Anaerobic Digestion and Biogas: The Potential of Biogenic Methane Production from Organic Waste, Landfills, Manure Ponds, Wastewater Treatment Plants, Seaweed Farms, and the Use of Bio-CNG

Anaerobic Digestion and Biogas: The Potential of Biogenic Methane Production from Organic Waste, Landfills, Manure Ponds, Wastewater Treatment Plants, Seaweed Farms, and the Use of Bio-CNG

The Sequential Process of Decomposition 

Anaerobic digestion refers to decomposition of organic matter by anaerobic bacteria in environments of low oxygen (reducing environments). A product of anaerobic digestion is methane, also called here biogenic methane, or biogas. Another product of anaerobic digestion of organic matter is compost, which is basically decomposed organic matter that can be used to fertilize plants and amend soils. Decomposition often begins with aerobic decomposition by oxygen-imbibing bacteria. It is the main means of decomposition in backyard composting where chopping up the matter, turning it to expose more to oxygen, and otherwise keeping it aerated, warm, and wet, speeds up the process. When oxygen is depleted and the aerobic microbes die off the anaerobic microbes can begin if conditions are right: low oxygen and water saturation. First the anaerobic microbes produce enzymes called cellulases that break up molecules into sugars, amino acids, and fatty acids. Then acetogenic bacteria ferment the previous products into alcohols and organic acids, including acetic, lactic, and formic acids. Finally, the methanogenic microbes convert acetic acid and methanol into methane gas, CO2, and water. Typically, the gases are 60% methane, 40% CO2 and small traces of water vapor and H2S. The gases have a tendency to rise with pressure drop via open cavities. In landfills and other non-closed loop anaerobic digestion facilities, methane collection systems have been devised to gather and pump out the methane (along with CO2 and other gases) through perforated pipes. The methane must then be separated from the other gases. Even organic matter in saturated soil and wetlands can produce biogenic methane, ie. “swamp gas.” Soil scientists give a ‘reduction sequence’ of chemical constituents that are reduced in “wet” anaerobic digestion: first oxygen is reduced, then nitrogen, then iron, then manganese, then sulfur (with accompanying rotten egg smell), then carbon, then finally microbial gas.

One can divide the conversion from organic waste to biogas into three stages: hydrolysis, acid formation, and methane production. Hydrolysis refers to organic chemicals breaking down in the presence of water. However, these processes do occur simultaneously with the products of one process making up the substrates, or food, for the microbes of the next. The goal for efficiency is to balance the degradation rates of each stage. Optimal synchronization of the processes leads to optimization of efficiency. This involves synchronizing the decomposition of multiple microbial species.

Horvath, etal, in a recent article in the Biofuel Research Journal note that:

“The degradation of organic matter requires a synchronized action of different groups of microorganisms with different metabolic capacities. Recent developments in molecular biology techniques have provided the research community with a valuable tool for improved understanding of this complex microbiological system, which in turn could help optimize and control the process in an effective way in the future.”

Mesophilic vs. Thermophilic Anaerobic Digestion

Anaerobic digesters may be operated at mesophilic temperatures (30-40 deg C) or thermophilic temps (50-60 deg C). Each utilizes different methanogenic bacteria optimized to function at the respective temperature ranges. Acid-forming bacteria can survive and thrive in a variety of temps but methane-forming bacteria require specific small ranges as well as specific pH conditions. Reactions in mesophilic systems typically slow below 35 deg C and bacteria becomes inhibited above 45 deg C so temp control is necessary. Thermophilic bacteria have an optimal temp range of 50-60 deg C. The higher temps can also kill pathogens that can kill the bacteria although the waste can also be pretreated to kill pathogens. However, thermophilic systems are more expensive to operate due to the necessity to add heat energy. Temp control is of primary importance, particularly in thermophilic operations. Heat requirements are typically fulfilled on-site by burning produced biogas or recovered heat from a CHP/cogeneration unit. Digestion vessels also need to be well insulated to protect against the effects of ambient temperatures. Where ammonia may inhibit decomposition thermophilic systems are less desirable. 

