By John D. Forcier, P.E., Lee Enterprises Consulting, Inc.
Special to The Digest
Farm-based anaerobic digestion systems have adapted in various regions of the world. The “European Model” relies on cash crops and manures with very high incentives and feed-in-tariffs. The “U.S. Model” is based on mostly or all cow manure from large farms (800 to 2,500 cows, or more). The “North American Model” utilizes cow manure and mixed organic feedstocks and is well-suited for the small to medium farm (200 to 500 cows). There are many advantages of the “North American Model” Farm-Based Mixed Substrate Anaerobic Digester Systems including:
Energy/Efficiency
These AD systems provide truly baseload renewable energy and are typically 92% available 24/7 at full output. The included CHP systems have ~40% electrical efficiency plus ~40% heat efficiency. The CHP heat exchangers capture waste heat from the generator set to be used for process heat and for heating nearby facilities, houses, greenhouses, etc. Their highly efficient and effective mixing systems force bacteria and “food” from feedstocks to fully intermingle for maximum biogas and energy output. There is a significant energy gain (3 to 10 times manure only) from adding off-farm feedstocks with the optimum recipe. Most importantly, there is a major reduction in dependence on fossil fuels for electricity and heat. For example, a 1MW system provides electrical energy for 700 homes plus thermal energy for 300 homes.
Environmental Improvements
Air quality is improved significantly, since odors are reduced >90% due to reduction in volatile solids. There is a reduced carbon footprint with carbon credits/renewable energy certificates available and with major reductions in production of carbon dioxide and methane from manure lagoons. There is a 99% reduction in pathogens and the resulting separated liquid provides a virtually odorless organic liquid fertilizer. Landfill capacity is increased due to organics diversion implementation as required by laws that have already been enacted in many states. The system provides for beneficial reuse of food waste and other organics that have been diverted from landfills while producing renewable energy and liquid organic fertilizer.
There are water quality improvements with reduced/improved storm water runoff including reduction of phosphorus and other nutrients going into lakes and streams. The small footprint of this system also minimizes the impact on the environment. With much lower percent solids, most of the liquid fertilizer soaks into the ground, therefore it is much less likely to reach nearby streams, even in large rainstorms. There is also improved nutrient management since the nutrients are in more soluble and much more readily available forms (for better crop uptake). Therefore, less fertilizer is used and much less chemical fertilizer needs to be brought in.
Financial Stability
This provides a base-line industry for the next generations of farm families. These AD systems are well suited for small to medium sized (200 – 500 head) farms with proven technology adapted for the mixed-substrate market. There is typically a 5 to 7 year payback on this base-load renewable energy system. Typical revenues and savings include: tax incentives, fixed energy rates from feed-in-tariffs, renewable energy certificates/carbon credits, tipping fees from off-farm feedstocks, energy savings/energy revenue from captured heat energy, significant savings on animal bedding costs and major savings from reduction in the purchase of chemical fertilizer.
Typical Feedstocks
Some typical feedstocks (substrates) for farm-based anaerobic digesters can include animal manures, pre-consumer and post-consumer food waste, certain farm crops, fats, oils and grease (FOG), de-packaged source-separated-organics and other organic byproducts. In many jurisdictions, there are bans prohibiting organics from going to landfills. Since many of these organics are excellent feedstocks, anaerobic digesters can provide an excellent alternate in converting those feedstocks to energy and organic fertilizers.
Saving Family Farms
In general, anaerobic digesters can save family farms by providing a base-line business for the next generations by providing an economical solution for baseload renewable energy, reducing the carbon footprint, providing inexpensive animal bedding and organic fertilizer, improving crop production, lowering costs, reducing odors and helping the environment.
