Q: What can be done to control odor?

A: If a biodigester is located in an urban area where neighbors may be impacted, the mixing lobby and digester components can be completely enclosed. An enclosed mixing lobby prevents odorous process air from escaping into the environment. The mixing lobby is ventilated with up to 2.6 air exchanges per hour. The process air is released to the atmosphere via a biofilter, which can use material such as wood chips or lava rocks.


Q:  What’s the difference between wet and dry fermentation?

A:   Both methods of anaerobic digestion follow the same biological process of hydrolysis, acidogenesis, acetogenesis, and methanogenesis.  The main difference is that wet fermentation has a feedstock with a lower solids content and is pumped as a slurry, whereas dry fermentation feedstock has less moisture content and structurally it can be stacked and loaded into the fermentation chambers with a front loader.  One method is not necessarily better than the other; the technology choice depends on the available feedstock.


Q: Compare the energy content of wood vs. biogas.



1 Chord = 128 Cu. Ft. = 3.62 m3 = 1.2 US Tons = 2400 lbs

Bone-dry = 9600 Btu/ Lbs = 180,000 Btu/ Cu. Ft.

20% Moisture = 6400 Btu/Lbs = 120,000 Btu/ Cu. Ft.


100% Methane = 20,000 Btu / lbm = 1000 Btu / Cu. Ft

55% Methane = 11,000 Btu/ lbm =  550 Btu / Cu. Ft


Q:  What is the gas utilization efficiency of a combined heat and power unit (CHP)?

A:  The CHP at the University of Wisconsin-Oshkosh (UWO) Biodigester 1 has an electrical efficiency of 37.3% and when heat is utilized, it is 87.75% efficient.


Q: How is each system monitored?

A: The biodigester performance is monitored 24/7. There are several statistic programs in the background which allow every type of analysis on gas yields, percolate numbers, gas quality, etc.

Q: What kind of energy output can we expect from each biodigester?

A: On average, each biodigester will generate 85 kW to 125 kW of energy.  This number will vary due to the feedstock, retention time, etc.


Q: What protocol do you use for determining Carbon Credits?

A: Possible carbon credits are determined by the actual amount of methane that comes from the organic material and is captured and combusted instead of being released to the atmosphere. The formula we use is:

Scf of methane * lbs of methane per scf (0.00426 lbs/scf) = lbs of methane/2204.6 lbs per metric ton = metric tons of methane captured and combusted

Metric tons of methane * 21 GWP (Global Warming Potential) = metric tons of CO2e (carbon dioxide equivalent).


Q:  Is the percolate tank anaerobic?

A:   Yes –it functions just like a mini digester: under anaerobic conditions, same microorganisms, etc.


Q:  What feedstock material is good for starting a biodigester?

A:  The pH level of the feedstock should be at least at 7.5 or higher.  For startup, material from a wastewater treatment plant or dairy manure should be used as an inoculum. Generally, the inoculum is solid and the percolate is a result of the solid breakdown in the fermenters. Dairy manure is the ideal starter.


Q:  What is the largest waste tonnage per year BIOFerm would consider before building a biodigester?

A:  Generally speaking, once the material gets above 50,000 tons/year, the logistics become pretty difficult to handle and it may make more sense to build multiple plants, i.e. if you have 100,000 tons/year, build two 14 fermenter facilities.


Q:  What’s the average gas production per ton of material?

A:  It will depend on the specific feedstock, but based on estimates, 1 ton produces ~3000 ft3 of biogas, and 1 ton produces ~220 kWh electric power.


Q:  What’s the average capital cost per ton of organic material processed?

A:   Based on the latest capital cost model, the average cost per ton of organic material is $340 (i.e. 8 fermenter $8,148,000/24,000=340). This is based on UWO $437/ton ($3.5M/8000), based on CGC $280/ton ($14M/50,000)./


Q:  What’s the average annual operating and maintenance (O&M) cost per ton of organic material processed?

A:  Based on latest O&M cost model, the average annual O&M cost per ton is $15 (i.e. 8 fermenter $357,648/24,000 tons = 14.90).


Q: What is the life expectancy of the facility, and what are some limiting factors?

A: The concrete structures making up the biodigester have a lifetime of about 20 years, the CHP is roughly 8 years. After 8 years, the CHP usually requires replacement of the generator, not other components. The better the gas quality, the longer the CHP life-time and the shorter the oil and filter change intervals; that is why BIOFerm incorporates biogas cooling remove the water and silicates from the biogas . Furthermore, our technology allows us to effectively reduce the Hydrogen Sulfide (H2S) to a level below 200ppm to save CHP maintenance costs and improve the quality of the resulting biogas.