Wet vs. Dry Anaerobic Digestion

There are two types of anaerobic digestion (AD): wet and dry. Wastewater treatment plants use the wet method. Food and yard waste are better suited to the dry method. Another factor in determining which method to use is percentage of organics in the waste stream. Dry AD can be done in a fully closed-loop system which I assume would not have any undesirable odors. The sludge material comes out as digestate and is finished into compost. The biogas can be converted into carbon negative CNG or used to run small gas turbines or gas generators to produce electricity.

Benefits of Anaerobic Digestion

The benefits of anaerobic digestion are:

1)      Production of biogas

2)      Production of compost

3)      Capture of methane that would have leaked into the atmosphere as a powerful greenhouse gas

4)      Reduction of space in landfills

5)      Waste management that can be monetized

6)      Biogas and Bio-CNG as a very low carbon form of renewable energy

7)      Combined with seaweed farming for food, compost, and local reduction of ocean acidity that could keep coral reefs from bleaching

8)      On-site or near-site waste treatment by anaerobic digestion can reduce waste transport costs and emissions from waste transport

9)      Prevention of manure waste from causing contamination by entering surface waters

10)   Reducing the amount of sewage sludge that must be landfilled (which also reduces tipping fees)

11)   Cutting down on odors at sewage wastewater treatment plants

12)   Reduction of synthetic fertilizer used on crops

13)   Help in meeting renewable energy standards

14)   Digested manure is odor-free, more liquefied, and can be pumped and used in liquid application systems 

It should also be pointed out that AD and composting, whether induced or natural, also produces CO2 as well as some nitrous oxide, which are greenhouse gases. They will be produced through decomposition but AD produces them faster and vents them and so affects global warming more in the short-term. Another potentially undesirable effect is that in digested manure the nitrogen is in ammonia form and has more of a chance of washing away in the field than undigested manure although both have similar nutrient values.

So there are many desirable side-effects to AD biogas production: the capture of biogenic methane in landfills that would otherwise leak into the atmosphere contributing to global warming, by-products of anaerobic digestion of organic matter such as compost, capture and use of methane from sewage treatment facilities and manure management facilities, and a significant reduction in landfilled waste thus reducing the dangers of toxic leachate, land use, and other undesirable aspects of landfilling. Bio-CNG is also a product that one can say is carbon zero and putting that into a natural gas vehicle (NGV) would also be less carbon intensive than using electric vehicles. However, the amount of available biogas will always be limited. It is not in any way a viable replacement for fossil fuels on a significant scale. Due to the spread-out nature of waste, particularly in landfills, the efficiencies of concentrating waste and capturing biogas will never be as optimal as hoped but they can be improved. Even so, the economics and feasibility of large-scale anaerobic digestion are being evaluated. With a price on carbon it is likely that their use would increase significantly. 

The Market for Anaerobic Digestion Systems

Anaerobic digesters are more common in Europe, with 8726 of them in Germany alone (as of 2014) with electric production capacity of 3904 MW which makes up about 4.3 % of their renewable capacity. There are only a few hundred large ones in the U.S. so far. One source has it that there were 1241 small anaerobic digesters at sewage treatment plants in the U.S. in 2015. This is probably an emerging market in the U.S., especially for waste management companies who can provide the transport and management of the waste and use the methane to power their vehicles and produce needed electricity. The American Biogas Council estimates there are 11,000 potential sites that include 8,000 dairy farms, 2,400 sewage treatment plants, and 450 landfills. Projects can succeed with private investment and without government subsidization, although small subsidies, at both state and federal levels would surely be welcome and more economic than many of the other renewable energy subsidies. The U.K. has announced significant investments going forward in AD and biomethane as the major part of their Renewable Heat Incentive (RHI). Germany does have considerable subsidies for biogas which also require any biogas facility to also process the solids, or digestate, into compost on-site. 

