Overview

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In general terms, digesters consist of the digestion tank as such, which is thermally insulated, plus a heating system, mixer systems and discharge systems for sediments and the spent substrate.Two relevant forces act on the digester. The external active earth pressure causes compressive forces within the masonry. The internal hydrostatic and gas pressures causes tensile stress in the masonry. Thus, the external pressure applied by the surrounding earth must be greater at all points than the internal forces. Round and spherical shapes are able to accept the highest forces and distribute them uniformly. Edges and corners lead to peak tensile stresses which can result in cracking.

Requirements

No matter which design is chosen, the digester (fermentation tank) must meet the following requirements:

• Water/gastightness - watertightness in order to prevent seepage and the resultant threat to soil and groundwater quality; gastightness in order to ensure proper containment of the entire biogas yield and to prevent air entering into the digester (which could result in the formation of an explosive mixture).
• Insulation - if and to which extent depends on the required process temperature, the local climate and the financial means; heat loss should be minimized if outside temperatures are low, warming up of the digester should be facilitated when outside temperatures are high.
• Minimum surface area - keeps cost of construction to a minimum and reduces heat losses through the vessel walls. A spherical structure has the best ratio of volume and surface area. For practical construction, a hemispherical construction with a conical floor is close to the optimum.
• Structural stability - sufficient to withstand all static and dynamic loads, durable and resistant to corrosion.

Material for the Digester

Inside plaster of the gastight section of a fixed dome digester[1]

• Acid resistance, in extreme cases when gas desulphurization with air oxygen up to a pH of 2 in the gas chamber.
• Consistency under the hydrostatic pressure at the bottom depending on the container height to to 1,2 - 1,8 bar
  
• Technical tightness with respect to (mbar 15) of the gas pressure in the headspace of the container.
• Traditional in-situ concrete wall (Ortbeton)
• Stanless steel (V4A, 1., X5CrNiMo17-12-2)
• Enameled steel panels

In industrialized countries, most of the new digesters are built of gas-tight concrete or steel. Additives are mixed into the concrete to render it gas-tight. If existing concrete vessels are used, their gas-tightness has to be checked. Often, they have not been built from gas-tight concrete or cracks have formed over time which allow the gas to escape.

It is important to check the digester and piping system for gas-tightness prior to putting the biogas unit in service. If leakage is detected only during operation, the digester has to be emptied, cleaned and plastered again. Rectifying a leakage before the initial filling is a lot cheaper.

In developing countries, digesters are usually masonry structures. The plastering has to be watertight up to the lowest slurry level and gas-tight from the lowest gas level upwards (gas-holder). The plaster has to resist moisture and temperatures up to 60°C reliably. The plaster must be resistant to organic acid, ammonia and hydrogen sulfide. The undercoat must be absolutely clean and dry.

Steel

Steel vessels are inherently gas-tight, have good tensile strength, and are relatively easy to construct (by welding). In many cases, a discarded steel vessel of appropriate shape and size can be salvaged for use as a biogas digester. Susceptibility to corrosion both outside (atmospheric humidity) and inside (aggressive media) can be a severe problem. As a rule, some type of anticorrosive coating must be applied and checked at regular intervals. Steel vessels are only cost-effective, if second-hand vessels (e.g. train or truck tankers) can be used.

Concrete

Concrete vessels have gained widespread acceptance in recent years. The requisite gas-tightness necessitates careful construction and the use of gas-tight coatings, linings and/or seal strips in order to prevent gas leakage. Most common are stress cracks at the joints of the top and the sides. The prime advantage of concrete vessels are their practically unlimited useful life and their relatively inexpensive construction. This is especially true for large digesters in industrialized countries.

Cement Plaster with Special Additives

Good results in water- and gas-tightness have been achieved by adding 'water-proofer' to the cement plaster. For gas-tightness, double the amount of water-proofer is required as compared to the amount necessary for water-tightness. The time between the applications of the layers of plaster should not exceed one day, as the plaster becomes water-tight after one day and the new plaster cannot adhere to the old plaster.

The following 'recipe' from Tanzania guarantees gas-tightness, provided the masonry structure has no cracks:

1. layer: cement-water brushing;
2. layer: 1 cm cement : sand plaster 1 : 2.5;
3. layer: cement-water brushing;
4. layer: cement : lime : sand plaster 1 : 0.25 : 2.5;
5. layer: cement-water brushing with water-proofer;
6. layer: cement : lime : sand plaster with water proofer and fine, sieved sand 1 : 0.25 : 2.5;
7. layer: cement screed (cement-water paste) with water-proofer.

