ANAEROBIC DIGESTION

Anaerobic digestion consists of a series of microbiological processes that convert organic compounds to methane. Methane production is a common phenomenon in several diverse natural environments ranging from glacier ice, sediments, marshes, rumen, and oil fields. The microbiological nature of methanogenesis was discovered more than a century ago. While several types of microorganisms are implicated in aerobic processes, anaerobic processes are driven mostly by bacteria.

Anaerobic digestion has long been used for the stabilization of wastewater sludges. In recent years, however, it has also been used for the treatment of industrial wastewater. This was made possible through a better understanding of the microbiology of this process and through improved reactor designs. Its advantages over aerobic processes are the following:

1. Anaerobic digestion uses readily available CO₂ as an electron acceptor as its oxygen source. It requires no oxygen, the supply of which adds substantially to the cost of wastewater treatment.
   
2. Anaerobic digestion produces lower amounts of sludge (3 - 20 times less than aerobic processes), since the energy yields of anaerobic bacteria are relatively low. Most of the energy derived from substrate breakdown is found in the final product, CH₄. As regards cell yields, 50% of organic carbon is converted to biomass under aerobic conditions. The net amount of cells produced per metric ton of COD destroyed is 20-150kg [44.1-330.75lbs], as compared to 400-600kg [882-1323lbs] for aerobic digestion.
   
3. Anaerobic digestion produces a useful gas, methane. This gas contains about 90% of the energy, and can be burned on site to provide heat for digesters or to generate electricity. Little energy (3-5%) is wasted as heat. Methane production contributes to the BOD reduction in digested sludge.
   
4. Energy required for wastewater treatment is reduced.
   
5. Anaerobic digestion is suitable for high-strength industrial wastes.
   
6. It is possible to apply high loading rates to the digester.
   
7. Anaerobic systems can biodegrade xenobiotic compounds such as chlorinated aliphatic hydrocarbons (e.g., trichloroethylene, trihalomethanes) and recalcitrant natural compounds such as lignin.

Some disadvantages of anaerobic digestion are as follows:

1. It is a slower process than aerobic digestion.
   
2. It is more sensitive to upsets by toxicants.
   
3. Start-up of the process requires long periods of time.
   
4. As regards biodegradation of xenobiotic compounds by cometabolism, anaerobic processes require relatively high concentrations of primary substrates.

Process Description
Anaerobic digesters are large fermentation tanks provided with mechanical mixing, heating, gas collection, sludge addition and withdrawal ports, and supernatant outlets. Sludge digestion and settling occur simultaneously in the tank. Sludge stratifies and forms the following layers from the bottom to the tip of the tank: digested sludge, actively digested sludge, supernatant, a scum layer and gas. Higher sludge loading rates are achieved in the high-rate version, in which sludge is continuously mixed and heated.

Process Microbiology
Consortia of microorganisms, mostly bacteria, are involved in the transformation of complex high-molecular-weight organic compounds to methane. Furthermore, there are synergistic interactions between the various groups of bacteria implicated in anaerobic digestion of wastes. Although some fungi and protozoa can be found in anaerobic digesters, bacteria are undoubtedly the dominant microorganisms. Large numbers of strict and facultative anaerobic bacteria are involved in the hydrolysis and fermentation of organic compounds. There are four categories of bacteria that are involved in the transformation of complex materials into simple molecules such as methane and carbon dioxide. These bacterial groups operate in a synergistic relationship in as much as group #1 has to perform its metabolic action before group #2 can take over, etc.

Group 1: Hydrolytic Bacteria
Consortia of anaerobic bacteria break down complex organic molecules (proteins, cellulose, lignin, and lipids) into soluble monomer molecules such as amino acids, glucose, fatty acids, and glycerol. The monomers are directly available to the next group of bacteria. Hydrolysis of the complex molecules is catalyzed by extracellular enzymes such as cellulases, proteases, and lipases. However, the hydrolytic phase is relatively slow and can be limiting in anaerobic digestion of waste such as raw cellulolytic wastes, which contain lignin. The use of BZT® Waste Digester can complete this breakdown faster because it contains the necessary bacteria and enzymes groups.

