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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Anaerobic Digestion PDF Print E-mail

 

1. Introduction to anaerobic digestion process
2. Biology of anaerobic digestion
3. Feedstock of anaerobic digestion
4. Procedures of Anaerobic Waste Fermentation
5. Process Engineering of Anaerobic Fermentation of Biowaste
6. Anaerobic Digestion Products
7. Market potential for products
8. Mass and energy balances
9. Parameters effecting anaerobic digestion process
10. Environmental impacts
11. Economic data
12. Applicability
References

 

1. Introduction to anaerobic digestion process

Anaerobic digestion (Figure 1) is defined as the biological process during which the organic material is decomposed by anaerobic microorganisms in the absence of dissolved oxygen (i.e. anaerobic conditions). Anaerobic microorganisms digest the input organic material which is converted through anaerobic degradation into a more stabilized form, while a high energy gas mixture (biogas) consisting mainly of methane (CH4) and carbon dioxide (CO2), is generated. Biogas is collected and utilized as a source of energy, since it can be combusted in a cogeneration unit and produce green energy. Apart from CO2, CH4 is also considered as a gas which contributes significantly to the greenhouse effect and hence to global climate change. The organic material can originate from industrial or municipal waste, agricultural residues or sludge generated from wastewater treatment plants (Pavlostathis and Giraldo-Gomez, 1991).

Figure 1: Anaerobic digestion flow chart (Wastesum, 2006)

2. Biology of anaerobic digestion

One of the key factors in the success of microbial-mediated processes is an adequate understanding of process microbiology, more specifically the study of microorganisms involved in organic waste decomposition and the subsequent by-product formation. The anaerobic fermentation process is much more complex than composting due to the involvement of a diverse group of microorganisms and a series of interdependent metabolic stages which demand meticulous process control for stable operation. The anaerobic digestion of organic material is accomplished by a consortium of microorganisms (bacteria) working synergistically in the absence of oxygen. These microorganisms use up the initial feedstock as an energy and biomass source through various biological and chemical reactions transforming the input organic matter to intermediate molecules such as sugars, hydrogen and acetic acid before finally being converted to biogas. The anaerobic digestion process can be generally classified into four distinct stages which are related to the biological and chemical phases of anaerobic treatement of biodegradable organic waste as shown in Figure 9:

 

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

 

Generally the organic input material is composed of large macromolecules which need to be broken down into smaller chemical components so that the anaerobic bacteria will be able to access the energy potential of the substrate. Hydrolysis is the first stage of the anaerobic digestion process in which complex and large organic polymers are decomposed and dissolved to constituent monomers (Ostrem, 2004). Therefore the hydrolysis stage involves the breakdown of complex organic molecules such as polysaccharides, proteins, and lipids into simple compounds namely sugars, amino acids, and fatty acids by extracellular enzymes (e.g. cellulase, protease and lipase) and then to soluble products of small enough size to allow their transport across the cell membrane. Hydrolysis can be a rate-limiting step in the overall anaerobic treatment processes for waste containing lipids and/or a significant amount of particulate matter (e.g., sewage sludge, animal manure, and food waste) (Henze and Harremoes 1983; van Haandel and Lettinga 1994).

In the hydrolysis phase, acetate and hydrogen are produced  which can be directly taken up by methanogens. Additionally, chemical compounds such as volatile fatty acids which acquire  greater chain length than acetate (e.g. propionic, formic, lactic, butyric, or succinic acids) must be further broken down into constituents that can be used by methanogenic microorganisms (Khanal, 2008). The biological process in which the products resulting from hydrolysis are further breaking down is called acidogenesis and it takes place by acidogenic bacteria. Volatile fatty acids are generated along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products. The specific concentrations of products formed in this stage vary with the type of bacteria as well as with culture conditions, such as temperature and pH (United Tech 2003). The third stage of the anaerobic digestion is the acetogenesis. During this stage, simple molecules which have been produced through the acidogenesis stage (e.g. acetate) are further degraded by acetogens to produce mainly acetic acid as well as carbon dioxide and hydrogen (Khanal, 2008). The final stage of anaerobic digestion of the biodegradable organic waste is the methanogenesis phase during which methanogenic bacteria use the intermediate products resulting from the preceding stages for the production of methane, carbon dioxide and water. Methanogens are sensitive to changes and prefer a neutral to slightly alkaline (between pH 6.5 and pH 8) (Wastesum, 2006). The remaining non-digestable organic material, which the anaerobic microbes cannot decompose along with the dead bacterial, constitutes the digestate.

 

Figure 2: The stages of anaerobic digestion (Wastesum, 2006)

3. Feedstock of anaerobic digestion

The feedstock material that is used in anaerobic digestion constitutes the most important initial parameter when considering the application of anaerobic treatment. Feedstock include any substrate that can be converted to methane by anaerobic bacteria thereby it can range from readily degradable organic waste (e.g. wastewater) to complex high-solid waste. Anaerobic digesters typically can accept any biodegradable material, but the level of biodegradability is the key factor for its successful application. Anaerobic microorganisms can dissolve organic matter to varying degrees of success. More specifically sugars which are short chain hydrocarbons can be used readily whereas the decomposition process of cellulose and hemicellulose compounds is significantly longer. Anaerobic microorganisms are unable to break down long chain woody molecules such as lignin (Wastesum, 2006). Therefore it can be inferred that the characteristics of the input material determine in a large extent the methane yield and production rates within the anaerobic digesters. In order to improve the methane potential in anaerobic digestion several techniques are being applied by determining various characteristics of the organic feedstock.  Additional variables such as solids content as well as elemental and organic analyses are considered as useful methods in the design and the operation of anaerobic digesters.

Another parameter that needs to be considered when it comes to anaerobic treatment is the moisture content of the feedstock material. Generally, the higher the moisture content of feedstock the easier its handling and conveyance since standard pumps can be applied instead of concrete pumps and physical means of movement which are more energy demanding practices. However, the increased moisture content of the organic input material results in higher volume and area that is required in comparison to the levels of biogas that are generated. Therefore, the type of anaerobic system that will be developed and applied is depended on the moisture level of the input material different. Another key consideration in anaerobic digestion is the C/N ratio of the initial substrate that is subjected to anaerobic decomposition. The C/N ratio represents the relationship between the amount of carbon and nitrogen present in the organic material thus regulating the nutrients take up of microbes and balancing their growth. Optimum C/N ratios in anaerobic digesters are between 20-30 (Verma, 2002). A high C/N ratio is an indication of rapid consumption of nitrogen by methanogens and results in lower gas production. On the other hand, a lower C/N ratio causes ammonia accumulation and pH values exceeding 8.5, which is toxic to methanogenic bacteria (Verma, 2002).

The impurities level of the organic input material is another key parameter when considering the deployment of anaerobic treatment. In cases when the substrate acquires increased quantities of impurities such as plastic, glass or metals, then a pre-treatment stage is required in order to increase the purity level of the feedstock and to prevent potential malfunctions and inefficiencies of the anaerobic digesters processes (Wastesum, 2006).

