Biogas is produced when organic matter is degraded in the absence of oxygen. This anaerobic decomposition (or anaerobic digestion) process occurs naturally in wetlands, lake bottoms, and deep in soils. Man made sites include dairy waste water lagoons and landfills. Biogas contains about 60% methane, 40% carbon dioxide, and trace amounts of nitrogen, hydrogen, and hydrogen sulfide.
In the waste water treatment industry, anaerobic digestion has long been used as a means of reducing the amount of organic matter which must be treated. Environmental concerns about the effects of many industry waste streams are resulting in stricter regulations. Industries are being forced to clean up their discharges. Since anaerobic digestion reduces the amount of organic waste and produces methane, a valuable fuel, it is becoming more and more attractive as a waste treatment alternative.
Your basic task is to design, fabricate, and test a simple waste digester and gas collection system. With this system you can examine various facets of the anaerobic digestion process.
Your digester may be made up of only the following materials:
Digester seed material (inoculum) - enough for approximately 1 liter/team
Digester feed stock
Two liter soda bottles - one per team
1/8" plastic tubing - (optional) possible use in gas collection system
Large heavy duty balloon - one per team
Bucket for mixing inoculum and feed stock
Aluminum foil - (optional) possible use in glass collection system
Baking soda - 16 oz.
pH paper range pH 4 to pH 10
The basic performance index for your digester is that it produce some gas. Since even dying digester will produce carbon dioxide gas for a short period of time, continued gas production means that the digester is producing methane. If the biogas burns, the methane content is at least 45% and the participants have successfully designed, constructed, and operated an anaerobic digester. If the biogas will not burn, the methane content is less than 45%. Therefore, the extra performance index for your digester will be that it produce gas that burns.
Any number of reasons could contribute to failure of the gas to burn. The digester may not have produced a sufficient amount of gas, a leak may exist in the gas collection system, or the digester may have acidified and not be producing methane.
The earth provides all its inhabitants from microorganisms, plants, and insects, to fish, birds and animals with a relatively stable environment. When you create your digester, you are creating an environment for the bacteria in it to digest the food that you provide. It is important to realize that the environment in which the bacteria flourish is a delicate one. You are not only responsible for feeding the digester but also for controlling its temperature and acidity.
There are two general options for feeding a digester, batch feeding and routine feeding. Batch feeding is where the digester is initially feed at startup time and then monitored for gas production. Once the methanogens run out of food, biogas production ceases. Batch anaerobic digesters are not usually used in industry but are used to evaluate the potential for different feed stocks.
Digesters operating in industry are much more likely to be routinely fed. These digesters also include a reactor and a gas collection system. However, the reactor is usually fed either continuously, daily, weekly, or at some intermediate time interval. As the digester is fed, an equivalent volume must be removed from the reactor in order to maintain a constant level.
Anaerobic digestion can be described as a two-stage process accomplished by several types of bacteria which flourish in the absence of oxygen. A description of each of these two stages follows.
Acidification: Acid-forming bacteria break complex organic wastes down into volatile fatty acids. Proteins are broken down into amino acids and further into volatile fatty acids. Carbohydrates are broken down into simple sugars and then into volatile fatty acids. Fats and oils are broken down to long chain fatty acids and then to volatile fatty acids.
Acetogenic bacteria use the volatile fatty acids and form acetic, propionic, and lactic acids. In addition, hydrogen and carbon dioxide gas can be released by the acetogenic bacteria.
Methane Production: Methane-forming bacteria (methanogens) use acids formed in stage 1 to produce methane. Other bacteria use hydrogen and carbon dioxide produced in stage 1 to form methane.
These two types of bacteria can become out of balance in a poorly maintained environment. If your digester solution becomes too acidic (indicated by a pH less than 7) then the methanogen bacteria population is low. However, if the digester solution is too basic, then the acetogenic bacteria population is too low. Bacteria tend to adhere to solids in the solution. Because of this, many digesters used to process organic waste stir their solutions periodically to ensure a mixture of bacteria throughout the solution.
