Klargester BioDisc: the same model is 18 people in the brochure and 24 on the manufacturer's own declaration

Most people meet a Klargester the way you meet a boiler you did not choose: it came with the house. Somewhere under the lawn there is a green fibreglass drum with a lid, and at some point a letter, a smell or a solicitor makes it your problem. The first thing worth knowing is that a BioDisc is not a tank. It is a machine. Inside it a pack of discs turns slowly on a shaft, driven by a motor through a belt, running in two nylon bushes, and the bacteria that do the treatment live on those discs as they lift in and out of the sewage. Everything expensive about owning one follows from that sentence.

The second thing worth knowing is that the paperwork does not agree with itself, starting with the number that decides which unit you need.

The same BC is 18 people, or 24

The domestic range is straightforward on the face of it. BA at 6 population equivalent, BA-X at 9, BB at 12, BC at 18, all glass reinforced plastic, the BA to BB at Ø1995 mm across and heights of 2160, 2460 or 2960 mm, the BC at Ø2450 mm and heights of 2825 or 3325 mm. Above that the range goes commercial: BD at 25, BE at 35, BF at 50, all 2450 mm wide and 3340 to 4345 mm long.

Then you look at the manufacturer’s own declaration of performance on the public register and the BC is listed at 24 population equivalent, with identical dimensions to the 18 in the product guide. One of those documents is a sales brochure and the other is the paper that accompanies the CE marking, and they differ by a third on the only figure that determines whether the plant is big enough. Nothing about the drum changes; only the claim does.

This matters because population equivalent is not a comfort estimate, it is the sizing input. If your household sits near the line between a BB and a BC, ask the supplier in writing which figure they are sizing to and which document it comes from. How household size converts into plant size is covered in septic tank size and calculated by the sizing tool.

For comparison, the same manufacturer’s septic tanks are declared against a different standard entirely, BS EN 12566-1: the Gamma range runs 2800 litres at 5 people, 3500 litres at 10 and 4000 litres at 13, and the Sigma range from 2800 litres at 5 up to 9150 litres at 47. A septic tank and a treatment plant are not interchangeable, and the difference decides what you may legally do with the outflow.

What the certificate actually tests

Every treatment plant sold in the UK carries a BS EN 12566-3 test, and sellers describe it in reassuring terms: that it proves the unit is watertight, structurally stable, durable, has sufficient treatment capacity and offers effective treatment. All of that is true as far as it goes.

The most useful account of how far it goes comes, oddly, from a rival tank supplier rather than from the standards body. It states that the EN test certificate gives only the average of the 38 test results, not the worst figures, so a unit can fail the regulator’s limits in up to 30% of the 38 test weeks and still hold a valid certificate. It states that there is no fail mark level, because the test only determines the percentage reduction in influent pollutants rather than an absolute outlet quality. And it states that the EN 12566-3 test only tests the smallest plant in the range, not every model.

Treat that as an interested party’s account of a standard, because that is what it is. But the practical instruction it leads to is sound and free: ask for the declaration of performance for the exact model you are buying, not a certificate for the range. That is also the document that just told us a BC might be 24 people rather than 18.

The rules that make servicing a legal condition

For a discharge to surface water in England, the general binding rules are short and quotable. The discharge must be 5 cubic metres or less per day in volume. The sewage must receive treatment from a sewage treatment plant. Discharges must not be within 30 metres of a public foul sewer. Discharges must be made to a watercourse that normally has flow throughout the year. Discharges must not be made to an enclosed lake or pond. Fall outside them and you need a permit from the regulator, quoted at £125, and the same figure covers a source protection zone permit where a private water supply is within 50m.

Then comes the rule that changes how you should read the rest of this article. Rule 10 says the system must be installed and operated in accordance with the manufacturer’s specification, and owners must have systems regularly maintained in line with the manufacturer’s instructions. A skipped service on a BioDisc is not simply a maintenance decision. It is the condition your discharge is legal under. The full set of rules, and what changes when you sell the house, are in the general binding rules and selling a house with a septic tank. These rules are England’s; Scotland registers with SEPA, Wales with Natural Resources Wales, and Northern Ireland goes through the DAERA portal.