 

The feedstock for an anaerobic digestion plant can be organic waste that has been separately collected and delivered to the plant ready for processing or, municipal solid waste or its fraction from a mechanical sorting plant in which the other fraction is the refuse-derived fuel. A further source of organic waste is ‘green wastes’ collected at centralized collection points. At least the purity of the raw material fed into the anaerobic digestion process dictates the quality of the product coming out at the end of the process. The range of application of the anaerobic digestion process is very broad. In principle, any organic material can be digested a list of which is given below (Rilling, 1994):

  • organic municipal solid waste
  • waste from central markets (e.g. fruit, vegetable and flower residuals)
  • slaughterhouse waste (paunch manure)
  • residues from the fish processing industry
  • food waste from hotels, restaurants, and canteens
  • bleaching soil
  • drift materials such as seaweed or algae
  • agricultural waste
  • manure
  • beer draff
  • fruit or wine marc
  • sewage sludge.

 

However, the treatment method for each waste stream might be depended on its moisture level as shown in Figure 3, since composting is widely used for waste containing high amounts of dry matter, whereas anaerobic digestion has turned out to be a good alternative for treating wet organic waste.

 

Figure 3: Suitability of waste for aerobic composting and anaerobic digestion (Kern et al., 1996)

 

4. Procedures of Anaerobic Waste Fermentation

The anaerobic treatment of organic waste generally follows specific steps as presented below (Rilling, 1994):

 

  1. delivery and storage of biodegradable organic waste
  2. preprocessing of the incoming biodegradable organic waste
  3. anaerobic fermentation
  4. storage and treatment of the digester gas
  5. treatment of the process water
  6. post-processing of the digested material.

 

Figure 4 shows the possible treatment phases carried out in anaerobic digestion process. Typically, all fermentation processes can be described as a combination of a selection of these treatment phases. The process technology demanded for the implementation of the different phases of the treatment varies significantly and depends on the anaerobic process chosen. In general, the gas production increases and the detention time decreases with increasing energy input for preparation of the material and the fermentation itself (mesophilic/thermophilic).

 

Figure 4: Possible treatment steps used in anaerobic digestion process of biodegradable organic waste (Rilling, 1994)

 

4.1. Delivery and Storage

The organic substrates are quantitatively and qualitatively recorded by weighing, are visually inspected at an acceptance station, and are unloaded into a flat or deep bunker or a collecting tank that serves as a short-term intermediate storage place and permits continuous feeding to the subsequent pretreatment plant.

4.2. Pre-processing

In the pretreatment stage pollutants and inert material are removed from the organic material, whereas the substrate is homogenized and conditioned. The pretreatment type depends on the specific system of the anaerobic fermentation process. More specifically, dry fermentation processes use dry preprocessing, in which sieves, grinders, shredders, metal separators, homogenization drums, ballistic separators, and hand sorting sections can be combined. On the other hand, in wet fermentation processes the organic substrate is additionally mixed with water, homogenized, and shredded. Organic material that has larger surface area is more easily broken down by the bacteria.

4.3. Anaerobic Fermentation

Once the pre-processing procedure has been elaborated any recyclable or unwanted materials is separated from the incoming waste, whereas the organic material is shredded and supplied to the digester. In case when organic waste with high water content, e.g. sewage sludge, is used as raw material the addition water is not required, whereas for dry substrates, e.g. household organic waste, water is usually necessary to be added in order to dilute the solids. Waste with low structure and high water content are best for wet fermentation. On the other hand, substrates with high structural strength can also be anaerobically decomposed through dry fermentation processes (RISE-AT, 1998). For anaerobic fermentation to take place heat is needed to be adjusted to the required process temperatures to about 35°C (mesophilic operation mode) or 50-55°C (thermophilic operation mode), and in some cases water addition is prerequisite. During substrate digestion the decomposition of the organic matter is carried out in the absence of oxygen i.e. anaerobically, in closed, temperature-regulated bioreactors. Depending on the process operation, the material consistency may vary between well-structured and fluid suspension organic matter. The output of the anaerobic digester is a wet, organically stabilized residue (digestate) and biogas. After dewatering of the digestate, a compost like material can be obtained by aerobic post-treatment. In addition, the wastewater produced during draining can be partly recirculated into the pre-processing stage to adjust the water content of the initial substrate, whereas surplus wastewater has to be treated accordingly (e.g. purified in specially designed purification ponds) and discharged. Biogas, which constitutes the main product of anaerobic, is used as an energy source. Biogas is generally used in decentralized fuel-burning power stations for the production of electricity and heat in order to cover the energy requirements of the fermentation process and thus enabling the system to operate in an energy-neutral manner. The excess energy is marketed by supplying the public heat power needs. In cases when only easily degradable organic waste components are used in the anaerobic digestion, the energy is produced with minimal technical expenditure, whereas the energy-intensive pretreatment stages can be omitted (RISE-AT, 1998).

4.4. Post-processing

The solid by-product of the anaerobic digestion has to be stabilized, sanitized and refined prior to its application for agricultural or horticultural uses. Typically, after dewatering and/or drying, the digestate is composted and matured in order to become a good quality, marketable compost. The biogas, after drying and, if required, purification, can be used as an energy source.

5. Process Engineering of Anaerobic Fermentation of Biowaste

In contrast to the commonly established aerobic treatment processes, the anaerobic fermentation method of biowaste is relatively young and dynamic. With great scientific expenditure, process developments and optimizations are being pursued, so it may be assumed that the technological potential of biowaste fermentation has not yet been fully exhausted. Anaerobic digestion is generally suitable for the biological treatment of readily degradable substances which acquire low structure and high water content (e.g. kitchen waste). The anaerobic fermentation processes of organic solid waste differ in number depending on (i) the biodegradation stages (one or two-stage), (ii) separation of liquid and solids (one or two-phase), (iii) water content (dry or wet fermentation), (iv) feed method (continuous or discontinuous), and (v) agitation method (Rilling, 1994). The most important characteristics of anaerobic digestion are presented in Table 7.

Table 7: Characteristics of anaerobic waste treatments (Rilling, 1994)

Stages of biodegradation

One-stage

Two-stage

Separation of liquid and solids

One-phase dry fermentation

Two-phase wet fermentation

Total solids content

25%-45%

<15%

Water content

55%-75%

>85%

Feed method

Discontinuous

Continuous

Agitation

None

Stirring, mixing, percolation

Temperature

Mesophilic (30-37oC)

Thermophilic (55-65oC)

 

As stated above the anaerobic fermentation of biowaste can be operated by one-stage or two-stage fermentation. In the one-stage process (Table 8) all fermentation stages (e.g. hydrolysis, acidification, acidification and methanogenesis) take place in one reactor; therefore, optimum reaction conditions for the overall process are not achieved, due to the different environmental requirements during the various stages of the fermentation. Therefore, the degradation rate is reduced and consequently the retention time increases. The basic advantage of one stage process operation is the relatively simple technical installation and operation of the anaerobic digestion plant, whereas the costs are lower. In two-stage processes (Table 8), the hydrolysis and acidification- acidification take place in one bioreactor, while methanogenesis is carried out in a separate reactors thus providing flexibility to optimize each of these reactions so that e.g. mixing and adjustment of the pH can be optimized separately, permitting higher degradation degrees and loading rates. In two-stage processes the retention time of the substrate is significantly decreased. However, such systems involve more sophisticated technical design and operation and subsequently higher costs.

