This site has a neat suggestion for digging a sewage pit especially for pet waste, regularly adding some Septo-Bac which helps to get the sewage breakdown started, and placing a plank over the top. Looks like a good system to me. Further down the page there is more advice on how to do it. One person dug a hole and buried a garbage can with the bottom cut out of it.

Pet Waste Composting

There are also some commercial composting products mentioned.

Helen