23 March 2022
Biogas production from manure and liquid slurry. Sustainable solutions for livestock and agricultural waste management.
1. Environmental situation
The concept of waste is defined in Spanish Law 22/2011 on waste and contaminated soil as ‘any substance or object that its possessor disposes of or has the intention or obligation to dispose of’’. Agricultural and livestock waste is one of the most serious environmental problems in modern communities, with slurry and manure being one of the most problematic in rural and agro-industrial settings. Catalonia, the great engine of the livestock sector in Spain, at the end of 2021 had a "census" of more than 25,500 livestock farms and 62.5 million head of cattle.
Another main problem of modern society is the growing demand for energy and the consumption of fossil fuels to obtain it.
Combining these two problems, an ideal use relationship can be established: use of waste as a means of obtaining renewable energy.
One of the most interesting cases at a technological and environmental level is the use of slurry and manure as a source of renewable energy to obtain biogas as fuel or form of energy, through the application of anaerobic biodigestion and biogas conditioning,
In addition, during anaerobic digestion, the nitrogen contained in the slurry is transformed into its ammonia form, the slurry once digested (called digestate) can be treated and used as fertilizer. Ammonia is fixed in the soil and is easily taken up by plants. If the slurry was not treated anaerobically, the nitrogen would remain in its oxidized forms (nitrates and nitrites) that cannot be fixed so easily in the soil and are dragged by the aqueous stream of the irrigation, generating an alarming increase in its concentration in the groundwater.
This process allows, in addition to obtaining renewable energy and preventing the infiltration of nitrates and nitrites in the water, the reduction of greenhouse effect emissions into the atmosphere since biogas, composed mainly of methane gas, is used in its conversion and is not emitted to the atmosphere.
2. Introduction to anaerobic digestion of manure, slurry and other agricultural and livestock wastes.
Anaerobic digestion, sometimes referred to as biodigestion, consists of the treatment and management of agricultural and livestock waste by conditioning, mixing and introducing it into a reactor in the absence of oxygen. This digestion results in the generation of biogas as a result of the biological process and digestate as the material into which the waste has been transformed. Both are profitable and usable as a form of energy and products for use in agriculture.
The most used techniques for the management of manure and slurry with consolidated technologies and proven efficiency are the following:
1. Direct agronomic use: however, this technique aggravates the problem of nitrogen infiltration into groundwater.
2. Transport of unprocessed manure or slurry: displacement of slurry from an area of high density of livestock to another of low density, being a mere palliative that does not solve the global problem.
3. Storage: the objective is the adequacy of its availability for agricultural needs or subsequent treatment.
4. Solid-liquid separation: generally applied as a pre-treatment of other intensive treatments. The most commonly used techniques are:
Rotary drum filter separation
Once the separation has been carried out, the subsequent treatment of the solid fraction is generally composting, for the liquid fraction treatments are applied such as: aerobic digestion with nitrification-denitrification, Anammox, artificial wetlands, etc.
5. Anaerobic digestion: biological process in which microorganisms degrade organic matter in the absence of oxygen, generating biogas and digestate. Nitrogen is also converted to its ammonia form.
2.1 Advantages of biogas generation with anaerobic digestion
The advantages of biogas generation compared to other renewable energies can be summarized in the following points:
- Biogas is generated continuously: when the biodigestion process is correctly designed, the feeding of waste into the reactor is constant and therefore the generation of biogas and subsequent energy is continuous and storable, unlike other renewables such as wind power or the solar one that only produces energy when there is wind or solar incidence.
- Biogas systems allow the production of “dispatchable energy”, that is, energy that is generated when it is needed or when a peak of higher energy value occurs. This “peak power” has a high monetary value when the energy is in a period of high demand. A well-planned storage of biogas will allow the use of these periods to increase the profits derived from the sale of energy. Biogas storage is generally carried out in tanks with specially designed covers.
- Although the generation of biogas and energy production respond to scale economy, the design of anaerobic digestion plants can be applied to very different farm sizes (heads of cattle), always applying a feasibility study (see section 9).
