As the world turns to greater use of renewable energy, for both heating and cooling and power generation, more attention has been given to anaerobic digestion (AD) as an alternative to landfill sites for disposing of organic waste.
In addition to addressing food waste, AD technology is becoming popular on farms for processing waste from livestock and crop production, as well as among retailers and food producers keen on reducing their energy bills.
AD is also becoming increasingly popular as a general means of reducing the amount of carbon dioxide and methane released into the atmosphere, with governments assisting in this through the provision of incentives.
What is anaerobic digestion, and how it can benefit consumers, businesses, and the environment?
Anaerobic Digestion and How It Works
AD is a process in which organic material, such as food waste, animal manure, and solids from wastewater treatment plants, is turned into biogas. This can then be used as a renewable fuel in vehicles and can generate heat and electricity. It can replace natural gas, be fed into the country’s gas grid, and help power the national electricity grid.
The AD process is driven by bacteria, which break down organic matter in an anaerobic (or no oxygen) atmosphere. In doing so, the bacteria produce biogas, which mostly consists of methane (CH4), along with other elements, such as carbon dioxide, hydrogen sulfide, water vapor, and small quantities of other gases.
The process mirrors a similar process that occurs in certain natural environments. As discovered in 1776 by Alessandro Volta, methane, sometimes called “marsh gas,” is produced in some soils, lakes, and sediments in ocean basins.
In an anaerobic digester, this biogas rises to the top of the digestion chamber, leaving the waste solids (digestate) to fall to the bottom. This solid material, which is rich in nutrients, can then be extracted and used in agriculture as fertilizer or animal bedding, or turned into a base material for bioplastics production. The biogas can be refined to create biomethane or renewable natural gas (RNG), which can be fed into the national gas grid.
Bacterial Culture and “Seeding”
AD begins with a process called bacterial hydrolysis, where the chemical bonds of the feedstock, particularly the insoluble organic polymers such as carbohydrates, are broken down. Acidogenic (acid-producing) bacteria then get to work on the sugars and amino acids, converting them into acetic acid, ammonia, carbon dioxide, hydrogen, ammonia, and organic acids. Lastly, methanogens (methane-producing bacteria) convert these products into methane and carbon dioxide.
Given that these bacterial communities take a while to establish themselves, the AD process is typically given a “jump-start” by introducing other materials such as cattle slurry or sewage sludge. This practice is known as “seeding.”
Some digesters can accept a number of different feedstocks through co-digestion. These co-digested materials usually include manure, food waste, crops specifically grown to produce feedstock for energy generation (energy crops), crop residues, as well as fats, oils, and greases from restaurants.
Anaerobic digestion is increasingly being recognized as an important renewable energy and environmental technology in several countries, including the US, UK, Germany, and Denmark.
This co-digestion process has specific benefits in that it can generate biogas from materials that would normally be difficult to digest or would yield low amounts of biogas when digested on their own.
Anaerobic digestion is increasingly being recognized as an important renewable energy and environmental technology in several countries, including the US, UK, Germany, and Denmark.
Currently, in the UK, 3.2 million tons of food waste are collected and transported to AD plants. There are currently 650 AD plants across the country. Many are accredited under ISO 9001 (referring to management standards) and ISO 14001 (specifically relating to environmental standards) from the International Organization for Standardization. They take different forms, being designed in different shapes and sizes according to the waste material (feedstock) that gets fed into them.
The Importance of Temperature
Alongside the various bacterial cultures, another important factor in anaerobic digestion is temperature.
In Sweden, for example, a project was enacted in 2017 to explore the effect of specific temperatures on AD in a wastewater treatment plant (WWTP) processing mixed sludge.
In a laboratory-scale research program, using temperatures of 34°C (93°F), 38°C (100°F), and 42°C (107°F), it was found that an increase in temperature to 42°C increased the likelihood of process instability, reduced methane yield and increased costs.
Lowering the temperature to 34°C increased the amount of sludge matter and lowered the rate of biogas production. Consequently, the project concluded that 38°C was the optimum temperature for AD at WWTPs.
Lowering the temperature to 34°C (93°F) increased the amount of sludge matter and lowered the rate of biogas production. Consequently, the project concluded that 38°C (100°F) was the optimum temperature for AD and WWTPs.
