Modeling risks associated with wastewater reuse in agriculture

According to the World Resources Institute, as of 2019, there are 68 countries experiencing at least medium-high water stress, with 17 experiencing extremely high water stress (Hofste, Reig & Schleifer, 2019). This is dire news, and any solution must balance the rate of human water consumption with the rate of water replenishment, with the overall reduction in freshwater usage being key for all scenarios. Two main ways to accomplish this balance are increasing agricultural efficiency and recycling wastewater (Hofste, Reig & Schleifer, 2019). Agricultural efficiency can be increased through the adoption of more advanced agricultural methods that use less water, such as aquaponics, which is the practice of growing fish and plants together in a soilless system. Aquaponics has the potential to be a reliable source of healthy food for places lacking the land required for traditional agriculture, such as urban areas and islands. However, little research has been done on factors that determine the well-being of an aquaponics system. Chapter 1 of this dissertation dealt with pathogens in aquaponics and revealed a complete lack of research on this subject. However, conclusions could be inferred from studies in the related fields of aquaculture and hydroponics. It was found that studies conducted in these systems tested ultraviolet (UV) irradiation, blue light-emitting diodes, media filtration, membrane filtration, heat, sonication, and three miscellaneous methods as pathogen control strategies. Across the pathogen control methods, water quality and flow rate were key factors determining the efficiency of disinfection. Chapter 2 explored how to model changes in pH for the purpose of prediction and decision-making. The linear regression model developed was able to provide short-term predictions for pH with an accuracy of 0.74 using the pH values of the prior two days, but improvements could be made through the incorporation of data that is collected continually, rather than daily or weekly. Though aquaponics is promising for small-scale agriculture, it cannot replace traditional agriculture. Thus, adjustments need to be made to make traditional agriculture more sustainable, such as irrigating with a combination of recycled wastewater and freshwater. One concern with this option is that wastewater can be contaminated with harmful pathogens or chemicals that can be inhaled by individuals downwind of the sprayed water. Chapter 3 modeled the risks from ammonia toxicity, as well as infection with non-tuberculosis Mycobacteria (NTM) and Legionella pneumophila, with the intention of determining whether it is safer to anaerobically digest and disinfect wastewater prior to irrigation or to avoid increased ammonia levels by not disinfecting. The results of this study indicated that NTMs pose a negligible risk, ammonia has minimal risk, and L. pneumophila can be a risk to individuals within 500 m of the active irrigator. It is therefore recommended that wastewater be anaerobically digested, or disinfected in an equivalent way, before spraying. Chapter 4 examined a similar question of the risk of respiratory infection with L. pneumophila to individuals living up to 7 km away from the irrigator. The model output found that low-pressure irrigation yields a risk of infection in excess of 10-6 infections/exposure event up to 1 km from the source, while high-pressure irrigators can cross this threshold up to 2 km from the source. The main conclusions of these studies were to utilize low-pressure irrigators, avoid irrigation on windy days, use personal protective equipment if work is required downwind of the active source, and irrigate on days where the weather conditions are conducive to bacterial inactivation. These chapters, when combined, serve to improve the functionality of water-reusing systems and quantify the risks associated with water-conserving measures