PCSWMM was used to explore different wastewater treatment and urban flood management scenarios for Phnom Penh, Cambodia. Currently, wastewater in Phnom Penh is treated effectively using sustainable, naturally occurring wetlands. Urban expansion is placing increasing pressure on these wetlands and PCSWMM results showed that infilling of the largest wetland by up to 22% could have a negative impact on treatment, but the system still would function. The alternative of activated sludge treatment is shown to costly and energy intensive. Increased pump capacity at the existing stations would reduce, but not eliminate, local surface flooding. A combination of remote sensing, GPS technology, on-ground field measurements, individual interviews with 178households, direct market approach and indirect avoidance expenditure approach were used to obtain an estimate of the economic value for ecosystem services provided by the Boeng Cheung Ek wetland.