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.