Integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) for efficient domestic wastewater management at point sources

Abstract

In most developing countries, a substantial proportion of wastewater (~50%) is discharged without treatment, causing several environmental and health problems. It is primarily due to the lack of treatment infrastructure and low-cost technologies that can be implemented at different scales and locations. Notably, the existing wastewater treatment paradigm is largely unsustainable because of its resource-consuming (such as energy) operations. Considering wastewater as a resource rather than waste, transitioning the existing treatment plants to resource recovery plants and following a decentralization approach are among the key interventions desired to achieve the sustainable wastewater management paradigm. Consequently, the need for cost-efficient and sustainable interventions at different scales has led to the emergence of integrated technology concepts involving conventional bio-based and emerging membrane- and microbial (bio)electrochemistry based processes. Considering the lack of affordable and efficient technologies for managing wastewater at point sources such as households, isolated off-grid establishments, resorts, etc., this thesis focused on the development of innovative technology based on the integration of Hydroponics and Microbial Electrochemical technologies (named as iHydroMET). Conceptually, it utilizes the fundamental components, such as porous bed matrix, electrodes or conductive materials, and plants, of these technologies to enable various physicochemical, biological, and bioelectrochemical processes to achieve wastewater treatment with concomitant resource recovery in the form of reclaimed water for non-potable reuse, valuable horticulture yield, and electricity. First, a proof-of-concept iHydroMET system design, consisting of three main components, namely modular reactor units, drip manifold, and wastewater reservoir, was successfully demonstrated for sewage treatment and resource recovery. iHydroMET systems with an optimized set of reactor components in wastewater-saturated (i.e., water-logged) and unsaturated (i.e., free drained) conditions showed a distinct performance due to the growth of specific microbial communities in different reactor zones. For instance, organics removal was slightly higher in the water-unsaturated system (93±3.6%) than in the saturated system (87.1±2.1%). Nitrogen removal and electric voltage output were considerably higher in the saturated system (42.4±4.6%; 111±8 V/reactor) than the unsaturated system (18.4±2.9%; 95±9 V/reactor). The enhanced organics and nitrogen removal and voltage output in respective conditions were due to the high abundance of polysaccharide-degrading aerobic bacteria (e.g., Pirellula), denitrifiers (e.g., Thauera), electroactive microorganisms (e.g., Geobacter), respectively. iHydroMET configuration was found suitable for cultivating both aesthetic plants (e.g., Vinca and Money plant) and leafy green vegetables (e.g., Spinach, Peppermint, Lettuce, and Basil) in the water-unsaturated reactor condition. The understanding from lab-scale studies was then implemented in assessing the practical applicability of iHydroMET for managing source-separated greywater and sewage in field conditions using two system designs chosen according to the space requirement and intended application. The first design, a wall-mounted system coupled with a slow sand filter unit, showed PhD Abstract Ravi Kumar Yadav promise for greywater recycling by achieving efficient removal of turbidity (90±0.7%), organics (85±4.5%), nitrogen (72.9±4.4%), and phosphorous (60.6±5.1%). The second tower-type system design achieved robust sewage treatment (organics removal: 405±34 mg/L.d at 90±3%; nitrogen removal: 33±4 mg/L.d at 45±5%; turbidity removal: 98.3±0.6%), substantial vegetable yield (65.6±7 g biomass/month) without compromising on the key nutrients, and stable electric power output (75.7±4.8mW/m2). Life cycle assessment revealed that electricity consumption and plastic system framework are the major contributors to global warming (0.3 kg CO2-eq) and human carcinogenic toxicity (0.05 kg 1,4-DCB), respectively, while plant cultivation contributes substantially to environmental benefits. Field tests suggest that iHydroMET can provide a sustainable wastewater treatment solution while maximizing the on-site reuse of recovered resources at point sources. Its modular and adaptable design according to space availability, low cost components, moderate land footprint (~1.4 m2/population equivalent), and minimal energy consumption (0.2-0.4 kWh/m3wastewater), which is much lower compared to the existing technologies make iHydroMET a cost-efficient, easy-to-implement and manage technology. Further work on developing its compact prototype using eco-friendly materials, followed by its performance validation at the pilot scale, would take it to a higher technology readiness level.