Why Should Storm and Wastewater Be Kept Separate?

Stormwater (also called surface water) is the term used for falling water (precipitation) occurring naturally in the environment – rain, sleet, melting snow and hail. As such, it contains negligible levels of pollution and doesn’t need to be filtered or treated before it reaches natural watercourses or underground aquifers. Returning the clean water to watercourses is very important as it helps wildlife to thrive by maintaining good water levels and flow.

Stormwater will readily soak into permeable surfaces such as loose earth and grassed areas, and will gradually find its way through soil and subsoil layers.

Where surfaces are impermeable – roofs, pavements, roads – stormwater cannot be absorbed by the environment and must be channelled away via drains. Stormwater drainage systems may divert surface water to watercourses, swales or soakaways, or where volumes are too high for the immediate environment to contain, to water treatment plants.

Wastewater (also called sewage or foul water) is the by-products of human activity. This ranges from domestic sewage produced by toilets, baths and washing machines to chemical waste generated by chemical and food processing, manufacturing and other industrial processes.


Wastewater typically contains organic and inorganic waste matter; both potentially present a hazard to health and the environment if left untreated, so wastewater must pass through a range of cleaning and filtration processes before it can be safely returned to the environment.

In some areas (including central London) storm and wastewater are mixed in common wastewater drains and sent together to treatment plants. In other areas, storm and foul water are kept entirely separate. Let’s look at the pros and cons.

Separating Storm and Wastewater – the Advantages

If storm and wastewater share a single drain system, the risk of flooding and overflow following heavy rainfall brings with it an increased risk of pollution. Separating stormwater from foul means that in the event of water flow exceeding capacity, floodwater will not be contaminated and sewage systems will not back up in houses. The additional capacity provided by a secondary system for stormwater also mitigates the likelihood of flooding in the first place.

By keeping stormwater separate from sewage, it can also become a valuable water resource for non-potable use without the cost of filtration and cleaning. With ongoing advances in water conservation technology, this ‘greywater’ can be used to flush toilets, irrigate gardens, create natural habitats and for a whole host of other applications in agriculture and industry.

Segregating storm and wastewater will also optimise the performance of wastewater treatment plants through a significant reduction in the volume of water being processed. While installing two separate systems does incur higher initial costs, the extended lifespan and higher efficiency ensure a solid return on investment in the long run.

Separating Storm and Wastewater – the Disadvantages

In some areas (including central London) storm and wastewater are mixed in common wastewater drains and sent together to treatment plants.

Separate storm and wastewater systems do have their disadvantages, particularly where a combined system is already in use over a wide area. To replace the entire drainage system for a whole city, town or even a neighbourhood is likely to be too costly and too disruptive.

The separation process can also create problems, and if separation is incomplete or not carried out properly it can result in poor sanitation, leading to issues for human health and environmental pollution.

Separated systems also require adequate operations and maintenance follow-up if the benefits of the upgrade are to be proved and the costs justified.

In Conclusion

Where a new development is being planned, it makes perfect sense to install separate storm and wastewater systems from the start. This allows features such as rainwater harvesting and recycling to be incorporated into the scheme and create an efficient, healthy sewage system.

Where an existing combined system is already in place, the benefits may be less obvious, particularly if the system is in good condition and working efficiently.  But if the system is outdated and in need of extensive repair or replacement, or if average rainfall increases or higher demand created by increased urban development are likely to place a strain on the system, then a segregated system may be the most cost-effective answer in the long term.

If you are considering running separate storm and wastewater systems on your property or development, the experts at Drainfast are ready to advise you on the best drainage products for your needs. They can even help streamline the installation and help control your costs with our material take-off scheme – call them on 01730 774000 or email them at [email protected] for prompt, friendly advice and support from one of the UK’s leading providers of specialist drainage supplies and related products.