Is it economically viable to create energy from waste at your location? For dairy farmers and others, the answer is increasingly ‘yes’. In recent years, major players in the dairy industry have announced sustainability initiatives promoting anaerobic digestion (AD) of manure to create biogas for renewable energy on the farm. Led by the Innovation Center for US Dairy, the 2008 US Dairy Sustainability Initiative presents a roadmap to increase business value for the dairy industry supply chain while simultaneously reducing greenhouse gases.

 

Currently fewer than 200 AD systems are operating on American dairy farms. A goal of the initiative is to increase the use of anaerobic digestion to at least 1,300 farms by 2020. The initiative is being supported by AgSTAR, a partnership of the U.S. Departments of Agriculture and Energy, and the Environmental Protection Agency (EPA) to promote biogas projects.

 

For dairy farmers, the benefits of using AD to create biogas from manure include:

• Potential new revenue streams

• Avoided waste disposal costs

• Increased opportunity for milk sales to sustainable buyers

• Odor control

• Environmental protection

• Energy independence

 

Wastewater engineers at Short, Elliott, Hendrickson (SEH) have been designing AD systems for decades to treat municipal waste, and industrial food and beverage waste. More recently, Dan Hedrington, an Agricultural Design Specialist/Project Manager who directs the SEH’s Agricultural Services department, has seen a sharp increase in inquiries from dairy clients. They are interested in converting animal waste and crop residue to energy, and cleaning bedding material for reuse.

 

“I still work on the traditional operational improvements for farms and ranches,” Hedrington explains. “People are still interested in improving waste transfer and storage, but I’m hearing more from people who don’t just want to pump manure onto their fields if there’s a way to cash in. It gets people excited when they think of turning a waste stream into a revenue stream.” 

 

Agricultural producers ranging from small family farms and ranches to large corporate operations are looking for ways to make their operations more efficient and increase revenue. “Some farmers and ranchers want to use their land beyond traditional agricultural purposes, and others want to make the most of what they have,” Hedrington says. “They’re all looking for bottom line improvements.”

 

And there’s a lot of potential for agricultural producers. Hedrington points out that, collectively, agricultural producers holders large amounts of biomass – both animal waste and crop residue. “That makes renewable energy an easy marriage for farmers and ranchers,” he notes.

 

Anaerobic digestion is a naturally occurring biological process that uses microbes to break down organic material in the absence of oxygen. In engineered anaerobic digesters, the digestion of organic waste takes place in a special reactor, or enclosed chamber, where critical environmental conditions such as moisture content, temperature and pH levels can be controlled to maximize microbe generation, gas generation and waste decomposition rates.

 

Anaerobic digestion is generally accomplished in a sealed environment with no oxygen present while maintaining an elevated temperature to accelerate bacterial digestion. Variations of this process can optimize waste treatment and gas production depending on the particular blend of waste. Anaerobic digestion systems can operate in three temperature ranges; psychrophilic, mesophilic, and thermophilic. In general, as the temperature increases, so does biological activity. For each 18˚F (10˚C) increase, the microorganism growth rate doubles.

• Covered Lagoons

• Plug Flow Reactors

• Complete Mix Reactors

• Fixed Film Reactors

• Upflow Anaerobic Sludge Blankets

• Anoxic Gas Flotation Processes

 

During the 1980s, high energy prices encouraged the technological development of a more efficient class of digester in Europe. Significant research and system operational data available for high rate systems comes from European sources. Increasing energy prices, concerns about long-term sustainability and the need to reduce greenhouse gas emissions have led to renewed interest in AD projects for many types of organic waste.

SEH engineer Todd Fryzek recently toured anaerobic digesters in Germany, Sweden and Denmark. There, digesters are designed to create biogas for direct use or to generate electricity. The European market for biogas from anaerobic digesters has been in place for more than 20 years. Fryzek says while the technologies are fairly consistent between countries, and dominated by Continually Stirred Tank Reactors (CSTRs), each has a different model for using the biogas.