The seven courses of plaster should be applied within 24 hours.

A disadvantage of cement plaster is their inability to bridge small cracks in the masonry structure as, for example, bituminous coats can do.

Bitumen (Several Layers)

Bitumen coats can be applied easily and remain elastic over long periods of time. Problems arise in the application as the solvents are inflammable (danger of explosion inside the digester) and a health hazard. Bitumen coats cannot be applied on wet surfaces. The drying of masonry structures requires several weeks, unless some heating device (e.g. a charcoal stove) is placed inside the digester for two to three days. Furthermore, the bituminous coat can be damaged by the up-and-down movement of the slurry.

Bitumen Coat with Aluminum Foil

On the first still sticky bitumen coat, aluminum foil is mounted with generous overlaps. A second layer of bitumen is applied on the aluminum foil. Gas-tightness is usually higher compared to the several layers of bitumen without foil.

Water-thinnable Dispersion Paint

These paints are free from fire- or health hazards. Most of them, however are not gas-tight and not resistant to moisture. Only those dispersion paints should be used which are explicitly recommended for underwater use and which form a gas-tight film.

Single- and Dual Component Synthetic Resin Paints

Synthetic resin paints form elastic, gas-tight coats which can resist rather high physical load. They are comparably expensive, their use seems only justified if the coating has to resist mechanical stress. This is usually the case with fixed dome plants. Measurements have given evidence that the masonry structure of a fixed dome stretches, though minimally, after filling and under gas pressure.

Paraffin

Paraffin, diluted with new engine oil, is warmed up to 100 -150°C and applied on the plaster which has been heated up with a flame-thrower. The paraffin enters into the plaster and effects a 'deep-sealing'. If paraffin is not available, simple candles can be melted and diluted with engine.

Masonry

Masonry is the most frequent construction method for small scale digesters. Only well-burnt clay bricks, high quality, pre-cast concrete blocks or stone blocks should be used in the construction of digesters. Cement-plastered/rendered masonry is a suitable - and inexpensive - approach for building an underground biogas digester, whereby a dome-like shape is recommended. For domes larger than 20 m digester volume, steel reinforcement is advisable. Masons who are to build masonry digesters have to undergo specific training and, initially, require close supervision.

Construction of the dome for a 30 m3 digester in Cuba

Plastics

Plastics have been in widespread use in the field of biogas engineering for a long time. Basic differentiation is made between flexible materials (sheeting) and rigid materials (PE, GRP, etc.). Diverse types of plastic sheeting can be used for constructing the entire digesting chamber (balloon gas holders) or as a vessel cover in the form of a gas-tight "bonnet".

Sheeting made of caoutchouc (india rubber), PVC, and PE of various thickness and description have been tried out in numerous systems. The durability of plastic materials exposed to aggressive slurry, mechanical stress and UV radiation, as well as their gas permeability, vary from material to material and on the production processes employed in their manufacture. Glass-fibre reinforced plastic (GRP) digesters have proven quite suitable, as long as the in-service static stresses are accounted for in the manufacturing process. GRP vessels display good gas-tightness and corrosion resistance. They are easy to repair and have a long useful life span. The use of sandwich material (GRP - foam insulation - GRP) minimizes the on-site insulating work and reduces the cost of transportation and erection.

Wood

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A further suitable material for use in the construction of biogas systems is wood. It is often used for building liquid-manure hoppers and spreaders. Wooden digesters require a vapor-proof membrane to protect the insulation. Closed vessels of any appreciable size are very hard to render gas-tight without the aid of plastic sheeting. Consequently, such digesters are very rare.

Further Information

References

Biogas for animal waste

1. Biogas Definitions

Biogas has been defined in various different ways;

1. Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a renewable energy source. (Wikipedia)
2. A type of biofuel naturally produced from the decomposition of organic matter when exposed to an environment without oxygen they free a blend of gases. (youmatter)
3. A renewable biofuel naturally produced by the breakdown of decomposition of organic matter, such as food scraps and animal waste, when exposed to an environment without oxygen.
4. Any gas fuel derived from the decay of organic matter, as the mixture of methane and carbon dioxide produced by the bacterial decomposition of sewage, manure, garbage, or plant crops.(dictionary.com)
5. Biogas is a gaseous mixture generated during anaerobic digestion processes using waste water, solid waste (e.g. at landfills), organic waste, and other sources of biomass. (climate technology centre & network)

NB;

In summary from the above definitions biogas is;

• A mixture of gasses i.e carbon dioxide, methane and hydrogen oxide.
• Biogas is a biofuel/gasfuel
• Biogas is a mixture of methane and carbondixide
• Biogas is a renewable fuel.

