Construction runoff: Why misunderstanding sewers can lead to pollution

In this article our technical team delve into why not all sewers are the same, and why understanding the pathway your construction site water takes is critical for managing environmental risk responsibly, particularly under regulations covering England & Wales.

In conversations about pollution from construction sites, this phrase is said again and again.

On the surface, that sounds reassuring. It suggests that the water is being carried away safely and responsibly. In practice, however, this statement tells us very little about environmental risk. From a pollution prevention perspective, it is almost meaningless.

The reason is simple: not all sewers are the same.

A sewer could carry water to a wastewater treatment works. It could discharge directly to a river. It might overflow during heavy rain. Each of these situations presents different environmental risks and involves permissions from different stakeholders.

Understanding the type of sewer involved is therefore a fundamental step in managing water responsibly on construction sites.

To make sense of this, it helps to step back and look at the problem using a simple environmental risk framework: Source – Pathway – Receptor.

Surface water sewers

A surface water sewer is designed to carry rainwater and runoff away from roads, roofs and paved areas.

Crucially, surface water sewers typically discharge directly into rivers, streams, or other surface water bodies, usually without any form of treatment.

From an environmental standpoint, this creates a direct pathway between a construction site and the natural environment.

If silty water, cement residues, or contaminated runoff enter a surface water sewer, they can be transported quickly into nearby watercourses. In many cases the discharge point may be only a short distance downstream from the site.

For this reason, surface water sewers represent one of the highest-risk pollution pathways associated with construction sites.

Understanding whether a drain connects to a surface water system is therefore critical before any pumping, dewatering, or discharge takes place.

A site is very likely to need a permit if discharging to surface water sewer. Our technical team have put together separate guidance on temporary discharge permits. 

Foul sewers

A foul sewer carries wastewater from buildings and facilities to a wastewater treatment works.
At the treatment works, the water undergoes a series of processes designed to remove solids, nutrients, and other contaminants before the treated effluent is discharged to the environment.

Because of this treatment stage, foul sewers may appear to offer a safer option for certain types of discharge. However, they are not an unrestricted disposal route.

Wastewater treatment works are designed to handle domestic sewage and certain permitted industrial inputs. High sediment loads, cementitious materials, or chemical contamination can disrupt treatment processes, damage infrastructure, or cause operational problems.

For this reason, any discharge to a foul sewer typically requires permission from the sewer undertaker. The water company may impose limits on flow rates, suspended solids, or other parameters before accepting the discharge.

Simply assuming that construction water can be sent to a foul sewer without consultation can therefore lead to operational issues and regulatory breaches.

Combined sewers

A combined sewer carries both foul wastewater and surface water within the same pipe network.
Combined systems are common in many older urban areas, where historic drainage infrastructure developed before separate sewer networks became standard practice.

Under normal conditions, combined sewers transport water to a wastewater treatment works. However, during periods of heavy rainfall the system may reach capacity. When this happens, excess water can be released through structures known as Combined Sewer Overflows (CSOs).

These overflows are designed to prevent flooding of properties and infrastructure by allowing diluted wastewater to discharge into nearby rivers or coastal waters during storm events.

From a construction site perspective, this means that pollution entering a combined sewer does not always remain within the treatment system. During storm conditions, that material may still reach the natural environment via an overflow.

The combined sewer is the responsibility of the local utility provider, and permission must be sought prior to releasing construction site run off into one.

Understanding the type of sewer is only part of the picture. Identifying the receiving watercourse is key to assessing and managing risk of pollution.

Water that leaves a construction site does not disappear when it enters a drain or sewer. It simply begins a journey through the drainage network until it reaches the environment.

That destination might be:

• a small urban stream or brook
• a major river
• an estuary
• or a discharge point downstream of a treatment works.

The sensitivity of that receiving environment matters. Some watercourses may already be under pressure from sediment, nutrients, or other pollutants. Others may support protected habitats or sensitive species.

Without understanding the receiving watercourse, it is impossible to fully assess the consequences of a discharge.

In practical terms, this means that responsible site water management requires looking beyond the nearest drain and asking a more fundamental question: Where does the water ultimately go?