During rainfall events the capacity of the combined sewer system may be exceeded.
A stormwater overflow (SWO) is a structure designed to divert excess flows from the sewer network, either directly or via a storm sewer system, to the receiving water. SWOs may also be referred to as combined sewer overflows (CSOs). Sewer overflows can occur on gravity sewer networks, or at pumping stations if the pump capacity is exceeded. Overflow structures are also provided for pump stations in the event of mechanical or electrical failure, which are referred to as emergency overflows (EOs).
An important distinction between SWOs and EOs is that SWOs discharge because of insufficient flow capacity during storm conditions, whereas EOs operate only where there is a mechanical or electrical failure. SWO discharges contain a mixture of raw sewage and stormwater and are thus a source of microbial pathogens, oxygen-demanding substances, suspended solids, toxic substances, nutrients and gross solids (US EPA, 2004). SWO discharges have been recognised as a potential cause of receiving water impairments including beach closures, contamination of drinking water supplies and reductions in chemical and ecological status.
The extent of SWO installations in Ireland is significant: based on best available data, approximately 1300 SWOs were under licence or under application as of 2014 (for agglomerations greater than 500 population equivalents). An assessment of the impact of SWOs was carried out under the Water Framework Directive (WFD) Urban Pressures report (CDM, 2009). Based on sewer network models, the report concluded that cumulative annual spill volumes were in the order of 5–10% of the total annual combined flows. An assessment of nutrient loads to Irish rivers estimated that 1% of nitrogen loads and 5% of phosphorus loads were attributable to SWOs. An assessment of performance against environmental criteria is an ongoing requirement for SWOs under the wastewater discharge authorisation for each agglomeration; however, SWO performance data are not currently available in many cases. This may be due in part to the challenges posed by the monitoring of SWOs, which can be located in remote areas or under trafficked areas, discharge intermittently, and be highly variable in terms of discharge volume and quality.
Intermittent discharges from SWOs are identified as a receiving water pressure in the WFD River Basin Management Plans. Hence, to contribute to attainment of WFD goals and to ensure compliance with the Urban Waste Water Treatment Directive (UWWTD), targeted improvements of SWO structures and associated sewerage may be required.
Report No. 240 – Technologies for Monitoring, Detecting and Treating Overflows from Urban Wastewater Networks aims to review (1) the characteristics of intermittent discharges; (2) the current legislative framework for SWO control in Ireland; (3) guidance and best practice for SWO assessment in the EU, the USA and Canada; (4) current and emerging technologies for SWO monitoring; and (5) technologies for SWO improvement. A methodology to prioritise SWO monitoring has also been developed, which is described in Chapter 5.