Substituting Residential Rainwater Harvesting and Greywater Reuse for Public Water Supply: Tools for Evaluating the Public Cost

Abstract

SUBSTITUTING RESIDENTIAL RAINWATER HARVESTING AND GREYWATER REUSE FOR PUBLIC WATER SUPPLY: TOOLS FOR EVALUATING THE PUBLIC COST Jennifer FergusonThe intent of this project is to provide tools for public administrators to implement and evaluate the cost of an alternative on-site residential water supply using rainwater harvesting and greywater reuse in their jurisdiction. These tools are then applied to the city of San Luis Obispo (SLO), California as a case study to demonstrate how rainwater harvesting and greywater reuse could be implemented to supply all residential potable and non-potable water needs, completely replacing the current centralized publicly-managed water system. Further, energy and direct fiscal costs of the alternative system are compared with the current system. A cost analysis is crucial given that sustainability is heavily linked to appropriately valuing a resource and increasing the visibility of same to the public. Pursuing sustainable water supply options is particularly important given critical water shortages and the need to decouple the energy/water equation in pursuit of reducing energy use and greenhouse gas (GHG) emissions.A decision tree and other tools were developed as part of this project for use by public administrators to determine the site-specific scope of an alternative residential water supply system. For example, a key question is the capacity of such a system to supply both potable and non-potable water needs. These tools were applied to single family (SF) residences in the case study city of SLO and resulted in an alternative residential system capable of completely substituting for public water supply. Implementation requires a major adjustment of indoor water demand from the SLO average of 55 gallons per capita per day (gpcd) to a 'best practice' water conservation mode of 27 gpcd, including a greywater reuse system for irrigation and toilet flushing.With demand held constant, the costs of the alternative on-site residential water supply system were then compared to the costs of the current centralized public water system for both the municipality and the consumer in SF residences in SLO. The public water supply costs were based on overall budgeted costs, including implementing a new project the city is partially financing for conveying Nacimiento Reservoir water to SLO. Consumer-billed costs include expected price increases proposed for the next year by the City largely due to the addition of the Nacimiento project. The volumetrically apportioned municipal water supply cost ($0.0049/gallon) is 37% lower than that billed to the consumer ($0.0078/gallon), but the wastewater processing cost for the City ($0.0125/gallon) is 39% greater than that billed to the consumer ($0.0076/gallon). Thus the combined water supply and wastewater processing costs for the City are only 4% greater than that billed to residential customers. It is notable that the City intends to significantly increase water prices billed to customers over the next several years which would shift t...

Greywater as a Method of Water Conservation in Arroyo Grande

“…Greywater systems provide a partial solution to hydraulic overload because it reduces the volume of wastewater that is being sent to a treatment facility. In addition to reducing environmental hazards of system overload, greywater also reduces processing costs for users by 31% when compared to municipal wastewater processing (Ferguson, 2009). …”