When Can You Discharge to a Combined Sewer? Navigating the SuDS Hierarchy

Understanding when you can discharge surface water to a combined sewer is one of the most common questions we encounter in drainage design. The answer isn't always straightforward, but it's crucial to get it right to ensure your development meets planning requirements and environmental standards.

Understanding Combined Sewers

A combined sewer system carries both foul water (from toilets, sinks, and appliances) and surface water (rainwater from roofs, driveways, and roads) in the same pipe network. While these systems were common in older urban areas, modern drainage design prioritises separating these flows wherever possible.

The key principle to remember is this: discharging to a combined sewer should be your last resort, not your first choice. This is where the SuDS hierarchy becomes essential.

The SuDS Hierarchy Explained

The Sustainable Drainage Systems (SuDS) hierarchy is a framework that guides drainage designers and developers towards the most sustainable solution for managing surface water. It follows a clear order of preference:

1. Rainwater harvesting and reuse

This is the top priority. Collecting rainwater for reuse in toilet flushing, garden irrigation, or washing reduces both mains water consumption and the volume of surface water requiring disposal. Rainwater harvesting systems can significantly reduce your drainage requirements whilst providing a valuable water resource.

2. Discharge to the ground (infiltration)

Where rainwater cannot be reused, infiltration is the next preferred option. Methods include soakaways, infiltration basins, permeable paving, and rain gardens. These solutions allow rainwater to return naturally to the ground, recharging aquifers and reducing flood risk downstream.

3. Discharge to a surface water body

If infiltration isn't viable, the next option is to discharge to a nearby watercourse, such as a stream, river, or pond. This mimics natural drainage patterns and keeps surface water separate from the foul sewer system.

4. Discharge to a surface water sewer

Where neither infiltration nor discharge to a watercourse is possible, connecting to a dedicated surface water sewer is the next best option. These systems are designed specifically for rainwater and help prevent overwhelming the treatment works.

5. Discharge to a combined sewer

Only when all other options have been exhausted should you consider discharging to a combined sewer. This is the least sustainable option and requires robust justification.

When Is Combined Sewer Discharge Acceptable?

You can typically discharge to a combined sewer when you can demonstrate that all higher options in the hierarchy are genuinely not feasible. This means providing evidence that:

Rainwater harvesting is impractical or insufficient

Whilst rainwater harvesting should always be considered, it may not eliminate the need for drainage entirely. You'll need to show that harvesting alone cannot manage the volume of surface water from your development, or that site constraints make it unviable.

Ground conditions prevent infiltration

You'll need to conduct infiltration testing to prove the soil is unsuitable. Clay soils, high groundwater levels, or contaminated land can all prevent effective infiltration. Simply assuming the ground won't work isn't sufficient; you need test results.

No surface water body is accessible

You must show there's no reasonable connection to a nearby watercourse. Distance, topography, land ownership, and environmental constraints all play a role here.

No surface water sewer is available

Check with your local water company whether a surface water sewer exists in your area. If one is available within a reasonable distance, you'll be expected to use it rather than the combined system.

The combined sewer has adequate capacity

Even if you've ruled out other options, the water company must confirm the combined sewer can accept your discharge without causing flooding or overwhelming the treatment works. You'll need their formal approval before proceeding.

Building Your Justification

Local planning authorities and water companies expect a clear, evidence-based justification for any combined sewer discharge. Your drainage strategy should include:

Consideration of rainwater harvesting opportunities and why they don't eliminate the need for drainage. A site investigation report showing ground conditions and infiltration test results, if applicable. Correspondence with the water company confirming the availability (or lack) of surface water sewers. An assessment of nearby watercourses and why connection isn't feasible. Calculations showing discharge rates and volumes. Confirmation of combined sewer capacity from the water company.

Without this documentation, your planning application or building control submission is likely to face delays or rejection.

Practical Considerations

Even when combined sewer discharge is permitted, you'll typically need to incorporate attenuation to limit flow rates. Water companies often impose strict discharge rate limits to prevent overwhelming the system during heavy rainfall.

Consider using attenuation tanks, oversized pipes, or blue roofs to store water temporarily and release it slowly. This not only protects the combined sewer but can also reduce your surface water charges.

Combining rainwater harvesting with attenuation can be particularly effective, reducing both the volume requiring disposal and your ongoing water bills.

The Role of SuDS in Planning Applications

Planning authorities increasingly expect developments to demonstrate compliance with the SuDS hierarchy. The National Planning Policy Framework and local planning policies emphasise sustainable drainage as a key consideration for new developments.

If your drainage strategy shows discharge to a combined sewer without proper justification, expect questions from the planning officer. They may request additional information or even refuse the application if they believe more sustainable options haven't been adequately explored.

Common Mistakes to Avoid

One frequent error is assuming that because a combined sewer exists in the street, you can automatically use it. This overlooks the hierarchy and the need for proper justification.

Another mistake is conducting inadequate site investigations. Superficial assessments or assumptions about ground conditions won't satisfy planning requirements. Invest in proper testing early in the design process.

Overlooking rainwater harvesting opportunities is increasingly problematic, as planning authorities expect developments to maximise water efficiency and minimise mains water consumption.

Finally, don't leave drainage design until the last minute. Discovering late in the process that your preferred discharge method isn't acceptable can cause significant delays and additional costs.

Getting It Right

Navigating the SuDS hierarchy requires careful site assessment, thorough investigation, and clear documentation. While discharging to a combined sewer is sometimes necessary, it should only happen after you've genuinely explored and ruled out more sustainable options, starting with rainwater harvesting.

Working with an experienced drainage designer from the outset ensures your strategy aligns with planning requirements and environmental best practice. We can conduct the necessary investigations, liaise with water companies, and prepare the documentation needed to support your application.

If you're unsure whether your development can discharge to a combined sewer, or you need help demonstrating compliance with the SuDS hierarchy, get in touch. With over 40 years of experience in drainage design, we can guide you through the process and ensure your project meets all regulatory requirements from the start.