4 Advice to Choose a Waste To Biogas

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The United States produces more than 70 million tons of organic waste each year. While source reduction and feeding the hungry are necessary priorities for reducing needless food waste, organic wastes are numerous and extend to non-edible sources, including livestock manure, agriculture wastes, waste water, and inedible food wastes. When these wastes are improperly managed, they pose a significant risk to the environment and public health. Pathogens, chemicals, antibiotics, and nutrients present in wastes can contaminate surface and ground waters through runoff or by leaching into soils. Excess nutrients cause algal blooms, harm wildlife, and infect drinking water. Drinking water with high levels of nitrates is linked to hyperthyroidism and blue-baby syndrome. Municipal water utilities treat drinking water to remove nitrates, but it is costly to do so.

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Organic wastes also generate large amounts of methane as they decompose. Methane is a powerful greenhouse gas that traps heat in the atmosphere more efficiently than carbon dioxide. Given equal amounts of methane and carbon dioxide, methane will absorb 86 times more heat in 20 years than carbon dioxide. To reduce greenhouse gas emissions and the risk of pollution to waterways, organic waste can be removed and used to produce biogas, a renewable source of energy. When displacing fossil fuels, biogas creates further emission reductions, sometimes resulting in carbon negative systems. Despite the numerous potential benefits of organic waste utilization, including environmental protection, investment and job creation, the United States currently only has 2,200 operating biogas systems, representing less than 20 percent of the total potential.

Introduction
 

What is biogas?

Biogas is produced after organic materials (plant and animal products) are broken down by bacteria in an oxygen-free environment, a process called anaerobic digestion. Biogas systems use anaerobic digestion to recycle these organic materials, turning them into biogas, which contains both energy (gas), and valuable soil products (liquids and solids).

Figure 1: Anaerobic digestion process (Graphic by Sara Tanigawa, EESI).

Anaerobic digestion already occurs in nature, landfills, and some livestock manure management systems, but can be optimized, controlled, and contained using an anaerobic digester. Biogas contains roughly 50-70 percent methane, 30-40 percent carbon dioxide, and trace amounts of other gases. The liquid and solid digested material, called digestate, is frequently used as a soil amendment.

Some organic wastes are more difficult to break down in a digester than others. Food waste, fats, oils, and greases are the easiest organic wastes to break down, while livestock waste tends to be the most difficult. Mixing multiple wastes in the same digester, referred to as co-digestion, can help increase biogas yields. Warmer digesters, typically kept between 30 to 38 degrees Celsius (86-100 Fahrenheit), can also help wastes break down more quickly.

After biogas is captured, it can produce heat and electricity for use in engines, microturbines, and fuel cells. Biogas can also be upgraded into biomethane, also called renewable natural gas or RNG, and injected into natural gas pipelines or used as a vehicle fuel.

The United States currently has 2,200 operating biogas systems across all 50 states, and has the potential to add over 13,500 new systems.

The Benefits of Biogas

Stored biogas can provide a clean, renewable, and reliable source of baseload power in place of coal or natural gas. Baseload power is consistently produced to meet minimum power demands; renewable baseload power can complement more intermittent renewables. Similar to natural gas, biogas can also be used as a source of peak power that can be rapidly ramped up. Using stored biogas limits the amount of methane released into the atmosphere and reduces dependence on fossil fuels. The reduction of methane emissions derived from tapping all the potential biogas in the United States would be equal to the annual emissions of 800,000 to 11 million passenger vehicles. Based on a waste-to-wheels assessment, compressed natural gas derived from biogas reduces greenhouse gas emissions by up to 91 percent relative to petroleum gasoline.

In addition to climate benefits, anaerobic digestion can lower costs associated with waste remediation as well as benefit local economies. Building the 13,500 potential biogas systems in the United States could add over 335,000 temporary construction jobs and 23,000 permanent jobs. Anaerobic digestion also reduces odors, pathogens, and the risk of water pollution from livestock waste. Digestate, the material remaining after the digestion process, can be used or sold as fertilizer, reducing the need for chemical fertilizers. Digestate also can provide additional revenue when sold as livestock bedding or soil amendments.

