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As detailed in the process flow diagram below, the concentrate ZLD process consists of softening by chemical precipitation followed by UF primarily to reduce carbonate and noncarbonate hardness, as well as some total organic carbon and color. Lime and soda ash are used to precipitate carbonate and noncarbonate hardness in two solids contact clarifiers, and a polymer is added to enhance settling. The pH of the softened water is adjusted and a scale inhibitor is added before water is transferred to the UF feed tank. The UF feed pumps transfer the softened water through automatic backwash strainers and a pressurized vertical UF system that removes the remaining suspended solids. The UF system was designed to provide 100 percent treatment capacity when one of the plant's four trains is offline for cleaning, maintenance, or repair. The UF elements are cleaned through permeate backwash, chemically enhanced backwash, and clean-in-place (CIP) operations. The chemically enhanced backwash uses sulfuric acid, sodium hydroxide, or sodium hypochlorite. CIP operations use citric acid, sodi- um hydroxide, or sodium hypochlorite. The UF system is housed in a pre-engineered metal building designed to accommodate additional UF membranes and other components to allow for a future expansion of the ZLD process to 1.8 MGD. After the softened water is filtered by the UF membranes, the UF filtrate is conveyed to the UF filtrate and backwash tanks. The filtrate from the UF filtrate and backwash tanks is disinfected using free chlorine before blending with the combined permeate and raw water bypass water directly upstream of the existing ground storage tank and converting to chloramines for final disinfection. In addition to the main ZLD process stream, two other process streams are associated with the concentrate ZLD process: a solids stream and a recycle stream. The system recovers over 99.3 percent of the concentrate. This occurs by collecting the five recycle/recovery streams and combining them in the equalization tank where they are recycled to the head of the ZLD process. The recycle streams include solids-laden softened water from the UF feed tank, UF pretreatment strainer back- wash, UF-neutralized chemically enhanced backwash water, gravity sludge thickener supernatant, and belt filter press filtrate. The recovery and recirculation of the various streams within the concentrate ZLD process are imperative for successful implementation. The ZLD system was designed to produce lime sludge with approximately 50 percent dry solids. Sludge removed from the solids contact clarifiers is transferred to a gravity sludge thickener. From there, solids removed from the gravity sludge thickener are pumped to the solids handling and dewatering system via rotary lobe pumps. Sludge is dewatered with two belt filter presses, which allows 100 percent redundancy. The dewatered solids are hauled from WTP No. 2 by the city's contract hauler, where they are mixed with recycled concrete for dust control and act as a binder in the production of a paver base. Project Challenges Recycle Streams. One of the project's biggest challenges was handling all the recycle streams needed to achieve 99.3 percent recovery. Because of each stream's varying volume, duration, and frequency, an equalization tank with a capacity of approximately 80,000 gallons was designed and sized to provide ample capacity for the flows. In addition to providing the requisite storage for the recycle flows, the equalization tank was also fitted with a mixing system that includes jet-type mixing eductors and a pumping sys- tem to keep solids suspended in the tank. Three recycle pumps for the equalization tank transfer the various recycle flows back to the head of the ZLD process, directly upstream of the solids contact clarifiers, where the flows are mixed with the influent concentrate. Even with the equalization tank's fairly large capacity, flow control of the recycle stream is required to avoid surges and upsets to the ZLD treatment process. As a result of the recycle flows, the process units are designed to treat flows higher than the 1.2 MGD concentrate flow to accommodate the recycle streams. For 1.2 MGD concentrate, the treatment process is designed to handle an additional 0.65 MGD of recycle flow. The equalization tank was installed below grade to allow for wateronline.com n Water Innovations THICKENINGANDDEWATERING 15