Today many research engineers are examining the anaerobic digestion process. They are attempting to improve process controls, improve specific digester designs, tailor digester design for specific types of organic wastes, and integrate the process with other waste treatment technology. One of the major problems encountered is the processing and handling of potential digester feed materials. This project introduces the anaerobic digestion process and various associated problems.
In addition, this project is an excellent example of how design engineers must deal with many disciplines. Areas covered include microbiology, chemistry, heat transfer, and fluid behavior just to name a few.
The primary goal of this project is to have students design a simple anaerobic digester and gas collection system. With this system they can then examine various facets of the anaerobic digestion process.
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 wastewater lagoons and landfills. Biogas contains about 60% methane, 40% carbon dioxide, and trace amounts of nitrogen, hydrogen, and hydrogen sulfide.
In the wastewater 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.
Three to four hours are recommended for the presentation of background information, initial design, fabrication, and setup period. After the initial setup, a feeding and gas monitoring routine can then be established. This could be either daily or every two to three days. Monitoring activities should take no more than 1/2 hour. More time should be allowed when students compare gas production and test the gas for its flammability.
The anaerobic digestion process is not instantaneous. Time is required for the bacteria population to grow and stabilize. The minimum amount of time recommended for this project is two weeks. However, once the digester is established, it is easy to conduct a variety of experiments. Temperature can be varied, different feed materials can be tried, or you can overfeed the digester and "kill it". As engineers do, you may wish to try improving your design.
The digester should easily fit on a card table size table. Power is not necessarily required, but may be desired if temperature control is attempted.
The following material list should allow for several successful digester designs:
Digester seed material (inoculum) - enough for approximately 1
liter/team
Digester feedstock
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 feedstock
Aluminum foil - (optional) possible use in gas collection system
Baking soda - 16 oz.
pH paper range pH 4 to pH 10
The digester must operate in a consistent temperature range. We suggest two methods for temperature control. For method 1, an inexpensive Styrofoam cooler, thermometer, and aquarium heater are used. The second method uses a large cardboard box, small lamp and thermometer. We recommend the first method since it is the safest and offers the best temperature control.
Teams should consist of 1 to 4 members. The most efficient team size would most likely be 2 to 3 members.
Each digester will require seed material (inoculum) to provide an initial bacteria population. The volume of seed material required is approximately 1/2 of the digester volume. It is best to obtain seed material from an operating anaerobic digester. Some wastewater treatment plants have an anaerobic digester and operators are usually happy to provide all of the digester effluent you desire.
If an operating digester cannot be located, the instructor can start a special digester to provide seed for the project digester. A description of this process is given in Appendix A. It should be noted that starting a digester from scratch will probably require three to four weeks. The time is required for bacteria population growth. Actual time invested is minimal.
Project participants should be given an introductory lecture which includes anaerobic digestion principles, application of anaerobic digestion, and an overview of several digester designs. Two to three hours should be allowed for initial digester design, fabrication, and setup. For several days in a row the digester should monitored for pH and gas production. In addition, if the digester appears stable it should be fed. This should require 1/2 to 1 hour. After several days the project participants should compare gas production and conduct tests to see if the biogas will burn.
Gas production is one measure of success, but even a dying digester will produce carbon dioxide 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%. 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 Anaerobic Digestion Process
Anaerobic (without oxygen) digestion can be described as a two-stage process accomplished by bacteria which flourish in the absence of oxygen. A description of each of these two stages follows:
1. 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.
2. 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.
Temperature
The anaerobic digestion process is carried out by a delicately balanced population of various bacteria. These 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.
Modes of Digester Operation
There are two concepts for digester operation. The first is batch digestion. Batch digesters are fed at start-up and then monitored for gas production. Once the methanogens run out of food, biogas production ceases. This type of digester is generally a lab scale digester used to investigate the biogas production potential of various feed stocks. A typical batch system consists of a reactor, which contains the seed and feed material, and a means of collecting and measuring the biogas produced. The amount of gas produced can be a measure of the potential for that particular feedstock to produce biogas.
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.
Types of Digesters
The routinely fed digesters can generally be classified as one of two types. Dispersed growth digesters allow the bacteria to flow with the digester solution. The bacteria population is depleted as old material is removed and the population grows with the addition of feed. Often this type of system is stirred so that the feed and bacteria will be evenly distributed. Thick slurries can be effectively digested using this technique.
Attached-film digesters use wood chips or plastic or ceramic media designed to provide a large surface area to which the bacteria can attach themselves. The bacteria population inside the reactor is more stable. This type of digester is effectively used with dilute waste streams.
