How to Choose the Right Domestic Sewage Treatment Plant?

10.29.2026

The right domestic sewage treatment plant matches your household size and your site’s discharge point. Get those two right, and most other choices fall into place quickly. Otherwise, you could face pollution problems or a costly second dig.

This sewage treatment plant buyers guide walks you through each decision. 

How to Choose a Sewage Treatment Plant: 7 Key Steps

Here’s how to choose a sewage treatment plant, starting with what matters most.

1. How Many People Will the Plant Serve?

Start by sizing your sewage treatment plant for house use based on the number of people it serves. Plants are rated in population equivalent (PE), which is roughly one person each. For new plants, sizing should follow British Water’s Flows and Loads guidance. Also, plan ahead, because an extension must not leave your plant undersized.

2. Where Will the Treated Water Go?

Whichever sewage treatment plant you choose, its discharge point decides the rules you follow. In England, the General Binding Rules set daily limits for each route.

Go over these limits, and you’ll usually need a permit or a sewer connection.

3. What Are Your Ground Conditions?

Your ground decides how deep the plant sits and how well the water drains. Before you pick a model, ask your installer to check these points:

• Percolation test results for the drainage field
• Water table depth during the wettest months
• Rock or heavy clay close to the surface
• Room for a tanker to reach the plant

High water tables make deep excavation harder, so a shallow plant can help. The Marsh Ensign-EL suits these sites, with a shallow and compact footprint.

Understanding sewage treatment plant regulations is important before installation, particularly when it comes to planning permission and the requirements for your property. Read our guide to planning permission for a sewage treatment plant to learn more.

4. Which Type of Plant Suits You?

Most types of sewage treatment plant use bacteria and differ in how they get air.

• Submerged aerated filter (SAF): bacteria grow on fixed media, fed with air by a blower.
• Activated sludge: bacteria float freely in an aerated chamber, then settle out later.
• Sequencing batch reactor (SBR): the plant treats wastewater in timed batches inside one chamber.
• Moving bed (MBBR): bacteria grow on small floating carriers that tumble in the tank.

If nitrate matters on your site, an SBR plant is worth a close look.

5. Is the Plant Certified to EN 12566-3?

An EN 12566-3 sewage treatment plant has passed independent tests for treatment performance. GOV.UK names BS EN 12566 as the standard for new small plants. Plus, British Water keeps a list of certified plants serving up to 50 people.

When you compare certificates, check:

• The PE range covered by the test
• Effluent quality results from the test
• Energy use recorded during testing

Ask for the certificate before you buy, and make sure the model name matches.

6. Do You Need Nutrient Removal?

Some sites need cleaner water than a standard plant can deliver. Natural England has required nutrient neutrality for new development across 27 river catchments. There, new homes must not add to the nutrient burden in local rivers. A nutrient neutrality sewage treatment plant helps by removing phosphate and nitrate at source.

However, Natural England began consulting on its first Environmental Delivery Plan in September 2026. Until a plan covers your area, check your council’s current nutrient rules.

Where phosphate is the main concern, a phosphate removal sewage treatment plant setup makes sense. Marsh offers three options:

• Phos-Lite: reduces phosphate without chemical dosing, using naturally occurring media.
• Nutra-Lite: an SBR plant that cuts biological nitrates.
• Marsh Gemini: combines both systems in one unit to save space.

7. What Will It Cost to Run?

Running costs last for the plant’s whole life, so compare them before you buy. Budget for these three costs every year:

1. Electricity for the air blower, which runs around the clock
2. Desludging at least once a year by a registered waste carrier
3. Servicing by a competent engineer, such as one from British Water’s accredited list

Check the power figures on each datasheet, because similar plants can use different amounts.

Start With Your Site, Then Pick Your Plant

The best domestic sewage treatment plant suits your site and holds the right certificate. Now that you know how to choose a sewage treatment plant, start with your discharge point. Then size it, and check its certificate before anything goes in the ground.

Marsh also offers wider drainage solutions for sites with pumping or rainwater needs. Contact the Marsh technical team with your site details for a sizing recommendation.

Frequently Asked Questions

Can one domestic sewage treatment plant serve two houses?

Yes, a shared domestic sewage treatment plant works if it’s sized for every home. For new developments, the sewer distance rule multiplies 30 metres by the number of houses. Agree on servicing and emptying costs in writing between neighbours before installation.

Can I install a plant near a protected nature site?

Often not under the General Binding Rules, which exclude surface water discharges near protected sites. These include discharges within 500 metres of a biological SSSI. Check your location on Defra’s Magic map before you choose a plant.

Do I need to register my plant in Wales or Scotland?

Yes, in most cases, as each nation runs its own system. In Wales, you register your plant with Natural Resources Wales. In Scotland, SEPA handles authorisation, so contact them before you choose a model.

Can I install a domestic sewage treatment plant myself?

You need Building Regulations approval for any new plant, whoever does the work. The plant must also be installed to the manufacturer’s specification. So book an experienced installer, and keep the commissioning paperwork safe.

What should I keep after installing my plant?

Keep the plant’s manual and its EN 12566-3 sewage treatment plant certificate together. Add every service and emptying record as you go. When you sell, the new owner must receive these details in writing.