Basically you have a sump after a settling tank. Hair, lint, body oil toilet paper, any grease creams etc. that can not break down will close the pores of the sand bottom and plug it up. You might be able to use a caustic to 'burn' it open. Your system probably is not allowed today, and you may not be allowed to use caustic etc.. You certainly don't want this stuff getting your your well or into its recovery area.