Water quality around Cha-Am, Thailand is of prime concern because of its extensive oceanfront beach area. Cha-Am uses an aerated lagoon system consisting of three ponds and a natural wetland to treat the municipal wastewater. A personal computer version of the storm water management model, PCSWMM, was used to simulate the effluent quality of the treatment system. Water quality samplings for total suspended solids (TSS), total Kjeldahl nitrogen (TKN), Escherichia coli (E. coli), chemical oxygen demand (COD), as well as evaporation measurements, were conducted on a bi-weekly basis for three months to calibrate the model. The four ponds were considered as four storage zones in the model. Based on the observed water quality data distribution, Monte Carlo simulation was used (1 000 iterations, 20 times) to get the most probable input concentration for each pond to determine the appropriate treatment fractions for the model. Data on daily inflow rates, pump operation and bathymetric survey also were obtained from the system operator as model input. The dynamic wave method was used with observed inflow rates to generate a continuous water quality simulation from 2015-07-19 to 2015-09-12. Observed mean treatment efficiency was 51.9%, 77.3%, 99.6% and 9.4% for TSS, TKN, E. coli and COD respectively. Observed concentrations at the outlet ranged between, 10 mg/L to 25.5 mg/L, 0.98 mg/L to 3.92 mg/L, 0.1 CFU/100 mL to 260 CFU/100 ml and 48 mg/L to 119 mg/L for TSS, TKN, E. coli and COD respectively. The treatment fraction approach in PCSWWM was able to accurately represent the outlet concentrations of TSS, TKN, E. coli and COD.