Domestos can be used in households with septic tanks when used as directed. When the disinfectant enters the drains it reacts with the dirt and grime to form a harmless salt. This is diluted enormously by water coming from other drains in the home resulting in a very weak salt solution going into contact with the bugs in the septic tank. The harmless nature of this solution has been confirmed in tests with traditional septic tanks. Care should be taken with the relatively new aerobic septic tanks marketed under such names as Envirocycle and Biocycle. Although Domestos disinfectant is expected to be as harmless in these systems as in traditional tanks sufficient tests have not been carried out on these systems. We therefore advise here that you follow the manufacturer’s instructions. Under any circumstances cleaning products (like Domestos disinfectant) should not be directly poured into septic tanks.