Definition Of Anaerobic Digestion Biology Essay

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Anaerobic digestion is a fermentative process involving the solubilisation and reduction of complex organic molecules, by a synergistic group of microorganisms, in the absence of oxygen. Anaerobic digestion produces stabilised Biosolids, reduces biomass quantity by partial destruction of volatile solids, and produces a useable gas as a by- product.

The composition of this biogas varies, and depends to a large extent on the nature of the feedstock, organic loading rate, the hydraulic retention time and the temperature of anaerobic decomposition. However, typically, biogas is composed of the following:

Hydrolytic bacteria are a group of heterotrophic organisms, typically strict and facultatively anaerobic in nature. The hydrolytic bacteria are responsible for the solubilisation of complex organic macromolecules, such as protein, cellulose, lignin and lipids, into smaller monomers (glucose, amino acids, fatty acids and glycerol), which can be directly utilised by the next group of organisms (acidogenic bacteria) (Bitton, 1994). Hydrolysis is achieved by extracellular enzymatic degradation of the substrate, and has been identified as the rate limiting stage of anaerobic digestion (Tiehm et al., 2001).

B. Acidogenesis

The acidogenic bacteria, are mainly obligate anaerobes, which convert sugars, amino acids and fatty acids to organic acids (volatile fatty acids, alcohols and ketones, (ethanol, methanol, glycerol, acetone), acetate, CO2 and H2. The resulting products of acidogenesis are ultimately determined by the types of bacteria present (Bitton, 1994).

C. Acetogenesis

The acetogenic population includes a number of bacterial groups capable of acetate production. The first group being sulfate reducing bacteria (SRB) (non obligate proton reducing bacteria), which produce acetate and hydrogen in a mixed culture (Marty, 1984). Numerous studies have focused attention on competition between SRB and methanogens in the anaerobic digester environment (Fukui et al., 2001). However, a compatible relationship can exist in nutritionally rich environments, such as sewage sludge and animal wastes (Ueki et al., 1992).

The second group is the obligate hydrogen producing acetogenic bacteria, which forms an important symbiotic relationship with the methanogenic population.

The third group is the homoacetogenic bacteria which produces acetate from fructose, but are also capable of oxidising hydrogen and reducing CO2 into acetic acid, acting as competitors to methanogens or as synotrophic donors of acetate, hydrogen and CO2 (Marty, 1984; Kotsyurbenko et al., 2001).

D. Methanogenesis

Methanogens differ significantly from other bacteria. The most significant characteristic of the methanogen group is their ability to generate methane under strict anaerobic conditions (Lange and Adring, 2001).

In the anaerobic digester environment, there are three substrates which are important to the methanogenic population; acetic acid (which yields approx. 70% of the methane produced), hydrogen and CO2 from which the remaining methane is formed. The bacteria responsible for methane production can be sub-divided into the groups based on their substrate preference:

Acetoclastic methanogens

Hydrogen utilising methanogens (Polprasert, 1996).

The acetoclastic bacteria convert acetate to methane and CO2:

(CH3 COOH CH4 + CO2 ). Acetoclastic bacteria have a very slow generation time (µ max ≈0.04 hr) in comparison to the acidogenic population (µ max ≈1 hr) (Bitton, 1994). This is the reason that, during process instability, volatile fatty acids (VFA) may accumulate, which inhibits the activity of subsequent synergistic bacteria, resulting in pH decline. The pH does not affect the acidogenic bacteria themselves, however, until it drops below 4.5 (Marty, 1984).

The hydrogen utilising methanogens convert hydrogen and CO2 to methane as follows: CO2 + 4H2 CH4 + 2H20. In doing this, hydrogen partial pressure within the reactor is kept low, aiding in the conversion of VFA and alcohols to acetate, by the acetogenic bacteria (Bitton, 1994).

Parameters for Optimum Digestion

Temperature

Bacteria are classified by three ranges of temperature at which they can grow. Generally, different strains of bacteria grow in the different temperature ranges. The ranges for growth are:

Below 0 oC to 15 oC (Psychrophilic)

From 15 oC to 45 oC (Mesophilic)

From 45 oC. to 70 oC (Thermophilic)

The rate of digestion is temperature dependant. In the temperature range below 45 oC digestion becomes slower as temperature decreases, and mesophilic digestion virtually ceases at about 15 oC. However, the bacteria are not inactivated and digestion will take place if the bacteria are allowed sufficient time to acclimatise to environmental conditions. Above 50 oC, in the thermophilic temperature range (65 oC to 70 oC), digestion takes place at an increased rate, as substrate utilisation is increased. To date, the majority of research has focused on mesophilic digestion. It is accepted, however, that the bacteria concerned in thermophilic digestion are different species than those of a mesophilic digester. In general, the optimum temperature for mesophilic digestion is about 35 oC and for thermophilic digestion 55 to 60 oC (Hobson and Wheatley, 1993).