Q:  What’s the typical composition of biogas?  Does it need to be conditioned?

A:  Methane                    45-70 %

Carbon Dioxide         25-55 %

Water Vapor             0-10 %

Nitrate                        0.01-5 %

Hydrogen                   0-1 %

Hydrogen Sulfide      20-200 ppm

Ammonia                    0.01-2.5 mg/m3

Methane quantity will increase with higher energy content feedstock.  When utilizing the biogas in a CHP, it needs to be cooled to remove water but no upgrades are needed. This water reduction in the biogas works by running the biogas through a gas cooling unit after collection and before it is routed to the energy application.  When biogas is cooled (to approximately 2 degrees C), the water vapor in the gas condenses and falls to the bottom of the gas cooling unit.  If utilizing the biogas as biomethane, additional upgrading/conditioning processes will be necessary (i.e. pressure swing adsorption).


Q:  Is there an application for small dairy farms to use AD?

A:  Yes, our German team has developed the all-in-one EUCOlino, a compact plug and play biodigester sized between 35 and 70 kW. This biodigester application is ideal for operations with limited space and organic waste stream.


Q:   Why does your system operate in the mesophilic range rather than thermophilic?  

A:   There are a diverse range of bacteria at the mesophilic range (~104 degF) to digest multiple feedstocks.  Thermophilic (~160 degF) conditions would only be feasible with a mono-feedstock.  Additionally, thermophillic conditions produce excess waste heat that can’t always be utilized effectively and it can be difficult to maintain high digester temperatures in cold northern winters. There also exists the challenge of additional ammonia being produced in the thermophilic range compared to mesophilic. Excess ammonia can act as an inhibitor to methane production.


Q:  What safety measures are in place at a BIOFerm biodigester?  Are explosions a concern?

A: Biogas is not an explosive gas, due to the level of water content in the gas as well as the percentage of CO2 which is not combustible.  Under normal operating conditions, the system is anaerobic – oxygen is only introduced right before the intake for the engine. All the electronics are rated to be used in classified areas where gas may become present in emergencies. If gas levels become too high, gas alarm sensors are present to detect dangerous levels.  Furthermore, the plant operator is normally outfitted with mobile detection unit which will bring attention to a problem if one should arise.

Q:  How do the doors on each fermentation chamber work?

A:  The doors stay locked by a pneumatic pin that stays in place until the composition of the gas in the fermenter environment reaches below the lower explosion limit of biogas, which is <4% methane. Operator activates purge cycle on the night before the chamber will be opened, which will introduce air into the chamber and exhaust mixture above roof for a time – once composition reaches that safe level, then the door will unlock.  Once door is open, air is constantly being exhausted from the fermenter. If a door is opened before the correct time, a limit switch activates an alarm that shuts the system down and ventilation is turned on


Q:  Is the building under high pressure?

A:  The gas is under very low pressure (under 1 psi).


Q:  How is thermal energy from the CHP utilized?

A:  Specific to our applications, the CHP unit functions as a hot water heater, and the hot water can be piped to adjacent buildings as in-floor heat, through convectors, or for any type of manufacturing/process use. Depending on the flow rate and insulation used, the distance can be up to a mile. If the heat generation is more appealing to a customer than electricity generation, we could install a biogas-fired boiler to heat the water instead of the CHP.  This could be less expensive than a CHP unit, but it depends on the value of the heat.


Q: How are the fermenter chambers heated?

A: Both via the percolate, and in-ground heating within the fermentation chambers.  The percolate is heated within the percolate storage tank.  This heating is provided by s heat exchange system both within the concrete of the floor, as well as on the interior wall of the tank.  This heat exchange system is in direct contact with the percolate, and is from the floor to half way up the wall.  This is heated by the CHP.  The in-floor heating of the fermentation chamber is within the concrete, and also provided by the CHP.


Q: Is there a limit to the organic loading rate?

A: The only process restriction or sensitivity is the organic load of the fermenter which can be up to 5kg oTS/m³ fermenter space and day but this number is really depending on the type of organic fractions whether it is highly volatile and there are also differences between fats, carbohydrates and proteins.


Q: Is the fermentation process sensitive to atypical feedstocks (FOGs), or high protein materials like meat waste?

A: Highly energetic materials like fats oil and grease can easily be added. Every time they should be mixed into the solid biomass.