diagram of anaerobic digester

Anaerobic digestion

Instead of waste simply ending up in landfills, or being incinerated, waste can be turned into renewable energy by use of anaerobic digestion (AD). Anaerobic digestion is the process of producing energy from agricultural waste (such as livestock manure, or other organic waste from farm and agricultural processes), wastewater, energy crops, and other crops typically used in biomass/ biofuel production. AD can be used for municipal, commercial, and industrial waste, in addition to agriculture; for converting organic matter  such as food processing waste, biodegradable organic material, sewage, and other food waste, into renewable energy. Use of AD can make farm waste, sewage, and wastewater, treatment facilities energy neutral or even energy positive; translating to huge cost savings for municipalities. The renewable energy generated through AD is can become energy for: the municipal power and gas grid, just the farm(s) with the AD units (as in on-site anaerobic digesters on farms), energy to heat and cool homes; or even fuel for the transportation sector (after being converted into a usable form for transportation fuel, as discussed below). Organic waste finds a constructive purpose in an AD biogas plant, as it is put in an anaerobic digester to transform the waste into renewable energy.

AD also produces numerous beneficial by-products (digestate by-products) for farms; such as animal bedding, as well as low-cost fertilizer or enhanced organic fertilizer (from digestate that has a large amount of nutrients and can enhance soil). The molecules of the organic waste used in an anaerobic digester are broken down by the micro-organisms in the anaerobic digestion plant into a useful form, like glucose. The “digested” raw material is then used to create biogas. The remaining material can then be harvested as nitrogen and phosphorus-rich digestate, which can be used as fertilizer. Remaining material can also be used in animal bedding. The biogas can then be purified (the gas can also be upgraded with hydrogen) for use as pipeline-quality synthetic natural gas for the grid. Biogas can also be converted into biomethane, and then liquefied natural gas (LNG), or compressed natural gas (CNG), for vehicles, tractors, other farm equipment, that has been modified to run on LNG or CNG (see below).

anaerobic digester
Anaerobic digesters

Anaerobic digestion uses micro-organisms (enzymes, bacteria etc…) in an anaerobic digester (such as AD in a municipal anaerobic digestion power plant, or an on-site anaerobic power plant, such as an anaerobic digester on a large farm). Anaerobic digesters break down organic material and create biogas (in addition to biomethane, LNG, and CNG, biogas can be converted into syngas– SNG, after being upgraded with hydrogen). Through anaerobic digesters, farms can be entirely powered by waste from their livestock, food waste, crop/ agricultural plant waste (waste normally simply disposed of on most farms), energy crops/ other organic biodegradable material, and wastewater.

The anaerobic process also occurs naturally in landfills; and the process of using methane produced naturally in landfills (for municipal energy grids) is one example of the use of biogas as renewable energy. Clean, renewable energy, in the form of biogas, is created through the process of methanation in a wide variety of AD plants, biogas plants, and methanation plants on farms, on landfills, and in municipalities, throughout the world. As an AD plant generates biogas (and/ or biomethane) that is often burned on-site to generate heat, energy, or both – the process of anaerobic digestion easily can be integrated into combined heat and power (CHP) plants as well.

In addition to AD, waste-to-energy is done in landfills, using landfill refuse and landfill gases (methane and CO2). The use of AD in a biomass plant and waste-to-energy are cost-effective ways to produce renewable energy. Waste-to-energy also leads to less landfill waste, and is a constructive way for farms, businesses, and municipalities, to dispose of waste; creating renewable energy for municipal grids, energy for farms, heat for homes, and even transportation fuel.

Biogas can be upgraded with hydrogen, creating biomethane. Like conventional natural gas, biomethane can be used as a transportation fuel; once it’s converted, in an additional process, to LNG or CNG. When biogas is used for transportation, as LNG or CNG – both fuels can be used in place of diesel, given modifications to the vehicles in question, there are tremendous greenhouse gas emission reductions.

The entire bus fleet in Oslo, Norway, is run on CNG produced, through a series of processes, 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.

anaerobic digestion power plant in cannock
Poplars AD plant, Cannock, England

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 for the national grid; as well as SNG and other products. The Poplar plant shows that a large-scale anaerobic digestion project is viable. AD has been successful in many commercial operations as well; from Indiana, to China and India, and in dozens of countries throughout the world.

Another large-scale anaerobic digestion facility with success in a commercial application is in Orlando, Florida, from food (and other) waste sourced primarily from the Walt Disney World Resort. The AD plant near Disney World is fed with approximately 130,000 tons of biosolids (waste from the amusement park’s operations) to produce renewable energy.

Please see:

Renewable energy: biomass and biofuel

Please also see:

Gasification – syngas from fossil fuels and environmentally friendly versions

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