Economic design of the drainage field after a digester tank

[saveasteading]

I'm supposed to be an expert in this but am hoping for some genius or experience on reducing the vast area required by the regulations for soakaways after the sewage digester.

 

Basically there will be a Marsh 10 person digester which removes about 95% of the nasties.

 

However the regulations require the same amount of drainage field as if it was the 45% of a septic tank, less a paltry 20%  because it is treated.

 

I even gave some advice on this, on BH,  recently, suggesting phasing the soakaway and perhaps leaving phase 2 until later.... perhaps much later, unless we can see that it is needed.

This house will be kept in the family so we aren't a developer passing the buck

Space is not an issue (if it was then the Env Agency often gives permission for infiltration tunnels.)

 

We haven't tested the percolation yet , but my guess from observation is 80 s/mm.

 

That gives us a crazy amount of trench approx 250m long, maybe more.

 

We don't want a reed bed. A drainage mound is worth a thought.

 

When I first used these digesters, as a new thing, the regs and BCO were perfectly happy with percolation through French drains as if it was rainwater.. ie 30m or so.

The tanks are very much better now but the regs have gained this ultra-cautious approach. As I read in a sales blurb... "written legislation has fallen behind the technology available"  Agreed.

 

They then say that while the legislation has not caught up, "local authorities and Building Control have'.   If only. Unfortunately I can see no sign of that unless the land is too small.

 

The digester company cannot offer  help. I'd have thought that saving customers money without affecting their own cost would be good business, but apparently not.

eg they make a special model to reduce phosphates even more, but that doesn't make any difference to the soakaway area.

The other manufacturers appear to say even less.

 

It's not only stinginess (otherwise known as efficient and sustainable design):  I simply don't think it's sensible or necessary.

It is very simple how digesters work, as a flush of the toilet almost immediately pushes the same amount out of the treated end of the tank. 3 or 5 litres? More with a shower. That will not travel far along a perforated pipe so all the phosphorous will be at one location, close to the tank, especially in summer when the ground is drier.

The regulations are assuming a day's worth all rushing through at once.

 

BTW in passing I have read

 

• The real issue is of preventing phosphates from reaching the aquifer.
• that an orchard is an excellent way of taking phosphates out of the water.  We will have fruit trees exactly there, but I doubt the BCO will be interested.

This is the same principle as reed beds where the grasses absorb phosphates greedily and we would harvest it annually and... I guess compost it.

• Willows are good too, but at some stage need to be reduced and the timber taken elsewhere.
• A drainage field is regarded as secondary treatment rather than as a dispersal mechanism.

 

Has anyone got a brilliant idea?

Anyone used a drainage mound? Does it have to be a mound rather then below ground? 

 

 

 

 

 

 

 

 

[SteamyTea]

I shall follow this, not because I want one, just that I find it interesting.

 

You say you don't that a 'reed bed' but is a small orchard not the same thing?

 

'Let my plops fertilise your crops'

[saveasteading]

11 minutes ago, SteamyTea said:

is a small orchard not the same thing?

 

Yes but it is not in the regulations. A reed bed is.

 

From Google Earth I can see historic views showing that the land was once a commercial orchard. Thus I know that there will be fragmentation of the clay from the roots, which will long have rotted, and that there was drainage to the field in herringbone layout. I'd also expect the trees and apples to have depleted the phosphates from the ground (this latter is not my area of expertise though: it's a guess) .  None of this will matter to the BCO.: I'm just expanding on  the particulars.

[SteamyTea]

I seem to remember someone saying that you have stricter waste water rules in Scotland.

Not an area I have looked into much.

[Gus Potter]

3 hours ago, saveasteading said:

However the regulations require the same amount of drainage field as if it was the 45% of a septic tank, less a paltry 20%  because it is treated.

The regulations can be  wrong conservatively.. leads to over design and you lose a bit of money, but they can also be horribly wrong! to the extent that it makes a building plot worthless. 

 

@saveasteading has the advantage of a large plot.. you can often get something to work here with a bit of imagination. 

 

 

[JohnMo]

This is our design - effectively a fail on perc test as stated in the text. This is a early revision of the drawings shown as a 10 person drainage field (final was a 6 person)

 

Big area, but simple design. Don't overthink it just follow the rules

 

[saveasteading]

@JohnMothats helpful thanks, and will be especially so if I find we have overcalculated the area.