Table 1. Compilation of advantages and disadvantages of the anaerobic digestion of slurry and manure.
|Degradation of organic matter that does not require oxygen, savings of the costs involved||Slower kinetics than aerobic treatment, more HRT is required.|
|Produces between 3 – 20 times less sludge than aerobic processes: 20 – 150 versus 400 – 600 kg biomass/m3CODconsumed.||High sensitivity to toxic shocks.|
|It produces biogas with enough methane content to be valuable as source of energy.||Sensitive to alteration of optimal operational conditions (temperature pH, load rate, stirring power, etc.)|
|The solid by-product or ‘digestate’ can be applied as fertilizer-||Requires dilution water.|
|It is efficient with high loads of BOD5.||The benefits obtained depend on the number of head of cattle available.|
|Biomass can preserve its activity even if the system has been stopped.|
|It reduces bad odors up to 90 – 100%.|
|Digestate sanitation: removal of 90 – 100% of ankylostomiasis eggs; 35 – 90% de ascarids or gut worms; 90 – 100% of blood fluke.|
3. Biological process of biogas production
During anaerobic digestion, the organic content of the substrates is degraded in the absence of oxygen and biogas (composed mainly of methane CH4 and carbon dioxide CO2) and a digestate (remaining material after the digestion of the substrates) are generated as final products. This degradation is a biological process that consists of several steps carried out by different communities of microorganisms: hydrolysis, acidogenesis, acetogenesis and methanogenesis. A schematic of these conversions is shown in Figure 1
During hydrolysis, microorganisms excrete enzymes to break down complex compounds (proteins, carbohydrates, and lipids) into their simpler components (amino acids, sugars, and fatty acids). During acidogenesis further degradation occurs generating volatile fatty acids (VFA) including acetic acid, alcohols, hydrogen H2, carbon dioxide CO2 and the transformation of nitrogen into ammonium NH4+. During acetogenesis, another community of microorganisms digests these products into more acetic acid, hydrogen, and carbon dioxide. Finally, during methanogenesis, methanogenic bacteria convert acetic acid, hydrogen, carbon dioxide and the remaining products into methane CH4 and carbon dioxide CO2 forming biogas. All these steps also result in new cellular material, thus generating excess sludge. However, anaerobic digestion is characterized by the minimum generation of sludge: 95% of the carbon contained in the substrates is transformed into biogas and 5% into biomass.
Composition and general features of biogas are gathered in Table 2.
Table 2. Composition and general features of biogas.
|Composition (it varies depending on the substrates)|
50-75% methane CH4
25-50% carbon dioxide CO2
2% nitrogen N2
0.5% oxygen O2
2500ppm hydrogen sulfide H2S
Saturated in H2O vaporTraces of other gases (ammonia NH3, impurities)
|Fuel equivalent of 1Nm3 of biogas with average composition of 70% CH4 + 30% CO2||0.3kg carbon / 0.8L gasoline / 0.6m3 natural gas / 6.8kW electricity / 1.5kg wood / 0.71L diesel / 26500 BTU|
|Explosion limit||6 - 12% de biogas in air|
|Smell||Rotten egg due to is content in sulfides.|
La composición del biogás es variable según el substrato o mezcla de substratos que se introduce en el digestor. A continuación se muestra una media del contenido en metano que tendría el biogás producido por diferentes tipos de estiércol si estos fueran procesados como monosubstrato, también se incluyen pastos y restos de cocina como los co-substratos que más comúnmente se encuentran disponibles en las granjas:
Tabla 3. Contenido estándar en metano del biogás producido por diferentes estiércoles
|Procedencia del estiérco l||Contenido estimado en metano CH4 del biogás|
|Poultry||60 – 80%|
|Pig||60 – 80%|
|Cow||55 – 75%|
|Kitchen food waste||70 – 80%|
|Wastewater treatment plant sludge||50 – 80%|
|Glycerin (glycerin water with 25% solids)||50%|
4. Co-digestion of manure, slurry and other livestock and agricultural wastes.
Co-digestion of manure and slurry is a way to increase the performance of the process and ensure its viability and benefit.