 

In the first reactor, organic fraction is hydrolyzed producing dissolved organics, organic acids, CO2 and low concentrations of hydrogen. The reaction rate in the first reactor is limited by the rate of hydrolysis of cellulose. In the second stage the highly concentrated water is supplied to an anaerobic fixed-film reactor, sludge blanket reactor, or other appropriate system where methane and CO2 are produced as final products. In the second reactor the rate of reaction is limited by microbial growth (Verma, 2002).

 

Table 8: Comparison of one- and two-stage processes

Process Operation

One-stage

Two-stage

Operational reliability

In the same range

Technical equipment

Relatively simple

Very complex

Process control

Compromise solution

Optimal

Risk of process instability

High

Minimal

Retention time

Long

Short

Degradation rate

Reduced

Increased

 

5.1. Dry and Wet Fermentation

According to the moisture level of the substrate the anaerobic digestion systems can be classified as dry fermentation processes or “high-solids systems” (dry solids content >15-20%) and wet fermentation processes “low-solids systems” (dry solids content <15%) (Jördening and Winter, 2005). However, there is no established standard for the cutoff point. Table 9 shows advantages and disadvantages of dry and wet fermentation. With the dry fermentation process, little or no water is added to the biowaste. As a consequence, the material streams to be treated are minimized. The resulting advantages are smaller reactor volumes and easier dewatering of the digestate thus less costly reactors. Operating with high solid content places higher requirements on mechanical pretreatment and conveyance (e.g. pumping denser material), on the gas-tightness of charge and discharge equipment and on mixing the substrate. Because of the low mobility in dry fermentation, a defined residence time can be reached by approximating plug flow, which is particularly important for the sanitization of the solid product in the thermophilic operation process. The degradation rates in dry fermentation processes are lower than in wet fermentation, due to the larger particle size and reduced substrate surface availability (Jördening and Winter, 2005).

 

When wet fermentation processes are used, the organic waste is ground to a small particle size and appropriate amount of water is added so that sludges or suspensions are formed. This enables the use of simple, established mechanical conveyance techniques (pumping) and the removal of interfering substances by sink–float separation (Jördening and Winter, 2005). At the same time, the organic substrate can be easily mixed within the bioreactor, allowing controlled degassing and defined concentration equalization in the digester which in turn optimizes the degradation performance of the microorganisms. The mean substrate concentrations and thus also the related degradation rates are lower than in plug flow systems, since for completely mixed systems the concentrations in the system are equal to the outlet concentrations. Mixing is limited by the shear-sensitivity of methanogens, however, mixing at a lower degree may result in floating and sinking layers. Homogeneity and a fluid consistency permit easier process control. However, by fluidizing biowaste, the treated mass increases (depending on the required total solids content of the substrate) with the consequence that the aggregates and reactors have to be made much larger. Fluidization and dewatering of the fermentation suspension are costly procedures, since they require considerable technical and energetic expenditures. However, if the degrees of degradation are the same, recycling the liquid phase from the dewatering step to the fluidization of the initial substrate, makes it possible to reduce the wastewater quantity to an amount comparable to that used in dry fermentation and to keep a considerable part of the required thermal energy within the system (Jördening and Winter, 2005).

Table 9: Comparison of wet and dry fermentations

(Jördening and Winter, 2005).

Process Mode

Dry

Wet

Total solids content

High 25-45%

Low 2-15%

Reactor volume

Minimized

Increased

Conveyance technique

Expensive

Simple

Agitation

Difficult

Easy

Scumming

Little risk

High risk

Short circuit flow

Little risk

High risk

Solid-liquid separation

Simple

Expensive

Variety of waste components

Small

Great

 

5.2. Continuous and Discontinuous Operation

In case when the anaerobic digestion process is in continuous operation mode, the bioreactor is fed and discharged regularly. Completely mixed and plug flow systems are operating in that mode during which sufficient substrate is fed into the reactor to replace the putrefied material as it is discharged. Therefore, in such systems the substrate must be flowable and uniform to allow its unobstructed movement, whereas steady provision of nutrients in the form of raw biodegradable waste enables stable process operation and constant biogas yield. Depending on the bioreactor type, design and the means of substrate mixing, short circuits may occur. In such occasions the retention time cannot be guaranteed for the whole substrate in completely mixed systems. In the discontinuous-batch operation mode, the digester is completely filled with raw organic material mixed with digestate provided by another bioreactor and then discharged after a specified retention time. Batch mode bioreactors are easier to design with a relative lower cost than plug and flow systems, while they are suitable for dry as well as for wet fermentation (Table 10) (Jördening and Winter, 2005).

Table 10: Comparison of continuous and discontinuous feed

Process Operation

Continuous

Discontinuous

Retention time

Shorter

Longer

Technical equipment

Complex

Simple

 

5.3. Thermophilic and Mesophilic Operation

The optimal process temperatures for methane fermentation are in the mesophilic temperature range (about 35oC) and in the thermophilic temperature range (about 50-55oC) (RISE-AT, 1998). Bioreactors designed to operate on mesophilic levels are heated to 30 to 40oC. This type of systems acquires high stability process, while small temperature deviations have minor effect on mesophilic bacteria. This is attributed to the fact that a broad spectrum of mesophilic methane bacteria exist that show low sensitivity to temperature variation. The main advantage of mesophilic process operation is the lower amount of energy (e.g. heat) required to be supplied and the subsequent higher net energy production (RISE-AT, 1998). On the other hand, thermophilic operation requires temperatures between 50 and 60oC. Under certain conditions the thermophilic process operation enables higher substrate decomposition rates with subsequent lower retention times. However, this operation type requires larger amounts of energy to maintain the process temperature and thus higher energy expenditure. Therefore, the net energy production is lower than mesophilic operation, whereas the temperature sensitivity of the thermophilic microorganisms reduces the process stability. In addition, under thermophilic conditions the sanitization of the substrate might be achieved for a fixed retention time; otherwise, sanitation has to be achieved in a separate treatment step or by composting (RISE-AT, 1998). Table 11 lists the advantages and disadvantages of mesophilic and thermophilic process operations.

