Extensive extraction from aquifers often leads to deterioration in water quality. This is usually seen as an increase in the soluble salt levels which, over time, has a detrimental effect on membrane performance. A limiting factor in the efficiency of membrane operation is the recovery rate that can be achieved, particularly with ‘brackish’ water operation. A high recovery rate causes an increase in the concentration of salts in the reject water. Eventually the solubility product of some of the scaling species is exceeded resulting in salt precipitation and scale formation.
The availability of antiscalants that are specifically designed to be active against individual scaling species is essential to allow the economic operation on increasingly challenging feed waters. Deep well extraction of groundwater often results in waters with a high level of sulphate ions. This paper describes the chemistry and morphology of calcium sulphate scale and its formation mechanisms. Calcium sulphate scale crystals occur naturally in three forms, anhydrite CaSO4, hemihydrate CaSO4~0.5H2O (plaster of Paris) and the dihydrate CaSO4.2H2O. Under the temperature conditions of membrane operation the dihydrate form occurs as blade like crystals that can damage the membrane surface. Cleaning is rarely successful making calcium sulphate one of the most damaging scaling species.
The processes involved in the development of the new calcium sulphate specific antiscalant “Genesys CAS” are outlined and its performance within a complex brackish water RO system with brine recovery is described. The plant had previously suffered from calcium sulphate scaling and membranes had to be frequently cleaned, and their overall performance declined sharply. The satisfactory use of a sulphate specific antiscalant resulted in the overall plant recovery rate increasing from 48% to 61%. Using the membrane manufacturer’s software, pump energy savings were calculated based on the increased recovery. These savings, combined with a rationalisation of chemical use and increased membrane life are used to provide a model of overall cost saving.
This case study is the first in a series of three papers covering sulphate, silica and phosphate specific antiscalant development and field-trial performance.
Cost saving case study using a calcium sulphate specific antiscalant