Anaerobic Substrates

Typically, there are five categories of organic residues suitable for anaerobic digestion:

1)      Sewage sludge

2)      Animal manures

3)      Food industry wastes, including slaughterhouse wastes

4)      Energy crops and harvesting residues, including algae

5)      Organic fraction of municipal solid waste (OFMSW)

Which method, additives, and pre-treatments are used is dependent on which substrates or co-substrates used.

Manure-to-Energy, Sewage-to Energy, Manure/Food Waste Mixes, and Septage Bioreaction

Manure-to-energy projects will likely be more common in the future, especially since manure management contributes 10% to U.S. methane emissions. There is some government subsidization for manure management and I suspect there will be more government incentives and subsidization to encourage manure-to-energy and other biogas capture and utilization facilities in order to help reduce methane emissions. Thus another benefit of anaerobic digestion is mitigation of methane leakage from manure ponds. This can be extended to sewage treatment facilities as well which are a smaller but still significant source of methane emissions. Gas from manure lagoons must be captured. Then impurities must be removed. One method to purify the gas is the Pressure Swing Absorption (PSA) gas conditioning system. These systems also require a combustor or a way to flare off gas when the system is down for maintenance unless there is some way to compress and store excess gas. Landfills have long flared significant amounts of methane from AD processes that occur naturally in the landfills. Designers and operators of these systems can receive significant subsidization through carbon credits for decreasing emissions. Potential to emit (PTE) studies must be done to determine such rewards. Combustion efficiency is a factor in determining PTE since more efficient combustion will burn more gas and vent less.

Human sewage waste can be diverted into three end products: biogas, compost, and water. Enhanced digestion and capture of sewage biogas can eliminate the need for flaring and reduce methane emissions from sewage treatment facilities. Human sewage sludge can also be mixed with yard waste to make organic fertilizer as the city of Los Angeles was doing to make TOPGRO fertilizer in the 2000’s. Sewage sludge and wet manure speed up decomposition of other organic matter. The bioreactor method of “wet tomb” landfilling can enhance anaerobic digestion by adding manure or human sewage in anaerobic conditions.

An apparently successful process is “septage bioreaction” where human waste from septic tanks is being used to accelerate landfill decomposition and formation of biogas. This is being tested in a Michigan landfill. This allows the landfill waste to break down faster freeing up space. Space-reduction in landfills is a benefit of anaerobic digestion. More biogas can be produced faster. Breaking down the septage in the landfill also makes it less likely to seep into surface water. Basically the landfill becomes the leach field. One issue might be increased methane leakage as the bioreaction produces more gas faster it may require more efficient and effective capture technologies. In any case the formula seems to be: trash + poop = more biogas energy faster.

In China the government and the Asian Development Bank are helping livestock farms set up AD facilities that use the biogas in on-site power plants running gas turbines or generators that make electricity, selling much of it back to the grid. The by-product of compost fertilizer has allowed some of these farms to reduce their use of synthetic fertilizer and pesticides. In some cases rice yields have increased as a result.

In Grand Junction Colorado is the U.S.’s largest sewage-to-energy project. About 8 million gallons of sewage-filled wastewater is processed per day. The U.S. is roughly a decade behind Europe in implementing such projects. In conjunction with this project the city is converting waste management vehicles and other city fleet vehicles to run on this renewable natural gas (RNG), or bio-CNG. In Northern Colorado food and other organic waste is also being collected and added to one of the largest digesters in the U.S. Manure is also added as a mix of food waste and manure is thought to be the best for these digesters. This trend is likely to grow and continue in the U.S. After the biogas is processed it is fed into the natural gas pipeline system. Overall though, such biogas potential is still quite small compared to fossil gas production from wells.

In California one of the largest digesters in North America is being built that can process 320,000 tonnes per year of waste. It is expected to produce the equivalent of 4 million gallons of diesel. The processed gas can be used for their 900 waste management vehicles (initially 80) with the excess put in gas pipelines.