 

Table 11: Comparison of mesophilic and thermophilic process operation (Jördening and Winter, 2005)

Process Operation

Mesophilic (35oC)

Thermophilic

Process stability

Higher

Lower

Temperature sensitivity

Low

High

Energy demand

Low

High

Degradation rate

Decreased

Increased

Detention time

Longer or the same

Shorter or the same

Sanitation

No

Possible

 

5.4. Agitation

For a high degradation activity of the bacteria, it is necessary to provide the microorganisms with sufficient degradable substrate, whereas the metabolic products of the organisms have to be removed (Dauber, 1993). The aforementioned requirements can be met by mechanical mixing or other agitation of the bioreactor’s substrate. Another way in achieving the required mixing of the organic matter is to install a water recirculation system, by which the process water, which ensures nutrient provision and the removal of metabolic products, trickles through the biowaste in the reactor (Rilling and Stegmann, 1992). Other processes use compressed biogas for total or partial mixing of the material.

6. Anaerobic Digestion Products

The main products resulting from the anaerobic digestion process are the biogas, the solid end product (digestate) and water.

6.1. Biogas

Biogas is a mixture of various gases. Independent of the fermentation temperature, a biogas is produced which consists of 60%–70% methane and 30%–40% carbon dioxide, whereas trace components of ammonia (NH3) and hydrogen sulfide (H2S) can be detected. However, the yield biogas depends on several factors such as temperature, pH and alkalinity, hydraulic and organic loading rates, toxic compounds, substrate type, and total solids (TS)/volatile solids (VS) content (Pavlostathis and Giraldo-Gomez, 1991). The caloric value of the biogas is about 5.5–6.0 kWh m–3 which corresponds to about 0.5 L of diesel oil. According to Symons and Buswell (1933) the yield and composition of the biogas can be estimated from the following equation, when the chemical composition of the substrate is known

 

 

Table 12 shows the mean composition and specific quantity of biogas as dependent on the kind of degraded substances. For anaerobic digestion of the organic fraction of municipal solid waste, an average biogas yield of 100 m3 t–1 wet biowaste and having a methane content of about 60% by volume may be assumed. The highest yield of methane is accomplished after the bacterial population has reached its peak and it begins to decrease due to the gradual depletion of the organic load.

 

Table 12: Mean composition and specific yields of biogas in relation to the kind of substances degraded (Rilling, 1994)

Substance

Gas Yield

(m3kg-1TS)

CH4 Methane content (Vol. %)

CO2 Carbon Dioxide Content

(Vol. %)

Carbohydrates

0.79

50

50

Fats

1.27

68

32

Proteins

0.70

71

29

Municipal Solid Waste (MSW)

0.1-0.2

55-65

35-45

Biowaste

0.2-0.3

55-65

35-45

Sewage Sludge

0.2-0.4

60-70

30-40

Manure

0.1-0.3

60-65

35-40

 

The biogas is generally stored in an inflatable bubbles located on top of the system while in other cases the gas is  collected  and stored in biogas holders near to the facility.

 

6.1.1. Electricity Supply

 

The mode of operation of a gas engine depends on type of its use e.g. covers peak load, covers basic load, supplies its own needs and only feeds the surplus into the network. The electricity supply mode is determined by the local conditions as well as the price of electricity. Different plant designs are needed for covering a constant basic load and for covering peak loads for certain periods of the day. Peak load covering requires complex and expensive gasholders for longer periods and larger and more expensive power stations. The worldwide ongoing system of promoting renewable energy, as from biogas, does not especially consider whether the power is generated for basic or for peak load and at what time of day the current is fed into the network. Therefore, biogas plants are normally designed to cover the basic load, although the produced power depends on the activity of the microorganism and, as a result, varies. Biogas plants are usually constructed at places, where the power network is not available and special efforts are required to connect the central heat and power to the public power network (Deublin and Steinhouser, 2008)

 

6.1.2. Heat Supply

 

Generally, the economics of biogas industry largely depends on the utilization and exploitation of the generated heat from biogas combustion. It must be borne in mind that the heat is produced over the whole year and not only in the winter, when it can be easily used. The heat could be used for the following purposes (Jördening and Winter, 2005):

  • heating swimming pools and/or industrial plants
  • heating stables
  • heating greenhouses
  • cleaning and disinfection of the milking equipment
  • transformation of warmth in cold e.g. for milk cooling.

6.2. Digestate

Digestate is the solid end product of the process and contains organic compounds which are not susceptible to anaerobic microorganisms attack (e.g. lignin). It also consists of the inorganic remains of the dead bacteria population that have been developed during the anaerobic treatment (Wastesum, 2006). Digestate is produced in three different physical states depending on the type of feedstock material and digestion process. The three forms are  namely fibrous, liquor or a sludge-based combination of the two aforementioned fractions. More specifically in the case where two-stage anaerobic system is applied, fibrous and liquor digestates are produced from the different digestion tanks. In single stage digestion systems the two digestate fractions are being combined and need to be further processed in case when separation of the two forms is required.

When the digestion is complete, the residue slurry is removed and dewatered to produce a liquid stream and a drier solid. The water content is filtered out and re-circulated to the digester, and the filter cake is cured aerobically, to form compost. The final product is screened for any undesirable materials, (such as glass shards, plastic pieces etc) before being used on the land and sold as organic soil amendment to condition and improve soil (Ostrem, 2004). It must be noted that the produced digestate may contain ammonia a compound which is potential phytotoxic to plants while testing for pathogenic microorganisms shall be provided especially in case where the time temperature regime in not sufficient for their inactivation. However, pathogen destruction can be guaranteed at thermophilic temperatures with a high SRT (Ostrem, 2004). Therefore, the digestate is generally composted after the digestion in order to produce high quality end product. It must be stated that anaerobic digestion does not reduce nutrient content (NPK value), making the digestate more valuable as a fertilizer (Mahony and O'Flaherty, 2002).

 

6.3. Wastewater

 

This wastewater resulting from the anaerobic digestion process comes  from the moisture content of the produced digestate that is treated (e.g. dewatered) as well as the water that is being produced during the biological reactions within the digesters.. The wastewater that is being collected is generally recirculated to the system aiming to adjust and regulate the water content of the feedstock material whereas its excess is treated accordingly. Typically, wastewater contains high levels of organic load which is mainly not biodegradable and often it is required to be further processed (Wastesum, 2006).

7. Market potential for products

The methane in biogas is utilized as a renewable energy source to co-generate heat and electricity, using generally a reciprocating engine and/or microturbine. The energy generated (electricity & heat) is used to supply the energy requirement of the system in order to operate in an energy sufficient and energy neutral manner whereas the excess electricity can be either provided the local grid or sold to potential suppliers . The biogas that is produced during the anaerobic treatment of organic waste is considered to be biogenic meaning that it does not contribute to increasing atmospheric carbon dioxide concentrations since it is not released directly into the atmosphere while the emitted carbon dioxide originates from organic sources with a short carbon cycle.

