Anaerobic Digester: How It Works [Inputs & Outputs]

Process Inputs & Outputs

Inputs
Feedstocks
Digesters typically can accept any biodegradable material, with the level of putrescibility (the degree to which it decomposes) being the key factor in biogas yield. While sewage and different types of manure are most common, they yield less than non-woody organic waste (grass and plants) which have not already had the energy content taken out. A wetter feedstock with a 20-30:1 Carbon:Nitrogen ratio is most efficient as it moves through standard pumps easier and improves biogas yield.

Outputs
Biogas
Biogas is comprised primarily of methane and carbon dioxide, with some small amounts of hydrogen. The methane in biogas is typically burned to produce heat and electricity, which can be used to power on-site facilities and/or be sold to the local energy grid (the energy generated qualifies as renewable from the utility’s perspective).
Scrubbed, concentrated biogas can also be used as an alternative fuel. This is used in some European countries, but has the drawbacks of high energy input for the conversion and higher costs relative to fossil fuel derived fuels, which includes the cost required to ’scrub’ the hydrogen sulfide in post-biogas production or during biogas production with the inclusion of ferric chloride (FeCl3) to the digestion tanks.
Digestate
Digestate is the solid remnants of the original input material to the digesters that the microbes cannot use and the mineralized remains of the dead bacteria used in the digesters. Digestate can come in three forms: fibrous (acidogenic digestate), liquor (methanogenic digestate) or a sludge-based combination of the two fractions. In two-stage systems the different forms of digestate come from different digestion tanks. In single stage digestion systems the two fractions will be combined and if desired separated by further processing.
-Acidogenic digestate is a stable organic material comprised largely of lignin and cellulose, as well as a variety of mineral components. This material is similar to compost, but with a greater lignin content.
-Methanogenic digestate is rich in nutrients and can be used as a fertiliser dependent based on the levels of potentially toxic elements (PTE) in the initial feedstock. Most non-industrial waste streams have low PTE levels.

Wastewater
Wastewater originates from the moisture content of the original feedstock that was treated and the water produced during the microbial reactions in the digestion systems. This wastewater requires treatment.