Return to WASTE DIGESTER DESIGN Home Page.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.
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.
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.
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.
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
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.
Monitoring instructions and flame test
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.
| Day | Amount of Feed (grams) | pH | Did gas burn? |
|---|---|---|---|
| 8.0 | No | ||
| 7.5 | No | ||
| etc. | Yes | ||
This project was developed by John Garcelon and Joe Clark.