In Sweden, manure, waste agricultural products and food scraps are used to generate biogas. The biogas is purified and stored for use in trucks and automobiles. In Denmark, it is most common to use manure and waste agricultural products to generate biogas. Then it is piped to small communities where it is used to generate electricity. Excess heat is channeled through a district heating system to local homes. In both Sweden and Denmark, the feedstock is pasteurized prior to digestion to insure pathogens are destroyed early in the process. 

 

In Germany, corn is grown on fallow land and directly incorporated into the biogas production system. Pasteurization is omitted. The biogas is cleaned and injected into the natural gas grid.

 

Any of these models can be implemented in the United States, depending on local needs and conditions. 

Small scale manure digesters are increasingly common in agricultural areas of the Midwest. AgSTAR estimates anaerobic digestion could be cost-effective on about 3,000 American farms. However, as of February 2009, only 125 AD systems were operating on American dairy farms. Most of these systems use a simplified approach to anaerobic digestion to process and treat livestock manure and produce gas, to meet some on-farm energy needs.

 

Hedrington says ag producers considering an AD to biogas system should look beyond the basics of the digester technology.   “There are several components of an AD system that should be assessed during the planning stage,” he notes. 

 

• Feedstock collection and delivery

• Feedstock storage and equalization

• Pretreatment (sand removal, pH adjustment, etc)

• Digester (technology types and combinations)

• Post-treatment and storage of solids, liquids, and gases produced

• Interconnections for power or gas injection

 

Manure, food processing wastes, dead animals and inedible residues are all potential feedstock material. Digesters typically can accept a wide variety of biodegradable input, but actual energy producing capacity depends on:

 

• The amount of energy stored in the material

• The presence of inhibitory or toxic substances

• Nutrient balance

• Moisture

• pH

 

Livestock producers have access to manure from dairy farms, beef cattle, poultry and swine operations. Manure-derived feedstocks are often disposed of via land application, a practice that is coming under increasing scrutiny in the upper Midwest. The manure producer must maintain enough cropland to handle the nutrient load of the manure, or arrange for other area farmers to accept the material for land application on their own fields. Currently there are few alternatives to land application, and many are impractical. Alternative disposal methods, such as landfilling or incineration are not universally available or suitable and may involve significant capital costs.

 

Key considerations for using manure biomass include collection, transportation, efficiencies of scale and competition for resources. A study funded by AgSTAR showed anaerobic digester technology can be cost effective for dairy farms with more than 300 cows. While modified digester installations are being studied for farms with less than 300 head, a centralized approach (processing waste from multiple farms at a single location) to manure management may be more viable.

 

Fryzek says digesters operating solely on manure might not generate enough energy to fund capital and operational requirements.

 

“Mixing manure with other organic substrates may be necessary to improve energy production rates,” he explains. “There are a variety of food and food processing companies which produce waste materials and by-products that are potential feedstocks for biogas production.”

 

Field crop residues including corn and soybean stover can be recovered post harvest. Other conventional crops including potatoes, beet pulp, wheat and barley straw, rice hulls and rice straw have been studied for anaerobic digestion potential. Waste products from dairy operations with fats, oils, and greases also may be good substrates.

 

The amount of energy stored in a biodegradable material can be predicted by the material’s chemical oxygen demand (COD) and/or volatile solids (VS) content, usually expressed as milligrams per liter (mg/L), or milligrams per kilogram (mg/Kg). The higher the COD/VS content of the waste, the more biodegradable the material and the higher the gas yields possible from the system. The rate of decay can be enhanced by carefully developing the raw materials mixtures and the technology used to control the process and collect outputs.

 

Inhibiting substances and conditions in anaerobic digestion will vary depending on the feedstock, pretreatment processes, and AD process. Anaerobic bacteria can readily breakdown short chain hydrocarbons such as sugars, proteins and fats, but longer periods of digestion are required for cellulose.

