Anaerobic Digestion
Anaerobic digestion is the process of digesting organic matter by bacteria in an oxygen free environment. This is a process that is found in many naturally occurring environments such as watercourses, waterlogged soils and the mammalian digestive tract such as a cow.
Bovine Digestion
Dairy cows are ruminants and feed on grass. Ruminants (cattle, cows, goats, sheep, deer, giraffe, yak, camel, llamas and antelope) have four stomachs as opposed to non-ruminants otherwise known as monogastrics (humans, dogs, pigs). The four parts of the ruminant stomach are rumen, reticulum, omasum and abomasums. In the first two chambers food is mixed with saliva and separated into layers of solid and liquid. The solids can clump together to form a bolus or cud which is regurgitated and chewed to further mix it with saliva and reduce particle sizes. The partly digested food is called chime or digesta. The hydrolysis of cellulosic and hemicellulosic materials is facilitated in these two chambers by microbes (bacteria, protozoa, fungi and yeast). The cud is re-swallowed and further digested by micro-organisms in the rumen which ultimately feed the cow. Cellulose and hemicellulose is primarily broken down into three volatile fatty acids (acetic acid, propanoic acid, and beta-hydroxybutyric acid). Protein and non-structural carbohydrates (pectin, sugars, and starches) are fermented. The omasum’s function is to absorb water and some nutrients from the digesta. The abomasums functions like the human stomach and secretes enzymes to further digest materials that were not broken down in the rumen. The digesta passes through the abomasums and into the small intestines where more digestive enzymes are added and nutrients are absorbed. From the small intestines the material ultimately passes out of the animal.
Microbial Anaerobic Digester
A microbial anaerobic digester is very much like a mechanical cow. Anaerobic decomposition is a complex process. Based on the input feed stock different strains of bacteria will hydrolyze polymers of carbohydrates, proteins and fats into their component parts. This process occurs in four separate phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The last stage of the process is where bio-gas is produced. The advantage of using anaerobic digestion, compared to composting, is the production of bio-gas which is primarily methane, a gas that can be used as a fuel.
A variety of factors affect the rate of digestion and bio-gas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135°F (57.2°C). Anaerobic bacteria communities that thrive at temperatures of about 98°F (36.7°C) are called mesophilic. Anaerobic bacteria that thrive at temperatures of 130°F (54.4°C) are called thermophilic. Hence methane production can occur at 98 °F and 130°F. Thermophilic anaerobic digestion proceeds faster than mesophilic decomposition. Other factors affect the rate and amount of bio-gas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion and retention times depend on all of the above factors.
Optimizing the Anaerobic Digestion Process
Reactor Design
The basic requirements for an anaerobic digester design include identification of a feed stock for a continuously high and sustainable organic load rate, a short hydraulic retention time (to minimize reactor volume and control costs) and finally to produce the maximum volume of methane gas. The digester shape must take into account protocols and practices for mixing and heat loss as well as maintenance. There are three main types of reactor design: 1) batch reactors, 2) one stage continuously fed systems and 3) two (multi) stage continuously fed systems. Batch reactors are the simplest and involve filling a tank with feed stock which is left for a period of time (hydraulic retention time) to rot. During retention time bio-gas is released. Once the feed stock has been digested it is removed and the tank filled again. A single stage continuously fed system uses one tank where all of the bio-chemical reactions take place. A two stage continuous fed system separates the hydrolysis / acidification and acetogenesis / methanogenesis reactions. Multistage anaerobic digestion systems improve on the stability of the process compared to one stage systems. Instability can be caused by variable organic loading rate, heterogeneity of feed stock or excessive inhibitors. In multi stage systems the first stage acts as a buffer that helps to homogenize feed stock material before passing it into the second stage containing the more sensitive methanogen bacteria. Multi stage digesters tend to cost more due to the need for additional tanks and maintenance but generally have better performance then single stage operations.
Mixing
Mixing of the contents in a digester reactor ensures efficient transfer of organic material with active microbial biomass to release gas bubbles trapped in the medium and to prevent sedimentation of denser particulate matter. Mixing can be performed on an intermittent basis using a variety of methods. Some mixing methods include the use of propellers, hydraulic pumps and bio-gas sparging. The choice of mixing method is related to the cost for installation and maintenance versus the increase in digester performance.
Immobilization of Microbial Bio-mass
Immobilizing bacteria to a support system reduces the amount of bio-mass that can be washed out of the digester. Immobilization of microbial bio-mass can involve the use of an inert or degradable medium. Digesters that use immobilized bacteria are called anaerobic filter (AF) reactors. If the support material is degradable it can be considered as part of the feed stock.
