Abstract: A tank assembly (1) is mountable on a tank truck for the collection of waste material from a portable chemical toilet and for the maintenance thereof includes an outer rigid housing (10) and an inner tank (14) enclosed in the housing (10) and provided with waste material loading nozzles (18, 19) and waste material discharge nozzles (17). The outer housing (10) includes an inner wall (31, 32) having a surface which is complementary and substantially mating and closely contacting the surface of the inner tank (14). The outer housing (10) and the inner tank (14) are made of plastic material. The inner tank (14) has a substantially spherical shape. The tank assembly (1) is light-weight, of low production cost and does not have corrosion problems.