Combined sewer overflow events are responsible for the release of untreated wastewater - monitoring them helps cut pollution and prevent basement flooding.

Why monitor CSOs?

Combined sewers transport wastewater and stormwater runoff to treatment plants, but heavy, intense or prolonged rainfall can overload them. A combined sewer overflow (CSO) diverts excess wet weather flow, enabling plants to continue operating and preventing upstream flooding. By doing this, however, untreated water is diverted and discharged into the environment, causing adverse human health, environmental and aesthetic impacts. 

Monitoring CSOs enables network operators to understand the hydraulics of the sewer system, allowing teams to respond in real time to identify and clear blockages and alert residents to potential basement flooding, and over the longer term to construct models to improve network performance.

How to monitor CSOs 

CSO monitoring is best achieved using a remote "smart monitoring" network, which can gather and transmit data autonomously. 

There are two main sources of data that can be monitored in a CSO: flow and level. 

Monitoring CSO flow

The flow of water in a combined sewer overflow provides information about the conditions in the network, and monitoring it gives operators ongoing insight into the performance of the system regardless of the weather conditions.

A flowmeter solution provides long-term, real-world data that can be used to construct models in order to understand system hydraulics and develop improvements and upgrades to the network. 

Monitoring CSO level

The level of water in a CSO is a similar indicator of network conditions, and a level monitoring solution may be used to derive information about overflow risk. 

Level loggers can be configured for use in two different ways: they can measure water levels, or they can monitor when the level reaches a certain point. 

The first method provides information similar to that provided by the flowmeter, supplying a range of continuous data that can be used to derive network performance. 

The second method is used to trigger alerts and early warnings of overflow events--once water reaches a predetermined level, the logger records the data and transmits an alert (and an image, if paired with a remote camera). This is particularly useful for identifying and removing network blockages to prevent flooding.

Event Duration Monitoring 

Event Duration Monitoring (EDM) is used in CSOs to determine the duration of an overflow event. In this application, a monitoring system logs when a certain condition is met - when a certain flow is reached, or a certain water level - and then logs again when that condition is no longer met. These start and finish points provide a duration for the event. 

EDM data provides valuable information that enables water network owners to understand the performance of their sewers, and helps regulators (such as the UK Environment Agency) to ensure that sewerage systems are compliant with relevant legislation.

Monitoring in explosive atmospheres

Due to their nature, CSOs can contain explosive atmospheres, so monitoring them must be done safely. 

In the EU, ATEX - the name given to the two European Directives for controlling explosive atmospheres - provides the necessary guidelines for working practices and equipment for use in explosive atmospheres. 

In order to reduce human exposure to explosive atmospheres, CSO monitoring should be done remotely, and any monitoring equipment housed inside the Zoned Area must have been tested and certified by an independent certification body to meet the requirements of the EU ATEX Equipment Directive.