So i've been thinking about the costs of cleaning the black tank as have been using the blue chemicals for the last month. The thing is the blue liquids are extortionate considering the quantity needed to keep up proper concentration in a black tank as they are priced at portaloo/cassette users not RF tanks. 1. Try the geo method 2. The supermarket shelves in France are crammed with chemicals for septic tanks as not many have sewers. As my tanks are constructed from the same materials as septic tanks, PVC, what is stopping me using these more reasonably priced "digesters"?