A geeky technical description of how this two-stage mixed AD process works:
The number of each step described below corresponds to the numbered component of the AD facility site plan shown here.
Step 1. Up to 15,840 gallons of organic material, the daily meal for the digester, are macerated and mixed in the preparation tank (1) within the AD building. A series of powerful chopper pumps do the work. Each ‘meal’ includes manure and other organics, like energy crops, food-processing residuals, and pending one last permit, pre- and post-consumer food residuals. The mixture is balanced for pH and consistency, heated to about body temperature, and can be pasteurized if necessary. Ferric chloride is added to reduce sulfur and prevent the formation of corrosive hydrogen sulfide gas. Meal preparation takes 1 – 4 hours.
Step 2. The meal is pumped to the 135,000-gallon hydrolyzer tank (2) that holds 106,000 gallons of feedstock and here the first microbial process takes place. In the presence of oxygen, microbes break down food particles and large (macro) molecules into small organic acids via the aerobic process of fermentation. The aerobic process also occurs in compost piles. This process produces some gases, like carbon dioxide. Because the tank is sealed, the microbes use up all the oxygen in the feedstock and after 3 – 6 days the hydrolyzed feedstock is pumped to the anaerobic digestion tank.
Step 3. The 410,000-gallon anaerobic digestion tank (3) holds 317,000 gallons of hydrolyzed feedstock and is topped by a gas bladder that can hold 93,000 gallons of biogas: a mixture of methane, carbon dioxide, hydrogen gas and water vapor. The second microbial process, anaerobic digestion, happens here in the absence of oxygen.Methane forming microbes (killed by oxygen gas) break the organic acids down into methane, carbon dioxide and hydrogen gas in a process that takes roughly 20 days. Efficient digestion requires complete mixing. An impeller moves feedstock around the tank in a very slow vortex. A chopper pump can move material from the bottom of the tank to the top or vice versa to achieve complete mixing. Any solid material falls and can be removed through a valve in the conical bottom of the tank.
Step 4. Biogas travels from the bladder in the anaerobic tank to the generating engine (4) (genset) where it is combusted to create electricity and heat.The electricity is sold to Green Mountain power, and the ‘waste’ heat is captured, transferred to the campus heating plant, and be used to heat campus buildings. At full capacity, the digester should produce 2.8 million kilowatt hours of electricity per year. Excess biogas is burned via a flare. Heat capture equipment will be installed this year but won’t be fully functional until next summer.
Step 5. Once digested, the slurry is pumped from the anaerobic tank to a small holding tank in the in the building. A screw press, or auger, solids separator (5) squeezes effluent into liquid and solid fractions. The solids have 35% moisture content and can be used for bedding cows, creating compost, or spreading directly on fields for fertilizer.
Step 6. The liquid effluent is pumped to the 115,000-gallon digestate storage tank (6) that holds about one week’s worth of material. From there the liquid fertilizer is either spread on fields or moved to a holding pond being constructed at the campus farm.