It is important to know the theoretical potential for biogas production of the most commonly applied substrates in the agricultural and livestock industry for biodigestion plants. This will allow selecting the different substrates that can complement the manure and establish a predictive calculation of the expected biogas production. These values are collected in Table 4.
Table 4. Theoretical production of biogas from several substrates.
|Substrate||Nm3 biogas/ton of substrate|
|Sludge from wastewater treatment plant||24|
|Cow manure (dry manure)||25 (73)|
|Dairy cow manure||172|
|Pig manure / slurry||10 - 30|
|Raw potato scraps||39|
|Fried potato scraps||508|
|Chicken manure (dry)||80 (240)|
|Green residue (leaves, flowers, gardening wastes)||175|
|Bakery and pastry waste||714|
|DAF floated fats||63 - 179|
|glycerin (glycerin water with 25% solids)||70 - 80|
The selection of a co-substrate should be based primarily on its availability for the farm: proximity and agreements with the supplier of the waste.
Co-digestion substrates must be carefully selected to promote biogas generation and methane content, avoiding its inhibition. The increase in the volume of biogas generated, the impact on the digesters (if they already exist and a new substrate is being added), the organic load, the hydraulic residence time HRT and cell residence time CRT, potential inhibitory compounds, fate to be considered should be considered. will give to the digestate and what its composition will be, nutrients contained in the co-substrate, economic impact.
To establish whether co-digestion is possible and favorable, and the appropriate amounts of the new substrate or multiple substrates, the following key questions need to be answered:
- Does the volume of co-substrate reduce the HRT below the design minimum? Does it increase the organic load above what is recommended?
- Does the co-substrate require any type of storage, conditioning or pre-treatment?
- Can the biogas storage, conditioning and use equipment withstand the possible increase in the production?
- Does the new substrate contain toxic substances for microorganisms that can inhibit biogas production?
- Does the composition of the digestate vary in terms of nutrients and how will its subsequent treatment and conditioning be focused?
- Will there be long-term availability of the co-substrate?
- Are there any legal/environmental limitations or regulations for the transport and use of the co-substrate?
The addition of a co-substrate or several co-substrates must allow reaching an optimal carbon-nitrogen C/N ratio established in the range 20 – 30, mitigate the inhibitory effects thanks to the dilution of the toxic compounds (VFA, ammonia, products of lipid degradation) and improve the kinetics of methane production.
Table 5 shows the typical values of the C/N ratio of various substrates
Table 5. C/N ratio of typical agricultural and livestock substrates.
|Substrate||C/N ratio range|
|Sludge from wastewater treatment plant||6 – 10|
|Cow manure||6 – 251)|
|Pig manure / slurry||3 – 15|
|3 – 20|
|Wheat straw2)||90 – 150|
|Vegetable leaves||10 – 80|
|Food waste||35 – 90|
|Gardening waste||100 – 150|
|Sawdust||200 – 500|
|Grass||12 – 25|
|DAF floated fats||20 – 45|
|glycerin (glycerin water with 25% solids)||200 - 300|
1)According to straw content; 2) Compounds such as straw have a very high lignin content, a hydrolysis process prior to digestion must be considered so as not to inhibit methane production, since lignin is a component that is not rapidly biodegradable and would be an impediment to the correct development of the digestion.
For example, an ideal combination would be the co-digestion of pig manure and vegetable waste: in this way, the high buffering capacity of manure is combined with the high C/N ratio provided by vegetable waste.
The great advantage of using cow manure in a biodigestion process is that it already contains an inoculum of anaerobic microorganisms adapted to digestion (fermentation).
The following table compiles details of the characteristics of the main substrates of applicability in the livestock and agricultural industry in specific units for the design of the biodigestion process.