Temperature
The bacteria can be very sensitive to changes in their environment. Temperature is a prime example. It has been determined that 35 degrees centigrade (95 degrees Fahrenheit) is an ideal temperature for anaerobic digestion. As the temperature falls, bacteria activity decreases and biogas production decreases. As the temperature increases some bacteria begin to die, once again biogas production decreases.
Insulation, heat exchangers, heating elements, water baths, and steam injection are all means which have been used to control digester temperature. Temperature control is an important consideration when designing digesters. The materials list includes two potential heat sources. Anaerobic digestion will occur even at room temperature. However, any method of maintaining digester temperature constant near 35 degrees centigrade will improve digester performance. Any novel means of maintaining temperature are encouraged. After all, practicing engineering design is the primary purpose of this project.
Alkalinity and pH Alkalinity is a measure of the amount of carbonate in a solution. Acidity or basicity of a solution is indicated by pH. An acidic solution has more hydrogen or hydronium ions than hydroxide ions. A basic solution has more hydroxide than hydronium ions. At a pH of 7 there are equal amounts of hydroxide and hydronium ions. A pH greater than 7 indicates a basic solution and a pH less than 7 indicates an acidic solution. Alkalinity is important because as acid is added to solution, carbonates will contribute hydroxide ions which tend to neutralize the acid. This is known as the buffering effect of alkalinity.
Just as the bacteria population responsible for methane production flourishes in the absence of oxygen and over a relatively narrow temperature range, it also flourishes over the narrow pH range of 6.5 to 8.0. As the acid-forming bacteria produce acid, the methane-forming bacteria utilize the acid and maintain a neutral pH. Since the reaction rate involving the acid-forming bacteria proceeds much faster than the reaction involving methanogens, a larger population of methanogens must be nurtured and maintained.
Digester start-up is an especially critical time. When the digester is initially fed, acid-forming bacteria quickly produce acid. The methanogen population may not be sufficient to consume the acid produced and maintain a neutral pH. If the pH drops below 6.5, the methanogen population begins to die and the bacteria population becomes further unbalanced. The digester acidifies and produces no biogas.
In order to allow the methanogen population to grow, digesters are initially fed very small amounts and are often buffered by raising the alkalinity. In addition, raising the pH to approximately 7.5 by adding baking soda also increases the alkalinity or buffering capacity of digester solution.
Feeding the Digester
Digesters are usually fed based upon three criteria: volatile solids, hydraulic retention time, and carbon:nitrogen ratio.
Volatile solids (VS) is a measure of the amount of organic matter in a material. If too much organic matter is added, the acid forming bacteria can convert the organic matter to acids before the methanogens can use the acid. The resulting acid accumulation will cause the digester to fail because the methanogenic bacteria cannot survive in highly acidic conditions. A safe VS loading rate for the digester would be 1 g VS/(m3-day).
Hydraulic retention time (HRT) is a measure of the amount of time the digester liquid remains in the digester. If 10 liters of a 200 liter reactor is added and removed each day, it would take 20 days to completely replace the reactor contents. Hydraulic retention time is crucial because if the feed does not stay in the reactor long enough for the entire digestion process to take place, biogas will not be produced. Since your digester will only be working for a relatively short period of time, HRT will not play a large role in designing your digester.
Just as a balanced diet contributes to a healthy person, a balanced diet helps maintain a stable, healthy bacteria population. Anaerobic bacteria commonly use carbon as an energy source for growth and nitrogen to build cell structure. Generally, 25-30 times more carbon is required by the bacteria than nitrogen. The bacteria most efficiently utilize feeds which have a carbon:nitrogen ratio of approximately 30:1. Dry dog or cat food have a carbon:nitrogen ratio close to 30:1.
Project participants should wear standard lab safety equipment, i.e. safety glasses and gloves, when handling digester seed and feed material.