Rainbows in the Lehigh Valley Gorge

[Charlie B]

Now I worked next to  a sewage treatment plant for 11 years. The head operator was a great guy. Our boss from upstate New York was down for a visit and I mentioned that Darwin was a man that really knew his s*(&.  My boss about lost it.  One of the operators had fish tanks that had gold fish in the water that had been treated and ready for discharge.  They would empty the sludge tanks into large areas lined with sand and allow it to dry and when it was deemed safe it was given away for fertilizer. 
There were some pretty big piles at times but I don't know if you want to get that carried away with your model.   
Some of these plants have exploded.  They capture the methane gas and use it in the plant. I have not read about what exactly they use it for. Sometime I will check on that.  I have a septic tank cleaning truck on my layout, a model of the one my father in law had in the late 50's and early 60's.  
Charlie