Septic tank advice

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.

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.

Chamber 1 - Settlement Tank

Chamber 2 - Aerobic Tank
An air pump is added to keep, the water circulating to keep all particulates suspended, and to provide oxygen to the good microbes to eat up all the 'food"

Chamber 3 - Clarifier Tank
The inlet is usually screen so as to keep out particulates, the water usually looks crystal clear.

Chamber 4 - Nitrogen Reduction
A high surface area media is added to house bacteria that will use up nitrogen and phosphorous. (think aquarium filters)





There is a member here on permies.com, who actually stores this water in a open tank and uses it to water his garden. Fish and other crustaceans actually showed up, mostly from birds.

Begin by checking with your local laws.