Table 6. Characteristics to consider in the design of the anaerobic process and production of biogas from agricultural and livestock substrates.
|Substrate||Total Solids TS (%)||Volatile Solids VS (% of TS)||Biogas production1) (m3/tonVS)||Hydraulic retention time HRT (days)||Undesired substances||Inhibitory or toxic substances||Common problems|
|Pig manure / slurry||2.5 – 8||70 – 80||250 – 500||15 – 40||wood chips, hair, sand|
antibiotics, disinfection agents
|Cow manure||5 – 23||75 – 85||200 – 320||20 – 30||hair, sand, straw, wood||antibiotics, disinfection agents||foams|
|Chicken manure||10 – 30||70 – 80||350 – 600||>30||stones, sand, feathers||high content in ammonium, antibiotics and disinfectant agents||ammonium inhibition, foam|
|Food waste||10||80||500 - 600||10 - 20||bones, plastics, pieces of metal||VFA, disinfectant acids||acidification, sediments, clogging and mechanical problems|
|vegetable leaves||80||90||100 – 3002)||8 – 20||dust||pesticides||-|
|Wheat straw||70||90||350 – 450||10 – 50||sand||-||foam, low biodegradability (requires hydrolisiss3))|
|Gardening waste||60 - 70||90||200 – 500||8 – 30||stones, sand, dust, remains of low biodegradability||pesticides, herbicide||foam, low biodegradability (requires hydrolisiss3))|
stones, sand, dust
|DAF floated fats||7||90||900 – 1000||25||requires co-digestion to improve C/N ratio|
|glycerin (glycerin water with 25% solids)||25||80||70 – 80||25||other process compounds|
requires co-digestion to improve biogas production
1)According to HRT; 2)According to draught level; 3)For substrates with very high lignin content, a previous hydrolysis step must be applied, thermal, chemical or enzymatic hydrolysis technologies are available.
4.1 Nutrients of agricultural interest in manure and slurry
It is interesting to consider the content of the different valuable nutrients (nitrogen, phosphorus P2O5, potassium K2O and organic matter) that are found in the different types of manure, since it allows knowing the needs for the supply of extra nutrients and their distribution. They will remain in the digestate, and they can be revalued as a product for agricultural application in the form of fertilizer. The nutrients provided by different types of manure are collected in Table 7.
Table 7. Nutrients of interest content in diverse types of manure.
|Dry matter DM (%)||Total nitrogen (%DM)||Phosphorus P2O5 (%DM)||Potassium K2O (%DM)||Organic matter (%DM)|
|Pig manure/slurry||2.5 – 8||8.0||6.0||5.2||68.3|
|Cow manure||5 – 23||1.8||1.7||3.1||66.3|
|Chicken manure||10 – 30||1.7||4.2||3.8||64.7|
1.2 Digestion of pig manure
Among all the collected substrates of animal origin, pig manure deserves a separate consideration. This is an excellent substrate for anaerobic digestion as it has a high buffering capacity and a high nutrient content. However, monodigestion of pig manure has disadvantages such as: low biogas yields (production of 10 – 30Nm3/ton with methane content between 60 – 80%) mainly due to its low C/N ratio, between 3 – 15, and its high moisture content (>90%). In addition, the high concentration of ammonium could produce toxicity and therefore inhibit the process. Therefore, the best option for the anaerobic digestion of pig manure is co-digestion. Ideal co-substrates would be, for example, plant remains (fruits, vegetables, etc.) and kitchen and food remains.
The typical distribution of the most used co-substrates together with pig manure are as follows:
Agri-industrial waste (vegetables, processed foods, etc.) = 54%
Biodiesel industry waste (glycerol, glycerin process water, etc.) = 13%
Organic fraction of urban solid waste = 8%
Others (accordingly to availability) = 25%
The suitable co-substrates to combine with the manure must be rich in carbon and with a very high content of biodegradable organic matter. The indicated co-substrates have a high C/N ratio, low buffering capacity and can produce high concentrations of FVA. In the mixture, the manure would provide their buffering capacity to maintain the stable pH within the appropriate range, in addition to reducing the concentration of ammonia due to its dilution with the other substrates, all this leading to the improvement of the production of biogas and methane.
A strategy to increase the production of biogas from manure and remedy the problem of high water content is to carry out a separation of the solid fraction (where all the nutrients are retained) and the liquid fraction prior to biodigestion. This separation is commonly done by centrifugation or filter press.
