Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/107143
PROJECTS with two-stage dual-media filtration has proven itself, it still is a challenge to meet the water quality targets for feeding RO systems. Water intake location is critical and preference is given to a deep intake. Drawing water from 20 m below the surface will guarantee a more stable feed water quality. This has a number of disadvantages: l The water temperature is typically 2 – 3 degrees lower than at the surface. This will increase the viscosity of the water and consequently the operating pressure. This leads to a higher energy consumption l The installation cost for the intake structure is higher as it needs to be extended out further and deeper. UF PRETREaTmEnT CaSE I Gas Atacama operates a combined cycle thermal power plant in Mejillones with an installed capacity of 780 MW. It supplies electricity to the residential market through the Northern Electricity Grid (SING) and to several of the largest mines in the northern region. The power plant uses SWRO desalination as the single source of water. Based on an extensive tender evaluation, the customer selected a dual-membrane desalination system: ultrafiltration pretreatment followed by RO desalination. The desalinated water is fed to a final demineralization step. Main contractor for this project was Proyectos y Equipos (Chile). The UF and the SWRO system was supplied by Unitek (Argentina). The desalination plant has been commissioned in June 2011 and has been in operation since. The feed water is characterized by seasonal variations in turbidity: during autumn and winter, the turbidity ranges from 1 to 5 NTU; during spring and summer it ranges from 3 to 35 NTU. Red tide events occur as well, typical frequency is 1 to 3 times per year with a duration of up to a week. Feed water SDI is 18 (SDI5), or even immeasurable. Figure 2 shows the fouling on the test membranes after 5 minutes of filtration. The ultrafiltration system (Figure 3) supplies a capacity of 285-350 m3/h. The level in the intermediate buffer tank controls the flow set-point of the UF system. This level will change, depending on demand of the SWRO system and of the UF backwash pumps. The pretreatment prior to UF consists only of straining, therefore the UF design parameters have been chosen conservatively. For a plant of this size the cost implications of a conservative UF design are relatively small. The cost savings on pretreatment prior to UF outweigh the additional cost of UF membranes. The UF performance has been very stable at these settings. All UF permeate SDI tests show SDI15 <2. UF design parameters: l Flux rate: 75 L/m2 hr l Backwash interval: 25 minutes l CEB interval: 12 hours l UF recovery: ± 90% The reverse osmosis has not been requiring a clean-in-place since start up. RO pressure drop and permeate flow capacity remain stable since the start-up. Figure 4 (page 24) shows the UF units (in the front) and the RO units (behind) in operation. In close proximity to the dual membrane plant, a second desalination system has been operated since 2000. This system uses conventional pretreatment: DAF and single-stage media filters. According to the information reported by the end-user, the added value of the UF pretreatment is: l Strong reduction of chemicals consumption Conventional plant: 25 ppm coagulant + Figures 2: SDI results of Feed Water Figure 3: UF System | 22 | Desalination & Water Reuse | August-September 2012 flocculant addition Dual membrane plant: 1 ppm coagulant, no flocculant l No sludge treatment/disposal necessary for the dual membrane plant. The UF backwash water blended with RO brine, meets all discharge limits l Low CIP frequency Conventional plant: Autumn – Winter, 1 CIP per month; Spring – Summer, 2 CIP's per month Dual membrane plant: none l Low cartridge filter replacement Conventional plant: every CIP Dual membrane plant: negligible. UF PRETREaTmEnT CaSE II This is a typical copper mine in Northern Chile with a port facility about 80 km northwest of the mine. The mine has been in operation for a number of years. The current supply of fresh water is by aquifer abstraction. This is a nonsustainable solution: the aquifer is being depleted over time. To safeguard the water supply, a new desalination plant is under construction. The project will be executed in two phases: Phase 1 will have a capacity of