Geraldine is correct. above ...

Geraldine is correct. above ground is the best. the sides should be like a dike. rise over run of 0.5 on both sides. with water proofing on the outside. inside throw in some rocks the size of soceer balls and the HRT should large. the rocks will provide geobacters. have vents to allow methane to escape. use them with occasional additions of saw dust. if you can add vent fans, even better. if you want bioaugmentation, try biosix biobuddy. only $600 and it will last 10 years.