Temperature
Anaerobic digestion can take place at temperatures below 20 C but most reactors operate at either mesophilic (optima at 35 C) or thermophilic (optima at 55 C) temperatures. The structure of the microbial community is different between the mesophilic and thermophilic reactors. Small changes in temperature can reduce bio-gas production.
pH and Buffering Capacity
The ideal pH range for anaerobic digestion is small: pH 6.8-7.2. The growth rate of methanogens is greatly reduced below pH 6.6. The optimal pH for methanogens is around 7.0. The optimum pH for hydrolysis and acidogenesis is between 5.5 and 6.5. This is the primary reason digester designers prefer multiple stages.
Buffering capacity refers to the ability of the reactor medium to resist rapid changes in pH. Generally it is the carbon dioxide (CO2) from bacterial respiration and bicarbonate ions that provide the resistance to pH changes. Buffering capacity is proportional to the concentration of bicarbonate. Buffer capacity is a more reliable method of measuring digester imbalance than direct pH measurements because the production of short chain fatty acids will reduce buffering capacity before the pH decreases.
Short Chain Fatty Acids
Short chain fatty acids are a key intermediate in the process of anaerobic digestion and are also capable of inhibiting methanogenesis in high concentrations. Monitoring of fatty acids specifically butyrate and isobutyrate has been demonstrated to indicate process stability. Generally the organic loading rate is important for controlling fatty acid concentrations.
Feed Stocks
Direct comparison of bio-gas produced from various feed stocks is difficult because of the wide variety of experimental conditions tested. In general bio-gas production from food waste is much larger than from animal waste. The primary reason is that energy has been extracted from feed by the animal before being discarded as waste. This may be the reason why co-digestion, the practice of digesting sewer sludge with agricultural waste, can improve methane production.
Pre-treatments and Additives
Pre-treatment of feed stocks can increase bio-gas production, reduce volatile solids and increase solubilisation. The use of pre-treatments is an advanced for of anaerobic digestion particularly useful for bio-mass feed stocks because they tend to be high in cellulose and lignin. Pre-treatment can break down these recalcitrant polymers physically, thermally, chemically or enzymatically. Alkali pre-treatment of 10% sodium hydroxide (NaOH) can increase degradation rate of plants and paper feed stocks. But, alkali pre-treatment in continuous reactors can generate toxic compounds ultimately reducing bio-gas production. Thermal and thermal chemical pre-treatments can facilitate particle size reduction and solubilisation of feed stock but increase both energy operational costs. Particle size is important for anaerobic digestion rate because it is related to substrate availability to bacteria.
Various additives can increase digester health and methane production. Cell lysate and enzymes are used to speed up the hydrolysis of feed materials which can result in 60% increase in methane production. The addition of certain metals to the feed stock can increase bio-gas production. The specific metals and amounts may depend on the type of digestion (mesophilic or thermophilic) and is considered to facilitate enzymatic activities. Many enzymes use metals as part of their three dimensional atomic structures very much like vitamins. Another additive procedure is seeding. Seeding involves adding bacterial communities to the digester either to reduce start up time or wash out. In advanced multi stage anaerobic digesters careful seeding can release hydrogen as well as methane. The difficulty with these processes is maintaining the microbial communities.
Monitoring and Control
Monitoring of anaerobic digestion is difficult and a complex multi-variable process. There are few reliable on-line sensors for measuring important parameters. Many of these sensors can foul. Many parameters can be measured using simple electronic sensors that have to be routinely cleaned. Parameters measured in liquid phase include temperature, pH, and conductivity. Measurement of gas composition can include hydrogen by electrochemical cells and methane and carbon dioxide by infrared. Monitoring of methane production is important consideration for trading in carbon credits. Software can be important for monitoring anaerobic digestion process. Generally various process parameters are collected on-line in real time and implemented into a mass balance based model which can predict out puts. The comparison of predicted and measured outputs forms the basis for an automated closed loop control system.
The primary objective for using sensors to monitor anaerobic digestion is to create a closed loop automated control system. Generally for anaerobic digestion control is exerted at the organic loading stage. A decision making module achieves control by operating actuators. The decision making module takes inputs from process sensors and uses them in a mathematical model of anaerobic digestion. The models may be based on linear, non-linear, fuzzy logic and artificial neural networks that are designed to predict process parameters given initial conditions. Comparison between predicted outcomes and measured parameters determines the action of the decision making module.
References
Curry, N. and Pillay, P. Biogas prediction and design of a food waste to energy system for the urban environment. Renewable Energy, 2011.