All the definitions are in agreement that biogas is derived, produced or generated from decomposed waste materials, organic matter, waste water, and solid waste or plant crops. In absence of oxygen.

Biogas is produced through the processing of various types of organic waste such as food waste, human waste, sewage, paper waste, manure, green waste, biodegradable plastic, and slaughterhouse.

Biogas production also helps in the easy disposal of organic wastes and is eco-friendly because combustion of biogas does not cause much pollution. It has high calorific value and is a non-polluting renewable source of energy

Biogas production starts from the arrival of feed-stocks at the biogas plant. A diverse range of solid as well as sludge-like feed-stocks can be used.

2. Materials Suitable for Biogas Production

• Biodegradable waste from enterprises and industrial facilities e.g. surplus lactose.
• Spoiled food from shops, homes, hotels and institutions.
• Bio-waste generated by consumers
• Sludge from wastewater treatment plants
• Manure and field biomass from agriculture

3. Types Of Biogas Systems

There are two types of biogas systems depending on the moisture content of the feed-stocks;

1. Wet biogas system&#; it&#;s the most common digester style. A wet digester or low solids AD system generally processes feedstock with less than 15 percent solids content
2. Dry biogas system &#; Dry digesters keep the substrates in a stackable form and remain in a pile during the digestion process. Food waste is mixed with green wastes such as yard debris for structure and porosity and is put into a long, rectangular vessel in a stack. The vessel is then sealed tight and warmed.

4. Types Of Small Scale Biogas Digesters

The table below gives a first comparison of the different types of biogas;

FactorsFixed doneFloating drumTabular designPlastic containerGas storageInternal Gas storage up to 20 m³ (large)Internal Gas storage drum size (small)Internal eventually external plastic bagsInternal Gas storage drum sizes (small)Gas pressureBetween 60 and 120 mbarUpto 20 mbarLow, around 2 mbarLow around 2mbarSkills of contractorHigh; masonry, plumbingHigh; masonry, plumbing, weldingMedium; plumbingLow; plumbingAvailability of MaterialyesYesyesYesDurabilityVery high >20 yearsHigh; drum is weaknessMedium; Depending on chosen linermediumAgitationSelf agitated by Biogas pressureManual steeringNot possible; plug flow typeEvtl Manual steeringSizing6 to 124 m³ digester volUp to 20 m³Combination possibleUp to 6 m³ digester volMethane emissionHighMediumLowMedium

1. FIXED DOME&#; A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable/ fixed rigid gas-holder and a displacement pit, also named &#;compensation tank&#;. Gas pressure increases with the volume of gas stored, i.e. with the height difference between the two slurry levels.

Advantages: 

• There are no moving or rusting parts involved.
• Its design is compact hence saves space and is well insulated.
• It&#;s a Source of employment during construction for both locals and skilled people.
• Has a long life span if well constructed and fixed.
• The underground construction saves space and protects the digester from temperature changes.
• Low initial and construction costs and long useful life-span;

Disadvantages: 

• Masonry gas-holders require special sealants and high technical skills for gas-tight construction
• Gas leaks occur quite frequently
• Fluctuating gas pressure complicates gas utilization
• Amount of gas produced is not immediately visible.
• Fixed dome plants need exact planning of levels.
• Excavation can be difficult and expensive in bedrock.
• Constructed and supervised by highly experienced biogas technicians.

• FLOATING-DRUM PLANTS&#; Floating-drum plants consist of an underground digester (cylindrical or dome-shaped) and a moving gas-holder.  Floating-drum plants consist of an underground digester (cylindrical or dome-shaped) and a moving gas-holder. The gas-holder floats either directly on the fermentation slurry or in a water jacket of its own.

They are chiefly used for digesting animal and human feces on a continuous-feed mode of operation.

They are used most frequently by small to middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-industrial estates (digester size: 20-100m3).

The floating-drum must not touch the outer walls. It must not tilt, otherwise the coating will be damaged or it will get stuck.

Disadvantages:

• The steel drum is relatively expensive and maintenance-intensive.
• Removing rust and painting has to be carried out regularly.
• The life-time of the drum is short (up to 15 years while in tropical regions about 5years
• The gas-holder gets &#;stuck&#; If fibrous substrates are used in the resultant floating scum.

