Septic tank for water storage?

We are renovating an old farmhouse in the south of Spain. As is there is no sewage system, we planned to instal compost toilets and use the greywater to irrigate the garden, but our lawyer says that we have to install a septic tank to get a permission to live in the house...

So we were thinking, that if we rigidly follow the law to the letter, but not to it's intent, we could install a septic tank, but be really bad engineers and install it "the wrong way" (guessing that the technician who goes thought the house for our housing permission won't check if it is connected to anything, just that it is there) and store water in it, our cistern only stores 10-20.000L so we would need an extra any way, so if we are forced to have a septic tank the money wouldn't be totally wasted. Only thing is, we don't know if the water stored in such a tank would be potable - assuming that we buy it brand new off course

Anybody knows if the plastic used in a septic tank is food grade?

Most of the plastic used in septic applications is ABS (acrylonitrile-butadiene-styrene), which is a fairly low outgassing plastic. As such, it is less likely to leach organics into the water than other plastics. It's certainly as safe or safer than PVC, which is commonly used in potable water plumbing applications.

Good luck with your deliberate obtuseness as to installation of a concrete box.

I'm based in Portugal, which probably has a very similar selection of plastic tanks to Spain.

When we've bought water tanks, the seller explained that there we three grades - only blue was suitable for drinking water, black was only suitable for septic, and other colours were 'washing water only'. A quick sniff of the black tanks convinced me that I wouldn't want to drink anything that had been stored in it.

Maybe I should get down to a ferreteria and smell for me self... Thank you