Good Example Of Anaerobic Digesters Research Paper

Anaerobic treatment systems (more popularly known as anaerobic digesters) in the industries are primarily used to treat waste effluents rich in organic matter. In anaerobic digestion, bacteria in the absence of oxygen, convert organic matter into useful products such as sludge and biogas. The sludge can be used as fertilizer, while the biogas can be used as fuel. This biogas is primarily composed of methane (CH4, about 60%), which is the more useful component, and other gases (Deublein and Steinhauser, 2008). The methane content is responsible for the heating value at approximately 50.0 MJ per kilogram.
For anaerobic systems to work, there are five requirements that needs to be met (Lettinga, 1995). These requirements translate to design considerations of the bioreactor. First is the high retention of viable sludge in the reactor. This positively affects the loading capacity of the digester. Second, there must be sufficient contact time for the bacteria and the wastewater treated. In engineering design, this is termed as the retention time of the reactor. Third, there must be high reaction rates. The kinetics involved in the anaerobic digestion process has to be considered. Another condition to be met is the acclimatization of the bacteria. The biomass is a complex mixture of different bacteria performing different roles. It has to be adapted to the characteristics of the wastewater it is treating. Last is the maintaining of favorable environmental conditions for the bacteria. This is done through preconditioning by pH adjustment, equalization, and preheating (if needed).
Anaerobic digestion is compared most of the time to aerobic treatment (in the presence of oxygen). Each has their own set of strengths and weaknesses. Beers (2010) discussed the main strengths of anaerobic digestion: (a) positive energy balance, (b) high loading capacity, (c) intermittent operation-capable (d) little excess sludge, and (e) less maintenance costs. Anaerobic digesters provide energy in the form of methane creating a positive energy balance. This methane can be either used for heating purposes or electrical power generation. Another strengths is the high loading capacity since it can accommodate highly concentrated wastewater at greater than 1,500 mg/ liter COD. Anaerobic digesters are also capable of discontinuous operations, which is very useful for industries during production shutdown. Little excess sludge is produced since the organic content has been converted to methane. Lastly, it has less maintenance costs in terms of pump and impeller parts which are used in aerobic systems.
In terms of weakness, anaerobic digesters require preneutralization or pH adjustment for the methanogenic bacteria biomass. Anaerobic treatment also does not substantially reduce phosphorus and nitrogen levels. Lastly, investment costs are higher due to the inherent design of anaerobic digesters.
The anaerobic digester designs used today include (a) CSTR, (b) trickling filter, and (c) UASB reactors (Driessen & Vereijken, 2003). In terms of design, the lagoon or CSTR type (continuous-stirred tank reactor) is the most simple. The design is similar to aerobic tanks however, the biomass involved is anaerobic. The loading capacity is low due to having no special sludge retention mechanism. Another design is the trickling filter type which uses a carrier material where the bacteria biomass grow and sludge is accumulated. This takes advantage of gravity for vertical movement along the reactor from up to down. However, this design is prone to dead zones due to uneven due to non-uniform solid distribution in wastewater. The most common design used today is the UASB (upflow anaerobic sludge blanket). UASB is based on the concept that anaerobic sludge has good settling characteristics when not exposed to excessive movement. Wastewater flows from bottom upwards. There needs to be even feed distribution and biogas production to allow efficient mixing and high reaction contact time. This can be improved by applying recycle stream of effluent. The UASB reactor design is still undergoing improvements. Expanded sludge bed reactors include the fluidized bed and internal circulation types. Both mechanisms are developed for higher wastewater treatment efficiency.

REFERENCES

Beers, Angela. (2010). Energy from Wastewater: How Breweries can tap Undiscovered Energy Resources. Retrieved January 18, 2015 from http://envirochemie.com/cms/upload/downloads-en/fachbeitraege/Energy_from_wastewater_from_breweries.pdf
Deublein, D. and Steinhauser, A. (2008). Biogas from Waste and Renewable Resources: An Introduction. Weinheim: Wiley-VCH.
Driessen, W. & Vereijken, T. (2003). Recent Developments in Biological Treatment of Brewery Effluent. Livingstone: The Institute and Guild of Brewing Convention.