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[[Image:Anaerobic_digestion_icon.png|right]]
In a digestion process, organic matter from human, animal or vegetable waste is broken down by microbiological activity, in the absence of air. This anaerobic process produces a combustible gas, methane, a source of (biogas) energy. The digestion process takes a couple of weeks to a couple of months after which the remaining slurry can be removed, either continuously or batch-wise. Several options are available, ranging from simple digestion techniques to technologically complex designs on a household or municipal scale. A domestic anaerobic digestion technique `fixed dome type' consists of a simple biogas tank with a flat bottom and a round chamber covered with a dome shaped concrete gasholder. The gas is captured in the upper part of the digester. Gas pressure increases with the volume of gas stored, pushing the slurry into a separate outlet tank (see illustration).
[[Image:Anaerobic_digestion.PNG|thumb|right|150px|[[Dry toilet |Dry toilet]], in Vietnam (for credits, click the picture)]]
{| border="1" cellpadding="5" cellspacing="0" align="center"
|-
! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdead;" | Disadvantages
|-
| valign="top" | Excreta `out of sight'. <br>
Net production of clean renewable biogas. <br>
Elimination of visual contaminants (e.g. toilet paper). <br>
Low need for operational control and maintenance.
| valign="top" | Gas safety risk. <br>
Slurry from digesters has to be removed and treated. <br>
Insufficient pathogen removal without appropriate post treatment of sludge.
|}
== Costs ==
* Investment costs vary greatly depending on the overall plant concept. Costs for biogas production increase with decreasing climatic temperatures. Life expectancy ranges from about 20 - 25 years.
* Investment domestic biogas plant in Nepal, (fixed dome types from 4-20m3) US$ 300 400.
* Maintenance costs (8m3 digester) US$ 5.50 US$ 8.50 per year.
* Operational costs - Negligible.
== Applying conditions ==
<ul>
<li>Digesters are best suited to warm climates. </li>
<li>They are most appropriate in rural areas where animal manure can be added to the process. </li>
<li>The digestion process is sensitive to both temperature and materials. Both need to be controlled. </li>
<li>Relatively high skills are needed for construction. Operation and maintenance, however, are simple for batch systems.</li>
</ul>
== External links ==
* General information about Anaerobic digestion [http://www.itdg.org www.itdg.org ]
* General information [http://www.snvworld.org www.snvworld.org ]
* General information [http://www5.gtz.de/gate/techinfo/biogas http://www5.gtz.de/gate/techinfo/biogas ]
In a digestion process, organic matter from human, animal or vegetable waste is broken down by microbiological activity, in the absence of air. This anaerobic process produces a combustible gas, methane, a source of (biogas) energy. The digestion process takes a couple of weeks to a couple of months after which the remaining slurry can be removed, either continuously or batch-wise. Several options are available, ranging from simple digestion techniques to technologically complex designs on a household or municipal scale. A domestic anaerobic digestion technique `fixed dome type' consists of a simple biogas tank with a flat bottom and a round chamber covered with a dome shaped concrete gasholder. The gas is captured in the upper part of the digester. Gas pressure increases with the volume of gas stored, pushing the slurry into a separate outlet tank (see illustration).
[[Image:Anaerobic_digestion.PNG|thumb|right|150px|[[Dry toilet |Dry toilet]], in Vietnam (for credits, click the picture)]]
{| border="1" cellpadding="5" cellspacing="0" align="center"
|-
! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdead;" | Disadvantages
|-
| valign="top" | Excreta `out of sight'. <br>
Net production of clean renewable biogas. <br>
Elimination of visual contaminants (e.g. toilet paper). <br>
Low need for operational control and maintenance.
| valign="top" | Gas safety risk. <br>
Slurry from digesters has to be removed and treated. <br>
Insufficient pathogen removal without appropriate post treatment of sludge.
|}
== Costs ==
* Investment costs vary greatly depending on the overall plant concept. Costs for biogas production increase with decreasing climatic temperatures. Life expectancy ranges from about 20 - 25 years.
* Investment domestic biogas plant in Nepal, (fixed dome types from 4-20m3) US$ 300 400.
* Maintenance costs (8m3 digester) US$ 5.50 US$ 8.50 per year.
* Operational costs - Negligible.
== Applying conditions ==
<ul>
<li>Digesters are best suited to warm climates. </li>
<li>They are most appropriate in rural areas where animal manure can be added to the process. </li>
<li>The digestion process is sensitive to both temperature and materials. Both need to be controlled. </li>
<li>Relatively high skills are needed for construction. Operation and maintenance, however, are simple for batch systems.</li>
</ul>
== External links ==
* General information about Anaerobic digestion [http://www.itdg.org www.itdg.org ]
* General information [http://www.snvworld.org www.snvworld.org ]
* General information [http://www5.gtz.de/gate/techinfo/biogas http://www5.gtz.de/gate/techinfo/biogas ]