San Antonio is another city turning sewage into biogas and this trend is likely to grow with small incentives and the desire to reduce necessary venting and flaring of naturally occurring biogenic gas in such facilities, which contributes to global warming. Another example is in Bergen County, New Jersey where the biogas is used to run on-site generators, since wastewater treatment and particularly pumping of sewage uses a lot of electricity. In the New Jersey example the company generates 40% of its electricity from the captured biomethane saving $500,000 per year. Other incentives include reducing the amount of sludge that must be landfilled and reducing odors. Incidentally, the reduction of odors was the original reason AD began to be used.

Biogas Channel had a nice video of an anaerobic digester operation on a farm in France that utilizes large digesters and a gas turbine and cogeneration/combined heat & power (CHP) system that produces both electricity and heat for a nearby foundry. The substrates, or feed for the digesters, are pig manure and crop residues from the farmer’s grain production, but mostly tomatoes waste from nearby farms. From the video it looks like an expensive and sophisticated operation with lots of thick steel to process and deliver the gas and utilize the waste heat. The system requires regular maintenance and can be monitored and operated remotely if necessary. Both the wet (sludge) and dry components of the digestate are utilized as fertilizer by the farm.

Studies with Additives to Increase Biogas and Methane Yields from Anaerobic Digestion

What waste substrates are used and in what ratios do affect both raw biogas (mostly methane + CO2) and methane yields. Of course, more carbon, or organic matter means more biogas and more methane. Thus determining the total organic carbon (TOC) of the total substrate stream can give an idea of potential yields. Addition of biochar, or biomass charcoal, is also being evaluated as a way to increase gas yields. Studies have indicated a 3-5% increase in yields with addition of biochar. Biochar is first made by pyrolysis where organic matter, typically wood, is heated or burnt in very low oxygen conditions in a commercial retort. Biochar has other beneficial uses as a fertilizer and as a way to fix carbon so it has become valuable. For these reasons its use to enhance biogas yields must be compared to its value for other purposes. Addition of bottom ash from municipal solid waste incinerators has also shown increased gas yields. Other additives, both organic and inorganic are showing promise as well: inorganic macro-nutrients like phosphorus, nitrogen, and sulfur and micronutrients like iron, nickel, cobalt, molybdenum, and selenium; ashes from waste incineration; compounds able to mitigate ammonia inhibition; and substances with high biomass immobilization capacity. Iron cations have shown very good results as their ability to accept and donate electrons can aid enzymatic catalysis. Additional biological aids (bioaugmention) including methanogenic microbial inoculants and enzymes that make particulate organic matter more soluble can also be important in increasing yields and generation time. Zeolites are another additive that has shown promise particularly as a means to sequester ammonia. I have used zeolite in my chicken coops for a similar function. However, too much zeolite in AD can lead to toxic accumulation of heavy metals. Nanomaterials are also being tested but they may also have some undesirable side effects. There are also other means of increasing yields and times such as pretreatment of the substrates (typically with biological inoculants) and altering operating conditions of the digesters (typically temperature). Much of the research into additives gets deep into the realm of decomposition chemistry. Another area of current research is pre-treatment of lignocellulosic biomass which is slower to breakdown than other organic matter but abundant in crop residues. Utilization of alcohols and weak organic acids is one promising method of enhancing cellulosic breakdown.

