Frequently asked questions

Q. How do I become more energy independent?

Energy independent means producing all of your own energy requirements.  There are many ways to start become more energy independent.  The simplest and lest expensive is to reduce, reuse and recycle.  The next steps become more difficult because it requires planning, patience and money.

No one becomes energy independent overnight.  It involves understanding what your energy needs are.  For example an average residential home consumes ~10Kh or 10,000 watts per hour.  Hence for the average residential home to become energy independent it would require producing 10Kw every hour.  This is known as a zero-energy, zero net energy, or net-zero energy building where the amount of annual energy used by the building is roughly equal to the amount of renewable energy created on-site.

The available solutions for producing energy include solar power, wind, geothermal, water, pyrolysis and anaerobic digestion.  Each solution has its own unique characteristics, pros and cons, and costs.

Q. What is microbial anaerobic digestion?

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.

Q. What is bio-gas?

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.

Q. How much do digesters cost to build?

The cost of a microbial anaerobic digester is based on its size or scale.  The size of a digester can be measured in output such as kilowats or feed stock input as tons per day.  A simple but rough estimate for the construction cost of a microbial anaerobic digester is $100,000 for every ton of feed stock processed per day.  For example a digester that converts 10 tons of waste into energy per day may cost $1,000,000 to construct.  Smaller systems may cost a little more, larger systems a little less.

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.

Q. What is an enzyme?

An enzyme is a biologic catalyst.  Enzymes tend to be large molecules made of strings of amino acids or ribonucleic acids (RNA).  The amino acids or RNA residues form a linear sequence which folds into three dimensional shapes.  Because the individual amino acids or RNA residues have unique biophysical properties (charge, hydrophobicity, side chains) when they fold into a three dimensional structure a surface topography is created that has associated chemistries.  The shape and chemistry on the surface of large molecules helps them perform specific functions.

One example of a protein enzyme is amylase and its function is to break starch down into glucose.  A stereo image of amylase is provided below.  If you can stare at the image while crossing your eyes a third image forms in your mind that is three dimensional.  Good luck.

Q. What is fermentation?

Fermentation is a process that converts sugar to acids, gases, or alcohol. It occurs in yeast and bacteria, and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation

Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product. French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes. The science of fermentation is known as zymology.

Q. What is a single or multi stage digester?

Single & Multistage Systems

Single Stage

Single stage systems can come in many designs, including continuously stirred tank reactor (CSTR) and plug-flow digesters, each with different modes of operation and differences in design and operation.  Generally, single stage systems are simpler than two-stage systems, and cheaper to construct and operate. 

Potential limitations of single stage systems are that conditions within the reactor will not be optimal for the various trophic groups of microbes. While it is true that two-stage systems can offer more optimal conditions to the methanogenic bacterial population, this is not to say that single stage systems are unreliable.  The methanogenic bacterial population can be managed by controlling the feeding rate, by thorough mixing of incoming wastes to avoid peak concentrations of potentially harmful contaminants, by co-digestion with other organic wastes to provide essential water content, buffering, nutrients and trace elements or by the addition of these  through the use of chemicals and nutrients.

Multiple Stage (Click on me)

Although single stage digesters maintain environmental conditions in which all the anaerobic trophic groups can function, different anaerobic trophic groups perform better under different environmental conditions.  This is the key concept behind two (or multi) stage digesters, where digestion is separated into stages allowing the provision of optimum environmental conditions for each bacterial group.

In multi stage systems two digestion stages are normally used.  In the first, hydrolysis and acidification (and some degree of acetogenesis) take place, and in the second stage the main biological process is methanogenesis, with some degree of acetogenesis also occurring. 

In the first stage, hydrolysis of complex substrates is normally the rate-limiting step.  Methanogenic bacteria have the longest doubling time of all the bacterial groups involved in AD and this slow microbial growth rate is normally the rate limiting step in the second stage.

As well as providing optimal environmental conditions in the second stage, some kind of biomass retention scheme is often designed, in order to keep as many active, well adapted methanogens in the digester as possible although it is possible to employ two-stage system designs that resemble two completely mixed reactors in series, or two plug-flow reactors in series. Methanogens can be retained to form higher cell densities in two ways.  The first is to raise the solids retention time in the digester by separating hydraulic retention time from solids retention time.  One way to do this is to employ upflow systems, where the waste flows upwards, through a layer or ‘bed’ of bacteria, and exits at the top of the reactor.  The liquid waste exits at the top, while the heavier sludge layer (containing high concentrations of bacteria) is retained (by gravity) towards the bottom of the reactor.  Another way is to filter the effluent from the second stage and re-introduce the solids to the reactor. The second way to retain biomass is to design a reactor with ‘support material’, which allows attached bacterial growth and thus retained biomass.  This extra biomass retention provides more efficient biological operation per unit volume of reactor, and greater resistance to potentially inhibitory substances.  The disadvantages of multi-stage systems are that they are often more complex, and usually more expensive than single stage AD systems. The implementation of digestion systems with ‘support material’ is however only advised for effluent type feedstocks with low suspended solids, otherwise digesters are prone to clogging.

Advantages of Multi Stage Systems

• Greater biological stability
• Greater ability to cope with fluctuating feedstock volume and quality
• Potentially higher throughputs due to optimal conditions

Disadvantages of Multi Stage Systems

• More complex control and operational requirements
• Potentially higher capital costs


Multi Stage Digester
Multi Stage Digester