Biogas Feedstocks
 

Food Waste

Around 30 percent of the global food supply is lost or wasted each year. In alone, the United States produced roughly 133 billion pounds (66.5 million tons) of food waste, primarily from the residential and commercial food sectors. To address this waste, EPA&#;s Food Recovery Hierarchy prioritizes source reduction first, then using extra food to address hunger; animal feed or energy production are a lower priority. Food should be sent to landfills as a last resort. Unfortunately, food waste makes up 21 percent of U.S. landfills, with only 5 percent of food waste being recycled into soil improver or fertilizer. Most of this waste is sent to landfills, where it produces methane as it breaks down. While landfills may capture the resultant biogas, landfilling organic wastes provides no opportunity to recycle the nutrients from the source organic material. In , the EPA and USDA set goals to reduce the amount of food waste sent to landfills by 50 percent by . But even if this goal is met, there will be excess food that will need to be recycled. The energy potential is significant. As just one example, with 100 tons of food waste per day, anaerobic digestion can generate enough energy to power 800 to 1,400 homes each year. Fat, oil, and grease collected from the food service industry can also be added to an anaerobic digester to increase biogas production.

Landfill Gas

Landfills are the third largest source of human-related methane emissions in the United States. Landfills contain the same anaerobic bacteria present in a digester that break down organic materials to produce biogas, in this case landfill gas (LFG). Instead of allowing LFG to escape into the atmosphere, it can be collected and used as energy. Currently, LFG projects throughout the United States generate about 17 billion kilowatt-hours of electricity and deliver 98 billion cubic feet of LFG to natural gas pipelines or directly to end-users each year. For reference, the average U.S. home in used about 10,812 kilowatt-hours of electricity per year.

Livestock Waste

A 1,000-pound dairy cow produces an average of 80 pounds of manure each day. This manure is often stored in holding tanks before being applied to fields. Not only does the manure produce methane as it decomposes, it may contribute to excess nutrients in waterways. In , livestock manure management contributed about 10 percent of all methane emissions in the United States, yet only 3 percent of livestock waste is recycled by anaerobic digesters. When livestock manure is used to produce biogas, anaerobic digestion can reduce greenhouse gas emissions, reduce odors, and reduce up to 99 percent of manure pathogens. The EPA estimates there is the potential for 8,241 livestock biogas systems, which could together generate over 13 million megawatt-hours of energy each year.

Wastewater Treatment

Many wastewater treatment plants (WWTP) already have on-site anaerobic digesters to treat sewage sludge, the solids separated during the treatment process. However, many WWTP do not have the equipment to use the biogas they produce, and flare it instead. Of the 1,269 wastewater treatment plants using an anaerobic digester, only around 860 use their biogas. If all the facilities that currently use anaerobic digestion&#;treating over 5 million gallons each day&#;were to install an energy recovery facility, the United States could reduce annual carbon dioxide emissions by 2.3 million metric tons&#;equal to the annual emissions from 430,000 passenger vehicles.

Crop Residues

Crop residues can include stalks, straw, and plant trimmings. Some residues are left on the field to retain soil organic content and moisture as well as prevent erosion. However, higher crop yields have increased amounts of residues and removing a portion of these can be sustainable. Sustainable harvest rates vary depending on the crop grown, soil type, and climate factors. Taking into account sustainable harvest rates, the U.S. Department of Energy estimates there are currently around 104 million tons of crop residues available at a price of $60 per dry ton. Crop residues are usually co-digested with other organic waste because their high lignin content makes them difficult to break down.

Biogas End Uses
 

Raw Biogas and Digestate

With little to no processing, biogas can be burned on-site to heat buildings and power boilers or even the digester itself. Biogas can be used for combined heat and power (CHP) operations, or biogas can simply be turned into electricity using a combustion engine, fuel cell, or gas turbine, with the resulting electricity being used on-site or sold onto the electric grid.

Digestate is the nutrient-rich solid or liquid material remaining after the digestion process; it contains all the recycled nutrients that were present in the original organic material but in a form more readily available for plants and soil building. The composition and nutrient content of the digestate will depend on the feedstock added to the digester. Liquid digestate can be easily spray-applied to farms as fertilizer, reducing the need to purchase synthetic fertilizers. Solid digestate can be used as livestock bedding or composted with minimal processing. Recently, the biogas industry has taken steps to create a digestate certification program, to assure safety and quality control of digestate.

