Technical evaluation of a small-scale reverse osmosis desalination unit for domestic water

Tunisian standards for drinking water tolerate a maximum Total Dissolved Salts (TDS) of 1.5 g/L. The domestic water presents usually a salinity greater than 0.5 g/L. In the last few years, several small capacity reverse osmosis desalination prototypes have been marketed. They are used to desalinate brackish water with TDS lower than 1.5 g/L. The performances of such type of RO units with respect to the Tunisia tap waters are needed. A technico-economical evaluation of small-scale (100 L/day) reverse osmosis desalination unit has been studied. Water pre-treatment is composed of three filtration operations. Water is pumped through the RO membrane with maximum pressure of 6 bars. Before use, the desalinated water is treated with UV light. The salinity and the temperature of the tested domestic water are located respectively between 0.5 and 1.3 g/L and between 12 and 29 degrees C. The pre-treatment allows eliminating all the suspension matters, as the turbidity and the Solid Density Index are reduced to zero FTU and surrounding one unit respectively. No chemicals are used in the pre-treatment, so membrane scaling can not be avoided if reject water presents a high scaling power. The supersaturation relative to calcium carbonate and gypsum were estimated for reject water. Their values indicate that the tested waters have no risk to scale the RO membrane. The recovery rate of the RO unit was evaluated vs. different operating conditions such as applied pressure, raw water TDS and water temperature. The small capacity unit was able to deliver a treated water of a 100 mg/L TDS with a conversion rate ranging between 25 and 37%. The water treatment cost was evaluated at 0.01 E/L which is roughly the tenth of that of bottled table water.