Biogas may require treatment to refine it in order to use it as an energy source. Hydrogen sulfide is among the main chemical components that need to be removed from biogas since it constitutes a toxic product and it is released as a trace component of the biogas. Environmental legislation puts stringent limits on hydrogen sulfide concentration of biogases. Therefore, gas scrubbing and cleaning is required when the levels of hydrogen sulfide in the gas are high. The primary challenge associated with the use of biogas as a fuel is the need for gas cleaning to ensure that the gas meets the quality requirements for the utilization equipment. Biogas cleaning is a capital-intensive, multistage operation that can also carry high maintenance costs due to media replacements and/or power costs. However, if the gas impurities are left untreated, they can increase the maintenance requirements of the equipment fueled by the gas and thus reducing equipment duration. Therefore, gas cleaning to reduce condensation, lower H2S levels, and removal siloxanes is a prerequisite for effective gas utilization. Any foam and sediments entrained in the gas stream are separated using a foam separator in the digester gas piping, while for scrubbing H2S from biogas the most commonly used methods include the use of iron sponge or chemical scrubbers and the addition of ferric (Fe3+) salts to the feed. Finally, for removing seloxanes there are two common types of systems (1) low-temperature drying systems and (2) graphite molecular sieve scrubbers.

 

Biogas can be used either for the production of heat only or for the generation of electric power (combined heat and power generation plants). Alternatively, a stirling engine or gas turbine, a micro gas turbine, high - and low - temperature fuel cells, or a combination of a high - temperature fuel cell with a gas turbine can be used. Biogas can also be used to produce steam by which an engine is driven, e.g., in the Organic Rankine Cycle (ORC), the Cheng Cycle, the steam turbine, the steam piston engine, or the steam screw engine (Deublin and Steinhouser, 2008). Another very interesting technology for the utilization of biogas is the steam and gas power station. Figure 5 shows the range of capacities for the power generators which are available on the market as pilot plants or on an industrial scale. The electrical efficiency indicates the ratio of electrical power to the total energy content in the biogas.

Figure 5: Capacity Range of engines in relation to their electrical efficiency

 

The generated current and heat can supply the anaerobic bioreactor itself, the associated buildings, and neighboring industrial companies or houses. The surplus energy can be fed into the public electricity network, and the heat into the network for long - distance heat supply.

The acidogenic digestate is a stable organic material comprised mainly of lignocellulosic compounds, but also of a variety of inorganic elements resulting from the dead bacterial population. The material can be used as compost or to make low grade building products e.g. fibreboard. The methanogenic digestate is rich in nutrients and can be used as a fertilizer dependent on the quality of the material being digested. Levels of potentially toxic elements should be chemically assessed. This will be dependent upon the type and composition of the initial substrate.

8. Mass and energy balances

A typical mass balance of the anaerobic digestion process is shown in Figure 6. From the diagram it can be stated that from 1 tonne of organic fraction of Municipal Solid Waste (OFMSW) that is treated, 120 kg is the produced biogas, 423Kg is the digestate, 437kg is wastewater, while the remaining 10kg is the inert material of the OFMSW that is removed and disposed to the landfill prior to the biological treatment process. It must be mentioned that the mass balance considers a water recirculation of 370kg to the mixing tank, from the dewatering process of the generated solid digestate. The recycled process water aims to adjust/regulate the water to solid ratio of the feedstock, while at the same time saving money and resources during the operation of the anaerobic treatment plant.

Figure 6: Typical mass balance for an anaerobic digestion system

(Ostrem K., 2004)

One tonne of waste produces between 80 and 130 m3 of biogas, depending on the process, as has been reported by several large AD design firms treating MSW (e.g. BTA, Valorga, WAASA, DRANCO, Linde, Kompogas).

The net energy output in anaerobic digestion systems using different raw materials varies depending on transportation distance, means of transportation, conversion techniques and needs for handling of raw materials and digested residues. For Swedish conditions, from a life-cycle perspective, it appears that for transportation distances up to 50 km, the energy needed for running the biogas systems typically corresponds to 30-50 % of the energy content in the produced biogas. All raw materials could be transported more than 150 km, some dry waste streams up to 700 km, before the energy balance turns negative. The higher the water content in the raw material, the more sensitive the net energy output is to the transportation distance. There is great variability in data on biogas yield from different raw materials, thus estimations about biogas yield strongly affect the net energy output. Despite inherent uncertainties, the overall conclusion is that the net energy input in the studied biogas systems normally significantly exceeds the energy output in the form of produced biogas.

Energy input in various handling and transportation operations for bringing different raw materials to the biogas plant and for transportation of digested residues, and the biogas yield from various biomass resources. Values within parentheses indicate interval found in the literature (EUBIA).

Table 13: Energy input and output from various biomass resources, (EUBIA)

 

A typical energy flow diagram of the product of anaerobic digestion is presented in Figure 7 in which the methane content of biogas is 60%[1], whereas the remainder of the gas is predominately CO2, with trace elements of other gases, such as H2S, NH3 and water vapor. According to Figure 7, 100m3 of biogas (60% CH4) acquire an energy content of 560kWh which can give 336kWh of thermal and 224 of electric energy, while the remaining 56.0kWh is attributed to energy losses. The energy generated can be used within the plant for heat or electric supply purposes or to sell the electric energy to the local electric network. Anaerobic digestion plants large enough to produce electricity in a cogeneration unit can be self-sufficient on the power generation from their produced biogas. The low temperatures required for anaerobic digestion (less than 110°F), allow the heat to be supplied entirely from the biogas as well.

Figure 7: Typical energy balance for an anaerobic digestion system (Ostrem K., 2004)

9. Parameters effecting anaerobic digestion process

Αnaerobic fermentation processes are affected by the changes in environmental conditions; therefore, it is important to examine some of the important factors that govern the anaerobic bioconversion process. These include organic loading rate, biomass yield, substrate utilization rate, hydraulic retention time (HRT) and solids retention time (SRT), start-up time, microbiology, environmental factors and reactor configuration. The following sections elaborate on these factors.

9.1. Organic Loading Rate

In anaerobic process the loading parameters are expressed in terms of organic loadings. More specifically, the organic loading rate of solid waste and organic sludge is based on volatile solids (VS), while for wastewater it is expressed in terms of BOD or COD. Conventional environmental engineering practice has been to express digester loadings on a weight to volume basis per unit time (kilograms of VS per day per cubic meter of volume - kg/d/m3). The stability of the anaerobic fermentation and the biogas production rate are dependent upon organic loading rates. In cases where organic loading rate is higher than normal, the digestion process often becomes unbalanced due to the excessive production of volatile acids to inhibitory concentrations. CO2 production under these conditions often causes foaming of the digester and contributes to operating problems. Maintenance of uniform or near uniform loading rates based on frequent or continuous additions of substrate to the digester yields the most consistent digester operation (Deublin and Steinhouser, 2008).

9.2. Biomass Yield

Biomass yield is a quantitative measure of cell growth in a system for a given substrate which is represented by the yield coefficient (Y), given by the following equation (Khanal, 2008).

Y =ΔX/ΔS

 

where

ΔX biomass concentration (mg VSS/L),

ΔS substrate concentration (mg COD/L).

