Six Reasons Anaerobic Digesters Aren’t as Environmentally Friendly as You Think

March 1, 2021

9 minutes, 18 seconds read

Six Reasons Anaerobic Digesters Aren’t as Environmentally Friendly as You Think

As organizations with large amounts of food waste start investigating how to dispose of the waste other than sending it to a landfill, reducing their carbon footprint is probably front of mind. When organic waste is sent to a landfill it decomposes anaerobically, producing methane (CH4), a gas that is 87 times worse for the atmosphere than carbon dioxide (CO2). To reduce emissions of this greenhouse gas there are three main alternatives, but these processes are not equal: 

Composting is the traditional way to create fertilizer and involves having a pile of organic material that is turned frequently so the material will decompose in the presence of oxygen. That is, the process is aerobic. On an industrial scale, this requires much space which typically means that such facilities are a long distance from where the waste is generated. This involves the driving of substantial distances by trucks that are inefficient in their fuel consumption.

Anaerobic digesters, which do not use oxygen in the decomposition process, are again typically large commercial facilities that accept organic material from a variety of sources. The organic material is broken down anaerobically to produce methane that is then burned to produce electricity and heat. Compared to the process this waste would undergo in a landfill, the captured methane means there is less of an impact on the environment. However, there are some disadvantages to this biogas production.

Aerobic digesters use oxygen in the decomposition process. The output of this process is mostly water and carbon dioxide. This process is similar to traditional composting and when done correctly it results in no smells. Because of its environmental advantages, many organizations are opting for in-house aerobic digesters. 

Six Ways Anaerobic Digesters are Worse for the Environment

It’s important to recognize that each of these disposal solutions has its own impact on the environment. However, if you’re going to make a decision to divert your food waste and reduce your carbon footprint, then it makes sense to choose the option with the lowest environmental impact. 

Consider these six reasons anaerobic digesters aren’t necessarily the most environmentally friendly option for the disposal of food waste.

1. Anaerobic digesters still pollute

Controlled anaerobic digestion is an improvement over sending organic material to a landfill, as it reduces the amount of methane sent to the atmosphere. Still, it’s not a perfect system. Vermont’s Department of Environmental Conservation explains that although anaerobic digesters capture the gas emitted through this process, the subsequent combustion of this gas into usable heat or electricity does generate air pollutants, including carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2). The use of internal combustion engines to burn this biogas also can generate formaldehyde emissions at higher levels than occur with other fuels. 

As the Vermont department puts it, the term renewable energy simply means the fuel source is regenerated at a rate equal to or greater than its use. It does not inherently imply that the energy has low emissions, is clean, or that there are air quality, greenhouse gas or climate change benefits attributable to use of this energy.

Aerobic digesters, on the other hand, produce CO2 and grey water that has been rendered safe to enter the sewage system or, in some cases, irrigate surrounding land. The CO2 produced by aerobic digestion is part of the natural carbon cycle and therefore has no impact on the environment. In an independent study conducted by Brunel University London, researchers found that aerobic digestion using LFC biodigesters in a community produces 73% less emissions than anaerobic digestion.

2. Rising risk of toxic spills-and worse

Low-level pollution isn’t the only disadvantage of this biogas production process. Anaerobic digesters also present a risk of much more serious environmental damage due to toxic spills. The news is rife with examples of accidents on farms that result in an overflow of slurry sludge on neighbors’ properties. 

In 2020, Big Ox Energy in Nebraska was cited by the Nebraska Department of Environment and Energy due to a range of issues including venting hydrogen sulfide gas and at least nine releases of liquid biomass, some of which ran into neighboring properties. In 2019, more than 10,000 fish were found floating in a UK river as a result of a spill of anaerobic digestate (the solid and liquid waste that cannot be digested). The UK’s Environment Agency has tracked rising incidents of serious pollution, in some cases increasing more than 50% year over year. 

These leaks are problematic enough, but the production of highly combustible biogas can occasionally result in more serious problems. In the most recent incident, in 2020, four people were killed in the explosion of a water recycling center after the methane output from the anaerobic digester was reportedly ignited. 

3. Odor is another anaerobic output

A number of factors can influence the odor emitted during the anaerobic digestion process, including the type of feedstock going into the facility and how feedstock and digestate are stored and disposed of. Michael Levey, CEO of environmental firm Global Advantech Resources Ltd., writes that although the anaerobic digester is a closed and controlled process, there is a potential to release highly odoriferous compounds if odor control solutions aren’t designed into the facility. 

Release of any of these compounds, especially hydrogen sulfide, in the local environment has the potential to cause considerable nuisance and results in complaints from residents and businesses nearby; hydrogen sulfide can be detected by at least half the population at a concentration in air as low as 0.47 [parts per billion].

