Manure
is already used as a fertilizer on most farms, providing cost savings on
fertilizer purchases and helping to build soil health on fields that receive
this valuable amendment, but are there ways to get additional value? One that
has received some attention over the years is manure to energy. In liquid and
slurry manure systems this has typically meant anaerobic digestion for
production of biogas, which is rich in methane. However, in states like Iowa,
these systems remain relatively rare. Why is this? While there are many reasons,
a good place to start is by exploring the economics of these anaerobic
digestion systems. We are going to take a look at one specific type of digester
system, the covered lagoon, to see how it may impact manure economics.
What
is an impermeable cover? It is a plastic film placed on top of the manure
storage that is impermeable to both gases and liquid. The idea is that this
cover will keep rainwater out of the manure, hold in gases so odor and ammonia
emissions are minimized, and capture methane made by the natural breakdown of
organic matter in the manure.
There
are several ways covers can add value to a farm. For instance, reducing the amount of rainwater
that needs to be hauled, holding onto nitrogen and increasing the fertilizer
value of manure, and the value of odor control. By capturing the biogas, it can
be used for the generation of electricity, heat, or additional processing (via pressure-swing
adsorption to separate the methane and compression into pipeline quality gas in
this analysis). In addition, doing so will add several new expenses for the
operation. These include the cost of the cover, the cost of cleaning the gas to
pipeline quality, and the change in manure application costs, as more land will
be required as the cover maintained more nitrogen value.
Figure 1. Impermeable cover on a manure storage that allows collection of biogas, keep rainwater out of the manure, and help minimize odor and ammonia loss, but is it ever cost feasible?
So
let’s start with some of the positives. Adding a cover to an outdoor manure
storage (a Slurrystore, earthen, or concrete storage outside the building)
would allow the design of a slightly smaller storage, as we’d no longer have to
size for rainfall and the 25-year, 24-hour storm (about 5-5.5 inches throughout
the state of Iowa). However, this has a minimal change on the construction
costs of the actual storage. However, putting a cover on a storage does offer the
potential for retaining nitrogen in the manure. In a typical deep pit storage,
7.8 pounds NH3 is retained per pig per year. Switching to an
impermeable cover would save about 5 pounds of NH3 per pig per year,
which on a 4800-head swine farm amounts to about $7,000 of nitrogen value every
year. This would increase our manure application costs slightly as more
nitrogen means more acres would be needed and manure would need to be moved a
bit further, increasing application manure application costs by about $2,700
every year.
One
advantage of the impermeable cover, it allows us to capture the methane the
manure is making, the value of this captured methane would be around $9,000
through direct sales, and if marketed correctly to collect the RIN (Renewable Identification
Number) credit which is granted for transportation fuels would add another
$26,000 in value from the methane.
Comparing
this to new expenses generated, we’d have an annualized installation cost
(assuming a 5-year life of the cover) of $14,000 and a maintenance cost of
around $3,500. Our largest new expense would be biogas cleaning, so we can
inject onto the pipeline, which would be around $24,000 every year.
When
you take all of these into account, this sort of system hovers right around the
break-even point on an annual basis. While I’m not saying we should all start
making biogas at our farm, it is nice to see that we are close to being able to
say, in the right situation, it could make sense on a farm.