Anaerobic Digesters

An anaerobic digester in Vermont

This  is an air quality perspective on the environmental impacts of anaerobic digesters used for electric power generation with an internal combustion engine.  Like most sources of renewable energy, these projects have positive and negative attributes.  With respect to air quality, unless these projects incorporate hydrogen sulfide controls and an engine exhaust emission oxidation catalyst, they are notable sources of air pollution.  

Do I need an Air Permit to install an anaerobic digester? 
In most cases, an Air Permit to Construct will be required before installing an anaerobic digester.  But permit applicability is based more on the end use of the biogas than the anaerobic digester itself.  For this discussion it is assumed the digester will utilize an engine to generate electricity since the economics and incentives to date have not favored other uses for the biogas.  The combustion of the biogas in an internal combustion engine, typically coupled to an electric generator, creates significantly more air pollution than burning the same gas in a boiler or flare.  The emission potential varies significantly from one engine make and model to another.  If you are proposing an anaerobic digester, be it a manure or food waste digester, you must contact the AQCD to determine permit applicability.  Most of the current farm based Cow Power digester projects were previously approved by the AQCD with a streamlined preconstruction permit approval process provided the projects utilized an approved cleaner burning engine and included a flare to burn the digester gas when the engine was offline.

In  most cases, new digester projects today will be required to reduce hydrogen sulfide levels going to the engine and equip the engine exhaust with an oxidation catalyst.  In order to prevent poisoning of the catalyst, it is expected that hydrogen sulfide will need to be reduced to less than 40 ppm.  The catalyst will be required to continuously achieve a minimum of 95% reduction of carbon monoxide and 85% reduction of formaldehyde.  These emission limits are consistent with those currently required in both CA and MA.  The AQCD is also evaluating the technical and economic feasibility of requiring the existing Cow Power projects to be retrofitted with similar emission controls should they not voluntarily reduce their emissions.  Such controls would significantly reduce the annual emission fees such projects currently pay.

What is an anaerobic digester?
An anaerobic digester refers to an airtight vessel where “anaerobic” bacteria (i.e. those that thrive in the absence of oxygen) are used to digest (e.g. decompose or breakdown) an organic, carbon based, solid waste slurry, such as cow manure or food wastes, into smaller molecular weight compounds with lower residual odor.  The anaerobic bacteria generate both methane (CH4, also called natural gas) and carbon dioxide (CO2) gases in near equal volume as they digest the waste material1.  In modern anaerobic digesters this biogas is captured and used for energy recovery, typically in an internal combustion engine coupled to an electric generator.  During the subsequent combustion, the methane is converted to carbon dioxide, releasing energy to drive the engine or provide heat for other uses.  The CO2 emissions are released to the atmosphere.

Only a small percentage of the manure is actually converted to biogas in modern anaerobic digesters.  Dairy cow manure is about 85 percent water and 15 percent solids and only about a quarter of the solids end up being converted to biogas2.  The residual solids left over after the digestion are lower in odor and may be dried and reused as cow bedding.  The liquid effluent, which is also lower in odor, retains some of the nitrogen and phosphorus nutrients and is stored in tanks and/or lagoons before being reapplied to the land. That liquid effluent still contains organic material and if stored long enough can continue to anaerobically breakdown releasing uncontrolled methane to the atmosphere.

Anaerobic digesters are not new and have long been used to digest sewage sludge at waste water treatment plants to reduce the odor of the sludge before disposing of it.  The resulting biogas is typically flared or burned to heat the digester itself.  Landfills are another example where anaerobic decomposition occurs and could be considered a type of anaerobic digester, although the landfill is not well sealed.  Larger landfills collect the landfill biogas using a series of gas collection wells drilled into the landfill and a vacuum blower to pull the gas out and route it to engines to produce electricity.  Anaerobic digestion would also occur at farms without digesters that store raw manure in lagoons or pits.  In these cases the biogas is released uncaptured and to the atmosphere.  It is important to note that manure management is just one component of greenhouse gases emitted by dairy farms.  Dairy farms generate almost 10% of the total greenhouse gases emitted in the entire state3.  Much of this is from the cows themselves due to ruminant enteric fermentation (a cow is essentially its own anaerobic digester).  Manure management accounts for approximately 20% of the greenhouse gases from a dairy farm3.

