Feb

Microbiology of a Mesophilic & Thermophilic Digester

The bacteria within a methane digester are solely responsible for both the break-down of organic matter and production of various gases. However just like plants these bacteria can only survive within a certain range of conditions , where a mix of key variables affecting their growth are said to be ‘ideal’. These key factors include pressure , temperature and pH of the environment.

In the case of a methane digester the biogas we are aiming to produce is approximately 60-70% methane and so the bacteria we want to breed in the digester are ‘methanogens’ or methane-forming bacteria. They will get energy by breaking down the organic matter and as waste gas they will emit carbon dioxide and methane gas as a mixture. Of course then we will be aiming to operate the digester at ‘optimal’ conditions for methane-forming bacteria.

Temperature Control

The key here which will have the greatest impact on digester design , is whether the methane-forming bacteria being produced in the digester are of the mesophilic or thermophilic type. The difference is at which temperature the bacteria operate at and the metabollic rate of each type.

Mesophilic bacteria are the most common methane-forming bacteria found in biogas digesters as they have a lower optimal temperature range at 30-35°C which means little or no heating of the digester is required depending on the climate. Mesophillic digesters account for 90% of biogas producing digesters simply for this reason , however the difficulty is in maintaining this constant temperature as below 30°C acid-forming bacteria will grow and lead to digester failure. And so it is important when considering digester design to look at ways of insulating to reduce or eliminate the heat input required to maintain mesophillic conditions. A Mesophilic digester should be careful not to fall below 32°C as methane production will slow significantly and volatile acid formation can occur , 35°C is the preferred temperature for optimum methane production.

Thermophillic bacteria grow in the 50-60°C range anything higher than that are known as hyperthermophiles. The major advantage of operating a thermophilic digester as opposed to a mesophilic is that the metabollic rate of the bacteria is significantly higher. This means that the retention time is shorter and biogas is produced at a faster rate , at 50°C the retention time for waste is approximately 10 days as opposed to 20 days at 40°C. The problem is retaining 50°C requires significant heating and mixing to ensure the temperature profile is uniform across the digester , if left unmixed small variations of temperature can occur in pockets that lead to the death of thermophilic bacteria. Thermophiles cannot tolerate fluctuations in temperature as well as mesophiles so variations should be kept within <1°C daily.

pH Control

Alkalinity is essential for proper pH control in a biogas digester as it serves as the buffer for any enzyme activity in the digester. acceptable levels for acid-forming bacteria are at pH 5.0 and below but any enzyme activity from methane bacteria will not occur at below pH 6.2. At start-up of a anaerobic digester pH will drop as volitile acids are produced in the sludge , then once the methane producing bacteria begin to consume these acids and produce alkalines the pH will stabilize . At around 5 days retention time the methane producing bacteria will rapidly consume these acids. A properly operated digester will fall within pH 6.8-7.2 , however the CO2 % of the biogas produced will have a huge effect on the pH of the system.

High alkalinity concentration has a stabilizing effect on digester , as dropping alkaline levels can serve as a marker for impending digester failure . The inability for methanogens to consume the voltile acids or produce methane and CO2 will lead to a rapid drop in pH which can be read first by dropping alkalinity concentration. Reasons for this could be presence of wastes that inhibit the methane producing bacteria or accumlation of acids due to incorrect temperature or other conditions.

Alkalinity is added to the system by introducing chemicals such as ammonium carbonate , sodium bicarbonate or sodium nitrate. The key is that any chemical added to a digester to control pH is done slowly, as bulk loading the digester can lead to unwanted conditions such as foaming , ammonium poisoning or too greater Oxidation Reduction Potential (ORP) which means the bacteria wont be able to produce methane as a reaction. If any excess alkalinity exists in the digester it can be neutralized by adding ferric cholride.

The key to pH control of a digester is knowing that pH is a measure of what has happened in the digester while alkalinity is a measurement of what is happening.

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