CLEARMASTERS
repair engineers
call-outs
after every visit
and tankers
in drainage
The basics
What is an ASP?
ASP stands for Activated Sludge Process. It doesn’t sound very nice, does it? It’s simpler than it sounds.
The ASP is a single round tank. Wastewater enters a central biozone, where a diffuser inside a draft tube bubbles air through it. That feeds the bacteria (the ‘activated sludge’) and keeps everything circulating. Treated water moves into the outer settlement zone, where solids settle and are drawn back to the draft tube for more treatment. Clear water leaves over a weir around the edge.
Your plant will also need regular desludging, and we can do both on the same visit. We service the other main makes too, including Klargester and WPL Diamond.
We do this work across Surrey, West Sussex, East Sussex, Kent, Hampshire and London, with tankers and engineers in each area most days.
Everything moves by gravity, so there are few moving parts. The blower is what keeps it all alive.
How it works
How a Rewatec ASP works
Hover over or tap the numbers to see what each part does.
- 1InletWastewater enters the central biozone.
- 2Biozone and draft tubeAir from the diffuser rises up the draft tube, mixing the activated sludge and feeding the bacteria.
- 3Blower and diffuserThe blower supplies the air. Its filter needs cleaning and the diffuser can scale up in hard water areas.
- 4Outer settlement zoneSolids settle out and are drawn back into the draft tube for more treatment.
- 5WeirClear water spills over the weir around the edge of the tank.
- 6OutletTreated effluent flows out, by gravity or through an optional pump.
What we service
What an ASP service involves
Based on the manufacturer’s service guidance, an ASP service includes:
Effluent sample
Checking the final effluent, and the discharge point for any signs of pollution.
Settleability test
Testing how the activated sludge settles, which tells us when the plant needs desludging.
Biozone surface
Removing grease balls, foam or anything that shouldn’t be there.
Blower and air line
Cleaning the air filter, servicing the blower and checking the air pipe isn’t crushed or kinked.
Diffuser
Checking the turbulence, and cleaning the diffuser, which can scale up in hard water areas.
Pump and valve
Where fitted, clearing the discharge pump, checking the non-return valve and testing the float switch.
Fault finder
Something wrong with your ASP?
Not necessarily time to worry. Here are common symptoms and their usual causes. Either way, it’s best to get an engineer to check.
| Symptom | Usual cause |
|---|---|
| Strong smell | Too many cleaning chemicals, heavy laundry use, drains not venting properly or not enough air. |
| Blower not running | Switched off at the isolator, a power cut or a blower fault. |
| Blower running, but no bubbling | The air pipe is blocked, squashed or split, or the blower diaphragm has split. |
| Biozone looks grey | Not enough air getting to the plant. |
| Biozone dark and smelly | Heavy laundry use, or the plant is overloaded. |
| Biozone pale with white suds | Lightly used, or surface water getting into the plant. |
| Grease balls in the biozone | Too much fat or grease going down the drains. |
On the day
What happens on an ASP service
We isolate the blower
Safety first: the blower is isolated before we work on it.
We check the treatment
Effluent sample, settleability test and a look at the biozone.
We service the air system
Filter, blower, air pipe and diffuser checked and cleaned.
We secure and report
Lids secured, and we explain what we found.
Why Clearmasters
Why choose us for your ASP?
The ASP is a simple, reliable plant when it’s looked after. We know what it needs, and we can desludge it on the same visit.
Ask us a questionAlways on hand
Planned servicing at a time that suits you, and 24/7 call-outs when something goes wrong.
Straight answers
We explain what we found in plain English, and give you honest advice on what needs doing now and what can wait.
Trained engineers
Our engineers hold current confined space entry certificates and work on treatment plants, pump stations and drainage every day.
Desludge when it’s due
We use the settleability test to tell you exactly when to empty it, rather than guessing.
What our customers say
Checkatrade
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Scores as of September 2026
Questions answered
Rewatec ASP FAQs
Is Rewatec the same as Conder?
Yes. The ASP was sold under the Conder name before becoming the Rewatec ASP. They work the same way, and we service both.
How often should an ASP be serviced?
Once a year for a domestic plant. Keeping to the recommended schedule helps prevent breakdowns, and your warranty may depend on it.
How do I know when my ASP needs desludging?
A settleability test on the biozone shows how much sludge there is. We do it on every service and tell you when it’s time.
Can I service it myself?
We don’t recommend it. There are electrical, mechanical and health risks, and falling into a tank is a real danger. Leave it to a trained engineer.
Is desludging part of a service?
No, desludging is done by one of our tankers and charged separately, but we can arrange it for the same visit. Your engineer will measure the sludge and tell you when it’s due.
Is servicing a legal requirement?
The Environment Agency’s rules say your plant must be operated in line with the manufacturer’s specification, and should be regularly maintained in line with their instructions by someone competent. If your discharge causes pollution, the first question you’ll be asked is when it was last serviced. A regular service is the simplest way to stay on the right side of the rules.
Got a different question? Call 03333 233566 or send us a message. You can also read the rules on GOV.UK.
Related services
Fully accredited & compliant
We're independently vetted and certified so you can trust who you're letting onto your site.
SafeContractor
Health & safety pre-qualification accreditation, verifying our processes meet recognised safety standards.
RISQS
Railway Industry Supplier Qualification Scheme — approved to work safely within the rail industry supply chain.
Achilles UVDB
Utilities Vetting Database membership, confirming we meet the utilities sector's supply chain standards.
Trading Standards Approved
Vetted and approved under the local authority's trusted trader scheme for honesty and fair dealing.
NADC Member
Members of the National Association of Drainage Contractors, the UK trade body for the drainage industry.
Registered Waste Carrier
Registered with the Environment Agency, so your waste is handled and disposed of fully within the law.