It's not a planning control issue, it's building control you need to
deal with - completely different departments of your local council.

BC care about things being built right and safe - PC, what it looks
like, will neighbours object etc.

If you have a new soakaway/processor it will have to conform to
current building regs - I *think* the Environment Agency certificate
(which actually involves testing the purity of the outfall) is only
required if your system (as mine did) discharges into a watercourse.

However if you have a specialist contractor, they will do all the
paperwork for you - you just pay the (quite large) bill.

I had a wrecked herringbone (soakaway) at my last house, used APCL who
retrofitted a processor to my existing tank, built an outfall into a
river, arranged the testing and supplied an Environment Agency
certificate.