Domestic ‘Sewage Treatment Plant’ replaces ‘Septic Tank’ — who knew?

I’m indebted to the clear exposition of a popular ‘domestic sewage treatment plant’ served at CrystalTanks, for much of the information in today’s entry, which was inspired by a chance overhearing of a friend’s woes during their new building project.

Well . . . if you happened to be working on a new build, or renovating an old house say, far away from the village mains services, then you’d know about the change in the law which prohibits installation of septic tanks and their associated soak-away, in favour of a private sewage treatment plant that acts in much the same way as the big industrial size plants. But if you had a septic tank and proper soak-away installed before about 2013, when the new General Binding Rules (Environment Agency) came into force, you’d probably not know the new rules.

Waste processing technology has come a long way in recent years, and so private sewage treatment plants are now de rigeur in new and renovated rural domestic properties that obviously can’t connect to mains sewer systems.

One of the most popular domestic sewage treatment plants is the ‘Vortex’ system, which is diagrammed above. This system costs around £2000 to purchase, and its installation would set you back another £2000, so in effect you’d be looking at the best part of £5000 when you add in the VAT. With reference to the diagram, the system works as follows:

  • Wastewater from the building enters the Vibro Screen box (1). Here coarse air bubbles are used to physically break down solid matter in the sewage and form a mixed liquor with the water prior to treatment.
  • The mixed sewage liquor flows into the Aeration Chamber for treatment (2). A bacterial culture is present in the Aeration Chamber which digests the pollutants in the wastewater.  The bacterial culture must have a constant oxygen supply and this is provided by a fine bubble diffuser at the base of the tank.
  • The mixed sewage liquor then flows into the Clarification Chamber (3) where it is able to separate into clear, treated sewage effluent and sludge.  The clear effluent is able to flow past the scum baffle and out of the tank.

Also included in this process are vacuum pulled feedback loops to take residual sludge and surface crud back to the Aeration Chamber for further processing. Additionally, the system requires an annual clean, much like a normal septic tank. Its electrical consumption involves only a small external compressor motor with no other moving parts in the plant itself. The clear effluent leaving the sewage treatment plant can, it seems, be emptied directly into ditches or waterways, without the need for an additional soak-away for further filtering, as the plant delivers clear effluent with only 15.2mg/litre of solids (vs the Environment Agency’s requirement of 30mg/litre max) and ammonia of only 0.4mg/litre (vs the EA’s required 20mg/litre max).

Since we seem to be in the process of renovating most of our ageing services anyway, I’d be sorely tempted, sometime within the next five years maybe, to replace our septic tank with such a sewage treatment plant, feeding into the soak-away for added environmental protection. But let’s cross that bridge, ford that effluent stream, when we need to, eh?

It’s enough to realise, now, that older septic systems are out-of-date, and newer technology has surpassed them; when the time comes to re-configure, we can at least be aware of the challenges. For this year, at last, I reckon this topic can finally be put to bed.

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