An anaerobic digester – also called a biogas digester or simply a digester – is an air-tight chamber that is used to produce biogas and a rich organic fertilizer called digestate. When you feed organic materials (also called feedstock, raw slurry etc.) like animal manure or plant wastes into an anaerobic digester, it will decompose in the absence of air to produce the two products previously mentioned.
The act of decomposing in the absence of air is called anaerobic digestion or simply digestion. So we can say that the anaerobic digester is digesting the feedstock fed into it. In this article, we’re going to focus only on the popular types of digesters.
Digesters are generally classified into batch-type and continuously-fed digesters base on the type of feeding/loading.
A. Batch-type Digester: In this type of digester, the feedstock is loaded into the digester and left there until anaerobic digestion is complete (after about 30-60 days depending on the type of feedstock). The digestate is then discarded and a fresh batch of feedstock is loaded. Batch digesters usually require high labour input.
B. Continuously-Fed/Continuous Digester: Here, the feedstock is continuously fed into the digester on a daily basis. As new feedstock is fed, digested ones flow out automatically through an outlet pipe.
Continuous digesters are not that labour intensive and gas production is not interrupted as in batch types. Most digesters operate in the continuous mode. Below are some types of Continuous digesters.
1. Bag Digester
This is the cheapest type of digester. It consists of a long cylindrical plastic bag with an inlet and outlet at opposite end. A third opening at the top is for the biogas outlet.
Another type of anaerobic lagoon called the covered lagoon has an impermeable cover floating on the manure. They are more cost-effective to build because they are not completely sealed.
Anaerobic lagoon systems usually have 2 lagoons: a primary lagoon which is covered or sealed to trap biogas and a secondary lagoon for storing digestate and manure contaminated runoff.
Anaerobic lagoons are normally used in dairy farms that clean their pens by flushing. The solid content of the manure should be less than 3% to avoid accumulation of sludge which will gradually reduce the volume of the digester.To avoid scum formation, fibrous solids should be screened before the wastewater enters the digester. The scum, which is a floating layer of organic matter, has a tendency to grow in thickness and eventually prevent biogas from reaching the gas storage chamber above it. The screened solid will not attract flies and does not have manure odour.
4. Complex-Mix Digesters: These are enclosed tanks with mixing systems that can either be by mechanical agitation, effluent recirculation or biogas recirculation. They might also have a heating system for heating during cold weather when methane production is very low or nil. This type of digesters can digest manure with solid concentration of 3-10% and are constructed with coated steel or concrete.
5. Floating Tank Digester (Gobar Gas Plant): This design originates from India. It consists of a gas storage tank that floatsin the digester chamber. The drum was originally made of mild steel butdue to corrosion problem, it was later replaced by fibre-glass reinforced plastic.The tank must be properly anchored to prevent it from overtopping due to high gas pressure. The digester chamber can be constructed with bricks or reinforced concrete (more expensive).
The floating tank design is able to maintain a constant gas pressure, making it convenient to channel the gas directly to gas equipment without the risk of overpressure.
6. Fixed Dome Digester (Drumless Digest): This one originated from China.It consists of an underground digestion chamber built with bricks, on top of this is a gas storage concrete dome. The digestion chamber and the gas storage dome form one unit.Fixed dome digesters can last for 20-50 years.
A cheaper version of the original Chinese fixed-dome design is the Deenbandhu model. In this model, the dome structure is constructed of brick masonry instead of concrete. One disadvantage of this model is the highly skilled masons required toaccurately construct the dome out of brick-in-cement masonry.
7. Fixed-film Digesters: The unique characteristic of fixed-filmdigestersis their peculiar very short retention time. It can be as short as 2-4 days! This means that they can treat very large volume of slurry in few days. The reason for this short retention time is due to the large number of bacteria that take part in the anaerobic digestion. Fixed-filmdigesters concentrate large number of anaerobic bacteria on a packing medium like plastics, stones, corals etc.
Fixed-film digesters are suitable for handling slurries with less than 2% solid content. This makes them suitable for treating flushed manure and wastewater. Fibrous solids should be removed prior to digestion or else they will clog the packing medium. However, higher levels of solids can be tolerated depending on biodegradability. It can also tolerate some fine sand grains in the wastewater.
8. Upflow Anaerobic Sludge Blanket (USAB)
The USAB is similar to the anaerobic filter since it involves concentrating bacteria on a medium. However in the USAB, the medium used to concentrate biogas-forming bacteria is the sludge blanket that covers the lower part of the digester. The slurry is fed from the bottom of the digester and it flows through the sludge blanket. This type of digesters is common in Europe where they are used to treat dilute sewer and industrial wastes.
References
1. Centre for Energy Studies Institute of Engineering. Advanced Course in Biogas Technology. Lalitpur: Biogas Support Programme (BSP), 2001.