Anaerobic digestion (AD) can be used for farms, businesses and municipalities as a productive solution to a growing waste problem throughout the world. In addition to AD, waste-to-energy is done in landfills using landfill refuse and landfill gases (such as methane). The use of AD in a biomass plant is a cost-effective way to produce renewable energy. AD also leads to less landfill waste and is a constructive way for farms, businesses and municipalities to dispose of waste. AD is the process of turning agricultural waste (such as livestock manure), wastewater, or municipal, commercial and industrial waste streams (such as food processing waste), into energy. AD uses micro-organisms to break down organic material and create biogas (biogas consists mostly of methane and CO2).
Instead of waste simply ending up in landfills, or being incinerated, waste can be turned into energy. Farms can be entirely powered by waste from their livestock, food waste and wastewater. Use of AD can make also make wastewater treatment facilities energy neutral or even energy positive, translating to huge cost savings for municipalities.
Organic waste finds a purpose in an AD biogas plant, as it is put in a digester, along with various types of micro-organisms (enzymes, bacteria etc…), to transform the waste into energy (methanation). The molecules of the organic material are broken down in the plant into a useful form like glucose. The “digested” raw material is then used to create biogas (and digestate which can be used as fertilizer).
The biogas can then be purified (and also optionally be upgraded with hydrogen) and turned into pipeline-quality synthetic natural gas for the grid. Biogas can also be turned into compressed natural gas (CNG) for vehicles. The anaerobic process also occurs naturally (as in landfills), in addition to the man-made construct in a biomass plant.
An anaerobic digester and biomass plant generate biogas (and/ or biomethane) which can be burned on-site to generate heat, power or both (so, combined heat and power – CHP). AD is mostly used by farms and wastewater treatment facilities for on-site electrical and heating generation, although it can also be used in a variety of other applications. Biogas can also be purposed as an energy source for the grid when purified, and turned into pipeline-quality synthetic natural gas (or turned into biomethane and used for heat or transportation as CNG). Also produced in the process is digestate, which is a source of nutrients that can be used as a fertilizer.
Biogas can also be upgraded with hydrogen, combining the outputs of a biogas plant and an electrolyzer, creating biomethane. Like conventional natural gas, biomethane can be used as a transportation fuel in the form of compressed natural gas (CNG), or liquefied natural gas (LNG). When biogas is used for heat or transportation, as biomethane, CNG, (biomethane – CNG – can be used in place of diesel, given modifications to the vehicles in question), there are tremendous greenhouse gas reductions.
The entire bus fleet in Oslo, Norway, is run on CNG from sewage treatment and organic waste, and they see a dramatic (around 70%) reduction in greenhouse gas (GHG) emissions compared to fossil fuel burning vehicles. Food waste and other waste processed through AD also brings the benefit of reducing GHG emissions substantially by reducing landfill waste. When AD is used for on-site electrical generation, energy generation for a municipality, farm or wastewater facility, GHG emissions overall are greatly reduced. Energy produced by AD has a very low overall carbon footprint.
The AD plant at Cannock, Staffordshire, England (called the Poplars AD plant) is an example of a successful, large-scale AD plant. The £24 million project treats commercial and industrial food and waste to create, through methanation, around 6MW of renewable energy, synthetic natural gas, for the national grid. The Poplar plant shows that a large-scale anaerobic digestion project is viable. AD has been successful in many commercial operations as well. For example in Orlando, Florida, food waste sourced primarily from the Walt Disney World Resort is fed through an anaerobic digester, producing enough electricity to meet the needs of over 16,000 homes.