Anaerobic digestion (AD) is a large-scale industrial processing method of using organic substrates and feedstocks to mainly produce methane. The main source of raw feedstock are animal wastes.
The digester is a large container which acts as a reactor. This is where the material is treated with heat and enzymes to encourage decomposition and to extract essential components.
A typical anaerobic digester is many layered. It operates as a continuous reactor where the layers become more solid with gravity. The bottom layer are stabilised solids which are removed at various stages. Above this layer is an active fermentative biomass above which is a supernatant liquid. Usually the supernatant is drawn off at times. Above the supernatant is a scum layer and above it is biogas. Biogas is drawn off as methane with carbon dioxide to be used in other applications.
As a process of many thousands of years, anaerobic digestion is actually natural involving decomposition and decay. In this particular industrial process organic matter is broken down to much simpler compounds under anaerobic conditions. Being anaerobic, organic reactions occur in the absence of oxygen to form carbon dioxide and methane.
Feedstocks For Anaerobic Digestion (AD)
Animal waste is one of the main feedstocks for an AD. Cattle manure for example is a major source of nitrogen and phosphorous. Dairy cattle in particular excrete 75% of all the nitrogen they ingest. The excreted manure contains 52% nitrogen in urine and 48% in faeces (Taiganides, 1977; Hohlfeld & Susse, 1985).
Cattle manure contains the following:
- total solids (g/l) 40.0 to 40.5
- volatile solids (g/l) 25.5
- organic nitrogen (g/l) 3.0
- ammonia nitrogen (g/l) 1.47-1.50
- phosphate (orthophosphate) (mg/l) 800-825
- pH – 7.6
The rest of cattle manure will contain undissolved and dissolved organic matter too including proteins, lipids, carbohydrates and various inorganic compounds.
Stages In Anaerobic Digestion (AD)
A number of materials make up the particulate organic substrates such as proteins, carbohydrates and fats. A series of hydrolysis reactions converts proteins into amino acids, carbohydrates into sugar and fats into fatty acids. Many of these components are then converted by reactions such as acidogenesis and acetogenesis to produce small-chain fatty acids such as propionic and butyric acid which then form acetic acid and hydrogen. Both molecules are substrates for producing methane.
Reactions Occurring In An Anaerobic Digester
Virtually all organic compounds are broken down into simpler monomeric compounds which serve as substrates for various microorganisms. A large variety of extracellular enzymes produced by these microorganisms are responsible for catalysing this breakdown. Most of these reactions are types of hydrolysis. So, cellulose in plants is converted to starch using cellulases, triglycerides are converted to fatty acids using lipases and proteins are broken down to amino acids with proteases.
Acidogenesis
The majority of soluble monomeric compounds formed by hydrolysis then undergoes fermentation by bacteria. In the overall reaction scheme, the products formed from acidogenesis are butyric acid and propionic acid which are intermediate compounds. Other compounds produced include acetic acid, ethanol, hydrogen and carbon dioxide. The two gases are lost from the anaerobic digester.
The bacteria which cause this type of fermentation are various Lactobacillus species which are often associated with reducing the pH as acids are produced.
Acetogenesis
Commonly termed the third phase of AD.
Acetic acid and acetate is produced in a mixed acid fermentation and through secondary fermentation of products generated by acidogenesis. Acetate is usually converted to methane. Hydrogen and carbon dioxide are also produced.
As a result of acetic and propionic acid formation in this phase as in phase 2, a large amount of protons are produced which leads to the earlier drop in pH of the aqueous medium. The optimum pH for action be acetogenic bacteria is at 6.
The bacteria are acetogenic and include the Acetobacter species. They are slow growing and sensitive to fluctuation in organic loadings and other environmental changes (Merlin Christy et al., 2014).
Methanogenesis
The final step in anaerobic digestion is the formation of methane. The methane is formed from two types of substrate: acetate and/or hydrogen and carbon dioxide. There is no formation of methane if any oxygen is present.
The bacteria causing methanogenesis are methogens and are classified as:-
- Lithotrophic or hydrogenotrophic bacteria which produce their methane from carbon dioxide and hydrogen.
- Acetotrophs or acetoclastic bacteria convert acetate to methane.
- Methylotrophs convert methanol to methane.
All the methanotrophs belong to the Archaea and are a very ancient group of bacteria. They are also classified as chemolithotropic methanogens and methylotrophic methanogens. Most of these particular microorganisms are found in the digestive systems in herbivores and in marshes or lake bottoms as well as in sewage treatment plants. They can only use C1 compounds and acetate as the only C2 compound.
Sulphate Reducing Bacteria
All fermenting microorganisms where ever they grow need soluble sulphur as a growth nutrient. Generally, bacteria use soluble sulphide HS-.
