| dc.description.abstract | Intensification of farming operations and increased nutrient application rates have led to higher
crop yields and greater food security. At the same time, widespread use of commercial
nitrogen (N) and phosphorus (P) fertilizers and large-scale livestock production have led to
unintended environmental consequences, including eutrophication of both coastal and inland
waters, threats to drinking water, and increased production of N2O, a potent greenhouse gas.
In the past, crop and livestock production were typically more integrated, allowing most
livestock to be fed by local crops, and most livestock manure to be applied directly to nearby
cropland. Under current intensive agriculture practices, however, there is frequently a spatial
decoupling of crops and livestock, leading to hot spots of manure production and a lack of
opportunities for cost-efficient and environmentally sensitive disposal. In recent years, there
has also been increased interest in the use of both farm and regional-scale bioreactors to convert
excess manure to energy, thus exploiting a renewable energy source and increasing the
potential to recycle animal waste.
In the present work, I develop a spatially distributed optimization approach to identify hotspots
of manure production, and, using both economic and environmental criteria, evaluate the
economic feasibility of (1) transporting manure for spreading on cropland to meet established
nutrient requirements, and (2) constructing biogas reactors to process excess manure in areas
where long-range transport is found to be infeasible. This work is focused on manure
redistribution, and potential for biogas construction at the continental US scale.
In order to identify the spatial disconnect between livestock and crop production, I developed
a gridded data set where each cell was 6 km x 6 km and calculated the crop requirements and
manure production in each cell. After finding the P requirements in each cell, I found that
530,000 tonnes of phosphorus in manure was located in areas where, if applied, it would be in
excess of the local crop requirements.
I then examined the feasibility of transporting manure from excess locations (cells) to other
locations to use as fertilizer by formulating an optimization problem to maximize the financial
benefits of transporting the manure. Savings from transporting manure was calculated as the
financial benefit from buying less mineral fertilizer minus the cost of transporting the manure.
The solution to this optimization problem shows that transporting manure was able to reduce
the excess phosphorus applied to fields by at least 88% with savings of up to $3 billion USD.
Finally, I examined the costs and benefits of using the remaining excess manure (after
transportation for fertilizer) as fuel to operate biogas plants. For this, I formulated an
optimization model to site biogas plants across the continental US such that net profits from
the biogas plants were maximized. Biogas net profits were defined as the money made from
selling electricity minus the annualized costs for constructing and operating the biogas plants
and transporting the manure to the biogas plants. The solution to this problem shows that
constructing and operating 387 biogas plants yielded a net profit of $100 million USD and
would utilize all of the manure remaining after transportation for fertilizer. This 100%
utilization rate of excess manure would have great environmental benefits in terms of removing
potential sources of non-point source pollution from farms that would otherwise be available
to runoff into waterways. | en |
