By John D. Forcier, P.E.
Anaerobic Digesters provide an excellent baseload renewable energy source. Cow manure has been digested in the cow’s four stomachs and domestic wastewater has been digested in the human digestive system. Therefore, those materials have limited remaining energy, but they still have excellent methane producing bacteria. However, by adding select external feedstocks, there is a significant energy gain with advanced AD features, proper operation and with the optimum recipe.
- Low Energy Base Feedstocks: Manures and domestic WWTF sludge have limited energy due to previous digestion.
- High Energy Feedstock Issues: These include limited local availability, appropriate pre-processing systems and recipe management.
- Permitting Requirements: These may include solid waste and air permits, maintaining nutrient management plans with additional feedstocks and the electrical interconnection.
There can be significant energy gains (3 to 10 times manure/sludge only) from adding select mixed feedstocks to the manure/sludge. Optimizing energy output can be attained by selecting the best-suited feedstocks, designing advanced AD features and by optimizing recipe management. You can utilize innovative designs to minimize project costs while meeting permit requirements.
Extra Energy With Higher Energy Feedstocks
Typical individual AD Feedstock Relative Energy Potentials include:
- Cow Manure including feed waste (~12% TS)= 1X (Note: This is the basis of 1X)
- Liquid Food Waste including yogurt and cheese whey and food processing waste (3% to 5% TS)= 7X to 3X
- Brewery Waste including high strength liquid waste and spent grain (6% to 25% TS)= 3X to 15X
- Ice Cream Waste (20% to 35% TS)= 7X to 20X
- Semi-Solid Food Waste Fruits & vegetables & de-packaged SSO and FOG (15% to 30% TS)= 9X to 12X
- Glycerin Bio-Diesel By-Product (35% to 75% TS)= 40X to 80X
- Wastewater Sludge (~6% TS)= 1X
- Wastewater Sludge Cake (20% to 30% TS)= 3X to 5X
- Septage (1% to 3% TS)= 3X to 1X
Extra Energy With Advanced AD Features
The combined energy values can be increased substantially by adding mixed feedstocks and by providing advanced receiving, preparing, feeding and mixing systems. Receiving systems should have separate receiving tanks for manure/sludge and for other feedstocks, each with mixing, chopping/slurrying systems and pre-heating. Separate tanks, with the same features, should be provided for higher energy feedstocks (glycerin, ice cream, etc.). All receiving tanks should be sized for 3 to 5 days of storage. The feedstock feeding systems should have batch-controlled pumps suitable for the respective feedstocks. The AD tanks should be sized for the appropriate detention time and include insulation, heating and timed, adjustable mixers sized to completely mix the AD contents. A desulphurization system should maintain the H2S levels below the CHP’s allowed limits. The process control system should include integrated controls and process instrumentation that are controlled by a PLC and a SCADA system with remote access.
Extra Energy With Optimum Recipe Management
The baseline materials of manure or wastewater sludge, which I refer to as “kindling”, form the basis of the recipe. Other key operational factors to consider include the temperature range, pH range and VFA/TA Ratio. Since the bacteria like to have consistency, besides the key factors above, the bacteria also prefer consistent energy level, with adjustments of only 5% to 10% per day. If you are going to make major changes in energy potential, including by adding high energy feedstocks, then these should be implemented over a 2 to 4 week period. Avoid overfeeding problems, since bacteria can get a “sugar high and crash”. This will often result in a temporary peak and then a significantly decreased output due to damaged or killed bacteria. Overfeeding can also cause major foaming and crusting issues, which will likely further reduce the energy output. Finally, monitor the key operational factors and adjust the feed rates/recipe to maximize the biogas and energy output.
1+1=3: Sample Farm AD System
- Baseline Materials-
- Manure from 1,000 cows @ 25,000 gpd (80%)
- FOG/Food Waste @ 35 tons/day (20%)
- Theoretical Yield (Individually)-
- 80% Cow Manure (0.80 x 1X= 0.8Y) [110 kW]
- 20% FOG/Food Waste (0.20 x 12X= 2.4Y) [330kW]
- Total 0.8Y + 2.4Y= 2Y [Total= 110 kW + 330 kW= 440 kW]
- Actual Yield (Combined)-
- Cow Manure= 240 kW
- FOG/Food Waste= 825 kW
- Total= 2Y x 1,065 kW/440 kW= 7.75Y [1,065 kW]
- Similar Results With WWTF Sludge-
- Note: WWTF sludge + mixed feedstocks can have similar results.
You can utilize the “Typical AD Feedstock Relative Energy” values to help you to select the best available feedstocks for your system. You can get extra energy from optimizing the mixing and pre-processing of the feedstocks. Designing an AD system with advanced AD features will allow you to take full advantage of the mixed feedstocks and their potential energy. Monitoring the key operational factors and optimizing the feedstock feed rates and recipe can help to maximize the biogas and energy output.
About the Author: John D. Forcier, P.E. is a member of Lee Enterprises Consulting, the world’s premier bioeconomy consulting group, with more than 100 consultants and experts worldwide who collaborate on interdisciplinary projects, including the types discussed in this article. The opinions expressed herein are those of the author, and do not necessarily express the views of Lee Enterprises Consulting.