CAPDET is a computer-assisted procedure for the design and evaluation of wastewater treatment facilities developed by the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency. The purpose of this study was to develop a procedure for applying CAPDET to the capital cost estimation of Canadian wastewater treatment plants without altering the source code. The proposed methodology is simple and efficient, requiring no additional data to that normally used in CAPDET.A total of 10 Canadian wastewater treatment plants were studied which included many of the treatment processes used in Canada. Six plants were used to develop the procedure while four plants were used to verify the procedure. The design flow for the selected plants ranged between 550 and 13 600 m3/d. Construction cost estimates generated using site-specific design information were compared with those obtained using the default database provided in CAPDET. The importance of the various unit costs on the total plant cost was also determined.Under the proposed methodology, unit costs are first adjusted to the date of construction using appropriate inflation indices. Wall concrete, slab concrete, and excavation costs are increased by an additional 15, 50, and 25%, respectively, to account for differences in construction practice. A final estimate reduction of 15% is performed to account for Canadian conditions.The procedure provides construction cost estimates that are within ± 20% of actual construction costs with a mean absolute error of 11% and an average error of 2.3%. Site-specific design data had little effect on the construction cost estimates with the exception of lagoon treatment facilities. Utilities and support facilities accounted for 20–30% of the cost estimates. The most significant cost parameters were the inflation index values and concrete and building unit costs. Key words: capital cost, construction cost estimation, wastewater treatment plant, mathematical modelling, calibration, verification.
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