[Digestion] AD for cow manure

Rex Zietsman rex at whitfieldfarm.co.za
Sat Jun 11 03:50:55 CDT 2011


Hi Steve,

It has been a long time since we last communicated. I hope you are well. I
see that you are still experimenting as ever. As you can see, it appears
that I am back in the AD business for my neighbour and another farmer in the
district. 

You are spot on with respect to being caught between two sizes - hence my
use of second hand 3 litre motors driving generators. Brent has come up with
a number of suggestions that I believe have merit and should be able to help
keep the overall cost down. So I will look at those for sure. The fact is
that all of the dairies have existing lagoons and if we can modify them, it
makes life a lot easier. I agree with your idea of pre-heating the charge
before feeding it to the lagoon. This means that there has to be a large
intake pit to take all of the washings in one go, heat them up and let them
go to the lagoon. I believe that the cost of this should not be exorbitant
and it will allow the lagoon to operate in the mesophilic range. 

Surprisingly, I have been involved with dry digestion over the past year
with the objective of generating biogas for transport purposes. The gas is
upgraded to about 92% CH4 before compression, chilling and liquefaction. The
bio LNG is then pumped into cylinders in cassettes. The cassettes are
mounted on trucks and the gas reduces the diesel consumption. The empty
cassettes are swapped out for full ones as required.  Clearly this is on a
much larger scale than farm digestion and the capital is much larger as
well. The requirement for the biomass raw material is that it must be able
to ferment. Essentially, the rule of thumb is: "if a cow will eat it, we can
ferment it". So your grass attempts are not far off the mark. How well does
the fixed film work?

Kind regards
Rex 


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






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