Sewage treatment plants and networks: climate change adaptation case study

Reducing risks to surface waters during heavy rainfall at sewage treatment plants and networks.

This future scenario outlines possible climate impacts, risks, adaptation measures and ongoing management measures at a hypothetical site in NSW.

Scenario

A Council owned sewage treatment plant, and associated network (the system) in the Sydney Region of New South Wales (NSW) has recently experienced periods of heavy rainfall. During this period, the capacity of the sewer networks was overwhelmed, and the treatment plant became flooded which impacted critical plant operations and compromised its ability to safely treat and release wastewater.

Council undertook preliminary investigations to better understand the system’s exposure to climate risk. The investigations revealed that the region has already seen a 40% increase over the past 20 years in short-duration high intensity rainfall events and is projected to worsen across the century¹. Council also found that there will be an approximate 7% rise in rainfall intensity with each degree of warming that is realised which would result in the system being even more vulnerable to extreme rainfall events².

Climate impacts

During the heavy rainfall event, the system experienced a number of impacts that resulted in overflow of unmanaged and/or untreated sewage and consequently a breach of their environmental license conditions. These impacts included:

• sewage networks and transfer pipelines being overwhelmed by a surge in water volumes from improper stormwater connections. This resulted in overflows of untreated sewage from maintenance holes, sewage pumping stations and emergency relief structures across the network and untreated sewage being discharged to surrounding waterways.
• power outages caused pump stations to go offline which resulted in untreated sewage backing up in the system and discharging into local waterways.
• increased volumes at the treatment plant which reduced the treatment efficiency leading to outflows that were not in line with licence conditions.
• The treatment plant later became flooded, which interrupted power supply. This rendered the plant inoperable and resulted in emergency discharge of untreated sewage flows to the environment.

Following the incident, Council identified that while specific components of the system were impacted in this rainfall event, there may be other components that could also be impacted by climate change. These rainfall events not only disrupt the service to users but can also contribute to pollution incidents such as nutrient loading in rivers, accelerating algal blooms, degrading water quality, increased human health risks, and a breach of environment protection licence conditions or environmental legislation.

Further, Council engineers noted that the existing system was designed using historical rainfall data and may not be able to manage current and projected rainfall trends. Without the adaptive capacity to manage increasing rainfall, sewage overflows and environmental pollution incidents could become more commonplace.

Assessing climate risk

To understand the climate change impacts on the system, and to prioritise climate risk management, Council undertook a system wide climate change risk assessment using climate projection data for rainfall, temperature, fire and extreme storms. While several climate hazards and potential climate related impacts related to the facility were identified, this scenario focusses on increased volume and intensity of rainfall, resulting in surface water pollution impacts.

The assessment was undertaken across the lifetime of the system using projected climate data for a medium and high emissions climate scenario. The system was found to be at high risk of polluting surface waters during extreme rainfall events.

Identifying adaptation measures

The climate change risk and adaptation assessment undertaken by Council revealed a number of adaptation options to improve the resilience of both their new and existing infrastructure to extreme rainfall events and to reduce the likelihood of associated environmental pollution incidents. These included:

• For existing facilities – Assessing existing sewer networks, pumping stations, and treatment plants against relevant standards and guidelines, such as the Australian Rainfall and Runoff Guidelines (ARR) 2019 v4.2 (or latest revision available), to understand the capacity of the system to abide by compliance requirements while conveying sewage under projected climate conditions. The ARR guidelines provide climate change factors for various climate scenarios, rainfall intensities, and durations and should be determined by a suitably qualified professional. The outcome of this assessment will inform if and where upgrade works may be required.
• For new or enhanced facilities – Council recognises that a proactive approach represents best practice and will therefore aim to design all new infrastructure in accordance with relevant standards and guidelines, such as the ARR 2019 v4.2 guidelines (or latest revision available). This will embed resilience into the system from the outset ensuring that flows projected under climate change can be treated to required standards and conveyed through the sewage network.
• Collaborating on systems-wide management – Council acknowledges the importance of collaboration and understands they will need to work alongside other asset owners and agencies to reduce stormwater inflows to the sewer network by improving quality assurance and quality control of new stormwater connections or through proactive measures, such as sealing manholes, relining pipes and managing existing illegal connections. Working together to rectify connectivity issues will aid in reducing the volume of water being conveyed during wet weather periods and reduce the likelihood of the system being overwhelmed.
• Prioritisation and investment in preventative maintenance of systems to ensure they are operating at maximum efficiency. Maintenance works of the network are required to minimise stormwater infiltration, blockages, breaks, or chokes which can ultimately overwhelm the entire system leading to unregulated overflows and increased environmental impacts.
• Developing and/or updating site-specific management plans, manuals and operational procedures including seasonal preparedness plans, flood risk management plans, asset management plans, and overflow management plans to manage extreme rainfall incidents. Council recognises this as a low-cost measure often only requiring an additional ‘climate’ consideration to an already existing system or plan. These plans must clearly specify roles and responsibilities to ensure accountability of implementation and maintenance of these documents. As the climate changes, the review and refinement of these administrative controls may need to occur at increasing frequency.
• Embedding redundancy in the system to ensure that if one part of the system goes offline during a peak rainfall event or if the treatment capacity is nearing its limit, the facility or process can remain functional preventing harmful discharges. This could include having backup power supply to maintain operation of critical processes or having alternative pipelines/valves to redirect flows if an issue is identified downstream.
• Repurposing existing assets could negate the need for major upgrade works. Implementation of dynamic flow control systems such as adjustable weirs, and smart valves, can allow for alternative pathways and utilise the existing network for additional capacity during rainfall events. Flows can be redirected to sections of the network unaffected by the extreme rainfall event. Lagoon based plants which have been converted to mechanical plants could also be repurposed as additional retention ponds during extreme rainfall events.

Ongoing management of climate impacts

The above adaptation measures focus on mitigating and managing an increased volume and intensity of rainfall resulting in potential pollution to surface waters. This represents a sample of the long list of potential actions the Council identified which will be included in a climate adaptation plan for the sewage treatment plant and associated sewage network.

An adaptation pathway will be developed to ensure that adaptation measures are prioritised and implemented based on key trigger points. Key trigger points will be identified based on when the climate change impacts may eventuate in environmental risk, as well as taking into consideration the age, conditions and criticality of the infrastructure and likely timelines for infrastructure upgrades. The adaptation pathway will enable the Council to address the high-risk, time critical climate change impacts now while remaining open to future adjustments as the climate evolves.

Climate risk management is an iterative process and should be reviewed periodically to ensure the risk assessment and adaptation pathway remain effective and appropriate into the future. By planning ahead and building adaptability into administrative controls and infrastructure, the sewage and wastewater industry can reduce its vulnerability, safeguard its facilities, and protect and/or improve existing environmental conditions.