Likely to want to put a septic tank in the place we're doing up. Locally, they're called a "Fossa" - and I can buy, I believe, a ready-made 2-chamber tank which you just connect up and bury, which might not be the best-ever solution but it's quick and easy and should avoid having to get the actual system approved for use, which might be a thing with something less "normal".
OK that was the background - my question is (providing they don't cost too much) how much benefit in terms of water treatment would there be to adding a second unit alongside it and feeding the first into it, giving 4 chambers instead of 2? It'd be not much more work to install. There's a watercourse alongside the property, so the overflow of the system is likely to end up in that one way or another, and I'm keen to get it reasonably clean.
Austin Shackles : email ans"at"ddol-las.net. Snail mail on request
Check the local laws, here they are very specific on how you do septic systems and they will report any system each and every time they empty it which will get the house condemned if it is not working correctly.
As to the question, none I would think. the first tank it to allow the sediment to come out of the water the second just holds the fluids until they drain away into the soil, actual processing as it were is done in the soil.