I have recently started dumping the bulk of my water trough waste into a clear plastic peanut butter jar. If I dump 90 some per cent of the water and hold back on the very last bit, it is reasonably clean. I dump this down the drain. The essentially damp sludge is wiped up with a couple paper towels. Usually the last little bit must be washed out, but it is minuscule. This method is free and much faster than evaporation. It is the best method I have tried for those who do not have access to a dumping area (especially apartment dwellers or those of us who work in basements).
Ken