Septic tanksDouble-chamber septic tanks with leech system. Our substrate is hard clay and does not percolate to allow seepage tanks. The two in the picture serve two rooms; there will be an identical installation at the other end of the building for the other two rooms and the outside WC/shower and laundry room. Black water flows into the first tank which overflows into the second tank. Grey water goes directly into the second tank. There are no kitchens so a grease trap is not required.jazzman