A quick glance at the consumer news indicates that communities are rife with complaints about odor from these facilities, an indication that odor control isn’t always a priority or isn’t easy to accomplish.

Previously, the disposal of food waste at the University of Nebraska Lincoln (UNL) followed a complex path in which the waste was hauled away by custodial staff, loaded onto a truck, and dumped in a landfill where it would produce greenhouse gases. 

The University took a major step toward sustainability when its Dining Services installed their first biodigester in November 2019 within the Selleck Dining Center. Impressed by the operation of the LFC biodigester, UNL installed a second one at the Cather Dining Center in June 2020. Combined, the food digesters have diverted over 44,000 kg (98,000 lb) of food waste from landfills and reduced UNL’s carbon footprint by 185 tonne of CO2e since installation.

The first step towards sustainability may be daunting, but the payoff is well worth the effort. Commercial kitchens can avoid fees while reducing cost and impact on the environment. Power Knot has launched the smallest commercial grade food waste solution for businesses interested in getting started with sustainability initiatives, the LFC-25 Biodigester. It is a fully enclosed automated biodigester that can safely dispose of 25-55kg of food waste per day.

Take the first step towards a sustainable kitchen here.

4. Anaerobic digesters may require more than food waste

In some cases, anaerobic digesters have turned to using specially grown feedstock in addition to food waste in order to generate the level of energy needed to power local homes.  An International Energy Policy report notes that livestock manure is the most common feedstock, but the biogas production yield is significantly lower than what could be obtained from crop residues. However, using these higher-yielding crops can lead to competition for this feedstock. For example, forestry residues can be a sustainable source of direct heat, while crop residues can be used for animal feed or to produce advanced biofuels, it states.

As a case in point, for the anaerobic digester at the Agricultural College in Sparsholt, England, to produce the planned output of 49,000 MWh each year, it would require 60,000 tonne (66,000 tons) of feedstock from grass and rye, which would be grown on 3,000 acres of farmland and then transported to the site. The use of this farmland and the resulting trucking required are very detrimental to the environment.

5. Anaerobic digesters depend upon transportation

Perhaps the biggest potential difference between anaerobic and aerobic digestion is where these processes take place. Anaerobic digestion typically happens in an offsite facility. There are some exceptions, as small-scale digesters are being installed in some rural areas on farms or large campuses where there is room for the biogas chambers. These on-site biogas reactors also continuously produce digestate that must be stored or, again, transported off-site.

In general, however, anaerobic digesters require transport of food waste to the off-site facility. As with facilities doing traditional composting, such facilities are a long distance from where the waste is generated. For organizations looking to reduce their carbon footprint, this extra transportation can be a detriment. One farm found that the cost of transporting these loads to and from the anaerobic digester consumed 220,000 litre (58,000 gallons) of diesel a year. 

A University of Delaware study found that, compared to the CO2 emitted by aerobic digestion, trucking waste to an anaerobic digester increased CO2 equivalents by 67% while trucking food waste to a compost facility resulted in double the amount of CO2 equivalents. 

Fully enclosed aerobic digesters, on the other hand, are easily installed onsite anywhere food is prepared

6. Anaerobic digesters are expensive

The cost of building and operating an aerobic digestion facility uses the earth’s resources that are not reclaimed when the facility is decommissioned. In a traditional power plant, the operator of the power plant pays for the fuel it uses. For example, the operator of a coal fired power plant pays for the coal to be delivered. However, in most instances, the operator of a biogas power plant charges the waste hauler to deliver the food waste, just as the operator of a landfill would do. These are charges that are passed back to the producer of the food waste or society in general. The study by Brunel University found that the lowest possible payback period for the LFC was 1.18 years versus that of 8.17 for anaerobic digestion facilities.

The advantages that set aerobic digesters apart

No process is perfect, but aerobic digesters can negate the risks of pollution, odor, and explosions. What’s more, the University of Delaware study noted above calls aerobic digesters that release effluent to the sewer an environmentally favorable pathway. This option came out ahead when compared to the environmental impact of disposing of food waste in landfills, through composting, or anaerobic digestion for a number of factors. It also found that microorganisms that travel through drains help play a role in cleaning pipes of grease buildup.

Aerobic digesters are across the board better for the environment, and increasingly better for your bottom line as well.

Eddie Rodriquez, warehouse manager for Ace Natural Inc. explained that when his company switched from anaerobic digestion to aerobic digestion the company was able to save both time and space. “Previously, we collected and wrapped outdated and damaged produce on pallets, stored it in the warehouse refrigerator, and shipped it weekly to a solids composting facility, at significant cost,” Rodriguez says. “Our [aerobic digester] eliminates these costs and frees `up valuable refrigerator space.”

If you’re ready to learn more about how an aerobic digester can help you save money and reduce your environmental impact, contact Power Knot today.