Desalination by batch reverse osmosis (RO) of brackish groundwater containing sparingly soluble salts

Batch RO desalination is a new approach to high-recovery, energy-efficient desalination. So far, however, batch RO has been tested mostly with pure sodium chloride solutions. An important application of batch RO is desalination of brackish groundwater which, besides sodium chloride, contains sparingly soluble salts. In this experimental study of a batch RO system, we used simulated groundwater (with total dissolved solids ranging from 1180 to 3637 mg/L) following compositions of samples taken from a location in Egypt and a location in India. The groundwater contained high levels of salts which could be expected to cause scaling on the RO membrane surface. For example, the Langelier Saturation Index (LSI) for calcite reached 2.6 in the brine. Nonetheless, the system resisted scaling throughout >100 h of operation. Membrane permeability remained almost unchanged, as demonstrated by tests conducted before and after the experiments. Induction time calculations showed that salinity cycling did not fully explain the scaling inhibition. Other anti-scaling mechanisms – such as periodic flushing, osmotic backwash, and feed flow reversal – were also likely contributors. At recovery of 0.8, hydraulic specific energy consumption (SEC) was <0.5 kWh/m³ and close to that obtained with sodium chloride solution at equivalent osmotic pressure.