Energy consumption and energy efficiency of high-pressure reverse osmosis: Effect of water recovery, number of stages, and energy recovery

Reverse osmosis (RO) brine is becoming a concern due to its environmental impact. One of the proposed solutions to manage RO brine is to treat it using high-pressure reverse osmosis (HPRO) to increase potable water production and achieve near-Zero Liquide Discharge (n-ZLD). However, HPRO energy consumption is considerably high compared to conventional RO which urges the need to optimize it and make brine desalination economically feasible. In this paper, we aim to discuss possible pathways to minimize HPRO energy consumption towards n-ZLD. First, we present the impact of the recovery rate (RR) of each RO stage on the energy consumption and the energy efficiency of HPRO for a targeted total RR = 90 %. To investigate the opportunity of nZLD, we compared the energy consumption of 2-stage RO and 3-stage RO for high water recoveries (RR = 90 % with 2-stage RO versus RR = 95 % with 3-stage RO). Our analysis revealed that, depending on the values RR of each RO stage, the energy consumption of 3-stage RO can be lower than that of 2-stage RO where it reaches a minimum of 4.62 kWh m-3 (compared 5.41 kWh m-3 for conventional two-stage RO with RR = 80 %). To decide on the feasibility of 3-stage RO for high water recovery, we performed an economic analysis to estimate the levelized cost of water (LCOW). Results showed that the choice of RR in each RO stage is critical to achieve minimum energy consumption, thereby, lowering the LCOW. Finally, we investigated further improvement of the system by introducing pressure retarded osmosis (PRO) to recover the osmotic energy from the HPRO brine. The energetic and economic analysis revealed that the viability of the process is strongly dependent on the performance of the PRO membrane, PRO feed water concentration, fouling mitigation strategy, and the choice of PRO pretreatment. This study offers insights for a better energy efficient and cost-effective RO desalination process.