New, More Efficient Methods of Anaerobic Digestion 

The biggest challenge to AD has been the slow growth rate of methanogenic bacteria, methanogenesis being the final stage of the process. Two new methods have come about to address this problem: High-rate anaerobic reactors and anaerobic membrane reactors (AnMBR). The high-rate reactors utilize up-flow anaerobic sludge blanket (UASB). UASB reactors can be one-stage or two-stage. There are about 1000 UASB reactors in operation throughout the world. They are efficient, cheap to operate, and have high methane yields. The two-stage system was a modification that separates the hydrolysis and acid formation stages. This is more applicable to municipal solid waste substrates with a high fraction of particulate organic matter. Membrane reactors utilize a barrier that allows passage of some components but retains inhibitory components. There are two different designs for MBRs: membrane placed in an external loop or submerged within the reactor. Other membrane methods involve producing syngas (CO2, CO, and hydrogen) as additional energy source for methanogenic microbes. Combining high rate systems like UASB with membrane systems is also being investigated. Metagenomic approaches are also being developed as each microbial community is sequenced and studied with a variety of substrates. It was found that the post-digestion period must be monitored to avoid regrowth of bacterial pathogens which could reduce the fertilizing quality of the resulting compost. The key to high-rate systems is retention of slow-growth methanogenic bacteria that has a tendency to be ‘washed out’ of the system. UASB and membrane technology have addressed this. The bottom line is that AD involves a complex microbial process where optimal function is dependent on substrate and operating conditions. Operating conditions can be tweaked for different substrates through microbiological techniques.  

Food Waste Management

Food waste management is now a big issue on the environmental radar. Food waste, particularly from big companies that generate large amounts of it is ideal for transforming into both compost and biogas through anaerobic digestion. The companies that generate the waste can set up digesters onsite or groups of companies in an area could share facilities. This is a trend that is likely to continue and pick up steam. One advantage over energy produced from raw waste is that one generally knows the composition of food waste and so it does not produce unknown amounts of toxic solids, liquids, and gases as does the decomposition and burning of raw garbage in WTE power plants.  Fats, oils, and grease from restaurants can also be added and seems to speed up the digestion process. By some estimates about 15% of solid waste is food waste. That is 15% that could be kept out of landfills and put into ADs. Another aspect of food waste management is wasting less. Some old food can be used for feeding the hungry – utilizing food that may be out-of-date but is deemed still good so that useable food is not wasted and people can benefit from it. Creative siting and gathering of nearby food waste streams can be set up to provide adequate waste streams of known overall composition to optimize digester production of biogas and compost of predictable quality. Some areas have started composting facilities, mainly for making compost to sell but AD can and should be added to such facilities. Indeed, it should be required since otherwise the commercial composters will be venting significant amounts of methane into the atmosphere.

Bio-CNG: Perhaps Currently the Most Net-Zero Energy Source But Digestion Also Accelerates Methane Generation Time 

With biogas one is capturing methane that is being emitted to the atmosphere through human activities that concentrate it: landfills, manure ponds, organic waste, etc. production does consume some power. Bio-CNG does require removal of impurities and some compression so it is not net zero but a natural gas vehicle (NGV) that runs on Bio-CNG compared to an EV that runs off of a mixed mostly fossil and renewable grid makes significantly less emissions overall since it would have been emitted to the atmosphere by being vented rather than burned and burning has far less global warming potential (GWP) than venting. However, by speeding up decomposition that which is burnt releases waste faster than slow leaking. Even so, there is a significant emissions benefit to utilizing anaerobic digestion, processing, and storing. The biogas can be utilized onsite for those industries that burn gas for electricity or in heating processes and can take advantage of efficient small gas turbine or generator technology. 

Small-Scale Home-Use Anaerobic Digesters and Farm-Scale Digesters

For the do-it-yourselfers there are even small biogas units that can turn food waste and manure into gas for cooking and compost for fertilizer. An Israeli company makes a portable unit. The biogas can be piped directly to a stove for cooking and claims fairly small amounts of food waste (1kg) can produce 7 cubic feet of biogas which can light a flame on high for an hour. With a daily input of 6 liters of organic food waste and/or manure one could cook for several hours a day, they say. I must admit I am a little skeptical. The units sell for less than a thousand dollars and could well be applicable for developing countries and DIY greens. It has been reported that hundreds of thousands of domestic digesters have been installed I the past 20 years in Nepal, China, Vietnam, and other countries and tens of thousands in some African countries. Such a technology is desirable in developing countries for both replacing highly polluting wood, charcoal, and dung cooking fires with biogas cooking fires and for producing organic fertilizer.

