Feasibility of achieving net-zero water usage through rainwater harvesting at big-box retailers

Abstract

Premise: Increasing use of the limited supplies of freshwater worldwide, along with threats to water supplies from climate change, have led to increasing calls for improved management of water resources. UN Sustainable Development Goal 6 (SDG 6) is intended to address this issue, with a goal of ensuring "availability and sustainable management of water and sanitation for all." A key target of SDG 6 (SDG Target 6.4) is to "substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity." Motivated by altruism and public-interest in contributing to the stewardship of critical resources like water, as well as self-interest in improving sales, reputation, access to capital, and employee recruitment and retention, among other benefits, businesses are seeking to take actions consistent with the UN SDGs. As some of the largest employers in the world, big-box retailers like Walmart and Target, are well-positioned to significantly contribute to and benefit from actions directed at addressing the UN SDGs. The present article addresses the feasibility of one action that big-box retailers might take to address SDG 6.4: seeking to achieve net-zero water usage through rainwater harvesting.Process: Decentralized rainwater harvesting systems (RHS) can significantly offset demands on centralized water supply systems, thereby preserving these limited supplies of freshwater. RHS are among the earliest water supply technologies and typically involve capturing rainwater as it falls on various surfaces, such as rooftops, and storing it in cisterns for later use. With their expansive rooftop areas and moderate water usage, big-box retailers would seem to have the potential to successfully employ RHS. While water demand is relatively constant, as a function of big-box retailer size and customer traffic, water supply varies as a function of local weather and precipitation amounts and frequency. The present article considers the case of deploying a RHS in a big-box retailer in Broward County Florida, where conditions would appear to be nearly ideal for such an application (significant rainfall and never a threat of freezing temperatures). If RHS is not feasible in this situation, then it would be even less viable in regions with more challenging local conditions.The design parameters included assumed values for big-box store employee headcount of 200, rooftop area of 130,000 square feet, and daily customer traffic of 1480 people (consistent with averages for these metrics at Walmart and Target stores in the USA). A database of eleven years of daily rainfall amounts in Broward County was used in a unique, nonparametric bootstrapping model that synthetically generated 1000 years of daily rainfall amounts and then used mass balance, sequent peak techniques, to find the smallest cistern size needed to supply the constant daily water demand, without failure, throughout the whole period. It is assumed that water needed for toilet flushing will be satisfied by reusing treated greywater.Outcome: The US Energy Information Administration (EIA) has found that water consumption in large retail stores is 45 gallons per day (GPD) per worker. With 200 employees in the big-box retailer this would mean total water consumed by workers is 9000 GPD. Data on employee (and customer) usage of toilets suggests that 24 percent of total water usage is for toilet flushing, leaving 6840 GPD of water required for other uses. This demand is assumed to be constant every day of the year.It is assumed that 92 percent of the total rooftop area, or 120,000 square feet, can be used as a catchment for rainwater harvesting. Based on this catchment area, a constant daily demand of 6840 GPD, and the database of daily rainfall amounts in Broward County, the nonparametric bootstrapping model determined that a 750,000 gallon cistern would be needed to satisfy the water demand with a reliability of 99.3 percent. A concrete cistern of this size would have an estimated cost of $500,000 and an expected life of 50 years. If cistern size were reduced by 33 percent, costs would decline by 20 percent, but the reliability of the RHS to meet water demands would be reduced to 58 percent.Conclusion: A new modeling approach finds that even under highly favorable conditions, including relatively consistent and high rainfall and no freezing temperatures, use of RHS to meet all water demands for big-box retailers in Broward County, Florida, requires a large cistern and significant costs. Given the relatively low prices for water from public sources in the region, the value of other benefits (including those related to improved reputation and reduced environmental stress) would need to be significant to offset the costs and make achieving net-zero water usage through RHS at big-box retailers feasible; even in a region that appears to be so well-suited for this application.