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An assessment of the applicability of anaerobically digesting spent grains from the brewing process at Miller Brewing Company, Milwaukee, Wisconsin / Matthew P. Shattuck.

by Shattuck, Matthew P.

Sewage sludge digestion

Waste products as fuel

Costs and Cost Analysis

73 leaves : ill. ; 29 cm.

Advisor: Dr. Francis Mahuta.

Dr. Carol Diggelman, Kenneth Kaszubowski.

Introduction -- Literature review -- Materials and methods -- Results and discussion -- Cost analysis -- Conclusion and recommendations.

Spent brewers grain is sold to local farmers as a feed product, and most of the breweries' spent grains are taken off-site as a wet slurry. That slurry is 15% TS, and of the 15% TS, 95% are VS. Breweries have a high degree of focus on sustaining spent grain revenue, and any changes to the net worth of the spent grain are noticed immediately. Currently spent grain revenue is trending downward at most breweries. The net worth of the spent grain is decreasing due to the introduction of corn gluten, a byproduct produced from ethanol facilities, and the reduction in the number of large cattle herds across the United States. These current trends will cause the spent grain net worth to decrease further in the future. In light of these trends, a major brewing company in the Midwest launched an investigation into ways they can sustain or increase their spent grain revenue.

In association with the Milwaukee School of Engineering, the brewing company launched an investigation into the applicability of anaerobically digesting spent grain to produce biogas as an alternative fuel source. The Biochemical Methane Potential Test was done to determine the amount of methane that can be produced per gram of spent grain VS. Bench-scale test results showed that 0.430 ± 0.027 liters of methane was produced per gram of spent grain VS. There was also a 41% reduction of TS and a 59% reduction of VS observed during the test. An estimated 2.4 million therms can be generated per year from spent grain anaerobic digestion at each brewery. Hydraulic retention times were as follows for spent grain VS added: 2 g VS/L ~= 14 days; 5 g VS/l ~= 25 days; 7 g VS/l ~= 27 days.

Based on the promising bench-scale test results, a present-worth cost analysis was performed to evaluate the economic feasibility of implementing full-scale anaerobic treatment of spent grains at all of the brewing companies' breweries located across the United States. Some breweries consume coal as their main fuel source, and as a result of the low cost of coal the applicability of increasing spent grain revenue at these breweries by anaerobically digesting spent grains to produce biogas was not economically attractive. The other breweries consume natural gas as their primary fuel source. Anaerobically digesting spent grains at these breweries to produce biogas could significantly increase the net worth of the spent grains. The natural gas consuming breweries where this process would be most applicable would be the Brewery #1 and Brewery #2. The Brewery #1 had an ROI of 13.16% and an NPV of $1,116,267 (with an interest rate of 5%), and the Brewery #2 had an ROI of 10.90% and an NPV of $598,184 (with an interest rate of 5%). The NPV and ROI figures are expected to increase in the future as a result of the expected net revenue decrease of the spent grains and the expected increase of natural gas prices.

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