Flue gas desulphurisation (FGD) plant has been recently installed at Longannet Power Station with the aim of removing sulphur dioxide (SO 2 ) from the flue gases. The FGD plant produces a reduction of around 94% of the SO 2 emissions, contributing to meet the current SO 2 emissions legislation. Therefore, the use of the FGD plant contributes to extend the life of the power station. However, due to the nature of the sea-water scrubbing technology, the FGD plant, it produces an acidic effluent which needs to be controlled to meet the water discharge limits. The aim of this study is to improve the FGD plant efficiency. A thorough study of the FGD process derives to an expression for the pH and dissolved oxygen concentration in the discharge water. It serves to review the influencing factors affecting the efficiency of the FGD plant. In addition, the approximated prediction model derives to a new SO 2 load to flume limit, which is less conservative that the one in use. This would permit an increase of the FGD plant efficiency, minimising costs. Tests are recommended to obtain the unknown parameters in the proposed SO 2 load to flume limit.