What are biodigesters?

A biodigester system utilizes organic waste, particularly animal and human excreta, to produce fertilizer and biogas. A biodigester consists of an airtight, high-density polyethylene container within which excreta diluted in water flow continuously and are fermented by microorganisms present in the waste. The fermentation process is anaerobic, i.e., it takes place without oxygen, and the bacteria responsible for decomposition are methanogenic (i.e., they produce methane, also known as biogas).

With the surge in bio-based activities around the globe, a new concept called bio-refining starts to emerge. IEA Bioenergy Task 42 on Biorefineries defines biorefining as “the sustainable processing of biomass into a spectrum of marketable products and energy”. A bio-refinery combines/integrates a series of biomass conversion technologies to produce a range of products and (base-)materials, such as food, feed, chemicals, materials, oil, gas, heat and/or electricity. The concept is similar to a conventional oil-refinery where multiple petroleum products and fuels are produced.

Agricultural lands (lands used for agricultural production, consisting of cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops) occupy about 40- 50% of the Earth’s land surface. Agriculture accounted for an estimated emission of 5.1 to 6.1 GtCO₂-eq/yr in 2005 (10-12% of total global anthropogenic emissions of greenhouse gases (GHGs)).

Manure coverage is the practice of covering the surface of manure with materials of certain thickness instead of the traditional method of piling up manure to be exposed to air. Manure coverage changes the amount of manure surface in contact with air. Due to some reactions, i.e., a series of physical, biological and chemical reactions, it can reduce GHG emissions.

Fertiliser and manure management in rice fields are important methane mitigation technologies. The fertiliser management mitigation option includes changes in: fertiliser types; fertiliser nutrient ratios; the rates and timing of applications; and use of nitrification inhibitors to reduce methane emissions by affecting methanogenesis in rice fields. Rice cultivation is responsible for 10% of GHG emissions from agriculture. In developing countries, the share of rice in GHG emissions from agriculture is even higher, e.g., it was 16% in 1994.