Domestic sewage treatment plants: everyone beats the limit, so stop shopping on BOD
- The discharge limits quoted in the trade are BOD 20, SS 30, NH4-N 20. Every plant on the market claims to beat them by a mile — Graf says 7 mg/l BOD, WTE says 7.3. Choosing on BOD is choosing on a race everyone already won.
- Running-cost claims vary by 5× — from “approximately £65 per year” to “£305.76 per year” — and every one of them comes from someone selling something.
- A plant needs electricity, a service contract and emptying. A septic tank needs emptying. That difference, not the brochure, is the decision.
- Unlike a septic tank, a treatment plant can discharge to a watercourse — gov.uk requires one for that. That is the real reason to buy one.
- Service contracts run “from £120” to £200 a year, and the emptying is on top.
Read this one twice, because a manufacturer says the quiet part out loud. WTE Ltd, on package plants: "All of the 3 stage package plants need emptying every 3 to 12 months", and then the warning — "Assume that ALL package (all-in-one tank) plants need emptying at least annually under normal use", because "Some manufacturers claim extended emptying intervals that are not really honest".
Now open the same firm's comparison table. Its own VORTEX is listed at "6-36 Months". Thirty-six months is an extended interval by anyone's reckoning, published by the company that just called extended intervals dishonest.
That is not hypocrisy so much as gravity: every seller's own plant gets the optimistic column. So carry the two numbers nobody is selling. The Environment Agency: "You must have your septic tank or treatment plant emptied at least once a year." British Water's code of practice goes further for a plant — "Domestic Sewage/Wastewater Treatment Plants – half-yearly", and quarterly if it is commercial.
The shape is then obvious. The industry's own code says twice a year; the regulator's floor is once; the marketing says every three. An owner who lives with one describes the real rhythm without being asked:
It has to be serviced then the sludge emptied annually.
Margiknot, gransnet.comService and desludge, every year, on top of the electricity. No interval on a datasheet changes that.
Something worth saying before a single specification: on domestic sewage treatment plants, there is no neutral data in Britain. No government publishes performance. No regulator publishes running costs. Every figure you will read — here included — was published by a company that sells plants, or by a company that sells a competing plant.
So this article does not rank them. It tells you which questions the numbers can answer and which ones they cannot, and where the industry is arguing about things that do not matter.
The BOD race nobody needed to run
Start with what the effluent has to achieve. The limits quoted in the trade are BOD 20, SS 30, NH4-N 20 — milligrams per litre.
Now the claims:
| Plant | BOD | SS | NH4-N | Claimed by |
|---|---|---|---|---|
| Legal limit (as quoted in the trade) | 20 | 30 | 20 | — |
| Graf One2Clean | 7 mg/l | 14 mg/l | 0.5 mg/l | Graf UK |
| WTE VORTEX | 7.3 | 15.2 | 0.4 | WTE Ltd |
| Tricel Novo | 11 mg/l | 16 mg/l | 8 mg/l | Graf UK, about a rival |
Graf’s own words for the One2Clean: “BOD5 (Biological Oxygen Demand) Efficiency: 7 mg/l”, “SS (Suspended Solids) Efficiency: 14 mg/l”, “NH4-N (Ammonium) Efficiency: 0.5 mg/l”. WTE’s for the VORTEX: BOD “7.3”, SS “15.2”, NH4-N “0.4”.
So a homeowner agonising over 7 versus 7.3 is optimising a number that stopped mattering at 20. It is the equivalent of choosing a car because it does 0–60 in 6.1 rather than 6.3 seconds, when the road has a 30 limit.
And notice who supplied the losing figure. Graf published Tricel’s numbers. That is not evidence, that is marketing. If Tricel published Graf’s, it would read differently.
The BOD race is where the industry competes because it is the only place where the products differ measurably at all — and it is the place where the difference has no consequence for you.
The running cost, where the claims go fivefold
Here is where real money hides, and here the sellers cannot agree with each other at all:
WCI Group: the Graf One2Clean at “approximately £65 per year”. Direct Drainage’s 2024 guide: aeration at “£127.02” for up to “6” people. TEKNEKA, working from a “125” watt blower: “0.84p” per day, “£5.88” per week, “£305.76 per year” — on electricity at “£0.28” per kWh.
Five times the difference, for a device whose job is to run a small air pump continuously.
The energy claims underneath are just as scattered. Homeseptic says the One2Clean uses “around 46kWh per person per year compared to 217kWh for most normal systems”. Kingspan compares its own BioDisc at “68,504” kWh over twenty years against an aerated system’s “103,368” kWh — “£23,976” against “£36,179” at their assumed tariff.
Why you would buy one anyway
None of the above is an argument against treatment plants. It is an argument against choosing one on the brochure. The actual reason to own one is legal and simple:
A septic tank may not discharge to a watercourse. A treatment plant may.
gov.uk is explicit: “You must use a small sewage treatment plant to treat the sewage if you’re discharging to a watercourse such as a river or stream.” If your outfall goes to a stream, the plant is not an upgrade — it is the only compliant option, and the general binding rules is the page you need next.