Pre-treatment of feedstock

While all organic compounds are biodegradable, the initial stages of anaerobic digestion are rate-limiting i.e. hydrolysis and solubilisation. However, there are a range of pre-treatment applications that can significantly increase the rate of biodegradation by physically, chemically or mechanically altering the microbial cell (cell lysis).

In the case of food waste digestion mechanical disintegration is typically employed to reduce the particle size to 12mm or below.

Potentially Toxic Compounds

Hydrogen ion concentration, and several other compounds such as heavy metals and chloro-organic compounds, affect the rate of anaerobic digestion, even at very low concentrations.

Other Potentially toxic compounds that might be present are oxygen and sulphide. Some oxygen may be introduced in the incoming feed stream, but it will be used for oxidative metabolism in the acidogenesis process. Sulphide can be formed in the process due to the reduction of sulphate. (Van Haandel and Lettinga, 1994).

Mixing

Although some natural mixing occurs in an anaerobic digester, because of rising sludge gas bubbles and the thermal convection currents caused by the addition of heat, this level of mixing is not adequate to ensure stable process performance at high loading rates. It is therefore necessary that a mixing system is installed to create an homogeneous environment throughout the reactor.

There are a number of techniques to ensure adequate mixing within the digester, such as gas injection, mechanical mixing, and mechanical pumping (Quasim, 1999).

Gas Injection: there are two main types of gas injectors, described as unconfined and confined. In the unconfined system, gas is collected at the top of the reactor, compressed and then discharged through diffusers situated at the bottom of the reactor or through a series of radially placed top -mounted lances. Mixing is achieved by the releases of gas bubbles which rise to the surface, carrying and moving the sludge. In the confined system, while gas is again collected at the top of the reactor and compressed, it is then discharged in the digester through confined tubes. There are two main types of confined system, depending on the type of gas distribution system required, The Gas Lifter System and The Gas Piston System. Both the unconfined and confined gas injection systems are suitable for use in digesters with fixed, floating and gas holder covers.

Mechanical mixing: such mixers are usually installed into a shaft tube to promote vertical mixing. It is necessary to ensure preliminary treatment is in place to prevent fouling of propellers by rags and other transient materials. Low speed turbines are usually employed for mixing. Mechanical mixers are suitable for use in digesters with fixed or floating covers.

External pumped circulation: Sludge is pumped out of the digester and then returned by recirculation. Sludge is usually removed mid way in the digester and can be returned to the base or surface of the reactor to break-up scum. This method requires high-energy input.

Numerous studies have been conducted on the importance of mixing in terms of digester performance, many of which having contradictory outcomes. For example, the importance of mixing was illustrated by Casey (1984), who outlined that inadequate mixing caused two main problems within the reactor:

Floatation of solids, due to biogas bubbles growing on the surface of the digesters solids which can cause buoyancy forces and thus solids form a surface floatation layer.

Settlement of heavier solids forming a bottom deposit layer.

Floatation is deemed to be the most detrimental to the digestion process, since the particles which float are centres of colonisation for methanogens. Casey also noted that significant mixing can occur in a full scale digester naturally without supplementation, due to biogas production and convective currents caused by heating, primarily due to the velocity of gas bubbles as they rise to the top of the reactor, thus mixing intensity is design dependent. Pilot scale digesters do not have the height required to ensure adequate natural mixing and thus inadequate mixing is apparent in such cases. However, Stroot et al., (2001) also noticed contradictory information on the topic of digester mixing, and so conducted a pilot study to determine the implications of variations in mixing regime for the stability of mesophilic anaerobic digesters. The test substrates included: the organic fraction of municipal solid waste, primary sludge, and waste activated sludge (WAS). Six digesters were operated to compare performance under continuous mixing and reduced mixing levels at various loading rates and solids levels. The main outcome of this research was that continuous mixed digesters exhibited unstable performance at higher loading rates (propionate accumulation), while the minimally mixed digesters performed well for all loading rates evaluated. Results also showed that an unstable continuously mixed digester was quickly stabilised by reducing the mixing level.