Advantages

• The floating gas drum can be replaced by a balloon above the digester.
• Readily available materials are used to construct the digester.

• LOW &#; COST POLYETHYLENE TUBE DIGESTER- The tubular polyethylene film is bended at each end around a 6 inch PVC drainpipe and is wound with rubber strap of recycled tire-tubes. With this system a hermetic isolated tank is obtained.

Advantages

• Easy to solve the technical problems arising in the polyethylene tube digester.
• Economically, it saves the producer costs of buying liquid gas, firewood and chemical fertilizers. Low cost biodigester.
• It helps minimize health hazards such air pollution
• Reduces environmental threats, due to decrease in use of chemical fertilizers and organic waste exposures.
• There is also reduced deforestation from firewood use for energy purposes.

Disadvantages

• Has a short lifespan meaning the biodigester is not long lasting.
• There is no reliable back up support.
• PTDs suffer from effects of variable temperatures.
• The bio digesters can only function properly and last a little longer if the farmer takes good care of the digester.

• BALLOON PLANTS&#; A balloon plant consists of a heat-sealed plastic or rubber bag (balloon), combining digester and gas-holder. The gas is stored in the upper part of the balloon. The inlet and outlet are attached directly to the skin of the balloon. Gas pressure can be increased by placing weights on the balloon. The useful life-span does usually not exceed 2-5 years. Safety valves are placed to regulate gas pressure.

Advantages

• Standardized prefabrication at low cost
• low construction sophistication
• Ease of transportation
• Shallow installation suitable for use in areas with a high groundwater table
• High temperature digesters in warm climates
• Uncomplicated cleaning
• Emptying and maintenance

Disadvantages

• Low gas pressure may require gas pumps
• scum cannot be removed during operation

5. Various Stages Of Biogas Production

Biogas is produced using well-established technology in a process involving several stages:

1. Collection, transport and processing of biogas feed-stocks&#; Raw materials or Feedstocks used in biogas production are delivered to biogas plants and the emissions put into account. Biowaste is crushed into smaller pieces and slurrified to prepare it for the anaerobic digestion process. Slurrifying means adding liquid to the biowaste to make it easier to process Feedstock processing and odor control also is put to consideration in this stage.
2. Biogas production, upgrading and injection into the gas network- s regards biogas production and upgrading, emissions from heat and electricity consumed at biogas plants, emissions from the production of chemicals used in the biogas process and emissions related to water consumption and wastewater treatment are taken into account
3. Biogas distribution logistics&#; Emissions related to biogas transmission in the gas pipeline network consist of methane emissions and carbon dioxide emissions from compressor stations and transmission pipelines. Containers are also used for biogas transport.
4. Emissions from biogas use (tank to wheel, TTW)-  In the final stage, the gas is purified (upgraded) by removing impurities and carbon dioxide.

6. Steps Used in Biogas Production

1. Hydrolysis

It involves the conversion of polymeric organic matter (e.g., polysaccharides, lipids, proteins) to monomers (e.g., sugars, fatty acids, amino acids) by hydrolysis secreted to the environment by microorganisms.

• Acidification

Acidification increased biogas and carbon dioxide production in five cases, increased methane production and reduced nitrogen production in four cases, and reduced methane content in biogas in four of five cases.

• Methane formation

In practice this means that microbes feed on the organic matter, such as proteins, carbohydrates and lipids, and their digestion turns these into methane and carbon dioxide.

7. Importance Of Biogas

Biogas systems protect our air, water, and soil by recycling organic waste into renewable energy and soil products. Biogas is beneficial in the following categories;

1. Waste treatment benefits
2. Natural waste treatment process
3. Mature technology
4. Smaller physical footprint (vs. composting)
5. Reduces volume of waste for transport, land application, (vs. not using digestion)
6. Very efficient decomposition
7. Complete biogas capture
8. Nutrient recovery and recycling
9. Energy benefits
10. Net-energy producing process
11. Multiple existing biogas end-use applications, including; heat-only, electric-only, combined heat & power, pipeline quality biomethane and transportation fuel.
12. Baseload/dispatchable energy source(vs. intermittent wind and solar)
13. Distributed generation (which means lower transmission / transportation costs and higher reliability)
14. Direct replacement for non-renewable fossil fuel
15. Environmental Benefits
16. Dramatic odor reduction
17. Reduced pathogen levels
18. Reduced greenhouse gas emissions
19. Platform for reducing nutrient runoff
20. Increased crop yield
21. Economic Benefits
22. Jobs (temporary/construction and permanent)
23. Turns cost item (i.e., waste treatment) into revenue-generating opportunity
24. Can operate in conjunction with composting operations
25. Improves rural infrastructure and diversifies rural income streams
26. Digestate produced by the system can replace synthetic fertilizer or bedding purchase