 

Substances toxic to anaerobic digestion include caustic or acidic chemicals which disrupt the pH; detergents; antibiotics; and other chemicals. At low doses, anaerobic bacteria can withstand the toxic substances, but elevated concentrations will reduce reatment efficiencies or even cause system upset.

 

Physical contaminants may also inhibit production. If the feedstock contains significant levels of sand, plastic, glass or metals then pre-processing may be required. If not removed, these materials will block the digesters, reducing efficiency and increasing equipment failures.

 

Moisture, nutrient balance and pH can also affect feedstock suitability. Moisture content of the feedstock mixture is a factor in determining the type of processing system. Some systems are better suited to liquid waste. Others are better adapted to higher solid contents.

 

The nutrient balance in a feedstock mixture refers to the balance of nutrients required for microbial growth. Nutrient balance affects biogas production rates and methane content. Carbon and nitrogen are the primary limiting nutrients for anaerobic digestion. The optimal carbon to nitrogen ratio for anaerobe microbial growth is in the range of 20:1 to 30:1. If a nutrient imbalance occurs, it can result in an unbalanced pH. Bacteria respond poorly to low pH. A well-balanced system is maintained between 6.5 and 7.5. Feedstocks with low pH are often buffered prior to injection into the digester.

 

The US Dairy Sustainability Initiative identifies energy audits as a foundation for evaluating opportunities to increase energy efficiency. In April 2010, the USDA announced it will fund approximately 1,000 farm energy audits in 29 states. The funding is available via the NRCS Environmentally Quality Incentives Program (EQIP). Additional support for energy audits may be available from your local utility company.

SEH’s Hedrington says a comprehensive energy audit is a key first step to understanding where, when, and how energy is used on the farm. 

 

“An energy audit goes beyond a review of the overall utility bills for electricity, natural gas, and other fuels. Specific areas where energy is used will break down into milking operations, heating, cooling, ventilation, lighting, water pumping, manure management, feed handling, and other miscellaneous activities,” he explains. A detailed audit can identify specific energy use times, peaks and rates of usage (continuous or sporadic).

The audit helps owners identify opportunities for energy efficiency upgrades, and pinpoints areas where biogas could replace or supplement equipment traditionally powered by the grid or petrochemical fuels (natural gas, gasoline, or diesel).  

“This allows farmers to make educated decisions about improvement and payback scenarios, and provides a baseline to refer to after making improvements. The documentation can serve as evidence to justify future grant applications and cost-share efforts with public agencies,” Hedrington notes.

 

Equally important, the results of an energy audit can provide an inventory of greenhouse gases. This is especially critical for producers connected to supply chains with major retail outlets such as Walmart, Safeway, Kroger or other entities advocating for improved sustainability.

Hedrington recommends a site-specific feasibility study be completed to establish a clear understanding of the potential waste to energy opportunity. Public agencies and private lenders often stipulate the study be conducted by a professional services consulting firm, if funding support is being requested.

 

• Site visit & review of facilities

• Preliminary screening of waste to energy options

• Substrate testing

• Design basis development

• Technical evaluation of AD options

• Evaluation of biogas use options

• Economic evaluation

• Impacts on existing operations

• Project reporting

• Schedule of future activities

 

Site visit & review of facilities: The initial site visit includes a detailed walk through, review of farm operations, discussion of the owner’s goals, review of permits and process flow diagrams, and any other existing relevant data. The visit also includes a detailed discussion of the project schedule and milestones.  

 

Preliminary screening of waste to energy options: A review of available waste to energy options is conducted so the owner can make a well informed decision specific to the facility’s unique characteristics (size, location, permits, logistical constraints, etc). Other technologies for waste to energy may include composting with heat recovery, direct combustion, pyrolysis, or thermal gasification. This will help the owner to avoid the hammer/nail syndrome – “if all you have is a hammer in your toolbox, the whole world looks like a nail.”