5. Process design of anaerobic digestion of manure and slurry
The anaerobic digestion process of manure and slurry (and co-substrates) is a simple and robust process that applies known technology and that can be optimized knowing in detail the characteristics of the substrate and adequate operating conditions. During the anaerobic digestion of manure or slurry, the organic matter is degraded, obtaining biogas of between 10 – 30 m3/ton of substrate with a methane content between 60 – 80%. As a result of the process, the manure and slurry are transformed into a stabilized and easy-to-handle final product called 'digestate', which has excellent properties as a fertilizer. For the anaerobic digestion of manure and slurry, two types of regimen are applied:
i) Mesophilic conditions: the reactor operates at temperatures between 30 – 45ºC. They have a greater margin of maneuver and their use is more widespread.
ii) Thermophilic conditions: the reactor operates at temperatures between 50 – 55ºC. The digestate generated is more sanitized and has less viscosity, which facilitates its subsequent treatment as fertilizer.
The retention time can vary between 15 – 40 days and can be carried out in one or two stages. In the first part, the hydrolysis process of the manure or slurry and the co-substrates is carried out, and in the second part, methanogenesis and biogas production take place. Depending on the nature of the substrates, one or two reactors will be designed for the complete digestion process.
The environmental effects of manure and slurry biodigestion are:
- The biogas generated is a fuel with a lower carbon footprint than fossil fuels (emissions of CO2-equivalent per m3)
- Reduction of bad odors: digestate has a much less intense odor than manure or slurry.
- A digestate is obtained as a more stable end product that involves much less contamination in the soil than untreated manure or slurry. Being a more homogeneous product in its supply of nutrients, the digestate allows their distribution in a more uniform way in the soil and in the vicinity of the roots of the plants, promoting their absorption and assimilation. Furthermore, anaerobic digestion does not modify the N/P ratio, it only affects the form of available N. Anaerobic digestion transforms organic nitrogen into ammonium, thus the concentration of ammonium in the treated manure or slurry is much higher (up to 20%) than in the raw material.
It is important to take into account how the slurry will be stored before it is introduced into the reactor. Waterproof ponds with sufficient capacity to collect slurry should be built, preferably outside the farm's sanitary enclosure and covered when possible. The slurry should not be stored for too long and it should be worked with recently generated manure to avoid uncontrolled degradation phenomena and volatilization of ammonia and organic compounds.
5.1 Operation conditions of anaerobic digestion
The most important operating conditions in an anaerobic digestion reactor are the following:
Temperature: the temperature must remain stable within the selected range (mesophilic or thermophilic) since the microorganisms that develop at different temperatures are sensitive to changes and a sudden increase or decrease in temperature can affect their activity and health.
Agitation: it can be done by mechanical methods or by recirculation of the generated biogas injected in the lower part of the reactor. Low mixing speed conditions allow the reactor to withstand disturbances that occur in the feed.
Volumetric Organic Loading Rate OLR: amount of volatile solids fed daily per unit volume of reactor. It is one of the main design parameters and with it the volume and type of the reactor are established. When there is an overload of OLR, saturation and irreversible failure of the system can occur, so it is crucial to periodically control this load and maintain the supply so that it stays within the design range of the reactor (not overfeeding or maintaining a deficit for too long)
Hydraulic retention time HRT: it also determines the volume of the reactor. Table 9 lists the optimal HRT ranges for each type of manure. By increasing the HRT, the degree of degradation of organic matter and the generation of biogas rise.
Humidity: it is recommended to work with humidity around 80%, value in which the production of biogas and methane is theoretically higher.
pH: it is essential for the correct operation and stability of the system. The pH should be kept between 6.5 – 7.5. The growth of methanogenic microorganisms is very sensitive to pH and decreases dramatically below pH 6.6. The decrease in pH is an indication of the accumulation of VFA and therefore appropriate measures must be taken, for example, stop introducing biomass or reduce its load. This parameter must be carefully controlled. Another way to control the state of the reactor is through the VFA/Alkalinity ratio, which should be kept around 0.4.