Farm-scale digesters are designed for manure and crop waste. German researchers have noted that farm-scale AD results in a large number of small digesters due to the need for them to be close to where the waste originates. Transport is financially feasible for large operations and even some of those count on producers transporting and dropping off waste to get rid of it cheaper than landfilling it. The economics are better for large digesters. Small digesters can better their economics by adding more crop residues which are typically available on-site or near-site, the German study found. All digesters, but particularly small-scale ones, can develop operational issues. Light materials can float to the top forming a crusty scum which can keep the gas from escaping. Heavy insoluble material like dirt can settle to the bottom and lead to incomplete digestion and odor problems. Agitation with pumps and stirrers or specific placement of heating pipes are methods used to control settling and scum formation. Impurities such as sulfuric acid can be corrosive. Methane is flammable and potentially explosive so safety can also be an issue. Regular maintenance is also an issue as is cleaning out the digesters periodically. Knowledge of the processes would be helpful.

Landfill Biogas

Landfills emit just below one-third of the methane from the U.S. There are 2000 active municipal landfills in the U.S. and even the older inactive ones still produce methane. Much of the methane is flared off. Landfill fires are difficult to extinguish and may burn for many years. As of 2014 there were about 600 landfill biogas projects in the U.S. to convert the biogas into electricity, usually to be used on-site. Landfills also produce or vent other toxic gases including dangerous H2S. Landfill gas to energy technology is inefficient and advocates of waste-to-energy (WTE) plants point out that WTE is a more efficient way to make electricity (up to 10 times as much) from municipal solid waste (MSW). WTE projects can also recycle metals. While WTE, or energy from waste (EfW) is without a doubt a better way to reduce methane leakage the WTE plants can also emit other seriously toxic gases and significantly lower local air quality, even with significant pollution control equipment installed. Advocates of landfills are often in conflict with advocates of WTE plants.

The Global Methane Initiative, founded in 2004, notes that there are five main anthropogenic sources of methane: landfills, coal mines, oil and gas systems, agriculture (including enteric fermentation from cows and other ruminants and manure), and municipal wastewater. Worldwide, rice cultivation is also a significant source. 

  

Seaweed Farming for Food, Biogas Feedstock, Compost, Sea Ecosystem Enhancement ,and Local Mitigation of Ocean Acidity

Australian scientist Tim Flannery, in his book, Atmosphere of Hope, notes the potential of seaweed farming to fix carbon via photosynthesis (the growth rate of seaweed relative to other forms of agriculture is massive). Much of the waste could be used in anaerobic digesters to make methane and compost. Another desirable side effect of seaweed farming is localized decrease in ocean acidification with the subsequent possibility of locally reducing coral bleaching and aiding the health of coral reefs. Seaweed farming also helps coral reefs by increasing local biodiversity by providing niche habitats for fish, shellfish, and invertebrates. It is a food source for herbivorous fish and sea life. Yet another benefit is the ‘bioharvesting’ of excess nutrients such as nitrogen that may have been introduced into the sea from freshwater having concentrated in the freshwater from agricultural runoff. This is also called nutrient bioextraction. The potential for seaweed farming in biological carbon sequestration is thought to be significant. Certain species are favored for this. Used in combination with AD and fish farming it may become an important carbon mitigation technology in the years to come. 