Renewable Natural Gas

Renewable natural gas (RNG), or biomethane, is biogas that has been refined to remove carbon dioxide, water vapor, and other trace gases so that it meets natural gas industry standards. RNG can be injected into the existing natural gas grid (including pipelines) and used interchangeably with conventional natural gas. Natural gas (conventional and renewable) provides 26 percent of U.S. electricity, and 40 percent of natural gas is used to produce electricity. The remainder of natural gas is used for commercial purposes (heating and cooking) and for industrial ones. RNG has the potential to replace up to 10 percent of the natural gas used in the United States.

Compressed Natural Gas and Liquefied Natural Gas

Like conventional natural gas, RNG can be used as a vehicle fuel after it is converted to compressed natural gas (CNG) or liquefied natural gas (LNG). The fuel economy of CNG-powered vehicles is comparable to that of conventional gasoline vehicles and can be used in light- to heavy-duty vehicles. LNG is not as widely used as CNG because it is expensive to both produce and store, though its higher density makes LNG a better fuel for heavy-duty vehicles that travel long distances. To make the most of investments in fueling infrastructure, CNG and LNG are best suited for fleet vehicles that return to a base for refueling. The National Renewable Energy Laboratory estimates RNG could replace five percent of the natural gas used to produce electricity and 56 percent of the natural gas used to produce vehicle fuel.

Federal Policies Supporting the Biogas Industry
 

The Renewable Fuel Standard

Production of cellulosic biofuel (in gallons)
by fuel type Ethanol Renewable CNG Renewable LNG 2,181,096 81,490,266 58,368,879 3,805,246 116,582,508 71,974,041 * 3,536,721 56,916,606 34,224,820 * As of July

The Renewable Fuel Standard (RFS) was created by Congress as part of the Energy Policy Act. The RFS requires the blending of renewable fuels into the U.S. transportation fuel supply. Currently about 10 percent of the gasoline supply is provided by renewable fuel, primarily ethanol. The RFS sets fuel volumes for a variety of fuel categories: biomass-based diesel, advanced biofuel, cellulosic biofuel, and renewable fuel as a whole. Each category has a required minimum reduction in greenhouse gases.

EPA approved biogas as a qualifying cellulosic feedstock under the RFS in . Cellulosic biofuels must be 60 percent less greenhouse gas-intensive than gasoline. Currently, most of the cellulosic fuel volumes are being met through the use of RNG as a vehicle fuel. Compliance with the RFS is tracked through renewable identification numbers (RINs) that can be traded, and RINs for cellulosic biofuels can earn RNG producers $40/MMBtu (as of September ). According to biogas producers, the RFS has become an important driver of investment in the industry.

As part of the approval of biogas, the EPA updated the RFS to allow biogas-derived electricity used as vehicle fuel to qualify for RINs, or &#;e-RINs.&#; However, as of , the EPA has not approved any producer requests to start generating e-RINs, despite biogas production already exceeding current transportation electricity demand.

The Farm Bill

Programs under the Farm Bill&#;s Energy Title (IX) have been crucial for growth in the biogas industry. Under the Farm Bill, the USDA&#;s Bioenergy Program for Advanced Biofuels provides payments to producers to promote the production of advanced biofuels refined from sources other than corn starch. The program currently receives $15 million per year in mandatory funding with $20 million available per year in discretionary funding through .

The Rural Energy for America Program (REAP) provides grants and loan guarantees to agricultural producers and rural small businesses to promote renewable energy production and energy efficiency improvements. The program has mandatory funding of $50 million per year through , and $100 million available in discretionary funds.

The Biomass Research and Development Initiative is a joint program between the USDA and DOE. With $3 million in mandatory funding through fiscal year and $20 million in discretionary funding through fiscal year , the Biomass Research and Development Board awards grants, contracts, and financial assistance to projects that stimulate research and development of biofuels and bio-based products. However, these programs have consistently seen reductions in funding through the appropriations process.

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Other Agency Programs

AgSTAR is a joint program between the EPA, USDA, and DOE. The program promotes the use of anaerobic digesters on livestock farms to reduce methane emissions from animal waste. The AgSTAR program supports the planning and implementation of anaerobic digester projects, and includes state and non-governmental partners.

The EPA&#;s Landfill Methane Outreach Program (LMOP) encourages the waste industry to recover and use biogas generated from organic waste in landfills. LMOP forms partnerships with communities, utilities, landfill owners, and other stakeholders to provide technical assistance and seek financing for landfill biogas projects.

