Under normal conditions, wastewater treatment plants can process these flows provided the combined volume remains within the plant's treatment capacity.
Many treatment plants connected to combined sewer systems are equipped with retention basins designed to temporarily store excess rainfall. Once the rain subsides, the stored water is gradually released for treatment.
However, during prolonged or intense rainfall events, or periods of rapid snowmelt, these systems can quickly become overwhelmed. When both treatment plant capacity and retention basin storage are exceeded, overflow structures discharge a mixture of untreated sewage and stormwater directly into nearby rivers, estuaries, lakes, or coastal waters.
The environmental and public health consequences can be severe. Sewage-contaminated waters may contain harmful bacteria, viruses, and other pollutants, affecting water quality, marine ecosystems, bathing waters, and beach sand. Such contamination can render recreational waters unsafe for public use.
In a recent video, a respected Professor of Microbiology expressed serious concerns that beaches affected by sewage contamination may not always be tested before being declared safe for continued public use. If accurate, this raises significant questions about public health protection and whether beach users could unknowingly be exposed to contaminated water and sand.
Modern sewer infrastructure avoids many of these problems by separating wastewater and stormwater into different pipe networks. Wastewater from homes and industries is directed to treatment plants, while surface water runoff is conveyed separately to rivers, lakes, or the sea. By preventing large volumes of rainfall from entering treatment systems, separate sewer networks significantly reduce the risk of sewage overflows during extreme weather events.
Key Point: Combined sewer overflows are not treatment failures caused by negligence; they are a predictable consequence of ageing infrastructure that lacks the capacity to handle increasingly intense rainfall events. The result is that untreated sewage can be discharged directly into the environment precisely when water quality is already under pressure.
Lady's Bay Waste Water Treatment Plant in Buncrana is a perfect example of this fundamental underlying design fault which has destroyed the town's most important asset.
This important issue should be front and centre for discussion of any proposal to locate a WWTP inside the environs of Lough Foyle when Irish Water and Donegal Co Cl return to Moville in the near future.
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