Feassibility of horticulture practices in ihydromet system along with domestic wastewater management

Abstract

The increasing resource demand and expansion of agricultural and other industrial activities for sustaining the booming human population in the last five decades have not only created a situation of resource scarcity but also set up a resource security challenge for the future. Wastewater, which was long regarded as a waste and an environmental nuisance, is now seen as a non-conventional source of resources in order to achieve sustainability and promote a circular economy. Hence, researchers all across the globe are putting efforts to develop integrated technology options that can not only cost-effectively and efficiently treat wastewater but also offer resource recovery options. In this context, our lab has developed an integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) for sustainable and economical wastewater management and resource recovery at the household level. The reactor unit, which is an integral component of iHydroMET system functioning, contains three key components, namely, bed matrix, electrode assembly and plant sapling. The positive takeaway from earlier studies with aesthetic plant (like Vinca) cultivation in the iHydroMET system configuration revealed the opportunity of testing different horticulture plants and thus providing different options to the consumers. Hence, my thesis focussed mainly on testing the feasibility of horticulture practices in the iHydroMET system configuration for the sustainable management of resources like water, nutrients, and food at a household level. Based on the local availability, fit to reactor configuration, and compatibility with hydroponics, four types of plants, namely, Spinach, Tomato, Chilli and Brinjal were selected. In order to test the adaptability of plants in the oxic and hypoxic/anoxic environments, three iHydroMET systems (each consisting of three major components: an effluent collection duct for housing 12 reactor units, drip manifold, and a wastewater reservoir with a submersible pump) were run in three different experimental conditions. These include two main experimental conditions, namely, Saturated reactor unit system with Wastewater (SW) and Unsaturated reactor unit system with Wastewater (UW), and one positive control condition, i.e., Unsaturated reactor unit system with Hoagland solution (UH). Individual reactor units hosted a specific type of plant sapling, which was embedded in the cocopeat. The plants were tested in triplicates in each experimental condition. 20 L of domestic wastewater was fed daily, while 10 L of Hoagland solution was replaced biweekly. These nutrient solutions were dripped into each reactor unit 15 ± 2 mL/min rate. All the systems were operated outdoors under ambient weather conditions from November 2021 to March 2022 months. The iHydroMET system achieved significant organics and turbidity removal within 24 h of operation with slightly higher in UW condition (COD: 86.7± 4% and turbidity: 98.8 ± 0.5%) as compared to SW condition (COD: 80.4 ± 3% and turbidity: 95.7 ± 0.9%). While TN removal efficiency achieved in the UW condition (14.5 ± 3 %) was lower than the SW condition (34 ± 7 %). The TP removal was very less (>25%) and showed direct positive relation with the plant growth in all conditions. Along with the wastewater treatment, the average plant growth and biomass accumulation was good, moderate, and stunted in the UH, UW, and SW conditions, respectively. Despite having the same feed, i.e., wastewater in the UW and SW conditions, the plants grown in the UW conditions showed better growth and development than in the SW condition. It was because SW reflects the waterlogged condition, which creates a stressful hypoxic condition in the root zone and results in stunted growth. Whereas for the same unsaturated experimental conditions in UH and UW, plants grown in the UH condition outperformed the plants grown in the UW condition because wastewater contained only about 20% nutrient strength compared to the nutritionally optimum Hoagland solution. In particular, the spinach showed excellent growth in wastewater without any nutrient supplementation. The average yield (leafy biomass) obtained in the UH, UW, and SW conditions was 22.8±3.2, 21.1 ± 5.9, and 4.4 ± 1.2 g wet weight/reactor, respectively. The iron content, a representative of nutritional value, in the spinach yield of UW condition (13.7 mg/kg) was found to be similar to that of the UH condtiton (13.4 mg/kg). Also, the yield was not found to be contaminated with E. coli. Hence, the spinach yield harvested in UW condition was comparable in terms of both quality and quantity to that of UH condition. Furthermore, substantial vegetative growth due to macronutrients availability while poor reproductive growth due to the unavailability of micronutrients was observed in the other test plants by day 75. The supplementation of lacking nutrients (K, Fe, Mn, and Mo on day 105) in wastewater not only restored and accelerated the vegetative growth of Tomato, Chilli, and Brinjal but also aided in flowering to fruiting transition. As expected, the tomato and chilli fruits count was recorded highest in the UH, followed by UW, and minimal in SW conditions. Brinjal was severely damaged by the natural pest. Overall, the plant growth/yield was affected by several factors such as the strength of nutrients solution, experimental conditions of saturated and unsaturated zones, seasonal environmental conditions, and natural pest attack. In conclusion, these findings suggest the better prospects for horticulture practice in the unsaturated configuration of the iHydroMET system than in the saturated condition.