Secunda Collieries experience a positive water balance. Because of the zero discharge principle applied by Sasol and because it is rather difficult to build storage dams, desalination using a tubular reverse osmosis pilot plant was investigated. The objectives were to prove the technical feasibility of the process and to obtain enough information:
to design a full-scale plant and
to subsequently do economic evaluations regarding operating and capital costs.
With pretreatment consisting of flocculation/clarification and dual media filtration, excellent results were obtained. Average standard flux was 700 ℓ/m2.d at pressures of ca 4 MPa and at a water recovery of 70% (which was the maximum that could be obtained even with the addition of scale inhibitor). The importance of operating the system at feed water temperatures of ca 25°C was demonstrated. Salt rejection based on conductivity was very constant at ca 96% resulting in permeate conductivity values of ca 20 mS/m. Fouling, which was mainly inorganic in nature, was reversible and could be removed by citric acid washes and regular replacement of sponge balls.
Treatment of mine water using tubular reverse osmosis is a technically feasible process. However, further research is needed to improve overall water recovery.