived significantly longer in sediment than in water (hazard ratio [HR] for water/sediment is 2.2; 95% confidence interval [CI], 1.79 to 2.71). Also, FIB E. coli survived significantly longer than FIB enterococcus (HR for enterococci/E. coli is 12.9 [95% CI, 8.18 to 20.37]) after adjusting for the sediment/water and lake/river effects. Differences in the area under the curve (calculated from log CFU or PFU over time) were used to assess pathogen and surrogate survival in relation to FIB. Despite sample type (sediment/water) and seasonal influences, survival rates of pathogenic Salmonella serovars were similar to those of FIB E. coli, and survival rates of L. monocytogenes and pathogenic E. coli were similar to those of FIB enterococci. Further investigation of microbial survival in water and sediment is needed to determine which surrogates are best suited to assess pathogen survival in agricultural water used in irrigation water for fresh produce. IMPORTANCE Contamination of fresh produce via agricultural water is well established. This research demonstrates that survival of fecal indicator bacteria, pathogenic microorganisms, and other bacterial and viral surrogates in freshwater differs by sample type (sediment/water) and season. Our work highlights potential risks associated with pathogen accumulation and survival in sediment and the possibility for resuspension and contamination of agricultural water used in fresh produce production. Specifically, a greater microbial persistence in sediments than in water over time was observed, along with differences in survival among microorganisms in relation to the fecal indicator bacteria E. coli and enterococci. Previous studies compared data among microbial groups in different environments. Conversely, fecal indicator bacteria, surrogates, and pathogenic microorganisms were assessed within the same water and sediment mesocosms in the present study during four seasons, better representing the agricultural aquatic environment. These data should be considered when agricultural microbial water quality criteria in fresh produce operations are being determined.

"Zooming" Our Way through Virtual Undergraduate Research Training: A Successful Redesign of the CONSERVE Summer Internship Program

The COVID-19 pandemic has had an enormous impact on education globally, forcing the teaching community to think outside the box and create innovative educational plans to benefit students at home. Here, we narrate how the undergraduate, laboratory-based Summer Internship Program of our CONSERVE Center of Excellence, which focuses heavily on engaging women and underrepresented minorities in STEM programming, took a turn from an in-person research experience to a fully virtual one. We share our challenges and how we overcame them. Additionally, we provide a description of our virtual internship professional development curriculum, as well as the creative research projects that our seven interns were able to achieve in an 8-week virtual internship, including projects focused on the microbiological water quality of recycled irrigation water; social media promotion, enhancement and marketing of online educational resources focused on water, microbial contamination, and food crop irrigation; decision support systems for using recycled water in agricultural settings; and the effectiveness of zero-valent iron sand filtration in improving agricultural water quality, to name a few. Upon evaluating our internship program, we observed that more than 80% of our interns were either very satisfied or satisfied with the overall virtual internship experience. Through this experience, both the educators and the interns learned that although a virtual laboratory internship cannot completely replace in-person learning, it can still result in a very meaningful educational experience.

Designing the Next Generation of Fe0-Based Filters for Decentralized Safe Drinking Water Treatment: A Conceptual Framework

The ambitious United Nations Sustainable Development Goal for 2030 to “leave no one behind” concerning safe drinking water calls for the development of universally applicable and affordable decentralized treatment systems to provide safe drinking water. Published results suggest that well-designed biological sand filters (BSFs) amended with metallic iron (Fe0-BSFs) have the potential to achieve this goal. Fe0-BSFs quantitatively remove pathogens and a myriad of chemical pollutants. The available data were achieved under various operating conditions. A comparison of independent research results is almost impossible, especially because the used Fe0 materials are not characterized for their intrinsic reactivity. This communication summarizes the state-of-the-art knowledge on designing Fe0-BSFs for households and small communities. The results show that significant research progress has been made on Fe0-BSFs. However, well-designed laboratory and field experiments are required to improve the available knowledge in order to develop the next generation of adaptable and scalable designs of Fe0-BSFs in only two years. Tools to alleviate the permeability loss, the preferential flow, and the use of exhausted filters are presented.

Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe0 specimens for water treatment are typically small in size. Before the advent of this technology and its application for environmental remediation, such small Fe0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe0 technology are demonstrated. The misuse of the terms “reactivity”, and “efficiency”, and adsorption kinetics and isotherm models for Fe0 systems is also elucidated. The immense importance of Fe0/H2O systems in solving the long-lasting issue of universal safe drinking water provision and wastewater treatment calls for a science-based system design.

Characterizing a Newly Designed Steel-Wool-Based Household Filter for Safe Drinking Water Provision: Hydraulic Conductivity and Efficiency for Pathogen Removal

This study characterizes the decrease of the hydraulic conductivity (permeability loss) of a metallic iron-based household water filter (Fe0 filter) for a duration of 12 months. A commercial steel wool (SW) is used as Fe0 source. The Fe0 unit containing 300 g of SW was sandwiched between two conventional biological sand filters (BSFs). The working solution was slightly turbid natural well water polluted with pathogens (total coliform = 1950 UFC mL−1) and contaminated with nitrate ([NO3−] = 24.0 mg L−1). The system was monitored twice per month for pH value, removal of nitrate, coliforms, and turbidity, the iron concentration, as well as the permeability loss. Results revealed a quantitative removal of coliform (>99%), nitrate (>99%) and turbidity (>96%). The whole column effluent depicted drinking water quality. The permeability loss after one year of operation was about 40%, and the filter was still producing 200 L of drinking water per day at a flow velocity of 12.5 L h−1. A progressive increase of the effluent pH value was also recorded from about 5.0 (influent) to 8.4 at the end of the experiment. The effluent iron concentration was constantly lower than 0.2 mg L−1, which is within the drinking-water quality standards. This study presents an affordable design that can be one-to-one translated into the real world to accelerate the achievement of the UN Sustainable Development Goals for safe drinking water.

Making Rainwater Harvesting a Key Solution for Water Management: The Universality of the Kilimanjaro Concept

Rainwater is conventionally perceived as an alternative drinking water source, mostly needed to meet water demand under particular circumstances, including under semi-arid conditions and on small islands. More recently, rainwater has been identified as a potential source of clean drinking water in cases where groundwater sources contain high concentrations of toxic geogenic contaminants. Specifically, this approach motivated the introduction of the Kilimanjaro Concept (KC) to supply fluoride-free water to the population of the East African Rift Valley (EARV). Clean harvested rainwater can either be used directly as a source of drinking water or blended with polluted natural water to meet drinking water guidelines. Current efforts towards the implementation of the KC in the EARV are demonstrating that harvesting rainwater is a potential universal solution to cover ever-increasing water demands while limiting adverse environmental impacts such as groundwater depletion and flooding. Indeed, all surface and subsurface water resources are replenished by precipitation (dew, hail, rain, and snow), with rainfall being the main source and major component of the hydrological cycle. Thus, rainwater harvesting systems entailing carefully harvesting, storing, and transporting rainwater are suitable solutions for water supply as long as rain falls on earth. Besides its direct use, rainwater can be infiltrating into the subsurface when and where it falls, thereby increasing aquifer recharge while minimizing soil erosion and limiting floods. The present paper presents an extension of the original KC by incorporating Chinese experience to demonstrate the universal applicability of the KC for water management, including the provision of clean water for decentralized communities.