A properly working sewage treatment plant should not create strong, persistent odours.
If you notice bad smells around the plant, drainage field or property, something may not be working correctly.
This guide explains the common causes and what to do next.
Quick Answer
Sewage treatment plant smells are commonly caused by lack of oxygen, failed aeration, sludge build-up, overloading, blockages, poor ventilation or lack of maintenance.
Odours should be investigated before the problem worsens.
Failed Aeration
Treatment plants rely on oxygen to support bacteria that break down waste.
If a blower or aerator fails, the biological process can become ineffective and odours may appear.
Sludge Build-up
If sludge is not removed, it can reduce capacity and disrupt treatment.
This can cause smells, slow drainage and poor system performance.
System Overload
Too much wastewater can overwhelm the plant, especially if the system is undersized or occupancy has increased.
Overload can reduce treatment quality and create odour problems.
Blockages
Blocked inlets, outlets or pipework can cause wastewater to sit where it should not.
This can lead to odours, alarms and backup risks.
Ventilation Issues
Poor or blocked ventilation can allow gases to collect or escape near living areas.
Ventilation should be checked as part of the diagnosis.
When Smells Are Urgent
Treat odours as urgent if they are strong, worsening, accompanied by alarms, slow drains, pooling water or sewage backup.
Read emergency sewage issues if there is overflow or backing up.
How to Fix the Problem
A professional inspection should check aeration, sludge levels, pumps, controls, blockages and ventilation.
Planned servicing is the best prevention; see the maintenance guide.
Book an Inspection
Alton Facility Services can diagnose odour problems and restore treatment plant performance across Hampshire and Surrey.
Need help? Contact our team or call 01420 561661.
Why smells should not be ignored
Persistent odour is often an early sign that the biological process, aeration system or drainage arrangement is not working as intended. Waiting until there is a complete failure can make the repair more expensive and increase the risk of pollution or complaints.
If the smell follows recent emptying, power loss, overloading, blocked outlets or long periods without servicing, the system should be inspected rather than masked with chemicals.
How AFS would investigate this on site
For off-mains drainage issues, we would normally start by checking the tank or treatment plant, inlet and outlet pipework, alarms, pumps, blowers, sludge levels, drainage field condition and any signs of surcharge or pollution.
If the issue looks compliance-related, we would also consider whether the existing arrangement needs septic tank conversion, a new sewage treatment plant installation, or planned treatment plant maintenance rather than another temporary repair.
When this becomes urgent
You should treat the problem as urgent if sewage is backing up into the property, wastewater is escaping outside, alarms are sounding repeatedly, electrical equipment is wet, or the system is close to overflowing.
In those situations, stop unnecessary water use, keep people away from contaminated areas and arrange professional attendance. Where the system needs emptying before repair, sewage tanker services may be required to control the level safely.
Useful related AFS services
The most relevant AFS services for this topic are sewage treatment plant installation, treatment plant maintenance, septic tank conversion and sewage tanker services. These pages explain the practical options in more detail.
Need practical help with this?
For sewage treatment plants, septic tanks, private drainage systems and tanker support across Hampshire, Surrey and nearby areas, AFS can inspect the system and recommend the practical next step. You can call 01420 561661 or send details through the contact form.
Call AFS on 01420 561661 or send the details through for practical advice.
Contact AFSFrequently asked questions
Should a sewage treatment plant smell?
A properly working and maintained treatment plant should produce little to no noticeable odour.
What causes bad smells?
Common causes include failed aeration, sludge build-up, overload, blockages and ventilation issues.
Can smells mean the plant is failing?
Yes, persistent odours may indicate poor treatment performance or a developing fault.
Can emptying fix smells?
It may help if sludge build-up is the issue, but other faults such as failed aeration must also be checked.
When should I call someone?
Call if the smell is strong, persistent, worsening, or linked with alarms, slow drains or pooling water.
Can AFS inspect a private sewage system before quoting?
Yes. For sewage treatment plants, septic tanks, pump chambers and private drainage systems, AFS can inspect the site, identify the likely cause and advise whether servicing, repair, tanker attendance, conversion or replacement is appropriate.