What it costs to buy, and what it costs to keep

Unit prices at one drainage supplier: BA £4,129.00, BX £4,755.00, BB £5,279.00, BC £7,114.00 against a former price of £7,949.70, BD £12,232.00, BE £16,971.90, BF £22,328.90, BG £29,593.30. A second supplier lists the BB at from £5,200.00 and the BX at from £4,756.00, so the unit itself is worth shopping. Gamma septic tanks from the same maker start far lower, £1,152.00 for 2800 litres before VAT, which is the price gap between a tank that settles and a plant that treats.

Installation is the larger number. One estimating source puts the unit at £2,500-£6,000, a standard installation at £8,500-£13,500 and a retrofit at £12,000-£20,000, and works a six person example to a subtotal of £7,450 with £596 overhead and £1,770 margin, £9,816 in total. Another firm quotes an upgrade at around £8,000 to £15,000, sometimes more, plus £500 to £1,500 to decommission the old tank. Commissioning is quoted from £495. The wider picture of replacement costs is in septic tank replacement cost.

Servicing is where the three-way disagreement lives. One firm starts at around £180 +VAT for a planned annual visit and £200 +VAT for an unplanned one. Another quotes between £100-£200 each. A third says from £150.00 to over £400.00 per service. Desludging starts at around £195 per minimum of 1000 gallons, and is listed at £195.00 a year for a BA, BB or BioFicient 1 and 3, and £293.00 for a BC. What a desludge involves generally is in septic tank emptying cost.

Parts are cheap next to labour, which is the useful part. A motor and gearbox is £249.00, down from £313.50. A capacitor is £31.35, a magnet for the alarm £16.94, a cover lock £5.50, a dosing cup £13.20. Nylon bearings are £37.99 at the drive end and £17.99 at the non-drive end, or £38.99 and £21.99 on a BC. Pulleys are £16.99 top and £49.99 bottom. A Greasomatic is £25.00, reduced from £49.66. When a quote for a bearing job runs to four figures, that is access and time, not components.

The bit the brochure leaves out: it is a machine

Read the installation and operation manual and the tone changes completely. It asks for at least 1 metre of clear, level ground all around the unit. It requires the shaft to be level within ±3mm, and ±5mm from side to side, specifically to avoid bearing stress. If the property is going to be empty it tells you to remove the motor and gearbox unit and the drive belt or chain and store them in a dry or heated environment. And on the BC it is blunt: once installed, the motor should be left on and running, and the motor must not be left non-operational for a period of 7 days or more.

That last line explains most of the horror stories, because the commonest way to own a neglected BioDisc is to buy a house that stood empty. Here is one, in the owner’s words.

He got the rotor out with a home-made pergola and two chain blocks, and found what the last service had left behind.

Two details in that thread are worth more than any brochure. The first is that the drive pulley sits in a sump within the water level of the outlet section, which is why the bolts rust solid and why the job is one to do after the tank is emptied. The second is the scale at the top of the range: a contractor on the same thread recalled lifting a disc assembly on a village-sized unit at an estimated 7 tonnes, with an 11-tonne crane at its limit.

The moving parts have their own failure pattern, and it is not dramatic. Bearings are quoted as lasting 3 to 5 years. Forum users note there is no tensioner and that the belt has stretched. The alarm is a reed switch mounted next to the motor and gearbox with the magnet attached to the rotor, so if the discs cease to rotate the alarm will activate: it tells you the discs stopped, not why. One owner on a fifteen-year-old unit shared between four houses described the fix that finally worked.

Be careful reading service menus, too. One firm’s advertised visit lists mechanical checks, air blower inspection, electrical testing and cleaning of diffusers. A BioDisc has no blower and no diffusers; those belong to a different kind of plant. A service list that generic is a sign the engineer books rather than the engineer.