In the UK, the rules governing discharges from septic tanks, etc. were tightened in 2015 with a final deadline for compliance sometime (Jan IIRC) 2020. I recommend that you get a qualified company to review the current arrangements and advise. There are potentially huge fines and costs associated with unlawful discharge of sewage; septic tank outflows no longer qualify for direct discharge into water courses, etc.
It's now quite a complicated set of regulations so DIY modifications are very high risk.