Operational optimization of closed-circuit reverse osmosis (CCRO) pilot to recover concentrate at an advanced water purification facility for potable reuse

Abstract

Closed-circuit reverse osmosis (CCRO) was piloted to treat RO concentrate from a potable reuse facility to increase the RO recovery beyond 85%. The study determined optimum operating conditions and maintenance requirements for sustained performance at maximum recovery including flux, cross-flow velocity, and membrane cleaning intervals. The CCRO pilot included a “side conduit” to displace spent concentrate without depressurizing the membrane elements. Performance was evaluated in terms of recovery, clean-in-place (CIP) frequency, and permeate quality. Adaptive control strategies were implemented to manage feed water quality fluctuations by operating in variable recoveries where cycle-to-cycle recovery was controlled by concentrate conductivity, feed pressure and volumetric recovery. An important contribution of this study is the long-term pilot dataset collected over two years of operation, which showed the treatment of RO concentrate by CCRO to be technically feasible and operationally sustainable. The pilot operated continuously with a CIP interval greater than two months and produced permeate that met potable reuse requirements. At theoretical full scale, CCRO could increase the facility RO recovery from 85% to 91% (92% was demonstrated in a short-term run). At the future expanded plant capacity of 130 million gallons per day (MGD) (5.7 m3/s), this corresponds to a production increase to 139 MGD (6.1 m3/s).