148 Organic waste recycling: technology and management
4.1.1 Production of an energy source
The production of an energy resource (biogas) from anaerobic digestion of
organic wastes is the most tangible benefit of biogas technology. Practicing
biogas production in rural areas can have several advantages such as relieving
demand on electricity, coal, oil and firewood, and those associated with
administrative/distribution network. Organic matters required for biogas
production are abundant and easily available. Decreasing the demand for
firewood spares the forest and furthers afforestation efforts.
4.1.2 Waste stabilization
The biological reactions occurring during anaerobic digestion in the biogas
digester reduce the organic content of waste materials by 30 - 60% and produce
a stabilized sludge which can be used as a fertilizer or soil conditioner.
4.1.3 Nutrient reclamation
The nutrients (N, P and K) present in the wastes are usually in complex organic
forms that are difficult for the crops to uptake. After digestion, at least 50% of
the nitrogen present is as dissolved ammonia which can be nitrified to become
nitrate when applied to crops (Equation 3.5) or is readily available to crops.
Thus digestion increases the availability of nitrogen in organic wastes above its
usual range of about 30-60%. The phosphate and potash contents are not
decreased and their availability of around 50% and 80%, respectively, are not
changed during digestion. Anaerobic digestion does not lose or destroy any of
the nutrients from domestic and farm wastes, but makes them more available to
plants. In addition to being used as a fertilizer, the biogas digester slurry also
acts as a soil-conditioner and helps in improving the physical properties of the
soil. The application of digester slurry to unproductive soils would eventually
improve the soil quality, or the useless lands can be reclaimed.
4.1.4 Pathogen inactivation
During the digestion process, the waste is kept without oxygen for a long period
of time (15-50 days), at about 35°C. These conditions are sufficient to inactivate
some of the pathogenic bacteria, viruses, protozoa and helminth ova.
However, biogas technology has some drawbacks. When compared with
other alternatives, such as composting, biogas production can be construed as
the only major advantage of this technology. Other advantages, e.g. waste
stabilization and pathogen inactivation, are fulfilled better by composting (a
comparative analysis between biogas technology and composting is summarized