as uk retired builder-i comment - cant not get into the septic EXCEPT VIA THE 50M/M VENT !! unless the o/f of foul water is exposed to open ,which should NOT BE !! should terminate to a soak away in the ground .
not a very good idea traffic island be interested how Thais deal when need to turn ????..!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
ow dear more chaos to come ????