used septic tank for irrigation?

Were replacing a wastewater tank with a septic system, and the old holding tank is no longer going to be used. Digging the thing up is going to add a lot of money to the project and we'd rather not just abandon it if we don't have to. We were thinking about replumbing for it to collect roof runoff for watering the garden. Were obviously not stupid enough to drink from this tank, but it would seem like this tank would still need to be cleaned before watering food plants with it. Any suggestions? It seems like this should work well enough but were worried it could turn out to be a really bad idea.

Thanks.

I plan to do this with the next tank I get for free. I will compost some stuff in it to start and then do a sort of manure tea from it. Any residual pathogens should be overwhelmed by the hot composting. My tenant operates an excavator and will stumble upon free ones. We plan to mount one atop a 15 ft tall hugelkultur mound so that we'll have some water pressure.

Not sure where you are at, but septic guys are licensed and have to follow the rules, which may mean removing the old one.

If that is not an issue, it seems like a decent use for the existing tank. It even has its own overflow protection if the lateral lines are left in place.

The concern would be whether it was less expensive and less maintenance to NOT use it. I say this cause an above ground tank can be gravity fed. The below ground will need a pump which means electrical to it. Buying an above ground tank may end up being less money and maintenance in the long run.

Thanks for the replies. Its good to see were not totally off our rockers here. Theres already a wire which runs concurrent with the pipe, so electric isnt a problem, though it will require buying a pump that can be dropped in. The roof would need to be guttered for an aboveground tank where we can run rock/drainage tile to the old pipe to usd the current one. Well have to talk to the septic people about the Regs, that could really monkeywrench the whole idea.