    

References:

These $900 Biogas Units Turn Organic Waste into Cooking Fuel and Fertilizer – by Amanda Froelich, in True Activist (Environment-News-Sci & Tech) Dec. 29, 2015

Breaking Down Anaerobic Digestion – by Megan Greenwalt, in Waste 360, Dec. 30, 2015

ABUTEC Aids in Transformation of Manure into Renewable Energy, by PennEnergy Editors, in Renewable Energy World.com, Dec. 30, 2015

From Poop to Power: Colorado Explores New Sources of Renewable Energy – by Dan Boyce, on NPR-All Things Considered, Jan. 1, 2016

One of the Biggest Digesters of Urban Waste in North America is in California – by Paul Relis, in Biogas Channel (News), March 18, 2016

Atmosphere of Hope: Searching for Solutions to the Carbon Crisis – by Tim Flannery (2015)

Massachusetts Provides Biogas and CHP Incentives – by Amy Barad, posted at Biogas Channel, March 31, 2016

The British Government Confirms Investment of 1.15 Billion Pounds in Anaerobic Digestion Projects Up to 2021 – by Matt Hindle, posted at Biogas Channel, Feb., 10, 2016

Michigan Landfill Using Septic Waste to Accelerate Decomposition, Create Renewable Energy – by Arlene Karidis, posted in Waste Dive, March 31, 2016

Michigan County Seeing Good Results In Using Septic Waste at Landfill – by Megan Greenwalt, in Waste 360, March 29, 2016

Garbage Land: On the Secret Trail of Trash – by Elizabeth Royte (Little, Brown, and Company, 2005)

PIOGA – Water Table Training (Where is the Water Table?) – Pittsburgh, Feb 2016

SAWS Turning Sewage into Cash Through ‘Biogas’ Program – article by Sergio Chapa, in San Antonio Business Journal, April 8, 2016

SWANApalooza: How the U.S. Can Utilize 133B Pounds of Food Waste – by Kristin Musulin, in Waste Dive, April 8, 2016

Unlocking The Energy Potential of Manure – An Assessment of the Biogas Production Potential at the Farm Level in Germany – by Mattes Scheftelowitz and Daniela Thran, in Agriculture 6 (2), 20, April 2016

Energy from Tomato Waste and Pig Manure in the West of France – by Jean-Luc Guigourese and Franck Cabioc’h, in Biogas Channel, May 4, 2016

Biochar as Additive in Biogas-Production from Bio-Waste – by Daniel Meyer-Kohlstock, Thomas Haupt, Erik Heldt, Nils Heldt, and Eckhard Kraft, in Energies, 2016 9 (4), 247, March 29, 2016

Recent Updates on Biogas Production: A Review – by Ilona Sarvari Horvath, Meisam Tabatabaei, Keikhosro Karimi, and Rajeev Kumar, in Biofuel Research Journal, June 2016

Next Steps to Reduce Methane Emissions from Landfills – by Matt Kasper, Center for American Progress, April 16, 2014

Landfill Methane: Reducing Emissions, Advancing Recovery and Use Opportunities – Global Methane Initiative, September 2011

Seaweed Farming – entry at Wkipedia.com

In the People’s Republic of China, Manure is Being Turned into Money – by Giovanni Verlini, at adb.org, June 20, 2016

BMW to Power South Africa Plant with Biogas from Manure – by Liezel Hill, Bloomberg, in Renewable Energy World, October 16, 2015

Bergen Utilities Converting Sewage into Valuable Energy Source – by James O’Niell, in The Record at New Jersey.com, March 29, 2015

Small-Scale Manure Digesters: Potential for On-Farm Heat and Energy – by Guy Roberts, Intervale Innovation Center, Burlington

Nebraska’s Potential for Energy Recovery from Animal Manure – by Nebraska Methane Workgroup 

The Role of Additives on Anaerobic Digestion: A Review – by Joaquim Vila, Joan Mata-Alvarez, Josep Ma. Chimenos, and Sergi Astals, in Renewable and Sustainable Energy Reviews 58 (2016) 1486-1499

The Wales Centre of Excellence for Anaerobic Digestion – website – walesadcentr.org.uk

Carbon Sequestration, entry at Wikipedia.com

Anaerobic Digestion: Biogas Production and Odor Reduction from Manure (G-77) – Penn State Extension (2016)