The other case is ground that fails: if your percolation test lands outside the Vp band of 12–100, a drainage field is not permitted, and a plant discharging to a watercourse may be what is left. Where there is no watercourse either, the last option on the list is a cesspit, and it is a worse one. Our percolation calculator tells you which side of that line you are on.
The bill nobody itemises at the showroom
A septic tank costs you emptying. A treatment plant costs you emptying plus electricity plus a service contract plus an engineer when the blower fails.
Service, as advertised: The Sewage Guy, “from £120”. Xoli, a Tay annual service package at “£200”.
And then the desludging, which is a different animal from a septic tank empty. One owner posted his itemised quote — jet vacuumation unit £682.00, waste disposal at a licensed site £333.75, field service engineer £392.00:
£1,400 + VAT seems extortionate but it's only been a year since the plant was installed so we have not needed to have this done previously so don't have anything to benchmark against
Strummer22, forums.moneysavingexpert.comPut the annual numbers together and the shape becomes clear: somewhere around £65–£300 of electricity, £120–£200 of servicing, and a desludge that can run into four figures. Against a septic tank, which owners on the same forums report emptying for £150–200 — sometimes, as one put it, twice in sixteen years.
That is the comparison worth making. Not 7 mg/l against 7.3.
How to actually choose one
- Establish where the effluent goes. Watercourse means a plant, full stop. Ground that passes the percolation test means you probably do not need one.
- Ignore the BOD contest. They all beat 20. Any figure a manufacturer publishes about a competitor is marketing.
- Ask the blower’s wattage. Then do the multiplication yourself. It is the only running-cost number that is not an assumption.
- Ask what the service contract includes and what it excludes. “From £120” is a starting price; find out what starts at £120 and what does not.
- Ask what a desludge costs and how often. Get it in writing before you buy, not a year after installation when you have nothing to benchmark against.
- Ask who services it locally. A plant is a machine with a maintenance dependency. A brand with no engineer within 50 miles is a brand you will regret.
England: "at least once a year". British Water's code of practice says half-yearly for a domestic plant.
So does WTE's, for its own plant — while the same firm warns that "Some manufacturers claim extended emptying intervals that are not really honest".
Frequently asked questions
What is the difference between a septic tank and a sewage treatment plant?
A septic tank settles; the ground finishes the job, which is why Approved Document H says a septic tank may “only be used in conjunction with a form of secondary treatment”. A treatment plant does the biology itself — typically primary settlement, then an aeration stage, then final settlement — and produces effluent clean enough to go to a watercourse. That last point is the whole reason it exists: gov.uk requires a treatment plant, not a septic tank, if you discharge to a river or stream.
Which treatment plant has the best effluent quality?
Practically all of them, which is why the question is a trap. The limits quoted in the trade are BOD 20, SS 30 and NH4-N 20. Graf claims its One2Clean does “BOD5 … 7 mg/l”, “SS … 14 mg/l”, “NH4-N … 0.5 mg/l”. WTE claims its VORTEX does BOD “7.3”, SS “15.2”, NH4-N “0.4”. Graf says Tricel's Novo manages BOD “11 mg/l”. Every one of those numbers clears the limit with room to spare — and every one comes from a manufacturer describing itself or a rival.
How much does a sewage treatment plant cost to run?
Nobody neutral publishes it, and the sellers disagree by a factor of five. WCI Group estimates the Graf One2Clean at “approximately £65 per year”. TEKNEKA works a “125” watt blower out at “£305.76 per year” on electricity at “£0.28” per kWh. Direct Drainage's 2024 guide puts aeration at “£127.02” for up to 6 people. All commercial. Take the middle, budget for the top, and add the service contract and the emptying.
What does a service contract cost?
The Sewage Guy advertises regular maintenance “from £120”. Xoli lists a Tay annual service package at “£200”. That is the servicing alone — desludging is a separate bill, and on a treatment plant it is a bigger one than on a septic tank.
Is a treatment plant cheaper than a septic tank in the long run?
Almost never, if a septic tank is legal on your site. A septic tank has no electricity, no contract and no engineer — it has emptying. A plant adds all three. The plant wins when a septic tank is not an option: your ground fails the percolation test, or you must discharge to a watercourse.
How much power does it actually use?
The honest answer is: a blower, running continuously. TEKNEKA's worked example is “125” watts, which they cost at “0.84p” per day and “£5.88” per week. Homeseptic claims the Graf One2Clean uses “around 46kWh per person per year compared to 217kWh for most normal systems”. Kingspan's own comparison puts a BioDisc at “68,504” kWh over 20 years against “103,368” kWh for an aerated system. Different plants, different maths, all vendors.
Researcher & editor, off-mains drainage
Writes independent guides on septic tanks, cesspits and sewage treatment plants for homes off the mains. Cross-checks the general binding rules and the Environment Agency, SEPA, NRW and NIEA against real prices, British Standards and what owners actually report on the forums.