We are in a motorhome - and we retain water when asked. The RV camp at Port Augusta request retention of water, we have seen professionally made portable units and a lot of 20l containers with a screw on lid - the only problem with that is people empty them into the dump point (CMCA type), which fills it quicker. They get it pumped out every two weeks, but if grey water is put in there, it will require more frequent emptying, which will force the sports club to put their price up - currently I think it is around the $5 mark per night. So where else do you empty grey water, if there is a septic system involved?