L25. Anaerobic Digestion
In the last two lessons we discussed the conversion of biomass to electricity and biomass to biofuel. In this lesson, we discuss another technology called anaerobic digestion. In this process we biologically convert organic matter (food waste, manure, human waste, paper products, or almost any biomass) into biogas. Biogas is the term used for the gas that is generated with anaerobic digestion resulting in about 60% methane and 40% carbon dioxide. Methane is the main component in natural gas. If you remember, the three fossil fuels are coal, petroleum, and natural gas.
Introduction
To get us started on this topic, please listen to this 4 minute report "Turning Food Waste Into Fuel Takes Gumption and Trillions of Bacteria". (4 minutes)
You can read along Links to an external site. with the audio if you want.
The Science
In past lectures we have discussed many ways in which we can convert biomass into useful energy. This biomass can come from a variety of sources including crops that are grown specifically for fuel, or waste products. These processes all use some combination of biological, chemical, or physical processes.
Biogas is also a fuel produced from organic material. It is formed using a biological process called anaerobic digestion. The results are a gas (not a liquid) that is a combination of about 60% methane, about 40% carbon dioxide, and some other trace gases including water vapor and hydrogen sulfide.
The beautiful thing about this process is that it occurs at temperatures between about 50° F and 130° F, and at standard pressures, unlike pyrolysis or gasification. The only critical parameter is that the biological process take place in the absence of oxygen, hence the term "anaerobic digestion" (anaerobic meaning "without oxygen").
Anaerobic digestion of organic material into biogas (methane, carbon dioxide, and other trace gases) is generally categorized by three main phases of microbial activity. Hydrolysis is the first step that breaks down the complex polymers (proteins, fats, and carbohydrates). Acidogenesis is the next step where sugars, amino acids, and long chain fatty acids are converted to volatile fatty acids. The final step is methanogenesis where methane is formed from these acids. For those scientists out there, the process is more complex and there are plenty of places to learn more about these processes.
Although the process can take place in a range of temperatures (50° F to 130° F), the most ideal temperature is about 100° F and is referred to as mesophilic conditions. AD at lower temperatures the process goes slower. At higher temperatures the process is quicker, but it is harder to control the balance of necessary microbial populations. In all cases, maintaining a constant temperature is critical to these systems, as specific bacterial populations develop based on temperature. In cooler climates, the digester needs to be heated to maintain the 100° F temperature to stay in that ideal mesophilic range. This heating is typically done using some of the biogas produced from the system.
Feedstocks or Substrates
Feedstocks used in digesters are typically biomass waste products such as human wastewater, manure from farm animals, food processing waste, paper mill waste, etc. In all of these cases, the feedstock is basically free as it has already been collected. However, there is still a significant cost to build and operate the digester.
Co-digestion as the term suggests, refers to mixing different organic material (feedstocks) together in the digester. Typically, the main feedstock in a digester is manure or human waste and then the other wastes such as food, paper, cardboard, etc. are mixed in.
Biomass can be grown specifically for anaerobic digestion. A common practice in Germany is to grow corn to co-digest with farm manure. This is economically viable for them because of biogas subsidies and their high cost of energy. In fact, on an EROI basis, growing corn and producing methane in a digester is better than using corn to produce ethanol. However, ethanol is a more valuable end product because it replaces liquid fuels. Any dedicated crop in the US will be used for biofuels or biopower (electricity), not biogas.
The graph below shows the Bio-Methane Potential (BMP) of some different feedstocks. BMP is a laboratory test that measures how much biogas or methane could be produced from an equivalent mass of feedstock. Note how manure has the lowest energy output. Human wastewater is even less productive because of all the water. Food scraps have almost 10 times the energy value of manure. The highest energy feedstocks are oils and fats. But as you remember, fat and oil waste are typically collected and used to generate biodiesel - an even more valuable fuel.
The Anaerobic Digestion System
The process occurs typically in one tank, but is operated as a batch system or as a continuous flow system.
- Batch Systems collect organic waste over a long period of time (e.g. one year) in a large covered storage basin. Biogas is produced throughout the year. When the storage is full, the effluent (co-product) is pumped out and applied to cropland as a source of fertilizer and organic material for soil amendment.