Adequate Metabolism Time

Kiely (1997), defined hydraulic retention time (HRT) as:Working volume of reactor( L)

Rate of sludge removal (L / D)

And solids retention time (SRT) as: Mass of solids in reactor (kg)

Rate of solids removed (kg/d)

Both parameters are of critical importance to the anaerobic digestion process, as the retention time must be higher than the generation time of the slowest growing microorganism in the system, to prevent wash-out of the active biomass, and must also be long enough to achieve the required degree of volatile solids destruction (Dohanyos and Zabranska, 2001). The methanogens have the slowest generation time, when compared to the other members of the anaerobic digester microbial consortia, ranging from less than two days to more than 20 days at a mesophilic temperature (Malina and Pohland, 1992).

In a CSTR without recycle, the HRT and SRT are equal, ranging from 15 - 30 days typically (Tchobanoglous et al., 2001). At the higher range of retention time, a safety factor against wash-out is permitted and maximum contact time is allowed, facilitating fermentation of slow digesting polysaccharide feedstocks.

Limiting Factors to Digestion Process and Process Control

Process instability may arise due to organic overloading, hydraulic overloading, toxins in the feedstock, and temperature fluctuations, which may result in the accumulation of intermediate by-products (VFA), leading to environmental changes within the digester and a shift in microbial populations. Considering the fact that methanogens have the strictest environmental and nutritional requirements, such instability often leads to a decline in numbers. With inadequate process control, souring of the digester is inevitable. Fortunately, due to the complex nature of the process, which involves a series of interdependent microbial stages of degradation, and due to a good understanding of the process biology; instability, should it occur, can be foreseen by regular analysis of process intermediates and by-products, allowing remedial action to be taken. The analytical parameters chosen may be indirect indicators, such as the concentration of a metabolite in the digester, or a direct status indicator, such as the number of active microorganisms in the system. The most common parameters used to indicate process stability are indirect, including pH, alkalinity, VFA, biogas production and methane yield (Bjornsson et al., 2001; Michaud et al., 2001). However, a number of other parameters are used to evaluate overall removal efficiency, mainly solids analysis and chemical oxygen demand.

pH

pH represents the acidic nature of a liquid or the total concentration of hydrogen ions within the liquid (Fifield & Hains, 2000). It is used extensively as a monitoring tool of the anaerobic digestion process, and was considered the most important parameter by Irish plant operators in a survey conducted in 2000 (Scahill, 2000). Typically, anaerobic digesters operate at a narrow pH of 6.8 - 7.2. This is indicative of proper balance between the material entering and that discharged from the digester (W.P.C.F, 1987). Should the pH decrease below 6.0, the methanogenic bacteria are inhibited, characterised by a decline in methane production and VFA accumulation within the digester (Dohanyos and Zabranska, 2001).

Alkalinity

The alkalinity of a liquid is a measure of its capacity to neutralise acids. There are three major classes of materials that contribute to the alkalinity of a liquid: hydroxide, carbonate, and bicarbonate. In anaerobic environments salts of weak acids such as acetic, propionic, and hydrogen sulphide also contribute to the total alkalinity of the liquid (Sayer, 1999).

Alkalinity within the digester is of major importance during the acid phase of the anaerobic digestion process, to ensure adequate neutralisation or buffering of intermediate acids, ensuring optimum conditions for methanogenic bacteria. Typically, anaerobic digesters may have an alkalinity of 2000 - 6000mg/l as CaCO3. However, should alkalinity fall below this level, a number of steps can be taken to alleviate imbalance. The first is to cease feeding until the pH increases to 6.8 (approx.), this action allows the methanogenic population time to consume the backlog of VFA. Secondly, chemicals may be added to increase the alkalinity, and thus the pH, within the digester artificially. Chemicals such as bicarbonates (which add bicarbonate alkalinity directly) or carbonate salts (which trap CO2 from the gas and convert it to bicarbonate) are utilised. The addition of chemicals in the bicarbonate form is preferred, as precise additions can be achieved, unlike the carbonate salts, which must be added in small steps (to prevent the accumulation of insoluble calcium salts) to allow time for gas equilibrium to occur between each addition (Dohanyos and Zabranska, 2001).

