In this paper, sensitivity and duality analyses have been performed on our earlier developed optimal water treatment cost model for Ghana. Linear Programming was used to formulate the model and tested with real data collected from Weija Water Headworks in Accra using Interior-Point Method to obtain solutions. The effects of variations of selected key parameters on the developed model have now been investigated. Marginal costs of water production in the selected water headworks have also been found. It is strongly recommended that all Water Headworks under Ghana Water Company Limited (GWCL) should employ at least one Operations Researcher to assist them in some of these post-optimality analyses.