Manhole cover contribution to the issue of CSO discharges

The environmental impact of sewage discharges to watercourses from combined sewer overflows remains a hot topic for both the local and national press particularly where discharges have been continuous for several weeks over the winter months.

These discharges are caused when the volume of flow entering the combined sewer system exceeds its capacity causing it to spill during periods of wet weather from the CSO to avoid flooding properties. These additional flows originate from groundwater infiltration, or from direct inflow during rainfall events from roofs, paved areas and roads.

Water Companies are faced with the major challenge of either increasing the capacity of the sewers by pipe upgrades, additional storage or active management of available storage in the network. Although these solutions reduce the discharges in the short term, a more sustainable approach is to remove the excess flows from the system as these also mitigate the effects of future increases in rainfall due to climate change.

Groundwater infiltration is a particularly difficult source to address. Although the technology exists to seal existing points of entry, solving the problem in one part of the system, causes the groundwater levels to increase and so enters through privately owned laterals which are at a higher level..

The removal of direct flows into the system from paved areas and roofs can be effective where there is a viable alternative, but identifying individual sources can be time consuming and subsequent ‘illegal’ connections can negate the advantages gained.

One source that is often forgotten is the prevention of water ingress into the foul sewer network via manhole covers, particularly parts of the catchment where there are separate systems. A manhole located in the highway is a potential point of ingress of surface water particularly where the slope of the highway surface directs runoff towards the cover. The rainwater flow into a manhole cover positioned near the kerb can exceed the capacity of a 150mm diameter pipe Ponding of water around the cover is a particular issue during periods of high intensity rainfall and flash flooding.

Leakage of water through a cover will be accentuated if either seating wear of the cover has resulted in lowering of the cover within the frame or the cover supporting structure has failed and the cover level is below that of the surrounding paved area. In these situations, a cover incorporating a seal or a secondary sealing plate could provide protection against this form of water ingress to the sewer system. A relatively simple and time efficient ‘drive by’ assessment could identify manhole covers that may be allowing water into the sewers. Replacement of the cover with one of the many low leakage manhole covers, available at a relatively low cost, may offer some effective reduction of unwanted water ingress into the sewer network.

The standard for ‘Gully tops and manhole tops for vehicular and pedestrian areas’, BS EN 124, does not cover design and performance for surface water ingress. To address this omission, the sewage undertakers and manufacturers have developed a Water Industry Specification (WIS) WIS 4-26-01 Specification for leak tightness of covers for manholes and inspection chambers to provide a testing and classification of manhole covers according to their ability to prevent ingress. This is currently available in as a draft for comment here.

WRc, as an independent test house, has the facilities to test manhole cover products to the WIS so that manufacturers can demonstrate the performance of their ‘leak tight’ products and the sewerage undertakers can be confident that their investment is contributing to addressing the environmental issues that concern us all.