Constructed Wetlands for Wastewater Treatment Plant Resilience

Ms Cayla Anderson
The University of British Columbia - Vancouver, Canada 

Research supported by:
Dr. Rachel Scholes
Assistant Professor, Civil Engineering, UBC

Abstract: The increasing frequency and intensity of extreme storms and flooding events due to climate change threaten traditional wastewater treatment infrastructure capacity. In the North American Pacific Northwest, large flooding events have resulted in untreated and partially treated municipal wastewater bypassing treatment infrastructure and polluting local water bodies. As a result, there is a need to improve the resilience of wastewater treatment infrastructure. Constructed wetlands for wastewater treatment are an attractive addition to traditional wastewater treatment trains as they can provide additional treatment capacity during high-intensity storm and rain events. Vertical flow constructed wetlands have been used successfully to treat shock loads of wastewater from combined sewer overflows.

To further investigate the resilience of vertical flow constructed wetlands against shock loads of partially treated wastewater, nine model wetland columns were constructed in the laboratory that were fed with synthetic, secondary municipal effluent wastewater. Twice over the fourmonth experimental duration, three columns were subjected to high-intensity flooding events, modelling a one-in-100-year extreme rain event for the city of Vancouver, British Columbia. Under baseline conditions (i.e. not flooding flow rates), the column wetlands operated with an average hydraulic retention time of 2.35 days. During the high-loading simulations, the retention time was decreased to 0.24 ± 0.1 days in the three high-load columns. Under baseline conditions, the wetlands removed 85 ± 14% of the influent ammonia concentration.

All other parameters measured remained relatively constant, with no observable removal of other nutrients like phosphates and sulphates. The high-loading events produced no observable effect on pH measurements, dissolved oxygen, electrical conductivity, dissolved organic carbon and phosphates and sulphates measured in the effluent. Ammonia removal efficiency decreased to 78 ± 14% during the high-loading events. However, treatment performance resumed shortly after the shock event.

Results from the study indicate that vertical flow constructed wetlands are resilient to changes in influent-loading due to flooding and high-intensity storm events. Mass removal rates of ammonia based on the column data were calculated per square metre to estimate treatment performance during a high-load event in a larger, full-size system. However, caution is advised when extrapolating data. The vertical flow constructed wetland systems studied are a resilient technology that can potentially improve the resilience of municipal wastewater treatment trains.