 

Substrate testing: If the preliminary screening indicates AD is the preferred option, SEH recommends moving forward with substrate testing early in the evaluation process.   Substrates may include manure, and other local organic waste streams with higher methane production potential (such as fats, oils, and greases). Potential substrates should be sampled and submitted to a laboratory for analysis of pH, tCOD, sCOD, TSS, VSS, NH3-N, TKN, nitrate-N, nitrite-N, phosphate, sulfate, alkalinity, oil and grease, and chlorides.   Beyond the aqueous samples, SEH also recommends conducting biochemical methane potential tests (BMP) to evaluate methane potential, identify inhibitory effects (such as from salts), and determine impacts of potential feedstock mixes. 

 

Design basis development: The design basis is the heart of the overall feasibility study. It presents the various process flows, water balance, heat sources, facility limitations, current operations and future expansion, permit limitations, and other parameters to identify the overall scope of the system. 

Technical evaluation of AD options: A variety of AD technologies (and potential staged combinations) exist including covered lagoons, plug flow reactors and completely mixed systems. Selecting the optimal technology will depend on costs, existing infrastructure, available feedstock mix and level of comfort with complexity. This early evaluation should assist the owner in avoiding potential pitfalls from dealing directly with equipment vendors with limited technology options. As Dan says “ask a truck dealer what type of vehicle you need – they will probably try to sell you a truck – and it’s pretty certain it will be the brand they sell.”

 

Evaluation of biogas use options: This component of the study presents the array of potential biogas uses such as fuel replacement, heat and/or power applications. Biogas conditioning (cleanup) is an important step in this evaluation and will address removal of moisture, hydrogen sulfide, ammonia, siloxanes, and other potential biogas contaminants.

Economic evaluation: An economic evaluation addresses short- and long-term cost impacts. Costs evaluated include equipment, installation, permits, engineering fees, long term operations and maintenance costs, parasitic loads, avoided waste disposal costs, avoided natural gas costs, potential carbon credits, valuation of potential co-products, and potential public funding assistance opportunities. The economic evaluation may present a simple payback evaluation for base, optimistic and pessimistic scenarios. The scenarios can vary based on changing power and fuel costs, waste disposal costs, the voluntary carbon market, and other factors.

 

Impacts on existing operations: The evaluation considers manure handling operations, air and water discharge permit impacts, discussion of stranded or redundant assets, discussion of piping modifications, integration of controls systems, impacts of continuing spill management processes and impacts to material balances.

 

Project report: A detailed report describes the results of the above categories. The report includes text narrative, photos, preliminary site layout, conceptual process flow diagram, calculations, and other related materials. An executive summary will be valuable for future funding and/or permit applications. 

 

There are thousands of resources available to help owners better understand anaerobic digestion and biogas to energy opportunities on dairy farms.

 

• Innovation Center for U.S. Dairy (www.usdairy.com/Sustainability)

• US Environmental Protection Agency AgSTAR Program (www.epa.gov/agstar/)

• US Department of Energy (www.energysavers.gov/your_workplace/farms_ranches)

• USDA Rural Energy for America Program (www.rurdev.usda.gov/rbs/busp/9006loan.htm)

 

State and local government agencies, the state public service commission, and the local public utility are valuable sources as well. Depending on the complexity and potential risks of a project, a professional service consulting firm is an excellent resource to address key engineering and planning issues

 

There are a myriad of opportunities on the farm to increase efficiency and energy independence, while reducing impacts to the environment.   A feasibility study evaluation and an energy audit should be completed early in the process.

 

SEH is working with clients in the dairy industry to help evaluate potential opportunities. If you would like to learn more about economically viable solutions for transforming waste streams to revenue streams, we encourage you to contact our team.   As Grandma used to say, “waste not – want not.”