An alternative to anaerobic digestion is aerobic decomposition by “aerobic” bacteria (i.e. those that thrive in the presence of oxygen).  Active composting or manure “dropped” in the pasture by grazing animals would be examples of aerobic decomposition.  In aerobic decomposition only carbon dioxide is emitted in gaseous form and the remaining solids are incorporated into the soil.  Methane generation is unique to anaerobic bacteria.  Even though anaerobic digestion generates methane, a much more potent greenhouse gas than carbon dioxide, it would ultimately be expected to results in a similar level of emissions of greenhouse gases after combustion as aerobic decomposition, assuming no methane escapes from the digester and assuming a similar amount of the solids are broken down anaerobically and aerobically. However, since the majority of large farms utilize manure lagoons that would be expected to release methane to the atmosphere at least part of the year, the addition of a digester to capture the methane can substantially reduce a farm's greenhouse gas emissions. With anaerobic digesters the intermediate creation and capture of methane allows for energy recovery that is not possible with aerobic decomposition or open anaerobic decomposition that may occur with manure stored in open pits or lagoons.

Why do we regulate anaerobic digesters?
Anaerobic digesters generate appreciable quantities of greenhouse gases, namely methane and carbon dioxide.  But since most of the gas is captured, it is the subsequent combustion of this gas that generates the air pollution that is regulated by the AQCD.  The combustion of the biogas in an internal combustion engine creates significantly more air pollution than burning the same gas in a boiler or flare, but all three produce air pollution.  Air pollution would include carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2) and various hazardous air pollutants.

Raw manure also contains significant amounts of sulfur, predominately from grasses used as feed.  A portion of this sulfur is converted to hydrogen sulfide (H2S) in the digester, reaching concentrations that can exceed 4,000 ppm in the biogas4.  Hydrogen sulfide has a characteristic rotten egg smell and is poisonous, corrosive and flammable.  However, since the digesters are sealed, little of this gas escapes and instead it is burned in the engine where it will form the air pollutant sulfur dioxide and sulfuric acid.  This sulfuric acid and any unburned hydrogen sulfide can cause serious damage to the engine.  Most conventional fuels have much lower sulfur levels. 

The use of internal combustion engines to burn biogas also generates substantially more formaldehyde emissions than would occur with other fuels or other combustion devices. According to the U.S. Environmental Protection Agency (US EPA), formaldehyde is ubiquitous and naturally occurring in the environment at low levels, contributing to asthma and eye and respiratory irritation.  At higher concentration, it can cause severe irritation and is considered a probable human carcinogen by the US EPA5.  Unless the high sulfur levels can be reduced, it impedes the ability to equip the engines with a catalyst to reduce these other air pollution emissions.  Several facilities are attempting to reduce the hydrogen sulfide in the biogas due to the engine damage and increased maintenance costs that are incurred with high hydrogen sulfide levels.  These measures include adding chemicals to the slurry to bind to the sulfur and prevent the formation of hydrogen sulfide, such as ferric chloride, or by scrubbing the hydrogen sulfide from the biogas after it is formed.  The reduction of the sulfur in the biogas has the added benefit of also reducing the sulfur dioxide air pollution emissions when the fuel is burned.

