Evaluation of decentralized alternatives for separate treatment and supply of indoor water: Fort Collins case study

Abstract

The replacement of aging water infrastructure in the City of Fort Collins, CO provides an opportunity to evaluate the applicability of dual distribution and decentralized water treatment alternatives in comparison to the existing conventional system. The purpose of an alternative approach is to meet future water demands and quality standards which can be achieved by treating less water through the separation of supply for outdoor irrigation and fire flow from potable demand. Energy consumption required for the production of potable water and deteriorating water quality due to water age provide motivation for this evaluation. Few existing projects have demonstrated the applicability of dual water supply and decentralized treatment at a city-wide scale. This study explores these alternative approaches separating supply of water for indoor use and how decentralized water treatment may integrate into such an approach. Four water treatment and distribution alternatives were considered in comparison to the existing system using a Multi-Criterion Decision Analysis (MCDA) tool with eleven performance metrics assessed from a triple bottom line of economic, social and environmental perspectives. Alternatives were defined as city-wide dual distribution, neighborhood-scale treatment with dual distribution, point-of-entry treatment, and separated irrigation. This study focused specifically on evaluation of dual water supply alternatives incorporating decentralized treatment which reduce additional distribution infrastructure and water age in comparison to centralized treatment. A common selection process for both neighborhood-scale and point-of-entry treatment was used to recommend the most applicable systems for decentralized alternatives. An ultrafiltration package system with chlorine disinfection was recommended neighborhood-scale system due to compact design, low chemical requirements, consistent water quality and amenability to remote monitoring. Activated Carbon/Kinetic Fluxion Media filtration with ultraviolet disinfection was recommended for point-of-entry treatment system based on low capital costs, simplistic operation, low chemical requirements, and small system size. Results of the evaluation illuminate key drivers which dictate the competitiveness of dual water supply with decentralized treatment alternatives with the existing centralized conventional model. The largest advantages are reduced chemical use, improved water age and quality, adaptability to new water management strategies and revenue opportunities from increased capacity at the existing treatment facility. Neighborhood-scale treatment and dual distribution incurs large capital costs while consuming substantially more energy due to pumping. Disruption associated with the installation of neighborhood-scale treatment facilities and dual distribution networks has a negative effect on society and intensifies environmental concerns for greenhouse gas emissions and stormwater pollution. Point-of-entry treatment increases the risk of rate changes related to drastically higher maintenance costs and personnel needs. Both alternatives are strongly affected by the lack of defined regulations for these approaches at a city-wide scale. Overall, dual distribution and decentralized water treatment alternatives were not economically competitive with the existing system and offered negligible social advantages. Environmental benefits were realized for both alternatives which can be largely attributed to improved water quality due to shorter water age. Dual distribution with central treatment alternative results suggest that separating potable water from irrigation and fire flow is a practical solution that may be competitive with conventional water production in a city-wide application. While dual water distribution for separate supply of indoor water may have some advantages over conventional systems, the decentralized alternatives do not appear to offer a competitive advantage compared to the existing conventional system.