Elsan Double Blue Double Blue is double concentrated so whilst it gives exactly the same unbeatable anti-bacterial performance as Elsan Blue, it provides twice the doses making it especially good value. And because it is concentrated, it also takes up less valuable storage space. Elsan Double Blue is also pleasantly fragranced, non-staining and suitable for all chemical toilets and cassettes. Simply add 30ml of Elsan Double Blue to the waste holding tank for every 10 litres of waste tank capacity, then add 2-3 litres of water. Conforms to British Standards 2893: 1989. Pack contains 2 x 2 Litres Elsan Double Blue.