Conclusion
 

Biogas systems turn the cost of waste management into a revenue opportunity for America&#;s farms, dairies, and industries. Converting waste into electricity, heat, or vehicle fuel provides a renewable source of energy that can reduce dependence on foreign oil imports, reduce greenhouse gas emissions, improve environmental quality, and increase local jobs. Biogas systems also provide an opportunity to recycle nutrients in the food supply, reducing the need for both petrochemical and mined fertilizers.

Biogas systems are a waste management solution that solve multiple problems and create multiple benefits, including revenue streams. The United States currently has the potential to add 13,500 new biogas systems, providing over 335,000 construction jobs and 23,000 permanent jobs. However, to reach its full potential, the industry needs consistent policy support. Reliable funding of Farm Bill energy title programs and a strong Renewable Fuel Standard encourage investment and innovation in the biogas industry. If the United States intends to diversify its fuel supply and take action against climate change, it should strongly consider the many benefits of biogas.

Author: Sara Tanigawa

You can feed your HomeBiogas system approximately 6 liters of food waste or up to 20 liters of animal waste daily. An equal amount of water should be added with the waste when you put it into the inlet of HomeBiogas.

Do Put Kitchen Waste

The appliance thrives on all types of food leftovers; meat, dairy, vegetables, fats, bones, pits, seeds and egg shells. You can put in rotten or moldy food as well. Avoid feeding large quantities of citrus fruit (orange, grapefruit, lemon) skin since it contains anti-bacterial oil, which could reduce the effectiveness of the digestion.

Do Put Animal Manure

You can feed the appliance with animal manure, especially that of any animal that chews its cud (i.e. cows, deer, camels, buffalo, goats and sheep). The animal manure should be free of stones, straw, and earth, which will not break down in the digester. You may also add dog or cat waste (without sand). We recommend limiting chicken droppings, since they contain a high level of ammonia and will alter the balance of the digester fluid.

Do Put Human Waste

Those who choose to put human waste into HomeBiogas should use the 2.0 model in conjunction with the Bio-Toilet. Human waste will break down and be turned into biogas just as animal manure would.

Don&#;t Feed Too Much Of These Things

-Not too much chicken feces (up to 50%)

-Not too much fats, oils, grease (up to 25-30%)

-Not too much citrus fruit (up to 2 citrus fruit peels a day)

-Not too much roots, stalks

-Not too much compostable diapers (1 per day, depending on brand)

-Not too much tissues

Do Not Feed

The following items will not break down or will break down slowly, clogging the digester: feathers, paper, cardboard, fur, coffee grounds, bleach, garden clippings, sawdust, wood chips, straw, chewing gum, nail clippings, lint, hair, soil, sand, &#;Biobags&#; for dog poo/produce, any shells, even shells from clams and oyster.

How Long Does It Take to Make Biogas?

After Activation

The appliance will begin to produce gas 1-3 weeks after the initial activation using animal manure. (Read more about first-time set-up here). After this initial gas, the bacteria inside of the system will work 24/7 to break down waste and produce gas, just like our stomachs do, so long as it is warm enough.

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Biogas with Food Waste

How quickly your food will break down and convert into biogas depends on various factors such as what type of waste you feed and the temperature outside. Different foods have different amounts of energy that will become available when the food is digested (either by your body or by a biodigestor).

In general, food waste has a higher energy-generating potential because it has not yet been digested by the stomach of an animal. For food waste, fats, oils, and greases (FOGs) are especially productive, though not recommended in large quantities. But overall, feedstock high in calories, fat, starch, and sugar usually produces more biogas than other foods.

&#; High biogas yield: bread, corn, peas

&#; Low biogas yield: fish, poultry, water, spinach Still, we have found that as a general rule: 1 kilo of organic waste = 1 hour of cooking gas.

Biogas with Animal Manure Biogas yields from highest to lowest:

• 1. Poultry*
• 2. Pig
• 3. Cow
• 4. Horse
• 5. Rabbit
• 6. Goat
• 7. Buffalo
• 8. Sheep

Dog, cat, and human waste also work to produce biogas but are limited in comparison to the amount that livestock manure can produce.

*Although poultry manure has a high biogas potential, it is hardly utilized for biogas production because it contains large amounts of ammonia. Its effective utilization as a biodigester feedstock requires that it go through an ammonia stripping process.

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