Denitrifying woodchip bioreactors are a best management practice to reduce nitrate–nitrogen (NO3–N) loading to surface waters from agricultural subsurface drainage. Their effectiveness has been proven in many studies, although variable results with respect to performance indicators have been observed. This paper serves the purpose of synthesizing the current state of the science in terms of the microbial community, its impact on the consistency of bioreactor performance, and its role in the production of potential harmful by‐products including greenhouse gases, sulfate reduction, and methylmercury. Microbial processes other than denitrification have been observed in these bioreactor systems, including dissimilatory nitrate reduction to ammonia (DNRA) and anaerobic ammonium oxidation (anammox). Specific gene targets for denitrification, DNRA, anammox, and the production of harmful by‐products are identified from bioreactor studies and other environmentally relevant systems for application in bioreactor studies. Lastly, cellulose depletion has been observed over time via increasing ligno‐cellulose indices, therefore, the microbial metabolism of cellulose is an important function for bioreactor performance and management. Future work should draw from the knowledge of soil and wetland ecology to inform the study of bioreactor microbiomes.

The small Mediterranean country of Malta, an island located between Italy and North Africa, has been facing and will continue to face water stress and scarcity in the coming years, receiving the designation as the most water stressed country in Europe. In the past, Malta’s water challenges were centered around water quantity concerns, but in more recent years, the severity of their water stress has been strengthened by the deterioration of the groundwater quality and quantity. Through the over-extraction of their groundwater, saltwater intrusion has steadily occurred into their freshwater aquifer systems, worsening their groundwater quality for both domestic and agricultural purposes. The objective of this paper is to conduct an extensive review on the history of Malta’s water problems and how Malta is working to combat its water scarcity concerns. Additionally, this paper investigates the impact of Malta’s water scarcity on its food and energy security challenges. Our study concludes that while Malta has obstacles to overcome in their pursuit of water security and sustainable development, the future remains hopeful, with several alternatives still available. Some of the alternatives explored in this review include wastewater reuse, increased awareness, use of alternative energy sources, rainwater harvesting, and implementation of nation-wide strategic water policies.

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HighlightsTechno-economic analysis was performed for multiple scales of bioreactors operated under a variety of conditions.The unit cost decreased as the bioreactor size increased.The unit cost increased in bioreactors with longer HRTs and bypass flow due to reduced treatment capacity.One large bioreactor was more cost-effective than multiple smaller bioreactors.Abstract. Woodchip denitrification bioreactors are a relatively new, edge-of-field technology used to reduce nitrate-nitrogen (NO3-N) from subsurface tile drainage. The removal rate of nitrate is influenced by many factors, including temperature, dissolved oxygen, and hydraulic residence time (HRT). The objective of this study was to conduct a techno-economic analysis (TEA) for four scales of woodchip denitrification bioreactors operating at three HRTs (2, 8, and 16 h), designed with bypass flow or with a low probability of bypass flow, to determine the cost to remove 1 kg of NO3-N at each bioreactor scale and at each HRT. Several assumptions were made: the flow rate required to achieve a 2 h HRT on a per m3 basis could be achieved at all scales, the same mass removal of NO3-N was achieved on a per cubic meter basis, and the 2 h HRT did not have any bypass flow at each scale. With these assumptions, the lowest unit cost was observed for the large-scale bioreactor sized to have a low probability of bypass flow at 16 h HRT, with a resulting cost of $0.74 kg-1 NO3-N removed. The highest unit cost was observed for the pilot-scale bioreactor designed with bypass flow to achieve a 16 h HRT at a cost of $60.13 kg-1 NO3-N removed. At longer HRTs with bypass flow, a greater percent removal of nitrate has been observed with a lower mass removal rate. By having a low probability of bypass flow in the design, a higher mass removal and percent removal of nitrate were observed, leading to the above results. Contrasting this trend, the total and annual costs were highest for the large-scale bioreactor and lowest for the pilot-scale bioreactor. However, it was determined that 783%, 280%, and 54% increases in total cost for the pilot-, small-, and medium-scale bioreactors would be incurred to implement the number of bioreactors (66, 24, and 4, respectively) required to treat the same volume of flow as one large bioreactor. These results can be used to inform future design decisions and inform stakeholders of the approximate unit cost of installing a denitrifying woodchip bioreactor over a range of expected field conditions. While a larger bioreactor with a low probability of bypass flow may represent a more cost-effective investment, the potential for unintended, negative byproducts needs to be considered in the design. Keywords: Denitrification, Nitrate, Tile drainage, Water quality, Woodchip bioreactor.