Used as an essential part of an integrated waste management system, an anaerobic digester reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is a source of renewable energy as it produces a methane and carbon dioxide rich biogas that can be used to replace the use of fossil fuels as a source of energy.
The method has found to be especially suitable to wet organic material and is generally used for effluent and sewage treatment. It is a simple process, which can significantly reduce the amount of organic matter. Almost any type of organic material can be processed with anaerobic digestion, including bio-degradable waste materials, such as waste paper, grass clippings, leftover food, sewage and animal waste. An exception to this is woody wastes, which are generally unaffected by digestion as anaerobes are unable to degrade linen.
In an anaerobic system there is an absence of gaseous oxygen and hence the anaerobic digesters are designed to prevent the gaseous oxygen from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from other sources and not the surrounding air. The source of oxygen for these microorganisms can either be the organic material itself or can be supplied by the inorganic oxides from within the input material.
Anaerobic digester is a machine that works on the principle that in the absence of oxygen freely (naturally) occurring bacteria will break down the manure. The digestion of the manure occurs in 4 basic phases, viz. –
- Hydrolysis
- Acidogenesis
- Acetogenesis
- Methanogenesis
Most of the remaining biogas is CO2, with a little amount of H2S and traces of other gases. The digested manure requires being stored until the land applied. The bacteria that produce methane flourish at around body temperature (95°-105°F), and hence the heated digesters are more efficient methane producers than non-heated ones.
Types
The anaerobic digesters can be primarily categorized into three main types –
- Covered lagoon digester
- Complete mix digester
- Plug flow digester
Covered Lagoon Digester
A covered lagoon digester is an anaerobic digester that consists of a manure treatment lagoon with an impermeable cover and is usually not heated. The cover traps the gas formed during decomposition of the manure. Covered lagoon digesters are primarily used for the liquid manure (less than 2 % solids) and need large-volume lagoons. Since the rate of methane production is dependent on ambient temperature with a covered lagoon system, these digesters are not considered as cost-effective to use the biogas for producing energy.
Complete Mix Digester
A complete mix digester is ideal for manure, which is 3-10 5 solids, like a swine manure or dairy manure collected by a flush system. These types of digesters process the manure in a heated tank below or above the ground. A mechanical or gas mixer sustains the solids in suspension. However, a complete mix digester is expensive to design and costs more than a plug-flow digester to maintain and operate.
Plug – Flow Digester
Plug-flow digesters have found to be suitable for ruminant animal manure that has a solid concentration of 11-14 %, such as cow manure amassed by scraping. A flush system for collecting the manure is not suitable for this system, as this would lower the total solid content of manure below the specified level. In manure that have low solids concentrations, like a swine manure, the solids cannot remain in solution and are tend to settle to the bottom of the tank, determining their digestion. A plug-flow digester requires minimal maintenance and has fewer moving parts.
Other Types of Anaerobic Digesters
Besides the three main types of anaerobic digesters, other types of anaerobic digesters are also available in the market that have been used for processing industrial waste as well as municipal sewage. Most of these digesters treat the waste stream with a low solids content, and hence have found several ways to speed up the process of digestion or increase the solids content. This helps in reducing the volume required for digesting thereby reducing the costs. Some of these anaerobic digesters include -
- Batch-fed reactor
- Suspended particle reactor
- Anaerobic pump digester
- Fluidized- and expanded-bed reactors
- Temperature-phased anaerobic digester
- Continuously stirred tank reactors with solids recycle
- Fixed-film anaerobic digester
- Anaerobic filter reactor
- Up-flow solids reactor
- Up-flow anaerobic sludge blanket reactor