So that it is 100% sealed, the case is best made of plastic. Polypropylene, polyethylene, fiberglass and their analogues dignifiedly proven themselves as building material for various needs. The fact is that the plastics are chemically and electrically inert. That is, neither aggressive drains of fecal masses, nor industrial waters will harm the KNS. Alkali, acid, salt, high and low temperature will also retreat before such an obstacle. Corrosion of metal products and cracking of concrete remained in the past. Now wherever possible, these building materials are intensively replaced with plastic. And where it cannot be replaced due to a non -stop process, there is a lining and plastic inserts there.