I have a septic tank that half my roofs rainwater runs into as does my neighbours.It does not cause us any problems.
We also have an exemption certificate that allows us to discharge 4 cubic metres per day from the tank onto the surrounding land........no problems for us and it saves us £500+ a...
Buy a multi fuel stove with a back boiler,and you can then remove the back boiler you have and sell it for scrap.They are a very heavy copper and worth about £50 scrap.I have a clearview stove with back boiler and they are great for hot water and running a few radiators.
There's lots wrong with it and you've been told that waste water does not run uphill.The pipe going through the back of that unit needs to be lowered.You need a new hole drilled lower down as you have been told above.