Costs
Sewage Treatment Plant Cost
What drives the cost of a sewage treatment plant up or down, including installation, running and servicing costs.
In short: a sewage treatment plant generally costs more upfront than an equivalent septic tank — reflecting the electrical and mechanical components involved — but avoids the land requirement of a drainage field, and carries ongoing electricity and annual servicing costs a septic tank doesn’t.
What’s normally included in installation
- The treatment plant unit itself, sized to occupancy (population equivalent)
- Excavation and installation
- Connection to the electrical supply and existing pipework
- Commissioning and initial testing
What can increase cost
- Larger population equivalent — bigger properties or higher occupancy need a larger unit
- No existing electrical supply nearby — running new power adds cost
- Difficult access — long excavator runs or hand-digging near structures
- Permit requirements — where discharge conditions require an Environment Agency permit rather than falling under the General Binding Rules
- Pumped discharge — where gravity discharge to the watercourse isn’t possible
Ongoing running and servicing costs
Unlike a septic tank, a treatment plant has ongoing costs to budget for:
- Electricity — the aerator runs continuously, adding a modest amount to your electricity bill
- Annual servicing — typically required to maintain warranty and compliance, checking mechanical components and effluent quality
- Desludging — usually less frequent than a septic tank, but still periodically required
When professional assessment is required
As with septic tanks, ground conditions, access and the specific discharge arrangement all affect the final figure enough that only a site visit produces a reliable quote. See our sewage treatment plant installation guide for the full process.
Guidance reviewed August 2026. Figures are general UK indicators, not quotes, and should always be confirmed with a site-specific assessment.
Is a treatment plant more expensive than a septic tank overall? + −
Usually higher upfront and in ongoing running costs, but it avoids the land and groundworks cost of a drainage field, and is often the only compliant option on constrained sites — so total cost of ownership can be comparable depending on the site.
How much does servicing a treatment plant cost per year? + −
It varies by manufacturer and specialist, and is generally a modest annual cost compared to installation — get this confirmed as part of any quote, since ongoing servicing is often a contractual requirement.
Do I need a new electrical circuit for a treatment plant? + −
Often a dedicated, weatherproof outdoor circuit is recommended. Your installer will confirm requirements for your specific unit.
Sources
- British Water — Code of Practice: Septic tanks and treatment plants