Wastes of the wastes, Part 1

We have covered several different types of wastes to energy through this blog, but what about the wastes that are produced from these processes.  For anaerobic digestion there are several main waste streams that much be handled correctly in order for the process to have minimal effects on the environment.  As mentioned before, there is a large amount of wet sludge that is left over from anaerobic digestion.  This wet sludge has had all of the volatile (vapor acting) molecules removed, which is the known as the biogas.  Although most of the solids in this process are digested and converted to methane and carbon dioxide there are some left over solids and water that make up the wet sludge.  Typically the wet sludge contains about 6% solids by weight.  Meaning that if the water and solids were to be separated from each other than the weight of the solids would be 6% of the total weight of the wet sludge.  While this wet sludge isn't particularly harmful or polluting it must be disposed of used in a certain way.  Most commonly the wet sludge is give to farmers so that they can apply this digested material to their crop land as fertilizer.  Remember cow manure, the feedstock to the anaerobic digester, is also a reliable and effective fertilizer used by farmers around the world.  There have been studies showing that not only does the application of wet sludge to field not stink as bad as fresh manure but that it is also better for the environment.  This is due to the fact that when fresh manure is applied to the fields the volatile compounds contained within the manure are released to the atmosphere. These compounds being methane, ammonia, and carbon dioxide. 

Another typical way of handling the wet sludge produced from anaerobic digestion is composting.  The wet sludge is dried and allowed to rest while the left over naturally occurring microorganisms continue to degrade the left over solids.  This compost can be used just as any other compost would be used.  The dried solids from the wet sludge have also been recycled and used as bedding for livestock.  The pictures below show the two different ways of applying both fresh manure and wet sludge to crop land.  

Liquid application of left over wet sludge from anaerobic digestion.

Fresh manure application.