Your numbers are much smaller than ours, but that will he because your percolation is very fast.  

8 times as fast percolation means one eighth as much area.

 

In my opinion, there is no chance that your drainage runs  more than 5m into the system before disappearing,   and the rest is redundant.

I don't want to build a needlessly huge trench.. that's bad design. 

55 minutes ago, JohnMo said:

Don't overthink it just follow the rules

That's not how i have spent my career. Always looking for better, more efficient solutions.  You know when they work because everyone thinks it's obvious.

 

Equally, I'm happy to be told of a reason why I'm mistaken. That's expertise. But for now I think it's a very conservative design set by bureaucrats who are not thinking it through.

 

 

10 hours ago, SteamyTea said:

stricter waste water rules in Scotland.

Identical. 

The current project is SE England.

At the Steading, the digger driver said he had lifted in about 10 digesters and all went to a rubble soakaway. So either he was mistaken or the bco's were equally ignoring the rules. Rules that inspire contempt are bad rules.

Edited 13 hours ago by saveasteading

[dpmiller]

12 hours ago, saveasteading said:

It is very simple how digesters work, as a flush of the toilet almost immediately pushes the same amount out of the treated end of the tank. 3 or 5 litres? More with a shower. That will not travel far along a perforated pipe

 

Are you sure about this for your chosen unit? Our Solido runs as an SBR plant and only discharges twice per 24hrs, immediately after a settling period. It pumps (well, airlifts) down to a fixed level each time so up to a few hundred litres in 20mins.

[saveasteading]

39 minutes ago, dpmiller said:

discharges twice per 24hrs, immediately after a settling period

Good point. That would indeed push half a day's stuff into the drain.

I've looked at, smelt and let settle in a jar,  the effluent from 4 installations. No strong colour, smell or  settlement but presumably there is still chemical content.

I don't think much sludge reaches chamber 3 and then that decomposes like the rest. I'll have a read about the Solido for interest.

[JohnMo]

3 hours ago, saveasteading said:

Your numbers are much smaller than ours, but that will he because your percolation is very fast.  

8 times as fast percolation means one eighth as much area.

Not really correct - we have a fail and you go to the same place in the design rules if it's a fail for too fast or too slow. So the same design would be applicable to yourself also. Ours effectively sits on a thick layer of gravel, tops with more gravel. Suspect as you say disappearing pretty quickly.  But our layout (due to elevation changes) has a limit switch operated out flow tank (with pump) from the treatment vessel.  So around 500L is pumped at any time, then most likely off for a few days.

[saveasteading]

6 hours ago, JohnMo said:

we have a fail and you go to the same place in the design rules if it's a fail for too fast or too slow.

That's interesting , especially for 'too slow'. It may be easier, cheaper and less disruptive to build a drainage mound if the topography allows, and it won't take so much space.

Thus ground that has slow percolation may be beneficial.... which is plain silly.

I think the documents need more nuance as well as awareness of reality. 

 

Does your pipe reach the top of the mound by gravity or do you need a pump? A pump built into a tank appears to cost an extra £500 or so.

 

I've also had a look at digesters that remove a lot more of the chemical which appear to be the main concern. The figures are very impressive. Unfortunately it costs more but still does not allow the reduction of the dispersal area.

[saveasteading]

21 hours ago, SteamyTea said:

stricter waste water rules in Scotland.

 

Stop Press.

 

I replied 'identical' but I was wrong.

I've spent the afternoon comparing the Scottish and English reg's because I had a feeling that might be confusing me.  The old fashioned way with a printout and a yellow marker.

 

First appearances show the same rules, calculations and options, referring to the same document, and with shared drawings.

 

BUT they are far from the same. I haven't checked out the implications of which is stricter yet but simply warning anybody who is involved at present.

 

[JohnMo]

34 minutes ago, saveasteading said:

Does your pipe reach the top of the mound by gravity or do you need a pump? A pump built into a tank appears to cost an extra £500 or so.

Sort of. Treatment plant and soakaway are around 400m apart, and although it should gravity feed SEPA insisted on a pumped solution. So we feed from treatment plant to another tank with a pump at the bottom.