Lifetime CostEffectiveness of Constructed Wetland and Rain Garden Facilities in Relation to Expected  Treatment Performance

Abstract: The purpose of this thesis is to investigate the cost effectiveness of two different stormwater control measure (SCM) facilities, namely a constructed wetland in Boden and a rain garden in Gothenburg (both Sweden), in relation to treatment efficiency and contributing catchment area. The study approach involved collection of data, construction, and maintenance activities with their respective costs, for the two stormwater control measures from commercial and literature data. In terms of costs, the construction costs of constructed wetland facility were provided but lacked for the rain garden facility. StromTac was utilized to model the pollutant load removal values for each facility. The pollutants focused on in this study are phosphorus (P), nitrogen (N) and total suspended solids (TSS). Using the total costs and expected pollutant load removal values, the cost-effectiveness of each facility was normalized and estimated for pollutant removal efficiency and stormwater volume. The cost-effectiveness focused on the stormwater volume introduced from the catchment area to the facility and on the pollutant load removal of the three pollutants. The findings highlighted that the rain garden facility has lower total costs value than constructed wetland, namely 1 680 000 Swedish kronor and 2 180 000 Swedish kronor for respective facilities. The rain garden facility has a lower area than constructed wetland facility.  The cost-effectiveness in terms of volume treated by the facility for 1 m3 of annual stormwater throughout facility’s lifetime is in favor for constructed wetland facility where constructed wetland facility has a value of 16 kronor, m3 while rain garden has 18 kronor, m3. The cost-effectiveness in pollutant removal for each facility shows that rain garden facility has a better value for two out three pollutants namely nitrogen and total suspended solids. The reduction efficiency values generated by StormTac shows that the constructed wetland facility has better reduction efficiency value than the rain garden facility in phosphorus, worse in nitrogen and equal for total suspended solids. A major factor that could contribute to the higher phosphorus reduction efficiency of constructed wetland in comparison to rain garden facility could depend on the fact that constructed wetland facility has a serial combination of different stormwater facilities. The following order of stormwater facilities is as follows: sedimentation pond, buffer strip and wetland. Further research could be conducted using actual construction costs and long term documented maintenance costs for both facilities to make the study more robust and accurate.