- Continuous Flow Systems are most common. Organic material is added each day, or several times each day, and the same amount of effluent is removed. Think of a continuous system as a full sink of water as the "digester". If you turn on the faucet, the sink will overflow with the same volume of water that was added. It won't be the same water you just added, but instead the water that was already in the sink. If you have a sink that holds 5 gallons and add one gallon of water each day - in theory it would take 5 days to "change out" the water in the sink. That would be considered a system with a 5-day hydraulic retention time (HRT). If you only added 0.5 gallons per day to your 5 gallon sink, the system would have a 10-day HRT.
- A typical continuous digester is designed to have an HRT of between 5 and 20 days. The length of HRT is a function of many factors, but is primarily related to the type of feedstock. This is a combination of the size of amount of waste to be treated (volume per day) and the size of tanks (total storage volume). The effluent from the digester is typically pumped or flows to a storage system before it is applied to cropland as a source of fertilizer and organic material soil amendment. These storage systems may hold a year's worth of effluent.
The "output" from the digestion process, in addition to biogas, is a slurry, sometimes referred to as "co-product". It is made up of all the material in the waste that was not converted into biogas. The volume of co-product is nearly the same as the volume of organic material going into the digester because just a small fraction of the material is converted to biogas. This co-product has almost all of the nutrients of the original feedstock and is typically used as a fertilizer (sometimes called a biofertilizer) for agricultural crops. This co-product is relatively odor free and has far fewer pathogens than the original feedstock.
Biogas History
It is thought that the first biogas plant was built in India in 1859. Street lamps in England were fueled by biogas in 1885.
Because the technology is rather low tech, biogas is produced and used throughout the world primarily for cooking. In China, it is estimated that there are 5 million small digesters in operation on small family farms, primarily used for cooking fuel.
I think it is important to highlight the use of this technology throughout the world so I am asking you to watch this short video about anaerobic digestion in the Himalayas. Pay attention to what kinds of problems anaerobic digestion solves for these people.
Anaerobic Digestion in the US
Currently, the U.S. has over 2,200 sites producing biogas in all 50 states:
- 247 anaerobic digesters on farms
- 1,269 at wastewater treatment plants - about 860 of these use the biogas they produce while others just flare the biogas
- 54 stand-alone systems that digest food waste
- 645 landfill gas projects (See discussion below)
For comparison, Europe has over 10,000 operating digesters and some communities are essentially fossil fuel free because of them. (ABC Biogas 101. American Biogas Council, 2016)
The estimated 2,200 biogas sites in the US contribute less than 0.5% to the US energy supply (EIA). However, the Biogas Opportunities Road Map Links to an external site. (2014) suggest that there could be over 13,500 sites producing biogas in the US if the 'economic conditions' were right. These systems could supply energy to about 7.5 million homes.
Uses of Biogas
Biogas has an energy density of about 21.5 MJ/m3 or 600 Btu/ft3. This is about 60% of the energy density of natural gas which is 95% methane. Remember biogas is only about 60% methane.
One of the big problems with using biogas is the presence of hydrogen sulfide (about 5000 ppm). Hydrogen sulfide at this level is lethal to humans and is also very corrosive on metal piping and plumbing systems or in any engine system. Hydrogen sulfide is also very odorous (the smell of rotten eggs), making it an undesirable trace gas in biogas for cooking.
Here are the primary uses for biogas:
- Direct heating - cooking or space heating. The problems with this include the odors and that in warmer climates there may not be a need for space heating.
- Inject into the natural gas pipeline system - By removing the carbon dioxide, moisture, and trace gases from the biogas, all that is left is methane. They sometimes refer to this as "biomethane". With these components removed, the gas meets the natural gas pipeline standards and can be pressurized and pumped directly into the pipeline.
- Conversion to electricity - is probably the most common after direct heating. This can be done by generating steam and using a steam turbine, but most commonly the biogas is burned in an engine that turns a generator to produce electricity. Heat from the engine can then be used for other processes (remember combined heat and power, or CHP?). Some designs are integrating anaerobic digestion systems with ethanol systems as the ethanol plants require large amounts of heat. This technology is well tested, but is only economically viable in places with expensive electricity, or in places where there is no electricity access.
- Powering vehicles. There are currently vehicles that use natural gas as a fuel source. These same vehicles could use biogas although it would not be as efficient.