Volatile Fatty Acids (VFA)

VFA such as: propionate, butyrate, iosobutyrate and acetate, are formed as intermediates during the anaerobic degradation of carbohydrates, proteins, and fats. Excess VFA can be inhibitory to the digestion process and must be managed. Typically, VFA concentrations ranging from 50 - 250 mg/l as acetic acid indicates a satisfactory balance between the methanogenic and acidogenic bacteria. However, should inhibition of methanogens occur, due to operational or environmental changes, a decrease in the rate of VFA destruction may occur, leading to accumulation within the system and a corresponding reduction in pH. Under conditions of imbalance, VFA's may reach concentrations of 2000 - 6000 mg/l and will not decrease until a neutralisation agent is added to increase pH to the required level (Sayer, 1999).

In terms of methanogenic bacteria, metabolism of short chain fatty acids are of vital importance. Acetate, for example, yields to approx. 75% of the methane produced during digestion. Propionate and butyrate are important VFA's also, not only for the reason that they may be further converted into acetate and hydrogen, ultimately yielding methane, but also because the accumulation of these intermediate acids in the undissociated form retards the growth of several microbial species, which may cause a subsequent decrease in methane production (Aguilar, 1995).

Propionic acid in particular has been proposed as a valuable indicator of process performance. It has been noted to accumulate within the digester when the process has been subjected to shock loading, overloading, or during start-up (Gujer, 1983). A study conducted by Inanc et al., (1996), proposed that propionic acid accumulation during process instability is due to shifts in the acidogenic bacterial population and end product distribution. Following pilot anaerobic digestion trials, microscopic examination of anaerobic biomass showed the bacteria present to be gram-positive rods, during periods of butyric acid accumulation, while during periods of propionic acid accumulation the bacteria were gram-negative rods. The study also suggested that the two- phase anaerobic digester configuration, where the acidogenic reactor is operated at a pH of 5 or less, could prevent propionic acid accumulation, as the propionic acid producing bacterial species were inhibited during pilot trials at pH 5.

COD (Chemical Oxygen Demand) and its Equivalence of Methane

COD is a measure of the amount of oxygen required to chemically oxidise organic matter in a sample. The amount of oxygen is measured directly in mg/l as the oxygen equivalent using a strong chemical oxidant. COD is analysed in 2 separate components, Total COD, which includes soluble and colloidal matter and Soluble COD consisting of the soluble fraction of anaerobic effluents which contains residual degradable and non or slowly degradable influent substrate, and intermediates products, such as; VFA, (Barker et al., 1999). Soluble COD in itself is an important parameter, as it discloses information in regard to the extent of hydrolysis and solubilisation carried out by the acidogenic bacteria. (Maharaj and Elefsiniotis, 2001)

The quantity of methane produced per gram of COD removed can be easily determined for mass balance estimations. The COD equivalent of methane is:

CH4 + 2 O2 CO2 + 2H2O

From the above equation, it can be determined that for each mole of methane consumed (22.4 L @ 0 oC), two moles of oxygen equivalent are destroyed (64g). Therefore, 0.35 L (22.4 L / 64g) of CH4 at 0 oC and 760 mm HG pressure (STP) is equivalent to 1 g COD destruction. At a mesophilic temperature however, the CH4 equivalence is 0.395 L at 35 oC and one atmosphere {5.6 ft. 3@ 0 oC and 6.3 ft. 3@ 35 oC of methane is produced / pound COD destroyed} (Speece, 1996; Michaud et al., 2001; Tchobanoglous et al., 2001).

Solids

Total solids refers to the material residue left in a vessel following evaporation and drying of the sample in an oven at the designated temperature (APHA et al., 1995).

The solids portion is further classified by ignition of the sample at 550 ï‚°C +/- 50 ï‚°C. The ignition process causes oxidation of the organic fraction of the solid matter, and is thus driven off by the extreme temperature (Volatile Solids). The ash remaining in the vessel represents inorganic matter or Fixed Solids (Tchobanoglous and Barton, 1991).

The extent of digestion is often measured by volatile solids reduction. Anaerobic digestion can achieve Volatile Solids destruction of 40 - 60% and an overall destruction in total sludge volume of 25 - 30 % (approx.) (Quasim, 1999).

Biogas Quantity and Quality

Digester gas analysis can provide valuable data on the process efficiency. Biogas composition depends largely on the raw material, organic loading and, time and temperature of decomposition.

Monitoring of the biogas quality can be used as a measure of digestion efficiency, as regular monitoring conveys deviations from typically obtained values of individual gas components, allowing the operator to take remedial action as soon as unstable conditions are noticed. (Polprasert, 1996).

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