Kelly, G., Tayfur, G., Dolan, C., Tanji, K. Physical and mathematical modeling of anaerobic digestion of organic wastes. Wat. Res. V.31(3),pp. 534-540, 1997.
Lu, Y., Lai, Q., Zhang, C., Zhao, H., Ma, K., Zhao, X., Chen, H., Liu, D., Xing, XH. Characteristics of hydrohen and methane production from cornstalks by an augmented two- or three-stage anaerobic fermentation process. Bioresource Technology V. 100, pp. 2889-2895, 2009.
Steyer, JP., Buffière, P., Rolland, D., Moletta, R. Advanced control of anaerobic digestion process through disturbances monitoring. Wat. Res. V. 33(9),pp2059-2068, 1999.
Ward, A.J., Hobbs, P.J., Holliman, P.J., Jones, D.L. Optimisation of the anaerobic digestion of agricultural resources. Bioresource Technology V. 99,pp.7928-7940, 2008.
ANXIETY QUESTIONS
· MORE ENERGY INDEPENDENCE – Is that an interest?
· CONVERTING WASTE STREAMS INTO ENERGY - Are you interested?
· NO WASTE TRANSPORTATION COSTS – How much would you save?
· ENERGY COSTS – Are you tired of the escalation of prices?
· HOT WATER & ELECTRICITY – Would you like to supply your own?
· GLOBAL WARMING - Is that a concern of yours?
· CARBON FOOTPRINT – Are you interested in learning how to reduce it?
· CARBON CREDITS – How you can participate in new markets – any interest?
CAPABILITY QUESTIONS
WHAT IF YOU COULD…?
· PRODUCE YOUR OWN HEATING & ELECTRICAL ENERGY-One ton food waste processed per day generates ~1000 kW and / or ~1.4 million BTUs
· REDUCE YOUR WASTE TRANSPORTATION COSTS
· REDUCE YOUR CARBON FOOTPRINT-Trade or sequester your carbon credits
· GLOBAL WARMING – No contributions
· ACQUIRE ENERGY PRODUCTION INFO & CONTROL THE PROCESS FROM YOUR SMART PHONE - How would you feel?
C2B’s Enzyme Enhanced Microbial Anaerobic Digester
· “Our waste-to-energy solutions can give you that capability.”
· Construction costs estimates $200,000 - $230,000
· Our enzyme technology is designed to be more efficient and less costly
C2Biotechnologies develops advanced, nature-based biotechnologies to convert waste to energy at its source and allow our clients to reduce their carbon footprints while being profitable, self-sufficient and self-sustainable
Anxiety Question
How can you become more energy independent? Are you interested in converting waste streams into energy? How much money would you save if you didn’t have to pay for waste transportation costs? Are you tired of paying and paying for energy? Interested in supplying your own hot water or electricity? Are you concerned about global warming? How can you reduce your carbon footprint? How can you participate in new markets trading carbon credits?
Capability Question
“What if there was a way, you could produce your own energy for heating or electric and at the same time reduce your waste transportation costs and carbon foot print. What if you could say I have reduced my carbon foot print; I’m not contributing to global warming. How would you feel if you could point to your mobile phone or tablet and not only show how much energy your producing but control the process at the touch of a screen?”
Feature Statement (feature becomes a benefit)
“Our waste-to-energy solutions can give you that capability.”
Enzyme Enhanced Microbial Anaerobic Digester (EEMAD)
Description
Microbial Anaerobic Digester ~8,183,952 BTU / Day
Operation Continuous
Stages 2
Enzyme enhanced Yes
Digester Tank Composition Plastic
Number of Tanks 6
| Tank | Number | Size (gallon) | Weight Empty (lbs) | Weight Full (lbs)* | Vol. (ft3) | Diameter (ft) | Height (ft) | |
| Stage 1 (acidification) | 1 | 4,600 | na | >38,180 | 740 | 8.5 | 13.0 | |
| Stage 2 (methane) | 1 | 11,200 | 3,600 | 89,600 | 1500 | 11.8 | 17.8 | |
| Slurry / Mash | 1 | 1,500 | na | >12,450 | 250 | 5.3 | 11.2 | |
| MeOH Storage | 2 | 2,500 | na | >20,750 | 225 | 5.0 | 11.5 | |
| Liquid Waste | 1 | 1,000 | na | >8,300 | 150 | 4.0 | 12 | |
Feed stock animal manure, kitchen waste, energy crop
Feed stock Volume 1000- 3000 kg / day [~2000 -6000 lbs/day]
Application Convert on-site waste to energy for on-site use
Fenced Area 4000 ft2, (50’ x 80’)
Building 540 ft2, (20’ x 27’), 1 door, 1 garage door,
Window(s) 5
Office area 96 ft2, (8’x12’)
Pad / gravel surface 1739 ft2, (37’x47’)
Finish
Tanks can be painted solid, geometric or custom design. Trees and shrubs can be strategically placed to enhance the appearance of your power house plant.
