True batch reverse osmosis prototype: model validation and energy savings

Abstract

In this study, we tested a bench-scale prototype of a true batch reverse osmosis (RO) system using a flexible bladder and a 2.5 in. (6.4cm) spiral wound membrane element. In theory, batch RO systems can achieve the lowest practical energy consumption by varying feed pressure over time. However, this is the first study to validate batch models by measuring the hydraulic work of both the high pressure pump and the circulation pump. The experimental measurements agree well with the model (error < 3%) after accounting for concentration polarization. We used the validated model to calculate the energy savings of true batch systems at higher salinities and recovery ratios. Previous studies assumed that a batch RO plant would operate at the same flux and steady-state feed salinity as a comparable continuous RO plant. In order to match the permeate production of a continuous RO plant, a batch RO plant must operate at an elevated flux to offset its intermittent permeate production. A batch RO plant will operate at a steady-state feed salinity higher than the plant’s intake feed salinity due to salt retention between batch cycles. As a result of these practical inefficiencies, the energy savings achievable by true batch systems are less than previously thought, but still significant at relatively high recoveries. At 50% recovery of seawater feed, a batch system could save 11% of the energy consumed by a continuous RO system while still maintaining the same level of permeate production. We have demonstrated the successful operation of a true batch system and shown that it can indeed reduce energy consumption. Keywords: batch reserve osmosis; true batch; energy efficiency; energy savings; system design