[Digestion] AD for cow manure

[Steve Rodda]

Rex Zietsman wrote:
> Hi all,
......

> In my area we have a large number of dairy farms milking >500 cows. These
> are sizeable operations and the manure they produce is worth pursuing.
> Typically they all have slurry dams and they spread their manure from time
> to time (much to our temporary dismay while the odour kills us!). The
> question is: what is an economical way of producing gas? ......

Hello Rex
You seem to be in the classical situation of being too big for ad hoc solutions but too small for 
automated low labour installations.

My suggestion, is there a way in which the dairy farms co-operate to run a central biogas installation?
Perhaps each farm has a de-watering plant and transports the 'cake' only to be digested in a dry
digestion type facility. On site the fluid faction can be treated in a simple manner to produce
biogas(if desired) eg sequencing batch method (fill, react, decant  on a daily basis).

The digestate is valueable (probably worth the exercise by itself especially for an organic farm)
note the fluid should also be used in conjunction, having the N component and (can't remember which) some 
of the P or K component.

In my opinion, heating is the major factor, for installations not located in the tropics. Recovered heat
from an engine is possibly the easiest heat source. However if you can keep the quantity of liquid down, 
pre heating the influent (to make up for the heat loss from the digesting mass) seems to work alright.

My suggestion for on farm digesters ie centralised digester is not an option -- most of your efforts will 
possibly have to be justifying the cost versus benefit, perhaps if you could figure a way for direct 
burning (on farm)of the gas coupled with the fertiliser benefit, a simple trench covered with a membrane 
might make an economic match.


I mentioned dry digestion. I think I have developed a methodology for digesting grass. Basically,
grass and foodscraps are allowed to soften in a vessel and then are leached (flooded method) with 
effluent from the digester. The leachate spends a day in another vessel where (I believe... hope I am 
correct) simpler acids are formed and then input into the digester.
Perhaps cow manure'cake' can be adapted to this method.
The digester is fixed film. Urethane foam cubes as carriers in the intermediate vessel.

kia ora
stev Rodda