Utilizing manure as fertilizer is one common approach to capturing the inherent value in manure and reducing the costs of off-farm inputs. But the approach has its limitations; particularly in situations where there is more manure than can be responsibly managed on the available land.

For dairy farmers with large amounts of manure, anaerobic digesters offer several ways of capturing value from the manure waste stream. While digesters themselves are expensive, the resulting energy production, in the form of electricity, combined with the coproducts, which can be utilized as fertilizer or livestock bedding, can help to offset costs. But the return on the investment may not be economically promising, depending on the cost of electricity, and the cost burden can be prohibitive for some dairies.

Digesting more

Utilizing anaerobic digesters simply for handling manure doesn’t capitalize on their full potential, said Joseph Usack, Ph.D. candidate in the Cornell University Department of Biological and Environmental Engineering. Usack is researching this unutilized potential, seeking a means of optimizing both the environmental and economic impact of anaerobic digesters.

Anaerobic digesters can process more than manure. In fact, using manure in combination with other organic waste stream products, such as food waste, produces more biogas. It is this biogas that is converted into electricity to power the dairy farm, via a combined heat and power (CHP) system. Excess electricity is often sold back to the grid for further economic gain.

When a dairy farm is able to accept organic waste streams from the community, “tipping fees” are paid to the farm, offsetting more of the cost of the digester. With the additional amounts of heat generated, the potential for increased electricity production provides additional economic incentive.

This co-digestion of manure and other organic waste products is a more efficient use of the digester, Usack said, as co-digestion produces “a considerable amount of excess heat” over that of digesting similar amounts of manure alone. But co-digestion can also cause increased nutrient loads at the farm. So there are limits to how realistic co-digestion can be from the environmental perspective.

But not all of the heat ultimately generated by the digester is utilized in the typical CHP system. A large percentage is lost to the environment. If this lost heat can be captured and put to use on the farm, the value of that original manure becomes optimized.

Cooling with heat

Capturing the heat commonly lost to the environment and ultimately using it on the farm adds yet another layer of value to the original manure waste stream. This step of energy transformation takes the lost heat generated via anaerobic digestion and captures it. The captured heat is delivered to an absorption chiller unit, where its heat exchanger can convert the heat to cooling energy. This results in a cooled liquid, which can be directed for use in the dairy.

One particular use of this cooling energy is to alleviate heat stress in the dairy herd. The cooling energy, when passed through cow waterbeds, can reduce the impacts of heat stress, preventing the loss of milk output and increasing herd comfort and health. There are other scenarios where cooling energy can be used within the dairy – bulk tank chilling, space air conditioning, milk line chilling and to make the manure-to-energy process via anaerobic digestion more energy efficient itself.

“Cooling is really important on farms,” Usack said. By using waste heat, an absorption chilling system – which comes with a high price tag – becomes more economical and adds to the economic gain from anaerobic digester usage. An absorption chiller is also a passive system, which does not require any additional electricity to operate.

Alternatives

Another potential scenario with anaerobic digesters involves the liquid effluent that is produced during the process. If this effluent were converted into bio-crude oil, via a hydrothermal liquefaction (HTL) process, it could both produce a renewable fuel and decrease greenhouse gas emissions from liquid manure effluent. Usack has collaborated with other researchers at Cornell University to explore the feasibility of such a system. This would not only allow dairy farmers to generate electricity for use on the farm, but also produce a storable biorefinery energy product instead of returning additional electricity back to the grid.

Aside from the benefits – both environmental and economic – seen when using anaerobic digesters, value can also be added to manure by otherwise capturing the greenhouse gas emissions associated with animal waste. Methane is a potent greenhouse gas. The volatile solids in manure readily convert into methane. Methane emissions increase when anaerobic waste management systems are used.

The rise in popularity of liquid manure storage systems has had an inadvertent effect of increasing the amount of methane emission released from the manure. This methane is combustible and can be destroyed by flaring if it is captured. If there is a system where farmers receive carbon credits for flaring methane, value can be added to the manure waste stream while simultaneously decreasing the amount of greenhouse gas emissions from the farm.  PD

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food system topics.