What is Anaerobic Digestion?

Anaerobic digestion is a natural process by which microorganisms break down or ​“digest” biodegradable material in the absence of oxygen.  This process occurs in locations such as swamps, bogs, and the inside of compost piles where organic matter is submerged in water or covered by many layers of material.  Anaerobic bacteria in these environments gradual break down organic matter, releasing heat and gases.   (Click here for more information)

 

The anaerobic digestion process has four stages:

1. Hydrolysis: Large polymers are broken down by enzymes.

2. Acidogenesis: Bacterial fermentation produces carbon dioxide, volatile fatty acids, hydrogen, and organic acids.

3. Acetogenesis: Further breakdown of volatile acids produces acetate, carbon dioxide, and hydrogen.

4. Methanogenesis: The intermediate products (acetate, formaldehyde, hydrogen and carbon) of the preceding stages are consumed by anaerobic bacteria and converted into methane, carbon dioxide, and water.

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What are Anaerobic Digesters?

Anaerobic digesters are air-tight, insulated vessels designed to facilitate the anaerobic digestion process for biogas production.  A controlled environment of specific heat, pH, solids content, recipe, and ‘feeding’ rates is created and consistently monitored to maximize biogas output.  Anaerobic digesters also produce digestate.

 

What materials do anaerobic digesters process?

Anaerobic digesters can process a variety of organic wastes, including:

• Wastewater Treatment Sludge

• Manure

• Food waste

• Organic Fraction of Municipal Solid Wastes (OFMSW) - Post-consumer - The portion of your trash that is plant or animal derived. 

• Source Separated Organics - Pre-consumer organic waste 

• Fats, Oil and Grease (FOG)

• Green waste - Grass clippings and landscaping residue (excluding woody material)

• Crop residues

 

What are the components of an anaerobic digestion system?

A typical anaerobic digestion system consists of:

1. A collection system to store and pre-process (pulp/grind/etc.) organic waste;

2. An anaerobic digester;

3. A system to refine and store biogas;

4. A biogas combustion system such as a CHP gas engine or boiler; and

5. A solids separator for digestate.

 

Do all anaerobic digesters operate in the same way?

While all anaerobic digesters use the same basic process, they vary in type and construction.  The design will ultimately be dependent on factors such as feedstock type, loading rate, required gas quality, and retention time.  As such, there are several types of anaerobic digesters available.

 

What is Biogas?

Biogas is the gas produced by the breakdown of organic material during anaerobic digestion.  It is composed primarily of methane (60-70%) and carbon dioxide (approx. 25%), as well as small amounts of nitrogen, hydrogen sulfide, hydrogen, and oxygen.

 

Biogas is combustible and can be used as a renewable fuel.  Though biogas can be used in its raw form, the contaminants present can damage equipment, so it is conditioned to remove the contaminants prior to combustion. 

 

Biogas is most commonly used to heat and power on-site and/or nearby buildings with the excess electricity being sent to the grid, but it can also be upgraded further to biomethane (97% methane) and sold or injected into natural gas pipelines.

 

What is Digestate?

Digestate is the solid material that remains after anaerobic digestion is complete.  It consists of material that anaerobic bacteria could not metabolize, such as lignin and cellulose, and is high in nutrients.  Digestate can be used as soil amendments/fertilizer, cow bedding, filler for composite plastics, and as a low-grade building material such as fireboard.

 

Why install an anaerobic digester?

• It generates renewable energy in the form of biogas which can be burned to generate electricity and heat, reducing the need for fossil fuels.

• The end product (digestate) is a high-quality compost and fertilizer which can be used in place of expensive inorganic fertilizers, which are energy-intensive to make and can cause many adverse effects such as water pollution.

• There is the potential for revenue from the Energy as gas or electricity; Carbon credits; and Digestate as cow bedding, fertilizer, and filler for building materials.

• It reduces odor at wastewater treatment sites, improving life for nearby residents.

• It improves air and water quality through reduced greenhouse gas emission and local air pollution, reduced CO2 emissions, and methane diversion. 

• It diverts organic waste from landfills, saving space and reducing emissions of methane, a greenhouse gas 23 times more potent than carbon dioxide.

• The construction and operation phases will generate jobs and revenue.

• It reduces the costs, pollution, and hassle associated with outsourcing waste by providing for local waste management.

• It will be an example of sustainability leadership in the growing area of renewable energy generation from source-separated organics (SSO).