8. Advantages & Disadvantages of Using Biogas

Advantages

• Biogas is Eco-Friendly.
• Biogas Generation Reduces Soil and Water Pollution.
• Biogas Generation Produces Organic Fertilizer.
• It&#;s A Simple and Low-Cost Technology That Encourages A Circular Economy.
• Healthy Cooking Alternative for Developing Areas.

Disadvantages

• Little Technological Advancement. An unfortunate disadvantage of biogas today is that the systems used in the production of biogas are not efficient.
• Contains Impurities.
• Effect of Temperature on Biogas Production.
• Less Suitable For Dense Metropolitan Areas.
• Flammable
• Highly toxic
• Potentially explosive

HOW TO EFFICIENTLY SERVICE AND MAINTAIN BIOGAS DIGESTER

9. How to Efficiently Service and Maintain Biogas Digester

Maintaining a biogas plant involves;

• Doing minor repairs to the equipment,
• Changing its oil as needed,
• Removing debris from organic matter that falls to the bottom of the tank,
• Solving problems in the process
• Removing debris from organic matter that falls to the bottom of the tank
• Breaking and removal of any scum formed at the top of the slurry.
• Leakage test trouble shooting along the gas line.
• Inspection of any clogged/blocked joints at the utilization point
• Check the air blower on monthly basis.

10. Benefits Of a Rigorous / Regular Maintenance Of Biogas Plant

Among other things, the advantages include:

• Prevention of technical failures or problems in the process
• Increase in the life cycle of the equipment
• Prevention of accidents and safer plant environment
• Optimization of a plant operation or of biogas production.

11. How To Stop Biogas From Smelling

The method comprises the following steps:

(1) Firstly coarsely filtering through a mechanical grid, separating biogas residues from biogas slurry and introducing the biogas slurry into a reaction tank.

(2) Adding ferrate into the reaction tank and performing oxidation and deodorization;

(3) Introducing the biogas.

12. How To Minimize Risks and Ensure Safety During Every Step Of Your Biogas Project

The operator and the plant designer have to take certain measures at every step of a project. The goal is to ensure safety and minimize risks.

1. Plant design

This step is particularly important to ensure the safety of the biogas plant. The operator and the plant designer have to pay attention to:

• The norms, guidelines and all other codes that apply to biogas plants
• The classification of explosion zones, since the electric system installed on the biogas must be suited according to the risks of explosion
• Avoiding confined space
• Potential risks that can happen during the operation of the plant

2. Project construction

• Make sure to plan the project rigorously
• Hire an onsite expert to insure the workers apply the health and safety measures that are established

3. Biogas plant commissioning

Commissioning of a biogas plant can be the most dangerous step of a project.

Accidents that can happen include:

• Structural failure rarely happens when the tank is being filled or the high-pressure pipes are being tested
• Accidental hydraulic discharge during the pre-operation test of the pumps and valves
• Lack of calibration of the health and safety equipment
• Higher risks of explosion when the air inlets are opened and the air reach biogas

4. Biogas plant operation

During this step, a lot of accidents and incidents happen. To avoid them, the operator must:

• Train every operator of the plant for the work in confined space, portable gas detection, process and equipment use
• Apply strict procedures for equipment locking
• Regularly verify health and safety equipment to make sure they are calibrated and offer precise measurements
• Do a visual screening of all equipment to detect leaks and verify the state of equipment
• Make sure all workers apply health measures to avoid pathogen diseases
• Train all plant workers for basic firefighting skills and CPR

13. Summary

The biogas production process is the same for all types of biogas, and it leverages chemical reactions that are 100% natural. By placing biomass (organic waste) in a digester, you enable bacteria to break down the organic elements and turn them into biogas, which can then be used to generate energy. 

It&#;s a zero-waste process that reduces the amount of waste that ends in landfills while permitting you to produce cleaner energy than by using fossil-fuel sources. 

The process can occur at a large scale in industrial plants, but it can also be adapted for domestic usage, so you can easily have a small biogas station and produce energy for your house&#;s needs.

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