Enzyme Enhanced MAD Features
Produce bio-gas which can be converted into electricity or heat
Energy can be used on-site
Convert waste into energy
Reduced your energy costs
Reduces or eliminates waste transportation costs
Reduces your carbon foot print
Generates carbon credits which can be traded or sold
Continuous two stage design is more stable compared to single stage operations
Use of small volume plastic tanks reduces capitalization costs
Use of multiple stage tanks allows process to scale with feed stock volume
Enzyme increases amount and quality of bio-fuel produced
Producing enzyme on-site reduces operational costs
Producing enzymes on-site increases logistical freedom
Process monitoring and control can be performed over mobile phone or in control room
C2B Remote monitoring and facility maintenance is available
Waste to energy art, tanks can be painted with any design or theme
On-site enzyme production can be used to produce liquid fuel ethanol
Fuel ethanol can be used to power combustion engines
MAD waste can be converted into fertilizer and compost
Fertilizer and compost can support green house agriculture
Operations require relatively small foot print
Enzyme Enhanced MAD Specialized Features
Farmer & Farmer Cooperatives
MAD to power on-site field operations
MAD to power green house operations
MAD to convert field / orchard waste into energy
Reduce or eliminate waste transportation costs
MAD waste to grow feed for livestock operations
MAD waste to grow feed for aquaculture
State & Federal support for alternative energy
Educational Institutions
MAD to offset energy requirements
State & Federal support for alternative energy
Municipal leasing with flexible options for alternative energy
MAD facilities for on-site student training in “Green Energy”
Potential for increased student enrollment because of green technology
A
B
Figure 1 CAD model of MAD.
Panel A. The CAD model was developed to operate on 1 ton of animal manure per day. The model displays a continuous two stage operation. The first stage involves liquefaction and acidification of dissolved feed stock and operates at temperature of 95 F, pH5.5 – 6.5 and hydraulic retention time of 3 – 4 days. Methane is produced in the second stage which operates at temperature of 95 F, pH 6.5 – 7.6 and hydraulic retention time of 4 – 10 days. One ton of animal manure is projected to produce ~883 ft3 of bio-gas or the equivalent of ~530,000 BTU. By harvesting the methane in the bio-gas and using it for on-site heating removes the equivalent of ~89 tons of carbon per year with this system. To increase the scale of production additional liquefaction and methanogenesis tanks would be required. Liquid waste from MAD is collected and stored in large tank already on-site and used as fertilizer. A control room is included and houses all of the electronics used for monitoring and control of the system. Mechanicals and slurry tank are housed inside to protect against environment, weather and pests. The tanks and building are situated onto of a cement slab. A fence is used to enclose the compound and secure methane storage. Panel B. An image of a similarly sized anaerobic digester is provided. This operation was designed by a company in India.
Figure 2 Comparison of estimated daily BTU productions between food and manure processed per day
Volume and composition of waste stream determines amount of energy produced and digester sizing. Food waste can generate 5X-6X the volume of bio-gas compared to animal manure.
Figure 3 Comparing estimated annual energy savings from manure or food waste processed per day
The value of BTU generated was based on New York State retail value. Projected estimates are based on converting energy to BTU at 50 % efficiency for 1 year.
Figure 4 Estimated kWh generated from MAD versus waste processed per day
Bio-gas generated from MAD activities can be used to power a generator to produce electricity. The information displayed here is based on converting bio-gas into electricity with 50% efficiency. The volume and composition of feed stock directly affect the amount of electricity that can be produced.
Figure 5 Estimated annual carbon foot print versus waste processed per day
The bio-gas produced from MAD contains methane which is a green house gas that is 21 x more potent than CO2. By capturing the methane produced from MAD and converting to energy reduces your carbon foot print.
Figure 6 Estimated MAD cost versus food waste processed per day
Cost estimates are based on using a dual stage continuous enzyme enhanced MAD process. MAD costs are generally related to the size and number of tanks used to construct project. The size and number of tanks required is determined by the composition and volume of waste processed per day. Use of plastic tanks instead of concrete reduces project costs. Using multiple methane producing tanks instead of using larger tanks reduces costs.