The WWTP project in this Swedish experiment showed that mesophilic (moderate temperature) systems are more appropriate for sludge processing by anaerobic digestion.
In general, anaerobic digesters conform to two types. Mesophilic digesters are the most common, and these typically utilize a temperature range of between 35°C (95°F) and 40°C (104°F). In contrast, thermophilic digesters (with heat-loving bacteria) utilize temperatures above 50°C (122°F), but they have higher operating requirements and are therefore less common.
Constant monitoring of the temperature of a digester is very important, and it may sometimes be essential to cool the digester for optimum efficiency. This is because the bacteria in an anaerobic digester are very sensitive to temperature and “temperature shocks” can reduce methane production.
Pressure can be another important factor in AD. Pressure helps increase the content of methane produced and reduces energy costs for biogas upgrading and injection into the gas grid. However, high-pressure digesters require large capital investments, and this has tended to restrain the research in this area.
Environmental and Agricultural Benefits of AD
Anaerobic digestion can help to reduce the amount of food waste, which is a major problem across the world, particularly in more developed countries.
According to one prominent UK AD operator, each ton of food waste fed into an anaerobic digester instead of going to landfill prevents between 0.5 and 1.0 tons of CO2 entering the atmosphere. By capturing organic materials and processing them, AD also helps to prevent methane from entering the atmosphere. This is beneficial because methane is viewed as being more damaging to the climate than carbon dioxide.
According to one prominent UK AD operator, each ton of food waste fed into an anaerobic digester rather than going to landfill prevents between 0.5 and 1.0 tons of CO2 entering the atmosphere.
However, several challenges remain to utilizing AD for food waste reduction.
One of these is the tendency of food waste to produce volatile fatty acids in the early stages of the process.
If the AD process is poorly controlled and unoptimized, food waste digestion can generate various intermediate compounds. These can give rise to foaming and low methane yield, reducing the efficiency of the process. The high cost of transportation and operation is another problem.
The quality of biogas determines how it is used. Biogas that has not been purified can come straight out of the digester and be fed into more hardy, less efficient, internal combustion engines, including those used in buses and coaches.
Biogas that has been cleansed of trace elements can be used in more efficient and more sensitive engines. The best quality biogas, treated to meet gas grid standards, can be distributed through natural gas pipelines and used in homes and businesses. Biogas that has been upgraded into biomethane or compressed natural gas (CNG) and liquid natural gas (LNG) can be used in cars and trucks.
The remaining digestate can be separated into solids and liquids, upon which the solids can be processed into fertilizer pellets or decomposed, while the liquids can be used as liquid fertilizer.
The benefits of these materials for the soil include: increasing soil organic matter content, reducing or replacing chemical fertilizers and pesticides, improving plant growth, reducing nutrient runoff and soil erosion, helping to prevent soil compaction, and helping the soil to retain water, which in turn reduces the need for irrigation.
However, in several countries, particularly the US, this procedure requires a soil and nutrient management plan before application.
AD Use in the Agriculture and Food Sectors
Anaerobic digestion is commonly used in the agriculture and food sectors by big companies such as Smithfield Foods, a big global pork producer, US retail company Kroger, and large food vendors such as sports stadiums.
In the UK, most on-farm digesters were constructed between 1987 and 1995. New incentives for renewable energy generation introduced by the UK government should lead to a revitalization of the AD approach.
An Important Clean Energy Segment
Considering all the available evidence across several countries, anaerobic digestion is clearly and widely understood as an important segment of the wider global clean energy industry. While, understandably, food waste should ideally be redirected to populations suffering food shortages, the current reality is that the world still wastes far too much food, which mostly goes into landfills.
Redirecting this into anaerobic digestion not only reduces the problem but also generates much-needed clean energy to help counter climate change. For similar reasons, it also makes sense to redirect organic farm waste and solids from WWTPs into anaerobic digestion, which has the additional benefits of reducing costs for both farm and water industry operations by reducing waste and generating an additional income stream.
So, expect to see continued growth in anaerobic digestion.
*Robin Whitlock is an England-based freelance journalist specializing in environmental issues, climate change, and renewable energy, with a variety of other professional interests including green transportation.