Volatile Fatty Acids VFA: an overproduction of VFA by fermentative bacteria can lead to a decrease in pH and system failure, this may be due to feeding overloads or to an inhibition of the methanogenic bacteria that consume VFA and produce biogas. It is very important to keep the feed within the OLR design range (especially with substrates that are very easily hydrolyzed) and to control toxic substances that may be entering through periodic analysis of the substrates.
Alkalinity: represents the buffering capacity of a substrate or substrates and prevents imbalances in the process against agents in the feed that can add acidity to the medium or slow down the biological process. Avoids the sudden drop in pH when there are FVA accumulations.
Redox potential: during anaerobic digestion the redox potential must be kept below -300mV for proper degradation of organic matter.
Nutrients: keeping the C/N ratio between 20 – 30 ensures the optimal functioning of the anaerobic system. A C/N value below 20 can lead to excess ammonia production, which will negatively affect methane production. In addition to carbon and nitrogen, microorganisms also need phosphorus and trace elements (sulfur, calcium, magnesium, manganese, etc.). A sufficient base ratio may be C:N:P:S = 600:15:5:1.
Inhibition agents: the main inhibitors are nutrients that, when present in excess, negatively affect the process and can stop it: ammonium and ammonia, hydrogen sulfide, light metal ions, heavy metals, benzene, phenols, etc.
5.2 Reactors for the anaerobic digestion of manure, slurry and other waste
Once the substrate or substrates, their characteristics and the appropriate operating conditions are known, then the type of reactor must be selected. Not all reactors admit the same substrates and operate in a similar way, therefore the selection of the reactor must be adapted to the substrates and conditions and not the other way around. There are four main types of reactors to carry out the anaerobic digestion of manure, manure and co-substrates from the agri-food industry: Covered Lagoon, Plug-Flow, Complete Mix and UASB. Table 8 shows a comparison of these four technologies. All the systems described are systems that work continuously, discontinuous systems are little applied in this field.
Covered lagoon: adaptation of the soil to form an artificial "lagoon" with waterproof material, covered with a tarpaulin that acts as a biogas storage chamber.
Plug-Flow: elongated and narrow tank, can be built underground in civil works or as an external reactor, the agitation is mechanical and the substrate moves in a forward direction, the stages of decomposition of organic matter to biogas are carried out throughout the reactor. Digestate is pushed by fresh substrate introduced at the feed head. A biogas collector cover is installed on top.
Complete mix: It can be built underground as civil works or with the installation of a tank, both with a biogas collection cover. The agitation is carried out either with an agitator or with the recirculation of part of the biogas that is re-injected in the lower part of the reactor.
UASB: stands for 'upflow anaerobic sludge blanket reactor'. It is a high performance technology, built as a cylindrical tank that combines in its structure the digestion chamber, the settler and the biogas collection system, it works with granulated biomass of high efficiency and resistance. These reactors are suitable for homogeneous and consistent feeds.
Table 8. Comparative of typical reactors for anaerobic digestion.
|Lagunaje cubierto||Flujo-pistón||Mezcla Completa||UASB|
|Geometrical configuration||Deep artificial lagoon||Rectangular elongated, underground or in outside tankrior||Cylindrical, underground or in outside tank||elongated cylindrical t|
|Maximum admissible solid size||Fine||Coarse||Coarse||Coarse|
|Common temperature of operation||Ambient, not controllable||Mesophilic||Mesophilic or thermophilic||Mesophilic or thermophilic|
|It does require heat input||No||Yes||Yes||Yes|
|Compatible with co-digestion||No||Limited||Yes||Limited|
|Solids separation previous to digestion||Recommended||Not needed||Not needed||Not needed|
|High||Reduced (I built below ground level)||Medium||Reduced|
|OLR (kgCOD/m3·day)||1 - 2||1 - 5||1 - 5||10 - 25|
|HRT*||>48 days||20 – 40 days||20 – 30 days||10 days|
|VS removal1)||35 – 45%||35 – 45%||35 – 45%||50 – 55%|
|Costs||<3 – 5%||7 – 15%||3 – 12%||2 – 10%|
1)Based on cow manure as reference.
