Urban wastewater systems provide an essential service to communities in developed countries. The widespread development of wastewater reticulation and treatment systems over the last 150 years has resulted in major improvements in public health, with reduced mortality and disease outbreaks, as well as significant reductions in environmental pollution.

Despite these improvements, urban wastewater systems, like any engineered system, can malfunction or fail when stressed beyond their historic.

• 1. Impacts: The “effects on natural and human systems” of extreme weather, climate events and climate change (ISO, 2019) are defined in this report as the direct (first order) effect from a climate-related risk.
• 2. Implications: The indirect (second/third order, and cascading) effects, resulting from the impact. This relates primarily to changes to environmental, social, economic and cultural values following an impact.

IMPACTS ON WASTEWATER CONVEYANCE SYSTEMS

Pipeline conveyance systems include systems where storm water overflow is conveyed in the wastewater pipe, termed ‘combined’, and systems where separate pipe networks are used to convey storm water and wastewater, termed ‘separated’. Conveyance within pipe networks can rely on either gravity to drive wastewater flow or using pumping to drive wastewater under a pressure or vacuum (negative pressure) system, thereby allowing conveyance uphill as well as down. Often pumped systems will form a small section of the network within a larger gravity system. The materials used in pressurised and vacuum pipes must be flexible and durable to ensure the pipes are fully sealed to maintain pressure.

IMPACTS OF INCREASED RAINFALL ON WASTEWATER PIPELINE CONVEYANCE SYSTEMS

The most significant impact of increased rainfall on wastewater conveyance systems was identified to be increased overflows, and blockages and breakages. Increased rainfall intensity and extreme events are likely to lead to increased occurrence of inflow and infiltration (I&I) into wastewater networks. This occurs due to storm water directly entering combined networks or infiltrating the sewer network through cracks, poorly constructed or corroded manholes and direct connections.

IMPACT OF CLIMATE CHANGE ON WASTEWATER PUMP STATIONS

Pump stations are part of the broader wastewater system and therefore many of the issues discussed above that affect pipes, also affect the pumping station, such as overloading under increased rainfall, salinity impacts from rising groundwater, and blockages arising from build-up of fats oils and grease. Increased wind severity and lightning strike associated with more extreme stormy conditions can damage exposed structures, cut off power supply or damage controls.

IMPACTS OF SEA-LEVEL RISE ON TREATMENT PLANTS

Reduced capacity for outflow and raised groundwater tables, all related to sea level rise, are likely to result in significant impacts on WWTPs. WWTPs are typically located on low-lying coastal land, to minimize the cost of collecting wastewater and discharging treated effluent, which makes them particularly exposed to coastal flooding from sea-level rise. Rising sea level also increases the risk of compound flooding (e.g. combined coastal, pluvial and fluvial events and/or raised groundwater levels). With it, the potential for direct damage to Wastewater Treatment Plants increases, which may cause disruption to WWTP processes, necessitate repair or protection works, and eventually may lead to abandonment of stranded assets.