 

What is a CHP Engine?

CHP stands for Combined Heat and Power.  A CHP engine generates electricity while capturing waste heat for additional use.  CHP technology performs very efficiently (90%), producing fewer air pollutants and CO2 than their conventional counterparts, which are 20-30% efficient. 

 

For more information: http://www.epa.gov/chp/documents/faq.pdf

 

How is electricity produced with biogas?

Biogas (Methane) is fed into an engine (typically a reciprocating engine or a microturbine) which turns an electric generator producing electricity.

 

Is New England’s climate and weather suitable to anaerobic digestion?

Yes.  Although anaerobic digesters require specific operating conditions, systems are designed to maintain a steady state conducive to anaerobic digestion in a variety of climates.  The heated and insulated tanks keep the anaerobic bacteria happy and thriving around 100 degrees Fahrenheit, no matter what the weather outside is!  The warm tanks and sloped roofs, ensure that the buildup of snow and ice isn't an issue. 

 

What is meant by Co-Digestion?

Co-digestion refers to the incorporation and processing of different types of materials in the digester.  It typically is the addition of ‘organic’ material (food waste, FOG, grass clippings, etc.) to the primary substrate (WWTP Sludge, Cow Manure, etc).

 

In the graph below, manure and sludge have a low methane output because they have already been ‘digested’ once and most of the energy has been used.  Incorporating additional high-energy feedstocks will thus increase methane production.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For more information: http://www.epa.gov/agstar/anaerobic/codigestion.html

 

What are Source Separated Organics (SSO)?

SSO are the organic component of the municipal waste stream.  SSO are separated out at the source for separate collection.  An example would be a restaurant putting all of their prep food waste into a separate container which it is then collected separately from the main trash and brought to a composting or anaerobic digestion facility.

 

Are there currently digesters in operation in Massachusetts?

• Pine Island Farm, Sheffield, MA.

• Jordan Dairy Farms in Rutland, MA.

• Deer Island Wastewater Treatment Plant, MWRA, Boston, MA.

• Greater Lawrence Sanitary District, North Andover, MA.

• Wastewater Treatment Plant in City of Pittsfield and City of Fairhaven, MA.

 

Can an anaerobic digester explode?

Yes, but so can a gas station…or a car…or the propane tank in your yard…or the tanker truck going down your street...which is why these all have safety regulations and parameters - this includes anaerobic digestion technology. 

 

If organic material and wastewater sludge that contain pathogens and toxins are fed to the digester, how does it affect human health?

Pathogens and toxins do exist in some organic material and wastewater sludge, but are not released into the atmosphere or land because the sludge and organic waste is not burned as an energy source, instead, anaerobic bacteria and bugs eat and break down the organic material at high temperatures, destroying pathogens.  Additional treatment to the digestate can include aerobic composting, pasteurization, and pyrolysis to remove leftover toxins and pathogens from the wastewater sludge further protecting the environment and your health. 

 

Will truck traffic increase? 

This will be investigated in the feasibility study that is underway, from what type of organic waste is available? how much and how often? what is the route from point A to point B? What is the proper transportation equipment? etc.

Currently, waste is being picked up curbside and being transported through many neighborhoods to faraway places (sometimes hundreds of miles) to but dumped in landfills or incinerated.  Keeping source separated organics (SSO's) local can reduce transportation costs as well as associated noise pollution.  In many cases, the truck traffic and fossil fuel used associated with the disposal of WWTP sludge and organic waste industrial can be far more frequent and much farther than local waste management opportunities.  
 

Will odor increase?

Anaerobic digesters actually reduce odor generation and migration.  Wastewater treatment sludge and organic material are pumped (or dumped) into specially designed airtight, insulated vessels, where the AD process reduces odor by decreasing the amount of volatile organic compounds (VOC's - the putrescible, odor causing matter) in the substrate.

 

If an expensive anaerobic digestion system is implemented, will the cost be carried by the taxpayers?

The installation of an anaerobic digester is indeed costly, but the economics are very dependent on the sizing, feedstock acquisition and contracts, methane production, digestate marketing, etc.  Also as a renewable energy system, an AD system can have a 5 to 7 year payback.  In lieu of additions to the town budget, there are other options including 3rd party ownership and operation that would require no capital expenses.   This option would essentially be a lease that is paid back with the energy savings, energy sales and by-product sales from the anaerobic digester system.