Environmental Life Cycle Assessment of Agriculture Water Reuse From Small Water Resource Recovery Facilities: Case Studies and Spatial-Time-Variant Modeling

Introduction
Small water resource recovery facilities (WRRFs) serving communities of less than 10,000 people serve an essential function of treating wastewater to protect public health and the environment but exhibit significant environmental burdens associated with their built infrastructure, electricity use, and direct operating emissions. These systems can play a crucial role in the Food-Energy-Water (FEW) nexus since they help treat water with potential for reuse and can provide nutrient value for agricultural systems. The aging infrastructure of many small WRRFs is needing replacement in the coming decades and will also in many cases require nutrient removal modifications to meet more stringent effluent water quality permit limits to protect freshwater ecosystems (USEPA, 2015). Developing information to determine the conditions in which solutions may be the most cost-effective and environmentally sustainable will be valuable to community leaders, consulting engineers, and state regulators (Verbyla et al., 2013). In analyzing these systems, technologies that employ agricultural water reuse to local cropland can offer a great option for many small communities and thus further research highlighting case studies of their benefits and suitable application could aid in the broader adoption of the technology. The goal of this research was to evaluate the environmental life cycle resource impacts of implementing an agriculture water reuse system for small complete retention lagoons. The objectives of the study were:
(1) to assess the environmental profile tradeoffs of non-discharge lagoons that reuse treated water for non-direct human food consumption agricultural crops;
(2) use historic climate and crop data to model the long-term resource use and environmental impact tradeoffs of operating agriculture water reuse systems for small community lagoons.
Methodology
Life cycle inventory data was collected from 7 existing lagoons within Nebraska that reuse treated wastewater for local agriculture land. Most of the case studies were for communities less than 2,000 people and represented a wide range of climatic conditions from the east to west across Nebraska being representative of semi-humid, arid, and semi-arid climates that influence design and operating performance of lagoon and agriculture systems. A freebody diagram of the agriculture water reuse system is shown in Figure 1. The life cycle inventory collected for the study is shown in Table 1 and included construction, operating electricity usage, water quantity, water quality, and marginal changes occurring on the agriculture land. To help compliment the case study research, a time- and spatial-variant model for agriculture water reuse systems based on water balances was developed to estimate water reuse potential of lagoons and the corresponding resource use and emissions. The design and operation of irrigation lagoons were modeled for locations across the diverse climatic environment of Nebraska using 30 years of historic climate and crop data. Figure 2 shows how the water reuse model was integrated with the LCA model to help estimate potential environmental impacts associated with their use. Table 2 lists the independent variables, source, time step resolution, and spatial resolution utilized for the water reuse model. This included evaporation and precipitation rates, as well as the estimated historic crop evapotranspiration rates. Data on existing lagoon and cropland locations and size were used in assessing the required built infrastructure of the systems. ArcGIS Pro was used for the geospatial assessment of proximity and spatial interpolation of weather data. Assessment of the required built infrastructure, excavation, operating energy use, nutrient value, and potential crop yield improvements are assessed. Background emissions were used from Ecoinvent v3.6 database and used with characterization factors from TRACI 2.1.
Results
Results of the study provide insight into the key factors impacting the sustainability of adopting an agricultural water reuse systems at small WRRFs. Modeling long term performance of these systems helps reveal the implication of water storage, supply, demand, and weather dynamics on water reuse systems. Figure 3 shows the carbon emission intensity of the agriculture water reuse lagoon case studies characterized by source of impacts. Six of the case studies were observed to use water reuse to convert existing dryland cropland to irrigated land whereas one site used water reuse to substitute existing groundwater pumping. In all case studies, adopting agriculture water reuse systems exhibited a net emission reduction. Benefits from excavation reductions, fertilizer reductions, and improvements in crop productivity offset additional resource use with water reuse infrastructure and operating resources. Figure 4a illustrates the 30-year modeled relative max yield percentage of a water reuse system case study that converted dryland cropland to irrigated. On average over time, the water reuse system allowed for 15% increase in crop yields and helped the agriculture land resiliency during a drought year. In Figure 4b, the modeled annual water application rate is plotted relative to differences in crop evapotranspiration and precipitation rates. In this case study, both crop water maximum water demands and effective lagoon water storage capacity were rate limiting factors in how much water is applied. While the quantity of water capable of application to agriculture land is dependent on the relative size of the systems, the effects of low precipitation rates both reduces lagoon water reuse supply capacity and increases that cropland water demands. In the case of systems that use water reuse to help offset existing groundwater pumping for irrigated cropland, environmental impact tradeoffs are largely influenced by relative lagoon/farmland sizing and proximity, and depth of groundwater pumping. Figure 5 shows the change in carbon intensity of adding agriculture water reuse systems to complete retention lagoons based on these variables. Figure 5a shows that for relatively shallow groundwater, using water reuse for agriculture land within a 3-6 km proximity will result in net carbon emission reduction. Figure 5b highlights that sites with significantly deep groundwater will still exhibit net environmental benefits from water reuse at proximities less then 10km. It should be emphasized that these generalizations were based on averages made across 30-years of historic weather data and elevation differences also must be accounted for on a site-to-site basis if they contribute significantly to the pumping head.
Relevance
Provided the large number of small WRRFs throughout the world, wide climatic variability, and need for more sustainable systems, this work helps provide case study data and a framework for analyzing the environmental resource impacts of adopting agriculture water reuse systems. With many small systems experiencing stricter nutrient water quality limits, many are needing to evaluate what technologies may offer the best choice in terms of cost, ease of operability, and environmental performance. Within the context of the FEW nexus, agricultural water reuse from WRRFs will play a key role within small communities and for which its environmental impacts appears highly site-specific, time variable, and can be constrained by both supply and demand dynamics.