How to repair lightweight blocks.

Surely a bio-disk is in a fibreglass tank buried underground?

Is this structure above or below ground and is it the actual containment vessel for human waste:

Stabilising rotten wood and porous blocks is something a resin application will do

Alternatively why not apply a strong mixture of sand and cement + ordinary stabiliser possibly over metal mesh, if its above ground.

It's the 30% above ground part that is visibly deteriorating. The other 60% is below ground and who knows what state it is in. As you say, the containment vessel is fibreglass. The actual biodisks are above ground, as is the sewer invert - a somewhat Heath Robinson arrangement. Not installed by me. But it is already built into solid slate at a lower level then the immediately-adjacent stream; and immediately-adjacent on the other side house footings. As a matter of interest, what do you mean by "ordinary stabiliser"?

Ah. Thnks. Tis all clear now. Firstly you do not need waterproofing, merely structural stability. I think Id go with wire mesh and render myself.,

'ordinary' stabiliser is e.g weak PVA that binds surface dust, is weather resistant and primes for paint

The real McCoy is a [two part] resin mix that sinks in a LONG way and turns solid. Very effective in stabilising rotting wood.

Useful ideas, thanks. I'm a bit biassed towards the minimal effort solution, rather than minimal cost or optimal appearance! Resin patching repeated as necessary appeals.

PVA isn't particularly weather resistant as it can re-emulsify in wet/damp conditions. SBR would be a better alternative

In construction, SBR stands for Styrene-Butadiene Rubber. It is a latex-based liquid polymer mixed with cement, mortar, or concrete to dramatically increase its flexibility, water resistance, and adhesion. It is widely used for external rendering, repairing floor screeds, and waterproofing wet areas like basements.

Can also be used for sealing surfaces prior to plastering.

A typical data sheet

Could sulphates/sulphites , even in gases, from the sewage be chemicalling degrading the thermolite? blocks?

Interesting thought - though the damage is mainly on the outside of the structure.

Thermalite are not suitable blocks for exteriors usage without a water resistant coating/

They absorb water and then freeze spalling happens

Indeed. And ferns grow on them while one is not looking. Brick would have looked a lot nicer, and ordinary concrete blocks would have been better than the lightweight ones. But we are where we are.

render and metal mesh. That's how we clad timber houses here

keeps the rain out ifpainted