START-UP PLANNING FOR A 10 MGD HIGH RECOVERY RO SYSTEM WITH DEEP WELL INJECTION

摘要

The East Cherry Creek Valley Water and Sanitation District (ECCV) and its partners in Colorado areconstructing a 10 million-gallon-per-day (MGD) brackish water reverse osmosis (RO) treatment facilitywith an ultimate capacity of 48 MGD. The plant provides a renewable water source to areas of southeastmetro Denver currently served exclusively by non-renewable groundwater. ECCV evaluated severalconcentrate disposal options, including surface discharge with wastewater effluent and to irrigationditches, evaporation basins, thermal-mechanical evaporation, and deep well injection. Objections fromlocal agencies, regulatory issues and costs resulted in ECCV selecting a combination of brineminimization and deep well injection as the concentrate management and disposal approach.The selected brine minimization process relies on pH suppression and scale inhibitors to replace thechemical softening process more commonly used prior to treatment by a secondary RO system.Eliminating the softening process significantly reduced the capital and operating costs of brineminimization system by avoiding the need for process equipment, tanks, and chemicals associated with asolids handling and disposal system. Limitations for operating the RO systems and deep injection wellrelative to feed water quality parameters are discussed in the paper and presentation. Bid costs for a 1.2-MGD brine minimization RO system, high-pressure injection pumps and a Class 1 injection wellincreased the system’s capital cost by approximately $6,000,000 or approximately $0.60 per gallon ofoverall treatment capacity. Operational costs are expected to increase approximately $0.20 per 1,000gallons of finished water for chemicals, power and membrane replacement.System startup began in November 2011 with the ultraviolet (UV) disinfection system used to treat wellwater by-passed around the RO system and blended with the RO permeate to reduce the corrosionpotential. After startup of the RO systems is completed in May 2012, the full system including lowpressure RO, high pressure RO brine minimization, UV disinfection and deep-well injection will beoperational providing the first large scale application of this approach in the municipal drinking watermarket. The ECCV staff, engineer, membrane system supplier and general contractor completedextensive planning to coordinate testing, training and initial operation of every process feature in theplant. The startup outline and contingency plans developed for weather, regulatory approval and sourcewater quality changes will be discussed in the presentation. Challenges met and lessons learned during theplanning process and startup will be the presentation’s primary focus.The complete process train was pilot tested in 2011 using a four-stage RO system, treating approximately44 gallons per minute of well water. The pilot system was operated continuously for six weeks atrecoveries ranging from 91 to 97 percent, with varying degrees of pH suppression (6.8 to 7.2) in the feedto the brine minimization system. The potential precipitation of solids in the concentrated brine due tosupersaturated conditions and the resulting potential for fouling of the injection well was a primary pointof investigation. Operational information and experience gained during pilot operations were used tooptimize the start-up strategy, develop process setpoints, and orient ECCV staff on system responses andprocess adjustments during startup.