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Energy Efficient Configuration of RO Desalination Units

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An important consideration during membrane treatment plant design is the projected energy consumption that will result from the selected process and system configuration. Minimizing energy consumption can have a significant impact to the overall operations and maintenance (O&M) costs of a new facility. Membrane desalination processes require high feed water pressure to overcome osmotic pressure, induce meaningful permeate flow and accomplish salt water separation. The energy requirement for a desalination facility is a composite value of raw water pumping, pre-treatment losses, high pressure pumping, post-treatment losses, permeate pumping and auxiliary needs. The configuration of wastewater reclamation systems includes additional treatment steps, as compared to conventional low pressure brackish RO applications. The additional treatment steps consist of membrane filtration pretreatment and extensive disinfection, that frequently includes advanced oxidation process (AOP) and contributes to increased energy use. The energy use, related to operation of RO membrane unit, is function of unit configuration: number of desalination stages, number of membrane elements per vessel, permeate recovery rate and potential use of energy recovery devices. Majority of RO membrane units in wastewater reclamation systems, operate at low permeate flux rate, driven by low net driving pressure (NDP). Therefore, low feed pressure is required, translating into low specific energy use. However, some wastewater reclamation systems utilize large diameter membrane elements and operate at very high flux rates. In these systems, higher energy of the desalination process is compensated by lower capital cost of the membrane unit and smaller expenses of membrane replacement cost. Incorporation of energy recovery devices in low pressure RO systems operating at high recovery rate provides small energy saving. However, if the energy recovery device is installed in an interstage position and provides pressure boost, it could improve permeate flux distribution between subsequent stages. The same device could be used to provide flexible product water output, up to 10% product flow increase. Therefore, the additional cost of interstage energy recovery devices will be compensated by reduced size of RO membrane unit.

The paper will provide analysis of various components of energy use in wastewater reclamation systems, effect of system configuration and process parameters. Comparison of economics of low and high permeate flux operation will be provided.
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Keywords: Energy Usage; RO System Configuration; Reverse Osmosis

Document Type: Research Article

Publication date: 2010-01-01

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