Can we incorporate climate change principles into wastewater infrastructure design?

As wastewater professionals, the changing climate has significant impacts on our work.

For example, with regards to storm events (i.e. the 25-year storm), projected increases in precipitation will alter the recurrence intervals, meaning that storm magnitudes observed now will be greater in the future (National scale) – this will have important implications on wastewater system design.

Through WSP’s Future Ready initiative, we have studied climate change trends to better understand the increased variability in the climate. We are now observing an increase in land surface temperature, as well as an increase in the heat content of the oceans. This in turn correlates to an increase in water vapour in the atmosphere, and a decline in Arctic ice during the summer months. All of this alters the water cycle; some areas will have more variability in precipitation. We can expect winters will become wetter, with more rain rather than snow, and extreme rainfall events will be twice as common by 2050.

NASA Earth Observatory

The integration of this knowledge into decision-making processes has yet to happen in earnest in the water and wastewater field. This may be due to the large degree of uncertainty in the data available, and to the increase in capital costs required to design and construct infrastructure capable of handling these changes. It might also be due to the lack of regulations forcing municipalities to clearly demonstrate they are addressing climate change.

The City of Barrie has taken a leadership role in trying to understand and engage with the best climate change data available to them. Barrie’s Climate Change Adaptation Strategy reviewed and modelled future climate data to inform a decision around sizing a new peak attenuation facility at their Wastewater Treatment Facility (WwTF).

To analyze the potential implications of climate change on wastewater infrastructure, a simple flow chart was established. This flow chart is in line with the Engineers Canada Public Infrastructure Engineering Vulnerability Committee (PIEVC) protocol as well as Infrastructure Canada’s Climate Lens general guidelines.

WSP was retained to identify a preferred solution for addressing peak flows in the City of Barrie’s wastewater collection infrastructure, and to enhance the capacity of the Wastewater Treatment Facility (WwTF) to accommodate peak flow events. It was determined that installing a single peak attenuation facility would be the preferred solution. Following this decision, climate change impacts on this new peak flow facility were further scrutinized to ensure the tank was adequately sized without being unnecessarily large.

There are many climate parameters which have the potential to affect water and wastewater infrastructure. Therefore, it is important to focus the analysis on the specific climate parameters which will directly impact the infrastructure being assessed. Table 1 below summarizes the variables and parameters which can be included in an analysis of water and wastewater assets with a brief description of their interactions.

Since the Barrie study’s focus was a new peak attenuation tank and to enhance the capacity of the WwTF to accommodate peak flow events, we focused on precipitation.

Based on the City of Barrie’s compilation of recent climate change projections, we were able to leverage from the projections for temperature, precipitation, heavy and extreme rain, water levels and water temperature.

Across Ontario, most municipalities have completed similar climate change projection reports, which provide an excellent design basis.

In our case, Barrie had utilized global climate models and emission scenarios and developed some very specific projections (below). Full details on the precipitation projections are presented in the report, including high and low Greenhouse Gas (GHG) emission scenarios.

Summary of Projected Climate Changes in the City of Barrie (excerpt from City of Barrie Climate Adaptation Report)

Critical to understanding the impact of climate change on the infrastructure is understanding the climate-infrastructure interaction. This is discussed in detail in the PIEVC protocol and is illustrated in the figure below.

Climate / Infrastructure Interaction (excerpt from PIEVC Protocol)

For sizing a peak attenuation facility, the focus is on the collection system-precipitation interaction, where precipitation enters the collection system in two ways, inflow and infiltration (I&I).

• Inflow refers to rainfall and snow melts that enter the wastewater collection system from direct connections, such as downspouts, cross sections, storm drains and maintenance hole covers.
• Infiltration occurs when groundwater enters the collection system through cracks and openings in maintenance holes, laterals, and sewer pipes. Potential issues that can result from high levels of I&I are capacity limitations in the wastewater collection system, increased flows to the treatment plant and potential dilution of the organic load to the plant which could affect the efficiency of the treatment process if improperly managed.

The level of I&I increases as the level of precipitation increases.

To assess capacity constraints in the wastewater collection system under existing conditions, a dynamic hydraulic model was developed and calibrated. The model is fully dynamic and includes the City’s sanitary trunk sewer system as well as all the city’s pumping stations and the treatment plant.

System performance was assessed using the calibrated model during dry and wet weather conditions. From the model, it was determined that the wastewater collection system has adequate capacity to convey peak dry weather flows and no capacity concerns were identified.

To assess wet weather performance, several design storm events (2-year, 5-year, 10-year, and 25-year storm) were used as input to the model. The storm events were also adjusted to reflect the potential impacts of climate change. For all these storm events, surcharge conditions were predicted in some sections of the wastewater collection system. In some cases, surcharge conditions were excessive.

Model adjustments were made to include flows from future growth in the years 2021, 2036 and 2041. The model results were used to assess storage requirements for the WwTF when heavy rainfall events occur.

Modelling results for future peak wet weather flows were determined. Based on a peak capacity of the plant, it was clear that an equalization tank is required to manage peak flows regardless of the rainfall distribution.

The tank storage requirements were determined to be between 250 m³ and 12,000 m³, depending on the rainfall distribution utilized in the analysis. The City is incorporating this analysis along with other practical concerns such as land availability, geo-technical conditions and capital costs. Current capital plans for the city include a 12,000 m³ peak attenuation tank.

The infrastructure we are designing and constructing today will be still in service over 100 years from now. We, as engineering professional, are obligated to weigh the effects of climate change in our designs. We can no longer omit this type of analysis in our design work, as there is extensive data and research available documenting future climate scenarios in Ontario.

However, designing and constructing infrastructure to address climate change impacts is not simple – and requires decision making under uncertainty. It also requires detailed cost-benefit and risk analysis. It is not just the addition of a new parameter or the incorporation of a new factor.

Based on the data available, every municipality can improve their climate resiliency with infrastructure specifically designed for these future changes.