Rainwater storage in multiple underground tanks
 

I am closer to finally moving storage of rainwater from barrels to underground. This will allow me to store more of it (I have 6x 220-liter barrels now), year round, and hook it up to the house's pumping (toilets, washing machine, garden) instead for carrying it in buckets.

So I mentioned to my Dad-in-law (who built our house 50 years ago) that I would like to dig up part of the yard to bury a tank which holds 5-6 cubic meters of water. He replied that there is already an old septic tank under the yard. It is 3-4 cu.m. and hasn't been used since we were connected to the city sewer umpteen years ago. If it turns out that the tank can still be used (not leaking too much water), then I could bury a much smaller tank to cover our needs.

The question which arises is - If I have more than 1 underground storage tank, how to balance the amount of water in them? I don't want any fully automated system with sensors, pumps, etc. A primitive, "passive" system would be ideal, but what would work if the tanks are different sizes and at different depths, and that I may not be able to easily add plumbing to the old tank?

In the US most septic tanks were (and many still are) cast from concrete. The inlet, outlet, and service cover are not sealed to prevent ground water from entering. If yours is concrete it probably wouldn't be good for washing clothes no matter how well you clean it. Waterborne bacteria makes your clothes stink. I'm dealing with that now.

As for balancing, you could put an overflow pipe with a check valve from the top of the new tank to the old tank. That way you could still use the septic tank for garden water. It would require more plumbing and a second pump though.