The sulphate-reducing bacteria (SRBs) metabolise sulphates and sulphonates by reducing sulphur with electrons or donating protons. Hydrogen is a key electron donor. These bacteria are found in anaerobic digesters because they can metabolise sulphates in an oxygen free environment.
The key product from sulphate reduction is hydrogen sulphide. Unfortunately, an excess of sulphides or dissolved hydrogen sulphide gas is toxic to most bacteria and it is certainly most toxic to those microorganisms living in anaerobic digesters. Hydrogen sulphide is especially toxic to methanogens.
The Factors that Affect Sludge Digestion
Temperature
Temperature is one process parameter which can be manipulated for optimum performance of an AD. The methanogens (methane-forming bacteria) are active in two temperature regions. The mesophilic bacteria live in the range from 30 to 35 ºC. The thermophilic bacteria live in the warmer temperature range of 50 to 60 ºC. Between the temperatures of 40 and 50 ºC, any ethane forming bacteria are inhibited.
pH
Ammonia is one of the key compounds in the performance of a digester. When the pH of the digester is 7.2 or lower, the formation of the ammonium cation NH4+ is present. When the pH is higher than 7.2, ammonia NH3 is favoured. A high or slightly alkaline pH is detrimental to AD performance because dissolved ammonia gas is toxic to bacteria, especially the methanogens.
Nutrients
All bacteria rely on macro- and micro nutrients for growth such as biomass formation. The main elements needed are phosphorous and nitrogen.
All the methanogens have unique enzyme systems not found in most other bacteria. Many of these need heavy metals such as cobalt and nickel as well as elemental nutrients such as sulphur and iron.
Mixing of Anaerobic Digesters
All anaerobic digesters need to be mixed. Its an operation that significantly influences the efficiency of the digestive process. Mixing brings bacteria into contact with fresh substrate and nutrients as well as reducing temperature fluctuations.
If mixing is poor or stops, hydraulic dead zones form that are extremely detrimental to the reaction kinetics involved in anaerobic digestion (Verhoff et al., 1974).
The methano-forming bacteria are very sensitive to very high shear mixing. If it is continuously high, the methanotrophs are in danger of being cleared from the fermenter and have been seen in the effluent.
The flow behaviour in AD has been modelled using computational fluid dynamics [CFD] (Wu et al., 2008).
Seeding
An AD is seeded with an adequate population of facultative anaerobes including methanogens. The ratio of 1:10 of secondary sludge to primary sludge is usually employed. In the absence of seeding an AD will not form an adequately sized population for fermentation. Seeding helps to produce a quick population growth.
Monitoring Anaerobic Digesters
Biogas is produced from anaerobic digestion of organic material using many different types of microorganisms. Methane is the most important gas. The most straightforward method is monitoring gases such as(H2, CH4, H2S and CO2). Fluctuation in these gases is a good measure of an anaerobic digester’s productivity (Spanjers & Lier, 2006).
Other measures include pH. If the pH is outside the range of 6 to 8 for example, there is fermentation (process) deterioration. Deterioration means loss of methane production and eventually collapse of the fermentation system (Liu et al., 2012).
Concentrations of volatile fatty acids (VFAs; mainly acetic, butyric and propionic acid) have been suggested as useful control parameters. These acids are indirectly linked to fermentation performance as measures of methanogen microorganism growth.
VFA accumulation can be interpreted as organic overload or inhibition of the methanogenic microbial communities (Madsen et al., 2011). Acidogenic microorganisms transform hydrolysis products into VFAs, while acetogenic microorganisms then convert VFAs into acetate, H2 and CO2. Methane is then produced by the methanogens (Gerardi, 2003).
Ideally, a monitoring system must have a set of sensors coupled to a treatment phase within a software program, where the measurements are carried out automatically with limited human intervention or expertise. The data can then be combined with numerical models to update an algorithm diagnosing the state of the digester and detecting erroneous working modes (Bernard et al., 2001).
Optical Sensors For Anaerobic Digesters
Optical-based chemical sensors (colourimetric sensors) appear to have the potential of providing such additional crucial information. They are low cost, require relatively simple instrumentation and straightforward sample preparation, and can be integrated within existing control systems. In general, there can frequently be a trade-off between sensitivity and robustness, and their use in bioreactors may be severely challenged by limited selectivity, repeatability, robustness and stability (Peris & Escuder-Gilabert, 2013).
One way is to apply artificial noses and tongues. Each sensor in the array is only partially selective but has distinctive responses to the various chemical entities of interest. Adding a multivariate chemometric tool results in quantitative responses for each entity. A few attempts have been made to apply artificial tongues, based on electrical or electrochemical sensors, for detection during AD (Buczkowska et al., 2010). However, the limited success of these technologies is due to the complex and poorly reproducible composition of process media, resulting in sensor contamination and biofouling.