*The typical HRT range based on the type of manure is shown in Table 9. It can then be concluded that the most suitable systems for the treatment of slurry are either the Piston-Flow or the Complete Mix.
Table 9. HRT for different types of manure.
|Cow manure||20 – 30|
|Pig manure/slurry||15 – 25|
|Chicken manure||20 – 40|
When the substrate or mixture of co-substrates exceeds 20% solids, there are two possibilities for its treatment depending on availability of water use: i) dilution of the feed until reaching the design % or ii) application of anaerobic digestion reactors high solids content, special technology and more complex operation
1.3 Anaerobic digestion system stability.
A properly operated anaerobic digestion system can provide many years of efficiency and economic benefits. However, it is important to take control and take actions to maintain the biomass in favorable living conditions: keep it properly fed and at the design temperature. When some instability occurs in the process, it can be detected by a drop in biogas production or a decrease in its quality (methane content). Monitoring and control of the system are of crucial importance for its maintenance, in this way instabilities can be quickly detected and actions taken to restore normal operation.
System instabilities are typically due to one or more of the following four basic factors:
- Hydraulic overload: occurs when the HRT of the reactor is lowered below the design minimum, that is, more volume of substrate is being introduced than it is capable of supporting, producing a wash out effect: slow-growing microorganisms such as methanogens are dragged with the flow. Feed must be decreased to restore normal system activity.
- Organic overload: occurs when COD or VS is introduced into the reactor at rates that exceed the methane conversion capacity of the microorganisms. It may be due to an undetected increase in the organic content of the substrate itself, so even introducing the same volume/quantity, the net organic load is higher. This generates an accumulation of VFA in the reactor, which inhibit the activity of methanogen microorganisms. The feeding must be reduced, diluted with other less charged substrates or with water to recover the normal activity of the system.
- Thermal stress: It happens when the temperature in the reactor changes suddenly and this change is maintained for a long period of time. It may be the consequence of some failure in the heating system, or that the fed substrate was at a temperature very different from the design temperature of the reactor. Special attention should be paid to the heating system in areas of extreme temperatures, especially in winter. The fault must be fixed as soon as possible or the substrate must be reconditioned.
- Toxic overload: It happens when elements or chemical compounds such as sulfides, VFA, heavy metals, calcium, sodium, potassium, dissolved oxygen, ammonia, chlorinated organic compounds, etc. are introduced ay concentrations that are inhibitory for the microorganisms. These compounds can be found extraordinarily in the substrate. Stop feeding the current substrate, and feed it with another if there is a possibility of having a similar substrate in the meantime.
1.4 Process scheme of anaerobic digestion of manure, slurry and other waste.
The anaerobic digestion system not only includes the reactor and its heating system, digestion involves a complete process of treatment and use of agricultural and livestock waste. The basic scheme comprises the following stages, represented in Figure 9.
i) Conditioning/storage of the substrates.
ii) Anaerobic digestion.
iii) Separation of digestate fractions by solid-liquid separation (centrifuge, screw-press, filter-press, DAF): liquid fraction (lagoon or direct use, concentrated fertilizer via Reverse Osmosis or Ultrafiltration membrane technologies, stripping and/or evaporation) and solids fraction (compost, fertilizer, soil amendment, animal bedding)
iv) Gas conditioning and applications: gas burners, boilers, combined heat and power CHP or co-generation engines (self-consume or exportation), LNG for vehicles, CNG or grid injection.
6. Applications and uses of biogas
The biogas must be subjected to a cleaning process to be able to use it and value it. These biogas cleaning systems allow the elimination of hydrogen sulfide H2S, water vapor, CO2 and to compress it for storage or direct use. Figure 9 shows the possible uses of clean biogas. In addition, intensive processes can be applied for the separation of CO2 and compression generating Biomethane: a gas similar to natural gas but of renewable origin.
Table 10 contains a summary of the cleanliness levels of biogas, its characteristics and uses.
