Zero-valent iron sand filtration can reduce human and plant pathogenic bacteria while increasing plant growth promoting bacteria in reclaimed water

Authors: SAPKOTA, AMY - University Of Maryland; KULKARNI, PRACHI - University Of Maryland; BUI, ANTHONY - University Of Maryland; BRADSHAW, RHODEL - University Of Maryland; Del Collo, Laura; HITTLE, LAUREN - University Of Maryland; HANDY, ERIC - US Department Of Agriculture (USDA); PAULSON, JOSEPH - Genentech; GHURYE, JAY - University Of Maryland; NASKO, DANIEL - University Of Maryland; East, Cheryl - Roberts; KNIEL, KALMIA - University Of Delaware; CHIU, PEI - University Of Delaware; MONGODIN, EMANUEL - University Of Maryland; POP, MIHAI - University Of Maryland; Sharma, Manan; Van Kessel, Jo Ann

Submitted to: Frontiers in Environmental Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary: Reclaimed wastewater represents an additional source of irrigation water for farmers growing fruits and vegetables as irrigation water scarcity becomes a more pressing issue in the U.S. Additional filtration of reclaimed wastewater may be required before use in irrigation to reduce levels of bacterial pathogens. Zero-valent iron/sand (ZVI/sand) filtration of reclaimed wastewater has already lowered levels of personal care product chemicals and antibiotics; mat may be utilized to reduce pathogen levels in wastewater and prevent transfer of pathogens to fruits and vegetables. In this study reclaimed wastewater was collected and passed through a ZVI/sand filter. Levels of E. coli in ZVI/sand filtered reclaimed water were significantly lower than in in unfiltered reclaimed water. Antibiotic-resistant E. coli were also reduced to non-detectable limits in ZVI/sand-filtered reclaimed water. ZVI/sand filtration also reduced levels of several species of human and plant pathogenic genera of bacteria in filtered reclaimed water compared to levels in reclaimed water as determined by microbial community analysis. Results also indicated that bacteria recovered from ZVI/sand filtered water showed exposure to oxidative stress, consistent with exposure to ZVI/sand filtration. Results of this research show that ZVI/sand filtration can improve the microbial quality of reclaimed wastewater for potential use as agricultural water. This work benefits farmers seeking alternative and supplemental sources of irrigation water.

Technical Abstract: The increasing use of reclaimed water for irrigation in areas lacking access to advanced wastewater treatment and reclaimed water distribution systems calls for an examination of irrigation-site-based treatment technologies that can improve the quality of this alternative water source. To address this need, we investigated the impact of zero-valent iron (ZVI)-sand filtration on the bacterial community structure and functional potential of conventionally treated reclaimed water utilized in downstream applications. Over a two-month period, reclaimed water was collected from a tertiary wastewater treatment plant in the Mid-Atlantic, U.S. and trucked to our greenhouse facility. The water was stored in rain barrels and then ZVI-sand filtered every five days. Filtrate was then subjected to enumeration, phylotyping, shiga toxin screening and antimicrobial susceptibility testing of Escherichia coli. Aliquots of filtrate were also DNA extracted, and purified DNA was subjected to 16S rRNA gene sequencing and metagenomic shotgun sequencing. The genera Dechloromonas, Desulfotomaculum, Leptonema and Thermomonas, which contain denitrifying and sulfate reducing species, commonly used in bioremediation, and known to increase the inherent reactivity of ZVI, were significantly more relatively abundant in ZVI-sand filtered reclaimed water compared to reclaimed water. The concentration of E. coli in ZVI-sand filtered reclaimed water was significantly lower compared to that of reclaimed water, and cefoxitin- and tetracycline-resistant E. coli were undetectable after ZVI-sand filtration. ZVI-sand filtration reduced the occurrence of human as well as plant pathogenic genera (Aeromonas, Mycobacterium, Shewanella, Acidovorax, Agrobacterium, Clavibacter) but increased the proportion of Azospira, a nitrogen fixing bacterial genera, in the microbial community. Our exploratory functional analysis showed a modest non-significant increase in the proportion of open reading frames for genes associated with iron uptake, oxidative stress, as well as defense and repair mechanisms after ZVI-sand filtration. These data indicate an iron rich environment in the filter causing an oxidative stress response by the bacterial community present in the reclaimed water. Our findings demonstrate that ZVI-sand filtration effectively filters conventionally treated reclaimed water potentially making the water suitable for agricultural irrigation. Longer-term, field-based studies are needed to evaluate the effectiveness of the filter in agricultural settings.