Water Treatment and Washwater System Improvements Needed to Optimize Treating Higher-Turbidity Source Water

摘要

The Coastside County Water District (District) supplies drinking water to 16,000 peoplernin Half Moon Bay, California and adjoining communities. Like many small waterrnagencies, the District wants to maximize use of its highest quality source water at thernlowest cost. The District currently treats surface water from two local watersheds andrnlocal groundwater as well as raw water purchased from the San Francisco Public UtilitiesrnCommission (SFPUC). The District’s local surface water supplies are subject to highturbidityrnevents during and following winter-time storms. The high-turbidity events alsorngenerally coincide with periods when the District’s surface water supply is most-availablernand demand is lowest.rnThe District owns and operates two surface water treatment plants, the Denniston CreekrnWater Treatment Plant (DCWTP) and the Nu?es Water Treatment Plant (NWTP), thatrnsupply potable water to the District’s customers. The existing direct filtration treatmentrnprocess and washwater handling system at the DCWTP is currently not capable ofrncontinuous long-term operation when the raw water turbidity exceeds 15 to 20 NTU.rnThe District evaluated four alternative pretreatment processes as well as modifications tornits existing washwater (WWR) and solids handling system as part of an on-goingrnprogram to maximize use of its valuable local surface water source.rnThe District’s available Denniston Creek source water turbidity data indicate that thernturbidity can be as high as 200 NTU. Based on both capital and operating costs and thernvalue of its seasonally available surface water, the District determined that treatingrnsource water with turbidity is as high as 50 NTU would provide the most cost-effectivernimprovements project. An assessment of the WWR system indicated that the existingrnfacilities would not be adequate to handle the greater quantities of both washwater andrnsolids that would be generated when higher turbidity water is treated. Water suppliersrnthat are considering increasing use of higher turbidity source water supplies shouldrncarefully evaluate the capacity of its existing washwater and solids handling systems asrnpart of planned improvements.rnThe estimated probable construction cost for the project is $3.2 million. The pretreatmentrnportion of the project accounts for approximately one-half of the estimatedrnproject cost. The other one-half of the estimated project cost is for the washwater andrnsolids handling systems. The relatively high portion of the project cost associated withrnthe washwater and solids handling system is due to the need for a robust system thatrncan handle both the washwater as well as solids that are removed from the raw water.