Available at: https://digitalcommons.calpoly.edu/theses/109
Date of Award
MS in Architecture
The 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 the cost analysis in favor of the alternative system.
The alternative system costs were based on operating costs (such as electricity) and the cost of the installed components of the system averaged yearly according to the life of the parts (10-50 years). The municipal cost for water supply ($0.0049/gallon) was 55% less than the cost for the alternative system ($0.0111/gallon), but the cost savings of wastewater processing using the alternative system ($0.0086/gallon) is 31% less than the municipal cost ($0.0125/gallon). The alternative systems savings are mostly a result of on-site greywater reuse for irrigation and indicate the scope of the immediate fiscal benefit to the municipality to substitute residential greywater systems for current public wastewater services. To calculate the overall cost difference in terms of both water supply and wastewater costs, the total costs used are as if all SF residences in SLO used the system. Overall, the municipal costs for combined water supply and wastewater processing ($4,137,598/yr) were 20% less than the alternative decentralized on-site system ($5,376,735/yr).
The cost to the consumer for current water supply through the public water system ($0.0078/gallon) is 29% lower than the proposed alternative system ($0.0111/gallon). The wastewater processing cost to the consumer ($0.0076) is also 11% less than that of the alternative system ($0.0086/gallon). On this basis, the consumer cost using expected billing rates ($4,137,598) is 23% lower using the public water system than the cost of the alternative system ($5,376,735). Expected water and sewer rate increases may skew these results in favour of the alternative system where it is viable for the consumer to completely replace their water system and remove their household off the public water system to their financial benefit.
The overall energy for water supply and wastewater processing used by the public water system was 34% lower (1,216,849 kWh/yr) than the alternative system (1,855,894 kWh/yr). The alternative on-site system’s electrically-driven pump is mostly responsible for this energy use and could be virtually eliminated by using gravity feed, as is common in many parts of the world currently using rainwater harvesting technology. Solar energy is also an effective solution to eliminate fossil-fuel based electricity.
From a fiscal perspective, the alternative system costs are inflated given that an expected drop in supply cost would likely ensue with the economies of scale gained if an entire city was purchasing equipment for the alternative systems. This could override the results of the study showing the alternative system’s 20-23% higher fiscal cost than current public water system. Considering the ‘no fossil-fuel’ energy alternatives and the expected significant drop in supply cost with large scale purchasing, the alternative system provides a promising alternative residential water supply for SLO.