In the summer of the year 2007 I didst research the effects of agitation intensity on the seperation of sand in anaerobic digesters for dairy cattle waste. In short- I got to play in cow manure all day long!
First, my 2 minute background blurb. Anaerobic digesters are, in a very simplified sense, a big airtight chamber you throw poo in and then sit back and let the naturally occuring microbes build up in numbers. Eventually they will start breaking down organics in the manure into methane gas through a process involving several different bacterial populations. The fun part is that this gas can then be harnessed. It can be burned for heat, or used to generate electricity via fuel cells, combustion, or some other means. Cool eh? And on top off all this, the effluent from the digester still has most of its nutrient value, smells alot less, and doesnt contain as much stuff that will readily volitalize when it gets applied to the fields, making it a little more eco-friendly. Obviously this is not an in depth background, but I wouldnt want to bore the layman.
Our particular digester design was influenced by a desire to come up with a better method of sand removal from the manure. Why is there sand in the manure? Well, that would be because it is commonly used as bedding for cows, it being their prefered bedding material (no they didn't tell me, but some people did studieds). Anyways, the cows have a habit of kicking lots of sand out of the stalls and into the allyways with the manure, on the order of 40 pounds per stall per day. This isnt all bad, as it helps keep the sand on top fresher, but it does create problems with digesters.
The sand is more dense than the rest of the manure, as a result it will settle out on the bottom of the tank. Over time, it fills up the volume of the tank, rendering it inoperable. Thus far, most efforts have been to Treat the manure to remove sand prior to digestion, our digesters (only lab scale at this point) are designed to remove the manure out the bottom of it.
Aside from helping out running a battery of tests on all manner of things digester related, my focus has been seeing what results in teh cleanest sand effluent removed. Idealy we would like to be able to re use it as bedding! (I know, gross, but if the microbes are good they will eat most of the goo on the sand).