Inhibitors of Anaerobic Digesters
Anaerobic digesters (AD) can be damaged by the presence of inhibitors in the feedstock. The types of compounds that affect ADs are:-
- ammonia and ammonium compounds
- metal ions – aluminium, calcium, magnesium, potassium, heavy metals
- organic compounds especially derivatized benzenes such as alkyl benzenes, phenolics, chlorophenols
- long-chain fatty acids e.g. olive oil as oleic acid, lauric acid
- lignins
Hydrogen Sulphide Management in Anaerobic Digestion
One of the most noxious gases that an anaerobic digester generates and must contend with is the formation of hydrogen sulphide. Virtually all feedstreams, when fed into an anaerobic digester contain sulphur-bearing compounds. These are usually in the form of sulphates and sulphonates. Under atmospheric conditions which implies, in the presence of oxygen, the sulphur is with few exceptions in these anionic forms.
In organic feedstocks there are also sulphur-containing amino-acids contained within proteins such as methionine and cysteine. There are very high protein levels in manures particularly from swine (pigs) and poultry which have a high sulphur input for anaerobic digestion. Sulphur also occurs to varying extents in foods such as egg (1.8 mg S/g), garlic (5.6 mg S/g), and onion (0.5 mg S/g) (Doleman et al., 2017).
In an anaerobic digester, these sulphur compounds are reduced instead of oxidised because of the absence of oxygen. The sulphates are converted into hydrogen sulphide which leaves the digester as a gas. In most digesters, biogas will contain H2S in the range from 0.1% to 3.0% (1,000-30,000 ppm).
Technologies Controlling and Managing H2S Production
Hydrogen sulphide must be reduced by a significant amount before any biogas can be used in generators, engines and other machines. There are three different approaches for treating biogas. These are through post-treatments, pretreatments and process regulation.
The gas is removed by both dry and wet oxidation processes. A variety of these have been costed for each particular application. Post-treatment technologies such as biotrickling filters and scrubbers are well developed with over 95% removal efficiency but they do not mitigate for H2S toxicity to methanogens (bacteria) within the AD.
The Benefits Of Anaerobic Digestion
Anaerobic digesters are economically viable because they turn noxious wastes which are environmentally damaging into more benign materials. It also generates carbon dioxide and methane which can be collected and used in fuel processes. The volatile content of the sludge is reduced.
The solids are dewatered and this solid material can be handled more effectively. The water component can be further treated. The level of pathogens is also significantly reduced.
The Issues Of Anaerobic Digesters
All fermentations including AD needs constant supervision to prevent them becoming uncontrolled. There is always the danger of producing explosive gases and to a great extent no AD is easily controlled. The fermentation is also slow and best operated continuously where possible until the sludge content builds up to an unmanageable level.
Maintenance and cleaning of the AD is not really feasible but must be managed to some extent.
References
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Doleman, J. F., Grisar, K., Van Liedekerke, L., Saha, S., Roe, M., Tapp, H. S., & Mithen, R. F. (2017). The contribution of alliaceous and cruciferous vegetables to dietary sulphur intake. Food Chemistry, 234, pp. 38-45.
Gerardi, M. H. (2003). The Microbiology of Anaerobic Digesters. John Wiley & Sons.
Hohlfeld, J.; Susse, L., (1985) Production and utilization of biogas in
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Liu, Y., Zhang, Y., Quan, X., Li, Y., Zhao, Z., Meng, X., & Chen, S. (2012). Optimization of anaerobic acidogenesis by adding Fe0 powder to enhance anaerobic wastewater treatment. Chemical Engineering Journal, 192, pp. 179-185 (Article).
Madsen, M., Holm-Nielsen, J. B., & Esbensen, K. H. (2011). Monitoring of anaerobic digestion processes: A review perspective. Renewable and Sustainable Energy Reviews, 15(6), pp. 3141-3155 (Article).
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Peris, M., & Escuder-Gilabert, L. (2013). On-line monitoring of food fermentation processes using electronic noses and electronic tongues: a review. Analytica Chimica Acta, 804, pp. 29-36 (Article).
Spanjers, H., & van Lier, J. B. (2006). Instrumentation in anaerobic treatment–research and practice. Water Science and Technology, 53(4-5), pp. 63-76 (Article).
Taiganides, E.P. (1977) Animal wastes. Applied Science Publishers L.T.D. London.
Verhoff, F.H., Tenney, M.W., Echelberger, W.F. (1974) Mixing in anaerobic digestion. Biotech. Bioeng. 16 (6) pp. 757-770
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