Table 10. Characteristics and uses of the different quality levels of biogas.
|Raw biogas||Clean biogas||Biomethane|
|Humidity (kg vapor / 100 Nm3 gas)||3.62||0.89||0.01|
50-75% methane CH4
25-50% carbon dioxide CO2
2% nitrogen N2
0.5% oxygen O22500ppm hydrogen sulfide H2S
50-75% methane CH4
25-50% carbon dioxide CO2
2% nitrogen N2
0.5% oxygen O2
50ppm hydrogen sulfide H2S
≥95% methane CH4
0.8% carbon dioxide CO2
2.4% nitrogen N2
0.8% oxygen O2<1ppm hydrogen sulfide H2S
|Potential uses||Not applicable|
Conventional gas burners, boilers, co-generation CHP engines (self-consume or exportation)
|Boilers, co-generation CHP engines (self-consume or exportation), LNG for vehicles, CNG or grid injection|
|Average energetic content (BTU/Nm3)||Sin valor||17000 - 26500||34100|
|Process for raw biogas cleaning||Removal of steam and H2S||Removal of steam and H2S and conversion to Biomethane eliminating CO2 and compressing the gas|
7. Digestate application
In addition to biogas, the other valuable product obtained from anaerobic digestion is digestate, which, subjected to a series of treatments, can be used as a fertilizer.
The digestate must first be separated into its liquid phase and its solid phase.
· Applications of the liquid phase of the digestate: direct use as fertilizer in the field; manufacture of concentrated fertilizer through advanced treatments (Ultrafiltration and Reverse Osmosis membranes, stripping and/or evaporation). This fertilizer can be applied on the farm itself or marketed with a high ecological value.
· Applications of the solid phase of the digestate: solid fertilizer with high nutrient content NPK; compost and soil remedies; cattle beds. The surplus product that is not applied on the farm can also be marketed.
The set of value, income and savings that the full use of all the products (biogas and digestate) of anaerobic digestion supposes is what defines the viability and profitability of installing this process.
In addition to these "tangible" products, revaluation can also be obtained in the form of CO2 bonuses. These CO2 bonuses are awarded based on the biogas generated and consumed as renewable energy, as this prevents its emission into the atmosphere, helping to mitigate the greenhouse effect.
8. When no to use anaerobic digestion
The use of anaerobic digestion is not applicable in any condition when other technologies could be a better option. For example, composting may be more convenient in cases with certain types of manure characterized as 'dry' or 'stackable' or those that have a very high content of 'straw-type’ materials.
When the manure or other co-digestion substrates have a high lignin content (wood, fibrous crop residues, leaves, etc.), the application of anaerobic digestion is not recommended (unless a pre-treatment of hydrolysis).
9. Feasibility assessment of anaerobic digestion
The installation of an anaerobic biodigestion process implies a significant financial commitment, therefore it is important to carry out a feasibility study before starting the project.
For smaller farms, the installation of a biogas plant can be an opportunity under certain conditions such as: environmental problems, odor problems, energy production incentives, subsidies, availability of co-substrates, etc.
Three main steps are generally established for the feasibility study, in which the following points are defined and analyzed in depth:
- Type of manure and accessibility to other wastes.
- Economic viability.
- Existence of normative and regulation supporting biodigestion installations.
- Type of animals and number of heads.
- Feeding methods and times.
- Method and frequency of manure removal.
- Water sources and amount used for manure removal.
- Existence of storage equipment or pre-treatment for manure.
- Current energy uses.
- Available space and ground disposition.
- Environmental considerations
- Revenue for the farm.
- Estimated CAPEX and OPEX.
3) Feasibility assessment
- Definition of substrate composition and substrate mix if applicable.
- Definition of recoverable products (energy, Biomethane, fertilizers, compost, animal bedding, etc.)
- Definition of technologies and suitable reactor type.
- Mass and energy balances.
- Economic projections to short, medium and large term.
IMPORTANT NOTE: The data shown in this article and especially the values of the characteristics collected in the tables are an established range based on bibliographic information, real published experiences and work carried out by Sigma. For a correct knowledge of the substrate or substrates handled, a complete study of the same must always be carried out by means of ATA tests 'Anaerobic toxicity assay', BMP 'Biochemical methane potential' and complementary analyzes.
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