Filter sand for our raised drainage mound

The raised drainage mound is pushing its way up the list of priorities, now we have a mere 9 months before we move north. We have been looking in the background for affordable supplies of filter sand with the right porosity. So far we have failed.

The guiding principles are that:

1. The effluent from the septic tank collects in a holding chamber
2. When the effluent in the chamber gets to a given height, a pump pushes the effluent up into the top of the mound and distributes it evenly along a grid of pipes with regular small holes.
3. The effluent drops through the filter bed at a rate that allows bacteria to act on it – not too fast, not too slow.
4. The filter bed slopes outwards as it goes down, so the effluent has a 7m x 10m area to percolate into the ground, where the clean-up continues.
5. We know the ground is quite ‘slow’ to absord water and that we have a high water table over the winter, so our filter sand needs to be similarly at the slow end of acceptable porosity. Otherwise we might get seepage overground from the base.

The problem we have is that all low-cost sources of material we have tried, contain fines that make them way too slow. Commercial filter sand is much, much more expensive and we really do not want to have to buy 70 tonnes of it.

Ric is looking at options – he got us to post samples from our two big piles of subsoil down to Cornwall. We took 5 different samples, drove in to Ellon, bought a jiffy bag and were a bit surprised to be looking at £13 to post. On asking, we discovered that below 2kg, we could send it second class post for £2.95 – we were a couple of hundred grammes over. I went out to where we had parked and emptied part of each bag into a council planter and got it in the post. On receipt, Ric did a grain-size analysis of them and experimented. He extracted a sample in the range 0.4mm – 1mm and got what looks like an acceptable flow rate. He is looking at extending this to 0.2mm – 2mm. He tried dry filtering only, but got too many grains of compacted fines which broke up when wet. He thinks we need to get our subsoil into a slurry before separating the fines. He is thinking about a tank where we feed water in from one side, or the base, drop a batch of material in, flush out the fines, then seive the residue. He thinks we might get a roughly 50% recovery. If it works it will elegantly solve the twin problems of our large piles of subsoil and the lack of affordable filter sand. If it turn out to be impractical to use our subsoil, we have the option of trying it on bulding sand or quarry dust. The downsides are, it will be labour-intensive, we would need the mini digger for quite a period and what do we do with 10s of tonnes of fines?