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Biomethanation (Anaerobic gasification) |
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Biomethanation (Anaerobic gasification)
Municipal solid waste is a heterogeneous waste and contains the following fractions:
1. Putrescible fraction
This is also called digestible fraction and contains biodegradable organic matter such as kitchen waste, vegetable market waste, paper, grass cutting, and yard trimmings. Putrescible fraction represents 40% of MSW in India.
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2. Combustible fraction
Also known as refractory organics, these are either slowly digestible or indigestible organic matter such as wood, plastics, rubber, and other synthetics. They represent around 20% of MSW in India.
3. Inert fraction
They are typically non-digestible and non-combustibles such as stones, sand, glass, and metals. They represent 15% of the MSW in India.
4. Remaining 25% is the moisture content.
The putrescible fraction is ideally suited to produce biogas and the remaining slurry is a good fertiliser. There aredifferent types of biogas technologies. MSW is subjected to mechanical segregation to obtain putrescible fraction, i.e. ROP. The digestible organic fraction thus obtained is kept as pulp in hydrolysis tanks for breaking them into smaller molecules. The hydrolysed pulp is then fed into anaerobic digestion tanks. Here it is digested anaerobically (in the absence of air), in the specially designed digesters.
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Under this active bacterial activity, the digested pulp produces the combustible gas CH4, and inert gas, CO2, The CH4 gas is then used to produce power through a biogas engine connected to an electric generator. The remaining digestate (slurry) is a soil conditioner of good quality and free from pathogens. With the help of a solid or liquid separator, organic fertiliser is obtained and the treated water can be safely used for irrigation.
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Depending upon the MSW quality, quantity and local environmental and climatic conditions, there are various commercially viable technologies available globally. In all the technologies the following steps are involved:
Sorting
The putrescible fraction is separated either manually or mechanically.
Particle Size Reduction
To provide maximum surface area to the bacteria, the particle size reduction is carried out by using screw cutting, milling, drumming, pulping, or shredding machines.
Digestion
The material is then fed into anaerobic digesters for gas generation.
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Post-treatment
The slurry or digestate is matured for two to four weeks to make an agriculture or horticulture quality fertiliser or soil conditioner. The core of the whole technology is anaerobic digestion. There are many technologies for effective digestion which differ from each other depending upon their digestion parameters. They are briefly explained below:
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Dry batch
The MSW is fed batchwise in the digester after inoculating it with the slurry of the previous batch. The load is allowed to digest for 20-30 days till maximum gas is recovered. Commercially it is available as BOCELL process. It has the typical disadvantages of instability and material handling, pertaining to batch process.
Dry continuous
DRANCO, V ALORGA, and KOMPOGAS are the commercially available technologies in this category. They operate at the solid concentration of 20%-40% and they achieve high loading rates and minimise the requirement of water. Recycled effluent is mixed with fresh charge of MSW during semi-continuous feeding. They are ideally similar for thermophilic digestions (50-55 °C).
Wet continuous
REFCOM technology uses the MSW solid concentration <10% using large amount of water. This is most ideal if mixed with sewage sludge, animal slurries, and industrial waste. To avail the large disposal of liquid, the effluent liquids are used to dilute the feed. This technology is usually not chosen if it is to be used only for MSW.
Wet multistage
BTA and PAQUES designs are commercial technologies available in this category. MSW is converted into slurry with water and fermented in the first stage. The slurry with volatile acids is then converted to biogas in the second stage using high rate anaerobic digesters such as the anaerobic filter or UASB (upflow anaerobic sewage blanket) digesters. The major disadvantage is the complexity in design and operation. Highly skilled manpower is also required.
Sequencing batch
This is a new concept known as SEBAC but yet to be fully commercialised. This is similar to the dry batch type technology except that leachate from the base of one digester is used to inoculate, and remove the volatile acid from other digester in the series. The digestion is usually allowed for 20-30 days in one digester and the digesters are filled every week in sequence so that continuous gas supply is available through the centralised gas collection system. High solid content over 30% is used in it.
The advantage of this design is that volatile acids are not allowed to accumulate and anaerobic bacteria keep on getting food continuously. The only disadvantage is the lack of continuous feed.
Status and cost
Anaerobic digestion technology using mix-waste, bio-waste, and manure is working on a commercial basis in at least 15 countries, mostly in Europe, with capacity ranging from 4000 to 220 000 tonnes per year. Only a few of them work purely on MSW but the number of plants working solely on MSW is increasing, and many new ones are on the pipeline. There are some promoters such as ENKEM, CICON, GENL, and NSTLER who have offered to take up such plants using MSW on BOO basis in India. On an average they have estimated the total cost of the plant at 80-100 million rupees per 100 TPD capacity of the system. It has been observed that the cost of anaerobic digestion plants have reduced with time and is expected to be most competitive as compared to the other options. The most attractive aspect is environmental gain, i.e. no pollution and soil conditioning.
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