Slate Beds
The equipment for this process consists of tanks with horizontal slabs of slate separated a few inches by stone blocks. The sewage is allowed to stand in the tank for about two hours, during which the suspended matter is deposited on the slabs and is digested by multifarious forms known as aerobic germs, i. e., germs requiring oxygen for the continuance of their proper vital function. The deposits are thereby reduced to harmless and inoffensive humus. Slate beds are dosed and rested alternately so as to give them an opportunity to replenish their supply of oxygen. Multiple units are therefore necessary. The effluent must be treated as a tank effluent. Fowler suggests that when filters are used to purify the effluent, “humus” tanks be provided between the slate and the filter to retain the solids washed away from the beds and somewhat to equalize the composition of the effluent passing into the filter.