 

The biomass yield per mole of ATP totals 10.5 g volatile suspended solids for both aerobic and anaerobic processes (Henze and Harremoes 1983). With respect to the metabolic processes of microorganism, the total aerobic ATP generation is 38 mol, whereas for anaerobic digestion it is only 4 mol ATP/mol glucose. Therefore, the biomass yield for the anaerobic treatment process is significantly lower compared to the aerobic one. Anaerobic degradation of organic matter is accomplished through a number of metabolic stages in a sequence by several groups of microorganisms working synergistically. The yield coefficients for different biological treatment processes and stages are presented in Table 14. According to the table it can be seen that the yield coefficient of acid-producing bacteria is significantly higher than that of methane-producing bacteria (Henze and Harremoes, 1983), whereas in the aerobic treatment process for biodegradable COD irrespective of the type of substrates the yield coefficient is fairly constant (van Haandel and Lettinga, 1994). In anaerobic digestion treatment process, the yield coefficient depends not only on COD removed but also on the different substrate conditions being metabolized as shown in Table 15. More specifically, carbohydrate and protein compounds have relatively high yield coefficients, since both microorganism groups, acidogens and methanogens, are involved in the decomposition process of the above mentioned substrates to produce methane. Therefore, the yield coefficients of the aforementioned compounds result from the summation individual yield coefficient of acidogenic and methanogenic bacteria. On the other hand, chemical compounds, such as acetate and hydrogen, have lower yield coefficients, since only methanogenic bacteria are involved in the metabolism of these substrates.

Table 14: Biomass yield coefficients for different biological treatment processes and stages (Young and McCarty,1969; Henze and Harremoes, 1983; van Haandel and Lettinga, 1994)

Process

Yield Coefficient (Kg VSS/kg COD)

Acidogenesis

0.15

Methanogenesis

0.03

Overall

0.18

Anaerobic filter (mixed culture) (carbohydrate + protein as substrate)

0.115-0.121

Anaerobic treatment process

0.05-0.15

 

Table 15: Biomass yield coefficients for different types of substrate (Pavlostathis and Giraldo, 1991)

Type of substrate

Yield Coefficient (Y)

(Kg VSS/kg COD)

Carbohydrate

0.350

Proteins

0.250

Fats

0.038

Butyrate

0.058

Propionate

0.037

Acetate

0.032

Hydrogen

0.038

The estimation of the attainable methane production depends on various parameters among which the organic matter composition, the granulation of waste, the proportions of the involved substrates, the level of microbial degradability of the biomass, the relationship between nutrients (e.g. C/N ratio) and the moisture and organic matter content. Additional factors which affect methane yield are related to the anaerobic digestion method employed. Among these factors are the number of stages, the temperature level (i.e. mesophilic, thermophilic), the retention time of the substrate in the bioreactor, the type and frequency of substrate agitation, and the quantity and frequency of the substrate addition. These parameters must be analyzed in a laboratory test as well as in a pilot scale to confirm the obtained results prior to the construction of a production plant. The degradability of the substrates, the biogas yield, the maximum recommendable volume load, the possible and practical substrate mixtures and the changes of the concentrations of certain materials are important if large scale anaerobic digestion plants are to be operated (continuous or batchwise mode) and they can be determined through laboratory test (Reher, 2003). Before a large - scale plant is constructed, the results from the laboratory test should be confirmed in a pilot plant for the preliminary test of the fermentation process. According to Reher (2003) and Tiehm and Neis (2002) a pilot plant should consists of a hydrolyzer, a methane reactor, and a storage tank which should be individually equipped with arrangements for maintaining moderate temperatures, and with filling and cleaning devices. The recommended measurements evaluated and monitored in the laboratory and/or pilot plant tests are the following (Deublin and Steinhouser, 2008):

 

  • Temperature
  • pH value and redox potential
  • dry matter, water content
  • Content of organic dry matter (Loss on Ignition)
  • Degradability as total content of organic acids/acetic acid equivalent and inhibitors
  • Salt content
  • Total content of N, P, K, Mg and S
  • Availability of plant nutrients such as NO3-, NH4+, P2O5, K2O, and Mg
  • Granulation (maximum grain size), gross density
  • Heavy metals (e.g. Pb, Cd, Cr, Cu, Ni, Zn, Hg)
  • Content of short - chain fatty acids, principally acetic acid, propionic acid, butyric acid, and iso - butyric acid
  • C/N ratio.

 

The above mentioned measurements give important information on the biogas yield, the level of nutrients, the extent of decomposition of the biomass which can be provided during the fermentation, the fertilization value of the residue, and also the preferable type, dimensions and mode of operation of the production plant.

9.3. Specific Biological Activity

Specific biological activity indicates the ability of microorganisms/biomass to utilize and metabolize the substrate. According to Khanal (2008) specific biological activity is usually reported as:

 

Specific substrate utilization rate = (kg CODremoval)/(kg VSS·day)

 

The anaerobic digestion process has a substrate utilization rate between 0.75–1.5 kg COD/kg VSS day, which is significantly higher than of composting (Henze and Harremoes, 1983; Khanal, 2008). The reason of this difference is due to the fact that oxygen transfer and diffusion limitation is not an issue in an anaerobic digestion processes as it is in aerobic treatment plants. Additionally, when high concentration of different substrates in close proximity through biomass immobilization or granulation is maintained, a good balance of synergetic relation between acidogens and methanogens can be obtained. Finally the improvement of the understanding of the trace nutrient requirements of methanogens has significantly increased the specific activity of anaerobic systems (Speece, 1983).

9.4. Hydraulic Retention Time and Solids Retention Time

Hydraulic Retention Time (HRT) and Solids Retention Time (SRT) and are two important design parameters in anaerobic digestion treatment processes. The HRT is the ratio of the reactor volume to the flow rate of the influent substrate. Therefore, it is the time that substrate spends in the bioreactor in contact with the biomass (Cecchi et al., 2003). The time required to achieve a given degree of treatment depends on the rate of microbial metabolism and subsequently on the type and composition of input organic material. Waste containing readily available biodegradable compounds such as sugar, require low HRT, whereas complex waste, e.g. lignin organic compounds, is slowly degradable and needs longer HRT for their decomposition.

 

SRT is the average residence time of solids into the reactor and it estimated by the ratio between the content of total solids in the reactor and the solids flow rate extracted from the reactor (Cecchi et al., 2003). Therefore, SRT controls the biomass in the reactor to achieve a given degree of waste stabilization, whereas it determines the permissible organic loading rate in the anaerobic process. If the quantity of biomass extracted from the reactor is equal to the biomass produced in the reactor, then the solids concentration in the reactor, as biomass, will be constant in a given time and it can be said that the reactor is operating in steady-state conditions. SRT is a measure of the biological system’s capability to achieve specific effluent standards as well as to maintain a satisfactory biodegradation rate of pollutants.

