Treatment and recovery of reverse osmosis reject by membrane capacitive deionization with integrated pretreatment

Membrane capacitive deionization (MCDI) with integrated pretreatment was demonstrated to be a feasible solution to the reverse osmosis (RO) reject from the water reclamation process. Pretreatment comprised of biological activated carbon (BAC) and microfiltration (MF), which aimed to remove dissolved organic matter and suspended solids prior to the MCDI. Considering salt removal efficiency and process efficiency, the optimal applied voltage and flowrate of the MCDI unit were determined to be 1.3 V and 0.33 L/min. Except for iron and phosphate, the removal efficiency of calcium, magnesium, potassium, sodium, ammonium, chloride, nitrate, sulfate ranged between 68.8% and 90.3%. Removal efficiency for the TDS and TOC were in the range of 72.6%-85.8% and 21.6%-28.7%, respectively. Our study demonstrates that the MCDI desalting performance was stable during the continuous 35 d operation and the recovery efficiency and rejection efficiency of MCDI are 75% and 97%. Cleaning-in-place (CIP) was carried out every 10 d continuous operation to prevent cell fouling. CIP causes expansion of the fouling layer and ion exchange reaction with the polysaccharide-calcium complex in the fouling layer, thereby reducing membrane fouling. The increase in cell pressure was controlled within 50% as compared to the starting value, which suggests that the cell fouling is well controlled by cell cleaning strategy and the measures to treat MCDI feed. Calcium phosphate was found to be the major scalant accumulated in the cell, which can be effectively removed by the acid CIP.