Uncertainties associated with wastewater treatment plants inflows can lead to either undersized or oversized facilities. Such uncertainties are attributed to the deviation of actual development patterns from plans. Accordingly, a minimum-flow guarantee is a suitable risk mitigation strategy. This research proposes a guarantee evaluation algorithm, and a new stochastic model using real-options theory, in order to simulate the wastewater inflow to the wastewater treatment plant. A wastewater treatment plant in Egypt is examined as a case study. The results enable the monetary evaluation of contractual clauses relating to plant expansion and minimum-flow guarantee.