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Friday, July 4, 2014

Anaerobic Digestion a Solution for Waste-to-Energy

This week we will record a show that delves into a new, mandatory composting law in RI that passed this legislative session.  The PROS and CONS (in summary):  Positive aspects include taking compost out of an overflowing landfill, thereby reducing the odor that pervades in the summer; decaying compost, as seen below in this article, provides energy (capturing the methane) or can be incinerated directly as a cheap source of energy;  compost makes great, natural fertilizer.  The negatives, when you take it out of the landfill, reduces the methane gas production and, the most common criticism echoed during the hearings surrounding the passage of the new law, is the added costs to major generators of compost (schools, hospitals, hotels, big companies) they suffer to separate and transport the compost. 

We will have proponents and opponents represented (as we do with each show and issue).  In the meantime we highlight here a positive story we ran this week on our main site-- a new, waste-to-energy plant getting built in RI at the site of one of their most innovative and clean industrial parks.  It will be interesting to see if this waste stream becomes valuable in RI, is easily separated and, ultimately, comes with a simple transpiration  system to make it a viable asset and creates a new industry, new jobs and economic growth:

Brian Keane and Peter Arpin

Our friends over at REI must have had a crystal ball because they have seen the future, and the future is changing waste into energy in their home, the State of Rhode Island. On June 19, the State of Rhode passed legislation that would usher in a mandatory food waste recycling.

The bill would require institutions that produce two or more tons of food waste a week to divert the waste from going to the state’s  Landfill by 2016. Instead, they must either compost the material on site, have it hauled away for agricultural purposes such as animal feed, or send it to a compost facility or waste-to-energy plant.

So what does this mean? Opportunities, and REI is helping you look at one of many, "Anaerobic  Digestion"

Anaerobic Digestion a Solution 
for Waste-to-Energy
By Larry Cosenza and John Hays


Microbial anaerobic digestion (MAD) is a process that uses bacteria in an oxygen free environment to convert organic waste into energy. Bio-gas is one product produced from the breakdown of organic matter in the anaerobic process.   Anaerobic digesters come in many different styles, shapes and configurations but, they all operate in the absence of oxygen, in a sealed vessel and at elevated temperatures.  Anaerobic digesters can simply be considered as mechanical cows that are fed organic waste and in the process make bio-gas and other products.  Bio-gas is a mixture of methane and carbon dioxide.  If carbon dioxide is removed from bio-gas the remaining material is called bio-methane and is nearly the equivalent of natural gas.  The other products that result from anaerobic digestion are remaining solids and nutrient rich liquid often referred to as digestate.

Bio-gas is Energy

The last stage of the waste to energy multi-step process is where bio‐gas is produced.  Bio-gas is a form of energy which is primarily a mix of both methane and carbon dioxide (CO2) gases.  In a digester, easy to digest carbohydrates such as food waste from grocery stores and restaurants can digest very rapidly and convert to methane quickly much like our bodies metabolize sugars and starches quickly.  Bio-gas can be burned as fuel in a generator, hot water heater or fuel cell as a means of getting the energy value out of bio-gas.  Burning the bio-gas to form energy is usually referred to as combined heat and power (CHP).  A rule of thumb is that ~100 cubic feet per minute of bio-gas, on a daily basis, can drive a 300 kilowatt (kW) engine which is enough power for about 300 homes in a year.  High temperature fuel cells have been developed to convert bio-gas into heat and electricity.  Bio-gas that has been enhanced by removing CO2 and hydrogen sulfide can be injected into natural gas utility grid or compressed and burned in vehicles.  For comparison, 1000 cubic feet of bio-methane has the same energy as 7.2 gallons of gasoline.

Financing & Construction

Anaerobic digestion is a relatively old globally utilized technology that is gaining acceptance in American markets.  The lack of performance of first generation dairy farm digesters, new digester technology, and capitalization costs have hindered USA market acceptance of anaerobic digestion.  Securing financing for a digester project can be challenging and has been primarily based on the owner's personal or corporate equity position, credit record and projected financial payback for investors.  Federal and State grants, loans and tax breaks can be an integral part of the financial backbone of a project.  Public Private Partnerships are gaining in popularity whereby private companies can bring financing to a project at municipal settings in exchange for a long term partnership.  Installation of an anaerobic digestion operation can follow an engineer, procurement, construction (EPC) path where the owner deals with one single entity for all issues pertaining to system design, process performance and construction. An EPC provider will often bring in a digester process provider onto their team for a system performance guarantee.  Alternatively, an owner can chose to split projects into multiple parts that contracts separately for system design, permitting, construction and the process and power generation equipment. This can put a heavier management burden on the owner but can allow for some additional financial transparency and hardware choice. Each of these models provides their own level of flexibility and accountability for all participants involved.

Rhode Island passed legislation in June of 2014 that requires large food dispensaries to divert food scraps from the landfill. For more information go to or contact John Hays at Check the REI event calendar for programs on Anaerobic Digestion;

About the authors:

Larry Cosenza, Ph.D is Founder & Chief Science Manager, C2 Biotechnologies, LLC - Larry received his PhD @ Boston University - he develops enzymes that are catalysts for the anaerobic digestion process. John R. Hays, Energy Savings Guide, i SAVE energies is an Energy Marketing Consultant for:  Anaerobic Digestion & other Sustainable Energies.

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