Stay away from using septic tank waste. Human waste in compost can cause serious health problems. Ie:gerardia (sp?) and other viruses that won't be killed by the heat of the composting. From what I have read, the viruses that are contained in human waste, used in compost, will be absorbed into the plants from the soil and then will be ingested causing illness.
There are laws regarding the sale of human waste compost, due to what I stated above. There are also warnings to not use it on food crops.
Now, for general lawn supplement, it would be great.
And I don't see any problem using the bacterial cultures for septic tanks on regular compost, it's only beneficial bacteria.
Just thought I would throw that out there.
HL