But I thought “renewable energy” means it is always clean and climate friendly?
It is important to note that the term “renewable energy” simply means the fuel source is regenerated at a rate equal to or greater than its use.  It does not inherently imply that the energy has low emissions, is clean or that there are air quality, greenhouse gas or climate change benefits attributable to use of this energy.  Some renewable energy sources may indeed be considered clean and green.  However, most energy sources have both positive and negative environmental impacts and thus renewable energy is a balance of these benefits and trade-offs.  Combustion generates air pollution.  The terms clean, green and renewable are often intentionally and unintentionally confused.  Vermont defines “renewable energy” in 30 VSA Part 3 Chapter 89 Section 8002

Any discussion of renewable energy needs to also include a discussion of efficiency.  The term efficiency has many meanings.  Energy efficiency refers to the efficient use of the energy.  Thermal efficiency refers to the efficient generation of the energy.  If the goal is to conserve energy, both are important.  For example, there is little value in using a high thermal efficiency boiler if the energy is then wasted or used frivolously such as heating a home with the doors and windows open.  Renewable energy by its definition does not take either of these efficiency terms into account.  However, if one of the goals of renewable energy is to reduce greenhouse gases to mitigate climate change then both energy and thermal efficiency are important.  The combustion of fuels to generate electricity, whether they are renewable fuels or not, has inherent thermal inefficiencies as one tries to convert one form of energy to another.  In most cases the combustion of fuels to generate electricity results in a majority of the energy being lost as unusable heat.  A biomass boiler making steam to drive a steam electric turbine may be 25% efficient.  An internal combustion engine may be 36% efficient.  A combined cycle natural gas turbine may be 50% efficient.  But the combustion of fuels to generate heat for space heating or process heating is likely to recover 80% or more of the energy.  Nonetheless modern society still requires the generation of some amount of electricity and combustion sources generate a substantial portion of this.  A final definition of efficiency important for air pollution is combustion efficiency.  How much of the fuel was completely combusted and how much was emitted out the stack unburned as air pollution or potential energy lost?  Poor combustion efficiency can result in significant air pollution emissions of various intermediate combustion byproducts including carbon monoxide, benzene, and formaldehyde.

With respect to anaerobic digesters, these qualify as renewable energy in Vermont6.  These projects manage manure in a manner that maximizes the generation of methane and then combust the gas in an internal combustion engine to generate electricity, which in turn converts the methane to carbon dioxide.  The generation of electricity with an internal combustion engine, as with many other combustion methods for generation of electricity, is an inherently inefficient use of energy compared to the use of the biogas for thermal energy.  However, the need for thermal energy tends to be seasonal in Vermont whereas electric usage is more consistent.  This is one reason for the subsidy for electric generation over thermal use for these projects6,7.   Thus while it is technically feasible to generate biogas for other uses, such as providing gas to the pipeline, the economics of such projects without electric rate subsidies has precluded such operations to date.  

The use of an internal combustion engine, especially ones without emission control catalysts, also tend to be much high polluting than other uses of the biogas.  It is important to note that renewable energy projects need not conflict with clean energy goals.  The AQCD is hopeful that hydrogen sulfide scrubbing technology being initiated by projects to prevent engine damage can also enable an emission control catalyst to be successfully used to substantially reduce emissions of other harmful air pollutants.  Green Mountain Power has proposed such a project in St. Albans that hopefully will demonstrate that anaerobic digesters can viably produce clean, renewable energy.

Do anaerobic digesters reduce emissions of greenhouse gasses?
The answer depends primarily on what baseline is used for calculating the greenhouse gas (GHG) emissions prior to adding a digester.  If the actual manure management practice at the farm prior to adding a digester included allowing the manure to anaerobically decompose in a pit or lagoon without capturing the methane, then the digester GHG emission reductions would likely be significant8,9,10.   If the baseline farm practice resulted in the methane being captured and flared, like is done with wastewater treatment plants, the GHG reductions would be limited to only those reductions, if any, from the displaced electric generation (e.g. more reduction if it displaces coal, less if it displaces gas, none if it displaces hydro, solar or wind)11.  This is because combustion in a flare or an engine both equally convert the methane to carbon dioxide.  If the baseline manure management practices minimized anaerobic decomposition through more pasturing of the animals, or prompt spreading of collected manure, then the digester GHG reductions would be less significant because the baseline practice largely avoided anaerobic conditions which generate methane, and instead favored aerobic carbon dioxide generation (which is approximately 25 times less effective than methane as a GHG).  Additionally, some of the carbon in the manure would be incorporated into the pasture soil.  With respect to food waste digesters, the greenhouse gas reductions would be expected to be minimal since that waste would typically have been aerobically composted or placed in a landfill that was already capturing methane.