 

According to Speece (1996) the HRT is considered in process design especially for complex and slowly degradable organic pollutants, whereas the SRT is a design deciding factor for easily degradable organics. In case of methanogenic bacteria, which are slow-growing microorganisms, special attention shall be given to prevent their washout from the reactor in order to achieve a longer SRT. Elevated HRTs require a bigger reactor volume thus increasing capital expenditure. An early attempt to maintain a long SRT irrespective of HRT was the use of the clarigester or anaerobic contact process, where the anaerobic sludge was allowed to settle in the settling tank and was then returned back to the reactor. A wide variety of high-rate anaerobic reactors have been able to maintain extremely high SRTs due to biomass immobilization or agglomeration. Such systems operate under short HRTs without any fear of biomass washout. The empirical HRTs for different anaerobic systems to achieve the same degree of treatment are presented in Table 16.

 

Table 16: HRTs of anaerobic systems needed to achieve 80% COD removal efficiency at temperature >20C (Van Haandel and Lettinga, 1994)

Anaerobic System

HRT (h)

UASB

5.5

Fluidized/expanded bed

5.5

Anaerobic filter

20

Anaerobic pond a

144 (6 days)

a : BOD removal efficiency.

 

9.5. Start-Up Time

Start-up is the initial operational period during which the process is brought to a point where normal performances of the biological treatment system can be achieved with continuous substrate feeding. Start-up time is important parameter in anaerobic digestion processes due to the slow growth rate of anaerobic microorganisms, especially methanogens, and their susceptibility to changes in environmental factors. Anaerobic treatment systems often need quite a long start-up time, which may weaken their competitiveness with aerobic treatment systems. A start-up time between 2–4 months is generally obtained at a mesophilic operational mode (35◦C), whereas under thermophilic conditions (55◦C) start-up exceeds 12 months because of the high decay rate of biomass (Khanal, 2008). The start-up time could be significantly reduced in case when the exact microbial culture for the waste treated is used as a seed, thus leading to increased generation time of the anaerobic microorganisms. To further reduce start-up time, substrate loading rates and environmental factors such as nutrient availability, pH, temperature and redox potential should be maintained at levels favoured by microbes during the start-up.

9.6. Microbiology

The microbiology of the anaerobic treatment system is much more complex than composting, since digestion involves a sequential multistep process in which a consortium of microorganisms working synergistically degrades the organic matter. The stability of an anaerobic treatment plants constitutes a challenge due to the sensitivity of anaerobic microorganisms, especially methanogenic bacteria, to potential changes of the environmental factors such as pH, temperature, redox, sufficiency of nutrients and trace elements. Special focus should be given to anaerobic digestion operation to maintain suitable for the microorganism conditions, since in case of system failure (e.g. unfavorable environmental condition and/or biomass washout from the reactor) it may take significant time for the system to return to a normal operating conditions due to the slow growth rate of methanogenic bacteria.

9.7. Environmental Factors

It has been pointed out earlier that anaerobic processes are severely affected by the changes in environmental conditions. Anaerobic treatment system is much more susceptible than the aerobic one for the same degree to deviation from the optimum environmental conditions. The successful operation of anaerobic reactors, therefore, demands a meticulous control of environmental factors close to the comfort of the microorganisms involved in the process. The effect of environmental factors on treatment efficiency is usually evaluated by the methane yield because methanogenesis is a rate-limiting step in anaerobic treatment of wastewater. Hence, the major environmental factors are usually governed by the methanogenesis. Brief descriptions of the important environmental factors are outlined here.

 

 

 

9.7.1. Temperature

 

Anaerobic processes, like other biological processes, strongly depend on temperature. There are mainly two temperature ranges that provide optimum digestion conditions for the production of methane – the mesophilic and thermophilic ranges. The anaerobic conversion of organic matter has its highest efficiency at a temperature 35–40◦C for mesophilic conditions and at about 55◦C for the thermophilic conditions (van Haandel and Lettinga 1994). Anaerobic processes, however, can still operate in a temperature range of 10–45◦C without major changes in the microbial ecosystem. Generally, anaerobic treatment processes are more sensitive to temperature changes than the aerobic treatment process. Temperature variations of ±3°C have minor effect on the fermentation (Winter, 1985). In the thermophilic range (between 55 and 65°C), a constant temperature level has to be maintained, since small deviations may cause a drastic reduction of the degradation rates and thus of biogas production. Igoni et al. (2008) and Tchobanoglous et al. (1991) proposed that the optimal temperature ranges are the mesophilic, namely 30–38°C, and the thermophilic 44–57°C (Igoni et al., 2008), respectively. It has been observed that higher temperatures in the thermophilic range reduce the required retention time.

 

9.7.2. pH

 

The optimum operating pH depends upon the anaerobic fermentation stage and subsequently on the associated bacteria namely acid-producing and methane-producing bacteria. During digestion, the two processes of acidification and methanogenesis require different pH levels for optimal process control. More specifically, acidogenic bacteria prefer a pH between 5.5 and 6.5, while methanogenic bacteria prefer a range of 7.8–8.2. In an environment where both cultures coexist (e.g. one stage process), the optimal pH range is 6.8–7.4. A favorable pH range for methanogenic bacteria is between 6 and 8 with an optimum pH for the group as a whole near 7.0. In case where the process takes place in a single bioreactor, methanogenesis is considered to be the rate-limiting step and it is necessary to maintain the reactor pH close to neutral. Normally, acid and ammonia production vary only slightly due to the buffering effect of carbon dioxide/bicarbonate (CO2/HCO3) and ammonia/ammonium (NH3/NH4+), which are formed during fermentation, and the pH normally stays constant between 7 and 8.

 

9.7.3. Water Content

 

Bacteria consume the available organic substrate in dissolved form. Therefore, the production biogas and the water content of the initial organic matter are interdependent. When the water content is below 20% by weight, biogas production is significantly limited, whereas increasing water content biogas production is enhanced, reaching its optimum at 91%–98% water by weight (Kaltwasser, 1980).

 

9.7.4. Oxidation–reduction potentials (ORP)

 

Methane bacteria are very sensitive to oxygen and have lower activity in the presence of oxygen thus reducing biogas yield. According to Mudrak and Kunst (1991) the anaerobic process shows a certain tolerance to limited, even continuous, quantities of oxygen. The redox potential can be used as an indicator of the process of methane fermentation, since methanogenic bacterial growth requires a relatively low redox potential. Hungate (1966) found that -300 mV is the minimum redox value, whereas Morris (1975) reported that the optimum ORP value for the growth of anaerobic microorganisms in any medium, is between -200 to -350 mV at pH 7. Finally, according to Archer and Harris (1986) and Hungate (1967) it is has been established that methanogens require an extremely reducing environment, with redox potentials as low as −400 mV.

 

9.7.5. Nutrients and Trace Metals

 

All biological treatment methods require nutrients and trace elements during waste processing. Nutrients and trace metals are not directly involved in waste processing and stabilization but they are the essential components of existing microbial cells growth and synthesis of new cells. Therefore, the presence of nutrients and trace metals provide the needed physicochemical conditions for the optimum growth of microorganisms. If the digested organic material does not have one or more of the important nutrients and trace elements, the waste degradability is severely affected, since microbial cells are unable to grow at optimum rate and to produce new cells.

