crooksey

nothing in the tunnel, 70mm+ rocks on the bottom if you follow the spec correctly. Its basically just a huge void, water just hits the rocks and drains away, basically a hollow soak-away with an air vent.

You can use infiltration tunnels with BCO and EA approval, its very quick to get it approved. Graf make the infiltration tunnels so BCO usually state they want a Graf plant to go with it.

40m isn't that long. It will be far easier to install a package treatment plant and discharge to the ditch, If it eventually goes into the sewer fair enough. But will save you huge time and headache, and the ditch will also drain naturally for a lot of its run (i imagine).

Personally I would stick with Graf. Nothing in the tank to fail, spares readily available and top class support from Graf.

Thorfun, your cabinet is an additional cost (that most go for). Cabinet placement is something that can make or break an installation. They can be moved (within reason) limited by distance from the tank etc.

No membrane, mix of pea gravel with some coarser stones for bedding/around pipe. Make sure every inspection chamber has a catch pit of at-least 100mm., regularly clean out the silt. It would also be advisable to have the runs jetted every 5 years.

Underground pipes that are not adopted are very rarely queried by building control. You have made provisions for rodding which is good, they will probably just want a pressure test once finished, especially considering the presence of groundwater after such a long dry spell.

I use Ecover everywhere, safe with the kids and no harm to my septic system or drain field. Buy the large 5LT drums online and if I am ever caught short its also in most shops.

Ross, just get the Graf and get the dosing system. You can even fit remote monitors to alert you of any failures. I have the 9PE personally and it is a great system. You can even control the discharge time, to suit. It has loads of capacity in there, for instance, I can empty two full baths, shower, washing machine etc and it leaves me loads of capacity.

Tanks come with a guarantee, if its lost its shape, its on them to prove its through no manufacturing fault. This is why building control exists, they would have had to guarantee and oversee the installation of a new tank is done correctly. I would suggest speaking to the small local builder and his building controller as a first point of call.

Agree with the above, Graf can supply a "dosing" system and you can also set "holiday mode" which reduces energy consumption. If the plant will be used in holiday mode, I strongly recommend a gravity outlet over a pumped (if possible) and adding a dual flap NRV between the water course and the pump. If you are not there in the event of extreme weather or a higher than usual water course, this will protect the plant.

As long as your proven percolation tests come back correctly, take photos and it will be fine. Only thing is, you will need to know the depth of the TP relative to the fall of the rest of the drainage runs. If you really wanted to get it in, sit the tank a bit deeper, get a pumped outlet and then put the drainage field in nice and high. Such a small job in the scheme of things, and hiring a 5t digger seems a bit of a waste when you probably have a bigger machine in when you start the build.

In theory a diaphragm should be replaced every 2-3 years. If your plant was under a service plan, this is the interval it would be changed at. I order the repair kits from ALI express and keep two in stock. £30 every 2 years isn't a huge expense.

You are over complicating this, when you dig the tank in, you will need a control box somewhere (usually hidden) for the compressor etc, so put the vent direct from the tank there.

Quite common for IC and pipes to fill with crud during the build phase, avoidable but common. Usually always cleaned out, they have used concrete risers and caps, so imagine pipes been installed with a laser level. Jetting and vac out of drains isn't as expensive as you would think.