CHALLENGES OF PLANNING AND INTEGRATING FLORIDA’S FIRST LT2-COMPLIANT MEMBRANE FILTRATION FACILITY INTO AN EXISTING SURFACE WATER TREATMENT SYSTEM

摘要

Marco Island Utilities (MIU) maintains and operates two water treatment plants. The North Water Treatment Plant (NWTP) that is a 6.7 mgd facility that softens and filters fresh surface water from Marco Lakes using lime softening, granular and membrane media filtration while the South Water Treatment Plant (SWTP) is a 6.0 mgd facility that desalts brackish water from Mid-Hawthorn Aquifer Wells using Reverse Osmosis (RO). Approximately 3 mgd of finished water is currently transferred from the NWTP for blending with RO permeate at the SWTP to help stabilize the finished water. Increasing well salinity is reducing the reliable capacity and increasing the treatment cost of the SWTP. A study of the treatment facilities determined that MIU must expand the treatment of surface water at the NWTP in order to offset this reduced SWTP capacity and meet increasing long-term finished water demands. An evaluation and pilot study of potential NWTP expansion options identified membrane filtration (MF), in-line coagulation, lime softening, ultraviolet light disinfection (UV), ion exchange (IX), and RO as potential expansion processes. Increasing maintenance cost, degrading source water quality, as well as current and upcoming regulations including the Florida Department of Environmental Protection (FDEP) "bird rule" and the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2) necessitated that MIU plan to upgrade the treatment of surface water at the NWTP. It was anticipated that within 5 to 10 years the existing filters would need to be replaced and that more surface water treatment capacity would be needed to offset reduced capacity of the SWTP because of increasing well salinity. An evaluation of potential treatment rehabilitation/upgrade options determined that rehabilitating the lime reactor, while replacing the existing filters and aging submerged ultrafiltration system with a new LT2-compliant MF system was a cost-effective option that would improve overall treatment reliability, reduce maintenance and operating costs, and provide a robust means to ensure long-term compliance with pending and future regulations. Membrane filtration will also ensure regulatory compliance given the changing source watershed that may increase pathogen loading and increased salinity and organic loading. The new membrane filtration system will provide several benefits including: 1. Provide the most cost-effective long-term capital and operating cost solution for MIU 2. Eliminate capital investment that provides only short-term improvements in plant reliability and treated water quality. 3. Improve finished water quality, operability and reliability of the NWTP process. 4. Use the best available technology to manage potential increases in source water pathogens independent of decreases in UV transmittance while ensuring compliance with upcoming regulations. The design, permitting and construction of the new membrane facility presented several challenges including siting the new facilities within a limited availability footprint, the need to carefully sequence facility construction and commissioning in order to maintain plant production, accommodating future expansion, and allowing simultaneous treatment of raw (Marco Lakes) and lime softened waters. Permitting was equally challenging given that the new membrane filtration system will be the first to be permitted in Florida under the LT2 regulations and given that current FDEP rules limit log removals for Giardia and Cryptosporidium to that less than required for LT2 compliance for treatment of Marco Lakes water.