 

9.7.6. Toxicity and Inhibition

 

Anaerobic microorganisms are inhibited by the substances present in the influent waste stream and by the metabolic byproducts of microorganisms. Ammonia, halogenated compounds, heavy metals and cyanide are examples of the former, while ammonia, volatile fatty acids, and sulfide are examples of the latter. Toxicants, components in the feed material causing adverse effects on bacterial metabolism, are responsible for the occasional failure of anaerobic digesters. With reference to investigations of Konzeli-Katsiri and Kartsonas (1986), Table 17 lists the limit concentrations (mg L–1) for inhibition and toxicity of heavy metals in anaerobic digestion.

 

Table 17: Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas, 1986)

Heavy Metal

Inhibition (mg L–1)

Toxicity (mg L–1)

Copper (Cu)

40-250

170-300

Cadmium (Cd)

-

20-600

Zinc (Zn)

150-400

250-600

Nickel (Ni)

10-300

30-1000

Lead (Pb)

300-340

340

Chromium III (Cr)

120-300

200-500

Chromium VI (Cr)

100-110

200-420

 

9.7.7 Volatile Fatty Acids (VFAs)

Volatile fatty acids accumulation during process imbalance directly reflects a kinetic uncoupling between acid producers and consumers (Switzenbaum et al., 1990). The volatile fatty acids concentration has been most suggested for monitoring of anaerobic digester (Hill and Holmberg, 1988; Lahav et al., 2002; Feitkenhauer et al., 2002; Mechichi and Sayadi, 2005). In a low buffered system, pH, partial alkalinity and VFA measurements are useful for process monitoring, whereas in highly buffered system only VFA is reliable for indicating process imbalance (Murto et al., 2004).

9.8. Reactor Configuration

The configuration of the anaerobic digester is of paramount importance in anaerobic fermentation processes. The relatively low biosynthesis rate of methanogens in an anaerobic system demands special consideration for bioreactor design. The selection of bioreactor types depends on the requirement of a high SRT/HRT ratio, in order to prevent the washout of slow-growing and sensitive methanogenic bacteria. Therefore, the anaerobic digestion performance of the bioreactors is based on their capability to maintain a high SRT/HRT ratio and thus to retain biomass. Another approach for reactor configuration selection is based on required effluent quality. Because of relatively high half-saturation constants for anaerobic microorganisms, continuous stirred tank reactors may not be suitable, as immediate dilution of the waste leads to low concentrations of organic matters, but still too high to meet the effluent discharge standards, which are below the range of anaerobic degradation. Under such conditions, a staging or plug flow type reactor would be more appropriate (Khanal, 2008).

10. Environmental impacts

The technologies used for the anaerobic digestion appear to be more ecological than those of composting. The three categories of greenhouse effect, acidification and heavy metals play an important role in the environmental impact assessment. Carbon dioxide emission cannot be prevented, if biogenic matter is degraded. On the other hand, methane is freed in nature as soon as biomass is piled up into heaps. For an aerobic treatment after anaerobic digestion, there is the disadvantage that the organic matter is well inoculated with anaerobic bacteria. Even if just a very small share of the organic matter is degraded during composting after anaerobic digestion, methane emissions may be larger than those caused by composting alone. As far as energy is concerned, digestion plants are very good from an ecological point of view, mainly because they do not need external fossil and nuclear energy. The production of renewable energy has positive consequences on nearly all impact categories, because of savings in or compensation for non-renewable energy.

11. Economic data

It is difficult to discuss in detail the economics of deploying an anaerobic digestion plant for biowaste, because of the many factors that affect the costs and the variation in circumstances and costs between different countries. When comparing systems costs, one must consider which of the following cost items are included in the analysis (1) Predevelopment costs (Siting and permitting, Land acquisition, Environmental impact assessment, Engineering planning and design, Hydrogeological investigation), (2) Construction costs (Infrastructure e.g. access roads, piping, utility connections, Cleaning and excavation, Buildings and construction, Equipment e.g. tanks, machinery, electronics, Labor), (3) Operating costs (Maintenance fees, Labor, Materials, Water and energy, Supervision and training, Insurance, Overheads, Wastewater disposal, Solid residuals disposal, Regulatory fees. Figure 17(a) presents the capital cost curves for European MSW digesters, while Figure 17(b) presents their maintenance and operational costs incorporating the above mentioned parameters (CIWMB, 2008).

 

Figure 10: (a) Capital cost and (b) M&O costs curves for European MSW digesters (CIWMB, 2008)

 

In general, when looking at the treatment cost per tonne of MSW for the large facilities built in Europe, it is clear that over the last few years the trend is for a reduction in overall treatment costs making anaerobic treatment systems more competitive. However, economies of scale mean that the complex industrial systems need to process many thousands of tonnes of MSW per year to have a reasonable treatment cost per tonne. According to the Carbon Finance Unit of the World Bank in 2008, the capital cost of an anaerobic digestion treatment system with a capacity of 300 tonnes/day is around 15-55 million € (319€ per tonne capital cost), while the operation and maintenance cost is 40-70 € per tonne.

Another source estimates the relevant treatment costs to 70 – 100 €/t (Neamt Master Plan, 2008). Based on recent EU data, the financial cost of anaerobic digestion in EU member states (including annualized capex, opex, maintenance expenditures and other specific costs or revenues) varies between 48 - 107 €/tonne for  the various end uses of biogas namely electricity, CHP, supply to the grid or as vehicle fuel (EC, 2010). Finally according to the different uses of biogas the report provided by ARCADIS & EUNOMIA (2010) suggests the following costs for anaerobic digestion:

  • AD with Electricity Only: capital costs 375€ per tonne, operating costs 37.50€ per tonne for facilities with a capacity ranging from 20,000 to 30,000 tonnes with appropriate post-treatment of the produced digestate.
  • AD with combined heat and power: capital cost of 478 € per tonne, and an operating cost of 38.75 € per tonne
  • AD with Gas Upgrading for Use as Vehicle Fuel: capital cost of 440 € per tonne and operating expenditures of 45.25 € per tonne
  • AD with Gas Upgrading for Use in Grid: capital cost of 275 € per tonne whereas for operating expenditures no data were available.

12 Applicability

It is considered that anaerobic digestion has considerable applicability for the case of Romania and Bulgaria, since both countries are characterized by very high content of biodegradable waste within municipal waste, as confirmed by the results of waste analysis that took place in both countries in the framework of the implementation of the BALKWASTE project. It should be noted that the separate collection of organic waste can contribute to large extent to improving the performance of this biological method for waste management and achieving higher yields of the produced biogas. In addition, in this way the quality of the biogas is also much better. It is also true that the energy produced from the application of the anaerobic digestion is an important output for any national economy.

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[1] methane content of biogas ranges from 50% to as high as 75%, though most plants report values close to 60%.