Sewage Treatment Plant self install

[Russell griffiths]

 

like the other photo in this post it’s all down to how high your water table is. 
having to hire box shuttering to install your tank can seriously damage your wallet. 

[flanagaj]

5 hours ago, MortarThePoint said:

 

Which system did you install? Did you have a concrete backfill?

I installed a Graf One 2 Clean.  We are fortunate as it's chalk bedrock at about 1m and the water table is about 60ft below the surface.  As a result we backfilled with 10mm pea shingle as per Graf's installation details.

[BadgerBodger]

Nice. I like that so many people see the same opportunities. STP install is very easy. Same for drainage field. Wouldn’t like to do it at this time of year though. Hate claggy boots. Here’s mine with drainage field. Marsh ensign pumped so I didn’t have such a deep drainage field.

 

 

3 days on the hottest day of the year.  digger, dumper, driver, plant, materials and yours truly just under 5700
 

not much better than muddy boots really!  
 

 

Edited Wednesday at 19:36 by BadgerBodger

[MortarThePoint]

46 minutes ago, BadgerBodger said:

Wouldn’t like to do it at this time of year though.

 

Silly of me not to have sorted it sooner, but am where I am

[saveasteading]

Some fundamentals.

Don't dig the hole bigger than necessary.

You have to stop an empty tank from floating. So a)if the water table is low then no risk.

b). There are several ways to hold it down and that isn't with total concrete backfill. It depends on the tank but the Graf pic above shows holes through the base for bars or ties.

 

Options are 1.  to backfill in stages with concrete against the tank and earth away from it. 2. Concrete slab as a base and tie the tank tied to it...then all fill is acting on the slab and it won't move. ..in which case gravel surround will work too.

You mustn't have anything against the tank which might puncture it eg fill with bricks in it.

 

But some tanks may have other requirements.

But some tanks instructions may be overcautious.

 

If the hole is filling with water during vjnsyruction it's going to be a good test. Dig a sump lower than base level and pump from there.

 

I wonder.... could those auger piles sold for decking and fencing bases be used as anchors.  Or platypus anchors?

[flanagaj]

@MortarThePoint - If you are concerned about whether to use concrete or not, I'd suggest spending sometime on the BGS web site GeoIndex - British Geological Survey you might find some bore hole related data for your area.  If the water table is many metres below the surface, then you could simply back fill with gravel as there is no risk of your tank floating out of the ground due to a high water table.  

[MortarThePoint]

I'd quite happily have an above ground tank if there was such a product for 7PE or more and no freezing risk.

[BadgerBodger]

@saveasteading the bigger hole point is a good one, I spent another couple of hundred on the gravel backfill because of the over dig. Only by 250mm all sides due to exposing for archaeologist investigation and relief drawings but nonetheless… 

[saveasteading]

14 hours ago, saveasteading said:

backfill in stages 

This is the way if mixing on site.

Position a board (600 high or so) at about 300mm off the tank, and pour your concrete or gravel in there. Backfill the rest, pull out board, repeat.

A small digger to fill the hole is preferable to barrowing.

Saves money on concrete or gravel, and in muckaway.

 

This process may work with ready-mix too , IF you have the labour to speed it along.

[scottishjohn]

On 15/10/2025 at 09:54, MortarThePoint said:

That is the rate I can get a 6T digger for including transport and insurance. About double that for a week, which I'd go for as I have other jobs I need it for and will consume the spoil and would get a dumper for. That's efficiency I haven't factored into the costing though

you doing well there

6ton is £380+vat for a week --but plus deliver=pickup --that adds another £220- 

 

 hence why I very quickly bought a 3ton  myself 

Edited Friday at 08:34 by scottishjohn

[scottishjohn]

On 15/10/2025 at 10:41, Russell griffiths said:

We all seam to answer with our opinions, but it’s never really answering the original question 

Dave said he did his in a couple of days, but his hole is tiny, the hole you are suggesting is huge and will have approx 

30 cubic m of clay coming out of it. 
 

dave said he mixed his concrete himself, again no problem but his hole is again tiny, yours is huge, your not hand mixing 10 cubic meters. 
 

you can use a wet mix if your hole is dry, if you have water in the hole you need a dry mix. 
 

I think this job is very different depending on ground conditions and tank used. 
 

@flanagaj has just done his maybe he could put up some more up to date and in context pictures. 

 

russel is correct --ground conditions control it all

 

 me I have free draining granite rubble etc iwth 45 degree babnk about 10metres away follwed by a 8o m drop into the quarry about 30 m,.s away 

 

so dig was hard work for digger but no need to do anything but backfill

 hole open for 3 days and it was raining and all water just drained away 

 

 

[saveasteading]

53 minutes ago, scottishjohn said:

bought a 3ton  myself 

I'm too suspicious that these will be on the point of clapped-out-edness.

We had a very good groundworker who we used for years, and he had lots of costs and delays for repairs, esp with tracks. One machine he abandoned on our site.

Any advice?

[markc]

Good used machines are hard to find, plant hire companies rarely get rid of good machines so you need to beware of track drives and slew bearings.

Leaking/chaffed hoses are easily replaced and rams can be generally be re sealed at reasonable cost but engines, pumps and slew bearings are expensive to replace.

[saveasteading]

9 minutes ago, markc said:

plant hire companies rarely get rid of good machines

It occurred to me that this would be the best source. They will get rid as soon as call-outs become too frequent and hirers complain.

They might be fine for a while with careful / caring use.

Where do they offload  (pun was not intended) them? Eg car leasers send to auction at 3 years, likewise main dealer trade-ins..

 

 

[ProDave]

On the point about filling, you should fill the tank with water as you backfill the hole, keeping the water level in the tank level with the backfill.

 

Our previous house we had bad advice from the installer and that tank is just backfilled with gravel.  It is okay if you ONLY have it pumped out in the summer preferably after a long dry spell when the water table is low.  It could end very badly if pumped out in the middle of winter.  we made sure the buyers of the house were aware of that.

[saveasteading]

4 hours ago, ProDave said:

tank is just backfilled with gravel

That's OK if there is a slab under it,  which takes the weight of thd gravel, and if the tank is tied down.

[martjulie]

Completely encased our tank in concrete just to be sure it stopped there, half filled with water then half outside with concrete, then filled rest and poured concrete, better not move, tank stands on a concrete base.