A chemical toilet is a toilet which uses chemicals to deodorize the waste instead of storing it in a hole or piping it away to a sewage treatment plant. Common types include an aircraft lavatory, some passenger train toilets and the portable toilets used on construction sites and at large gatherings. They can normally be identified with a blue-colored dye in the bowl water as chemical toilets are usually used for short term periods and because of their high purchase prices, they are mostly rented rather than bought, often including servicing and cleaning.