Group 2: Fermentative Acidogenic Bacteria
Acidogenic (i.e., acid-forming) bacteria convert sugars, amino acids, and fatty acids to organic acids (e.g., acetic, propionic, formic, lactic, butyric, or succinic acids), alcohols and ketones (e.g., ethanol, methanol, glycerol, acetone), acetate, CO₂, and H₂. Acetate is the main product of carbohydrate fermentation. The products formed vary with the type of bacteria as well as with culture conditions (temperature, pH, redox potential).

Group 3: Acetogenic Bacteria
Acetogenic bacteria convert fatty acids (e.g., propionic acid, butyric acid) and alcohols into acetate, hydrogen, and carbon dioxide, which are used by the methanogens. This group requires low hydrogen tensions for fatty acid conversion; and therefore a close monitoring of hydrogen concentrations is necessary. Under relatively high H₂ partial pressure, acetate formation is reduced and the substrate is converted to propionic acid, butyric acid and ethanol rather than methane.

Group 4: Methanogens
Anaerobic digestion of organic matter in the environment releases 500-800 million tons [453.6 - 725.75 metric tons] of methane per year into the atmosphere and this represents 0.5% of the organic matter derived from photosynthesis. The fastidious methanogenic bacteria occur naturally in deep sediments or in the rumen of herbivores. This group is composed of both gram-positive and gram-negative bacteria with a wide variety of shapes. Methanogenic microorganisms grow slowly in wastewater and their generation times range from 2 days at 35°C [95°F] too as high as 50 days at 10°C [50°F]. About two thirds of methane is derived from acetate conversion by methanogens. The other third is the result of carbon dioxide reduction by hydrogen.

Factors Controlling Anaerobic Digestion
Anaerobic digestion is affected by temperature, retention time, pH chemical composition of wastewater, competition of methanogens with sulfate-reducing bacteria, and the presence of toxicants.

Temperature
Methane production has been documented under a wide range of temperatures. In municipal wastewater treatment plants, anaerobic digestion is carried out in the mesophilic range at temperatures from 25°C [77°F] to up to 40°C [104°F] with the optimum at approximately 35°C [95°F]. Thermophilic digestion operates at temperature ranges of 50-65°C [122°F-149°F]. It allows higher loading rates and is also conductive to greater destruction of pathogens. One drawback is its higher sensitivity to toxicants. Because of their slower growth as compared with acidogenic bacteria, methanogenic bacteria are very sensitive to small changes in temperature, which leads to a decrease of the maximum specific growth rate while the half-saturation constant increases. Thus, a mesophilic digester must be designed to operate at temperatures between 30°C [86°F] and 35°C [95°F] for their optimal functioning.

Retention Time
The hydraulic retention time (HRT), which depends on wastewater characteristics and environmental conditions, must be long enough to allow metabolism by anaerobic bacteria in digesters. Digesters based on attached growth have a lower HRT (1-10 days). The retention times of mesophilic and thermophilic digesters range between 25 and 35 days but can be lower.

pH
Most methanogenic bacteria function in a pH range between 6.7 and 7.4, but optimally at pH + 7.0-7.2, and the process may fail if the pH is close to 6.0. Acidogenic bacteria produce organic acids, which tend to lower the pH of the bioreactor. Under normal conditions, this pH reduction is buffered by the bicarbonate that is produced by methanogens. Under adverse environmental conditions, the buffering capacity of the system can be upset, eventually stopping the production of methane. Acidity is more inhibitory to methanogens than of acidogenic bacteria. An increase in volatile acid levels thus serves as an early indicator of system upset. Monitoring the ratio of total volatile acids (as acetic acid) to total alkalinity (as calcium carbonate) has been suggested to ensure that it remains below 0.1.

Toxicants
A wide range of toxicants is responsible for the occasional failure of anaerobic digesters. Inhibition of methanogenesis is generally indicated by reduced methane production and increased concentration of volatile acids.

BZT® Waste Digester not only contains the right bacteria and enzyme concentrations to start the whole process faster but can be an invaluable tool to use in normal maintenance to keep the system in balance and to re-establish systems that have experienced upsets.

BZT® Waste Digester provides you a high tech, all natural answer to a more efficient operating system.