Organic matter + water (anaerobes) CH4+CO2 +new biomass +NH3 +H2S +heat
This process is commonly used to treat wastewater sludge and organic wastes as it reduces the waste volume, produces valuable products and reduces the emission of methane and CO2 from land fill sites. The end product of digestion is the nutrient-rich solid which can be used as a fertilizer [50].
There are two basic types of anaerobic digesters;
(1) Batch: Batch digesters are the simplest. The process involves loading the waste into the digester and starting the digestion process. The retention time depends on temperature, pH and other factors. Once the digestion is complete, the residue is removed and another batch started.
(2) Continuous: Continuous digesters involve regular feeding of waste into the digester to continuously produce biogas. This type of the digester is suitable for large-scale operations.
The following examples involve treatment of organic waste which may be adaptable to hatchery wastewater with various levels of solids;
(1) Covered lagoons: The organic waste is covered by a pontoon or other floating cover. This digester is suited for manure waste with 2% or less solid content and requires high throughput to provide enough solids for the bacteria to produce gas. It is better for warmer regions, where digester temperatures can be easily maintained.
(2) Complete mix: The organic waste is added into a silo shaped tank, then heated and mixed for anaerobic digestion. It is suited for organic waste with 2–10% of solids.
(3) Plug flow: This method comprises a cylindrical tank in which the end products are released from one end while fresh organic waste is fed in from the other end. Hot-water is piped through the tank to maintain the digester temperature. It is suited for organic wastes with 11–13% solids.
(4) Fixed film: A tank is filled with a plastic medium that supports a biofilm. It is suited for 1–2% solids in an organic waste and a short retention time (2–6 days) [51].
(5) Plug-flow type polybag digesters (polydigesters): These are prototype digesters developed for organic waste treatment systems [49] and operate on the same principle as a continuous flow digester.
Since the 1970s, underground anaerobic digesters at various scales of operation to process rural organic wastes have been used in China. In these systems, a cylinder shaped reactor made from concrete and brick with a cement lid are used. Waste is manually fed into the reactor through a port connected to the base of the reactor The heavy lid prevents gas leaks [52] and pressurises the methane produced enabling it be piped to various areas for domestic or commercial use.