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. A discussion of the volatile solids measurement is in Appendix B. The VS of various materials is found in Appendix C. 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 kg VS/(m3-day). Optimum VS loading rates usually vary between 1 and 4.
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. Another way of considering HRT is that if the reactor is horizontal with the input on the left, output on the right, and not mixed, it would take twenty days for that which was put in on the left to come out on the right. 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. Dispersed growth digesters often require HRTs of 20 to 30 days for optimum methane production. Attached-film digesters on the other hand can often have an HRT of 4 to 6 hours. As mentioned earlier, attached-film digesters work well for dilute waste streams. In industry, the type of digester used depends upon the characteristics of the waste stream being treated.
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 ration of approximately 30:1. A table presenting the carbon and nitrogen content of some common materials is in Appendix D.
Specific digester start-up instructions
The reactor is considered the portion of the digester which contains the liquid or slurry. Half of the reactor volume should be filled with seed material from an operating digester.
Feed material should be prepared following the guidelines given previously. Feed material should fill 1/4 to 3/8 of the reactor volume. The remaining 1/4 to 1/8 of the reactor space is empty head space.
Note: Less headspace means that biogas produced will be less contaminated with air and will result in gas with a higher methane content. However, unless a digester is continuously mixed, scum tends to collect at the top. If an insufficient amount of head space is available, this scum can expand and block the gas collection connection.
For start-up no more than 1/5 of the recommended daily volatile solids loading should be used. Feedstocks could include table scraps, cow manure, dog manure, dog food, honey, sugar, or virtually any organic material. Organic materials with simple structures will be more easily converted to acids and the digestion process will occur sooner. If simple organic materials such as honey or sugar is used the initial VS loading should be reduced to prevent acidification of the digester. Very complex organic materials such as wood chips would take and extremely long time to digest and should be avoided. Water is used to dilute the desired amount of volatile solids to the desired volume.
Once the feed and seed material is combined, the pH of the digester start-up mixture should be raised to 7.5 or 8 by adding (sodium bicarbonate) baking soda.
If an attempt is being made to control the digester temperature, the reactor should be placed in the controlled environment, i.e. in the water bath or hot box.
Monitoring instructions and flame test
If the designed system is effective and the feed recipe was appropriate, biogas production will most likely be observable the following day. Biogas production may take longer if temperature control is not utilized. The digester could be allowed to go unmonitored for one or two days if necessary. Bear in mind the eagerness of project participants to observe their successful results.
Estimate the volume of gas produced. Gas collected under pressure (in a balloon) will occupy less volume than an equal amount of gas collected at atmospheric pressure. In addition, when the pressure of the collection system is higher, more of the gas will remain in the digester solution.
Flammability of the biogas can be tested by allowing small amounts of biogas out of the gas collection system near a match. Standard safety precautions should be taken. Wear safety glasses or a face shield. Do not point the biogas stream at any one. Even if all of the biogas produced were to escape at once, only a small (perhaps the size of a soft-ball) would instantaneously flash and expire. Best control of the flame test would be if the biogas were routed through the 1/8 inch plastic or tygon tubing with the match at the end of the tubing.
Lack of gas indicates that either the gas collection system is ineffective, the digester has not been maintained at a stable temperature long enough for biogas to be produced, or the digester has acidified. Check the pH of the liquid contents of the reactor. If the pH is greater than 6.5, the digester is most likely healthy. If the pH is less than 6.5, the digester has acidified or "pickled". If the pH is greater than 5, the digester may be resuscitated by adding baking soda to raise the pH back to 8. If the pH is less than 5, the digester most likely cannot be resuscitated and the reactor contents should be discarded. A new mixture of feed and seed material should be prepared.
The digester can be operated as a batch digester or as an intermittently fed digester. The feeding routine can be established by project participants.
Once gas has been collected and the flame test has been conducted, the project could be concluded. However, it is at this point that the project truly has the potential to develop a life of its own, directed by the participants. Feed stock recipes can be varied. The digesters can be underfed or overfed. Temperature or pH can be varied. Virtually anything which comes to the minds of the participants can be tried and its effect upon the digester observed.
Once the project has been concluded, the digester solutions can be disposed of via the sewer system. A toilet is the most appropriate disposal site because some feeds may tend to clog sink drains.
Results
Since this design project takes several days to monitor each design's performance, we suggest that you have each team monitor it design's progress using a table. The following table provides a suggested format.
Project participants should wear standard lab safety equipment, i.e. safety glasses and gloves, when handling digester seed and feed material.
This project was developed by John Garcelon and Joe Clark.