Do anaerobic digesters reduce phosphorus water pollution?
Anaerobic digestion by itself is not expected to result in reductions in phosphorus loading to soils, surface water, ponds or lakes.  In order to obtain phosphorus reductions, diet manipulation or further processing of the manure to separate and concentrate the phosphorus is required. Examples of separation techniques include: chemical precipitation, a decanter centrifuge, a screw press, and/or a dissolved air flotation (DAF) system. Separation techniques to remove phosphorus are not dependent on first digesting the manure but existing digesters already have infrastructure to separate solids from digester effluent using existing screw presses.  Some portion of the phosphorus remains in the solids removed by the screw press, but the bulk of the phosphorus still remains in the liquid effluent discharged from the screw press.  Further removal of phosphorus from this liquid effluent can be performed using a DAF system.   

Any collected phosphorus must not be placed back on the land in the watershed without a proper Nutrient Management Plan to ensure it is properly applied where it can be stored in soils or taken up by vegetation.  As of 2017 none of the 15 large manure digester projects in Vermont reduced their phosphorus loading due to the use of an anaerobic digester.  A lack of viable markets for the collected phosphorus has hindered phosphorus removal from manure to date.  For further information on phosphorus and water pollution please see the DEC Watershed Management Program Clean Water Initiative.


1 Biogas typically ranges from 50% to 70% methane and 30% to 50% carbon dioxide by volume.  National Renewable Energy Laboratories. U.S. Department of Energy.  Biogas Potential in the United States. October 2013.
2 Renewable and Alternative Energy. Biogas from Manure. Penn State Extension Service.
3 Vermont Greenhouse Gas Emissions Inventory Update 1990-2012. VT DEC. June 2015.
4 Removal of Hydrogen Sulfide (H2S) from Biogas Using Zero-Valent Iron. Journal of Clean Energy Technologies, Vol. 3, No. 6, November 2015
5 Facts About Formaldehyde. U.S. EPA. Accessed April 4, 2017.
6The Viability of Biomethane Digesters in Vermont.  Middlebury College and Green Mountain Power.  2016. 
7 Most of the existing Vermont manure digester projects were funded in substantial part by federal grant money from the 2009 American Recovery and Reinvestment Tax Act (ARRTA) Section 1603.  These grants ranged from $432,900 to $904,117 per farm.  For subsequent projects, grant money had also been available from the U.S. Department of Agriculture (USDA) Rural Development’s Rural Energy for America (REAP) program, authorized by the 2014 Farm Bill.  These grants had combined up to a maximum of $800,000 per project but have since been substantially reduced to no more than $300,000.  In addition to the grants, three farms participate in net metering programs that enable them to offset their electric bill at retail electric rates.  Most farms are paid a 20 year fixed feed in tariff rate of 14.5 cents per kilowatt under the Vermont Standard Offer program with additional revenue from either the 4 cent voluntary GMP Cow Power program or the sale of the Renewable Energy Credits.  These projects also generate quality cow bedding material from the residual solids that can offset bedding costs and create additional revenue by selling excess bedding.
8 Estimating Greenhouse Gas Reductions For a Regional Digester Treating Dairy Manure. U.S. EPA. Accessed April 4, 2017.
9 Quantifying Greenhouse Gas Fluxes in Agriculture and Forestry: Methods for Entity-Scale Inventory. USDA OCE. Accessed April 4, 2017.
10 Zaks, D. P., Winchester, N., Kucharik, C. J., Barford, C. C., Paltsev, S., & Reilly, J. M. Contribution of Anaerobic Digesters to Emissions Mitigation and Electricity Generation Under U.S. Climate Policy. Environmental Science & Technology, 45(16), 6735-6742. doi:10.1021/es104227y. 2011.
11 Emissions & Generation Resource Integrated Database (eGRID). U.S. EPA. (2017, March 02). Accessed April 10, 2017