Need some advise.

I am installing an outdoor toilet.

I will dig a hole beside toilet and put in a plastic sewage tank directly under toilet.

Firstly, should I put some concrete in the hole for the tank to sit on or is it OK just to put tank in hole & back fill the hole?

Secondly, the inlet/outlets are 4" diameter. So I will fit a short piece of 4" pipe from the shitter to the tank inlet but on the outlet, which will run into the main village drain, is it really necessary to fit a 4" pipe, as this will be just the water overflow from the tank?

I was thinking a 2 1/2" pipe would be plenty big.

Personally I'd go with a concrete pipe that fits the tank outlet despite it (theoretically) being only water. The concrete pipe is cheap but watch if you run it under an unsurfaced driveway, it's not that robust.

 

You will need something to stop the local wildlife moving in to the pipe and if it's a long run some inspection pits for when it gets blocked (if you don't, it will).

 

Are you running the grey water from the shower and sinks direct to the outflow? There's no need for it to go through the septic (actually it's a bad idea) and it should help keep the pipe clear.

No shower, just a sink to wash hands after using shitter.

I was going to use blue plastic pipe

If it's really that small no reason not to go with the smaller pipe, but is the cost saving that great, you can use the cheapest thin wall pipe.

 

I would look at the concrete pipe too.

Just thinking that plastic pipes would be easier to lay/join, as opposed to concrete pipes. Also easier to man handle.

It will do the job just fine :)

 

Our septics are on concrete pads at the bottom of the holes, check what the instructions say for yours.

Guess it wont do any harm to put 6" of concrete at the bottom of the hole.  

Concrete base for the plastic septic tank definately. Backfill around the tank with gravel/small stone/sand. If sand backfill in layers wetting the sand thoroughly. Don't compact it.

 

Oh yeah, when backfilling, do it in layers and fill the tank with water, keeping backfill and water level roughly the same - stops the tank collapsing......

 

but you probably knew that.

 

As Crossy says, go for 4" outlet, blue HDPE 13.5 pipe, won't cost much more and you never know what will end up floating down it.

Nice one grollies, I hadn't thought of the HDPE, cheap as chips in the farm supply places :)

 

OK, many thanks, will go for the 4" pipe, putting a few concrete manholes along the way-only 250Baht a go.

Concrete base also.

I have noted that the plastic tank has, along with the 4" inlet/outlets,  what looks like a 1" threaded hole at the top of the tank, any ideas what that is for?

Whats the reason for not using the soil to back fill?

The water from the sink, should that be discharged into the tank or tied into the pipe downstream of the tank, on the outlet pipe?

The small hole is for a vent pipe, lead it to the surface somewhere the odd niff won't matter.

 

The sink and other grey water would normally be plumbed to the outlet side of the tank. If it's just a hand wash it won't matter if a little extra water goes into the tank.

 

Soil, unless it's very fine, tends not to be very easy to get evenly around the tank and can lead to stress points and punctures, remember the tank relies upon contact with the surrounding ground for its strength when full of water.