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SELECTING WASTE FOR DIGESTER

Lesson 3.

This technology can be applied at the household level, in small neighbourhoods or for the stabilization of sludge at large wastewater treatment plants. Biogas reactors provide energy for cooking, lighting and heating as well as fertilising sludge for soil improvement. In rural areas, they are often used for the digestion of animal manure, kitchen waste (and only optionally toilet wastes).


Often, a biogas reactor is used as an alternative to a septic tank, since it offers a similar level of treatment, but with the added benefit of biogas. However, significant gas production cannot be achieved if blackwater is the only input. The highest levels of biogas production are obtained with concentrated substrates, which are rich in organic material, such as animal manure and organic market or household waste. It can be efficient to co-digest blackwater from a single household with manure if the latter is the main source of feedstock. Greywater should not be added as it substantially reduces the HRT. Wood material and straw are difficult to degrade and should be avoided in the substrate.


Biogas reactors are less appropriate for colder climates as the rate of organic matter conversion into biogas is very low below 15°C. Consequently, the HRT needs to be longer and the design volume substantially increased.



Advantages

  • Generation of renewable energy
  • Small land area required (most of the structure can be built underground)
  • Can be built and repaired with locally available materials
  • No electrical energy required
  • Combined treatment of animal, human and solid organic waste
  • Conservation of nutrients
  • Long service life
  • Low to moderate capital costs; low operating costs


Disadvantages

  • Requires expert design and skilled construction
  • Substrates need to contain high amounts of organic matter for biogas production
  • Incomplete pathogen removal, the digestate might require further treatment
  • Limited gas production below 15°C
  • Requires seeding (start-up can be long due to the low growth yield of anaerobic bacteria)


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