Economics of Anaerobic Digestion
In general, the economic conditions are not favorable for anaerobic digestion without some government subsidy. Biogas plants are expensive to build, operate, and maintain. The product they produce must compete with very cheap natural gas. Because of this, most biogas plants must be subsidized in some way. Without getting into specifics, these incentives include variations of the following:
- Not really a subsidy, but biogas plants charge a fee for the organic material they collect. With solid waste (garbage), the cost of disposal is recovered by what is known as a "tipping fee". This "tipping fee" is not like a 'tip' for your waiter at the restaurant, but rather a fee per ton of waste that is dropped off (tipped off) at the landfill. This tipping fee is somewhere between $25 in places with lots of land and not much garbage (like Iowa) to $100 per ton in places like New York. In addition to this fee, the waste must be transported to the landfill. The hauling cost and tipping fee are paid by the waste hauler and they are always looking for the best deal. Biogas plants also charge a tipping fee so haulers may choose to go to the biogas plant rather than to a landfill. Typically, the income from "tipping fees" is more than the income from the electricity produced from the biogas plant.
- Regulations at a city (such as New York in the story we started this lesson with) or state level could mandate that no organic wastes can go to landfills. This would force the garbage haulers to take organic material to biogas plants - essentially allowing them to charge whatever is necessary to digest the waste.
- Electricity customers in some areas wish to pay more for "green energy" so the power companies will contract with biogas plants to produce this green energy. Customers pay extra for their electricity and this gets passed through to the biogas plant. These are often referred to as Renewable Energy Credits or RECs. They can be part of a state or regional Renewable Portfolio Standard.
Landfills Generate Biogas
In the US, we generate about 254 million tons of Municipal Solid Waste (MSW) each year. MSW includes all the stuff in the pie chart on the right. It is all stuff we through out in our garbage cans - both residential and commercial. It also includes construction waste. About 60% of that is organic waste like food, paper, yard trimmings, etc. If we divide this organic waste per person, it is about 1000 lbs per person per year!!
About 52% of the MWS is sent to one of the approximately 3,500 landfills in the US. About 35% is recycled or composted, and about 13% is burned to make electricity - similar to what is done with wood pellets. (EPA Advancing Sustainable Materials Management Links to an external site.)
The breakdown of the organic waste (e.g. food, cardboard, wood, yard waste) in these landfills generates biogas - just as it does in an anaerobic digester. This is referred to as Landfill Gas or LFG. In fact, methane released from these landfills is the third largest source of human-related methane emissions, accounting for about 18% of total US anthropogenic methane emissions (EPA Links to an external site.).
Fortunately, LFG can be captured and used just as biogas from anaerobic digesters. The EPA estimates Links to an external site. that there are 652 operational LFG collection systems in place in the US as of July 2016 with another 415 more viable project sites. Again, biogas generation from these systems can be used for direct heating, generating electricity, or powering vehicles.
You may have seen biogas collection systems on landfills. A large piping system is placed directly in the landfill. The pipes take the methane to the surface so what you see is a series of vertical pipes on the tops of landfills. These systems capture between 50 and 99% of the methane generated in the landfill with the collection efficiency as a function of the type of landfill cover and collection system used (Source
Links to an external site.).
Conclusions
Remember the first story about digesters in New York being used for food waste? It sounded like a pretty good option. However, I would submit the following for you to consider:
- Is there energy involved in going house to house to collect the organic waste and bringing it to the digester?
- If this energy for collecting the organics was included in the energy balance would there be more energy being used than going into the system (EROI greater than 1?)
- What if the digester was located near a large source of food waste, like at the potato chip factory, rather than a digester that is used for organic waste hauled from individual homes?
- If landfills also generate and collect methane, why not just keep the food waste going to the landfill?
- Many wastewater treatment plants have anaerobic digesters so why not just send all the organic waste to them?
- Neighborhoods where these digesters are located will experience increased truck traffic and possibly odors. Many communities don't want them ("Not In My Back Yard").
Final Thoughts
Here are some advantages to AD systems:
- They can use a wide range of organic substrates - converting to a reliable renewable energy source.
- The energy output is almost continuous, unlike wind or solar that produce energy intermittently.
- AD is a rather simple technology.
- AD systems can be located anywhere where there is organic waste.
Here are some reasons why AD is not the perfect solution:
- The end product is not a 'liquid fuel' so is less versatile for many applications.
- Biogas is a low energy density fuel.
- Biogas is not pure methane so it requires 'upgrading' before it can be injected into natural gas pipelines. Upgrading is the removal of moisture, carbon dioxide, and many trace gases - especially hydrogen sulfide.
- Biogas production is more expensive than the fossil fuel energy it competes with (natural gas).
- There may be issues of odor and other nuisances around the biogas plant.