A commonly used term in the global water industry, CSO stands for Combined Sewer Overflow in water treatment.
 
In order to understand what a CSO actually does, we must first understand what a combined sewer is. In a combined sewer both wastewater and surface water amalgamate and are transported in one single pipe to a sewage treatment works for processing.
 
If there is a particularly heavy period of rainfall, a combined sewer is often unable to cope with the additional amount of surface water it is required to handle. If this happens, it can result in a sewage overflow as there is less room in the system for the wastewater. This is where a CSO comes into play.
 
A CSO is specifically designed to divert excess flow and prevent flooding upstream. As a consequence of implementing a CSO however, untreated wastewater is released into the environment which can then have the adverse effect of creating a pollution in a river or lake. Such pollutions can be hazardous to both human health and wildlife.

CSO monitoring is a process adopted by water companies to prevent flooding and reduce pollution.
Every time a Combined Sewer Overflow is activated, it has the potential to result in raw sewage spilling out into a river, stream or lake. The Environment Agency and UK Government have strict policies in place to limit the use of CSOs. Water companies can be heavily fined for spills or pollutions, just one of the reasons why CSO monitoring has become increasingly important, particularly here in the UK.

By implementing CSO monitoring, a water company has the opportunity to use a forecasting model (water level and weather patterns) that will facilitate the prediction of water levels. CSOs should be a last resort and they are usually only applied during a heavy rainfall event when a combined sewer must cope with increased amounts of surface water. So, if a water company is able to predict what will be happening in its wastewater network a few days ahead, it is possible to prevent spills and pollutions.
For example, CSO monitoring can involve the placement of sensors in various locations throughout the network. These sensors can send an alarm warning of a potential blockage, for example if levels begin to rise unexpectedly. Maintenance teams can then clear the blockage and prevent a spillage before it happens. This type of CSO monitoring is highly effective and provides timely prevention to deliver smart sewer network monitoring.

A CSO report (Combined Sewer Overflow report) is a generated when smart network monitoring is in place in a wastewater network.
 
All wastewater service operators in the UK monitor their wastewater networks to varying degrees; having monitors and sensors in place to measure flows and levels is commonplace. Such monitoring enables a water company to analyse and predict what is happening in the network at any given time.
 
As a result of monitoring, reports can be generated as required. One such report, for example, could be a CSO report i.e. a report that highlights and outlines the ‘activity’ seen by a specific CSO within the network.
 
A CSO report would contain details of when a CSO was activated such as during a heavy rainfall event. It would provide feedback on each CSO and its usage.
 
In recent years, in response to an increasing reliance on CSO, the UK Government and Environment Agency introduced a number of measures designed to get water companies to reduce their dependence on storm overflows. One such measure is the implementation of CSO reports.
 
Wastewater service operators now have a clear duty to publish data on their annual storm overflow operations and CSO reports will provide such data and form an important part of this reporting.

A CSO (Combined Sewer Overflow) works by diverting the excess flow of sewage and surface water during a heavy rainfall event.
 
In a combined sewer, wastewater from residential homes and businesses merges with surface water. That mix of wastewater and surface water ends up at the sewage treatment works where it is treated and processed.
 
If there is a storm or heavy rainfall, the combined sewer has to handle increased amounts of water. A such, to prevent flooding upstream, a CSO is implemented, and the combined sewage and surface water is discharged into a river or stream thus reducing the overall pressure in the network.
 
The consequence of a CSO is that untreated wastewater is released into the environment, which could have an adverse effect on the environment, polluting rivers and streams and increasing the risk to human health. Therefore, it is imperative that that water companies carry out CSO monitoring programmes to gather data, analyse and fully understand what is happening in the wastewater network at any given time.

England has a combined sewage system made up of hundreds of thousands of kilometres of sewers, built by the Victorians, in many urban centres.  This means that clean rainwater and waste water from toilets, bathrooms and kitchens are conveyed in the same pipe to a sewage treatment works. During heavy rainfall the capacity of these pipes can be exceeded, which means possible inundation of sewage works and the potential to back up and flood peoples’ homes, roads and open spaces, unless it is allowed to spill elsewhere. Combined sewer overflows (CSOs) were developed as overflow valves to reduce the risk of sewage backing up during heavy rainfall.

Overflows of diluted sewage during heavy rainfall are not a sign that the system is faulty. Combined sewer overflows (CSOs) are a necessary part of the existing sewerage system, preventing sewage from flooding homes and businesses.