Design of a 14.8 MGD High Recovery Rate RO Plant for Reclamation of Municipal Wastewater Effluents Utilizing Advanced Membrane Technology

The process of reclamation of secondary or tertiary municipal wastewater effluent utilizing advanced membrane technology is well established. Numerous large capacity systems operate worldwide with a standardized configuration with very reliable performance. This configuration of an advanced wastewater treatment plant (AWWTP) consists of membrane filtration feed water pretreatment, followed by reverse osmosis and advanced oxidation processes (UV with addition of hydrogen peroxide or hypochlorite). The product water, after disinfection and post treatment stabilization, is usually injected into an aquifer for indirect potable use or utilized for industrial applications. In some limited cases product water is added to a potable water network in direct potable use. The RO systems are designed to operate at permeate recovery rates of 75% – 85%, configured as two or three stage units with 6 to 7 elements per vessel. As an AWWTP usually treats only a fraction of the effluent from the municipal wastewater plant, the RO concentrate and other residual streams are often disposed of by sending it back to the wastewater plant. In the last few years in the US, the cost of disposal of the AWWTP waste stream to the wastewater reclamation plant has risen steeply, resulting in a significant contribution to the annual operating cost. This expense stimulated efforts to reduce discharge form the AWWTP through increased recovery rates of the membrane filtration feed water pretreatment unit and the RO unit.
Currently, a number of AWWTP’s are being designed to operate at a total recovery rate over 90%. The increase in recovery rate of the membrane filtration unit requires implementation of a secondary system for recovery of backwash and membrane cleaning waste streams.. The recovery rate of the RO unit is mainly limited by the presence of scaling constituents in the feed water; calcium, barium, sulfates, phosphates and silica. The increase in recovery rate of the rRO is achieved through the application of a more effective chemical pre-treatment process and innovative membrane configuration. In some cases, hybrid salt reduction membrane technology is being applied to reduce flow rate of the RO concentrate stream. The paper will discuss different process alternatives that were considered for design of 14.8 MGD high recovery commercial AWWTP project, located in southern California and awarded recently to a Shea/Tetra Tech/Biwater consortium. The paper will discuss the importance of process parameters that have to be considered in process optimization and provide evaluation of economic impact of high recovery design on capital and operating cost of wastewater reclamation projects.

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