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Six Reasons Anaerobic Digesters Are Not Green

Six Reasons Anaerobic Digesters Are Not Green – Let’s be Honest

Anaerobic digestion systems doubters have published articles in which they claim 6 scientific reasons that they say show that Anaerobic Digesters are not “green”.

Let's be honest, if true, it would mean all those advocating the development of anaerobic digestion, with its biogas and other beneficial outputs, should think again.

In this article, we admit there are problems with anaerobic digestion plants. No emerging technology was ever perfect. And, some of the criticisms refer to old “first generation” AD plant technology which is being progressively phased out anyway.

So, read on for a more balanced view including counter-arguments to place the problems people identify into perspective.

What's the Big Deal? – Why Do Organic/ Food Waste Disposers Seek to be Green?

Most food waste producers globally still send their waste to landfills. As companies with a lot of food waste look for alternatives to landfills, minimising their carbon footprint is likely a priority.

Organic waste decomposes anaerobically in landfills, releasing methane (CH4), up to 87 times worse for the atmosphere than carbon dioxide (CO2). There are three basic ways to lower this greenhouse gas, but they're not equal.

These are (aerobic) composting, anaerobic digestion, and aerobic digesters. Aerobic digesters are a relatively new idea developed in the US which pre-condition food waste creating a liquid slurry that is then discharged to public sewers.

What are Composting, Anaerobic Digestion, and Aerobic Digesters

Each can be characterised as follows:

1 – Composting involves turning a pile of organic material so it decomposes in oxygen. It's aerobic and emits CO2. This needs a lot of land on an industrial scale, hence such facilities are usually far from where garbage is created. This entails fuel-inefficient trucks driving long distances emitting more CO2. The compost piles need turning plus shredding and screening processes are used to produce a friable homogenous output of small particle size suitable for use as compost. All this equipment adds to CO2 emissions.

2 – Anaerobic Digesters don't use oxygen in the breakdown process and are also complex major commercial installations. Anaerobically breaking down organic material produces methane, which is burned for electricity and heat. It is increasingly upgraded to provide biomethane which can be injected into natural gas grids and fuel transport vehicles. The trapped methane reduces environmental impact compared to a landfill. Biogas generation has some drawbacks.

3 – Aerobic Digesters decompose using oxygen. Mostly water and carbon dioxide are produced. When done correctly, this method smells like traditional composting. Many firms choose compact little in-house aerobic digesters for doubtful environmental reasons. They are relatively simple to operate and are effective in odour avoidance, but would appear to do little more than to liquidize the food waste to allow it to flow down a sewer.

Six Reasons Anaerobic Digesters Are Not Green

Six Environmental Problems with Anaerobic Digesters

Each disposal method has an environmental impact. To lower your carbon footprint and divert food waste, you need to choose the option with the least environmental impact.

The detractors say that the following list shows that anaerobic digesters aren't the most environmentally friendly way to dispose of food waste. We don't think that is true!

1. Anaerobic Digesters Contaminate

This biogas-generating process causes low-level air pollution.

Garbage landfill truck
Reducing organic waste in landfills is the highest priority to limit climate change.

Diverting organic waste to anaerobic digestion reduces the putrescible waste in landfills and decreases methane emissions from landfills. It's not ideal. The “gas engines” used are simply reciprocating internal combustion engines which burn the raw biogas from the on-site Anaerobic Digestion Plant or the landfill gas emitted via this process.

Burning of this gas, in this way, into useful heat or electricity generates air pollutants, including carbon monoxide, nitrogen oxides, and sulphur dioxide (SO2). Burning biogas in internal combustion engines can also increase formaldehyde emissions.

Renewable energy means the fuel source is renewed at a rate equivalent to or greater than its usage. It doesn't necessarily mean the energy has minimal emissions, is clean, or has air quality, greenhouse gas, or climate change benefits.

But that's old technology! The AD industry has moved away from that technology.

Those planning an AD plant would be wrong to invest unless they research the technology they will be using and that includes taking responsibility to ensure they complete due diligence checks on the true sustainability of their projects.

Most do.

The great majority of new AD plants are installed with biogas purification and have been since 2017.

The result is practically pure methane and is known as biomethane. Biomethane which with further quality checks, maybe some added hydrocarbon to raise the calorific value, and once pressurized; is both a clean-burning vehicle fuel and a natural gas equivalent for injection into the gas grid.

So, let's be honest! Does the current generation of anaerobic digestion facilities, which “upgrade” biogas, contaminate in the way we described? No, they do not.

2. Anaerobic Digesters Are a Toxic Spill Danger That is Rising

Fish kills in a polluted riverAnaerobic digesters risk substantial environmental damage from hazardous spills. In the news, there are mishaps on farms that pour slurry sludge onto neighbouring property.

Big Ox Energy in Nebraska was cited in 2020 for venting hydrogen sulphide gas and releasing liquid biomass onto surrounding properties.

In 2019, a leak of anaerobic digestate caused 10,000 fish to float in a UK river (the solid and liquid waste that cannot be digested). The UK's Environment Agency has recorded rising pollution events, some up 50% year over year.

But, let's be honest!

If any farm business, with or without an AD plant, fails to train operating staff in slurry and sludge handling, sooner or later they will cause pollution.

Slurry spreading of the farm sludge, which is the alternative, also possess similar dangers, if not done right, to watercourses and fish.

Image shows a view of explosion smoke and fire.
Explosion risks at AD Plants need to be taken seriously.

In addition, biogas is highly flammable biogas which can cause extra difficulties. In 2020, four persons at a water utility company sludge storage tank were killed when methane from an anaerobic digester exploded at a wastewater treatment plant.

Full details of this accident have yet to be published. Nevertheless, it has been made clear that this explosion was not in any part of the sewage works which is part of the anaerobic digester.

3. Anaerobic Outputs Include Odour

The type of feedstock and how the feedstock and digestate are stored and disposed of might affect anaerobic digestion odour.

Michael Levey, CEO of environmental firm Global Advantech Resources Ltd., says that although the anaerobic digester is a closed and controlled operation, it can produce highly odoriferous substances without odour control methods.

Smelling a bad smell cartoonHydrogen sulphide may be detected at trace levels present by at least half the population (at 0.47 [parts per billion]) in the air.

A short look at local news shows that neighbourhoods ay report odour complaints from these facilities, indicating that odour control isn't always a priority or is difficult to achieve.

But, think about the alternative.

Let us not forget that food trash was formerly carted away by a waste collection company, loaded onto a truck, and thrown in a landfill.

Nobody can pretend that landfills don't smell and they also produce enormous quantities of greenhouse gases.

Case Study Puts the Lie to “Anaerobic Digesters are Not Green”

Dining Services erected its first biodigester in the Selleck Dining Center in November 2019. UNL constructed a second biodigester at Cather Dining Center in 2020.

Since installation, the food digesters have diverted over 44,000 kg (98,000 lb) of food waste from landfills and reduced UNL's CO2e emissions by 185 tonnes.

4. Anaerobic Digesters Need Food Waste

Scraping food waste off plate - cartoonAnaerobic digesters may use specifically cultivated feedstock in addition to food waste to power local houses. Livestock dung is the most prevalent feedstock, however, its biogas yield is lower than crop wastes.

It can be tempting to use higher-yielding crops as a feedstock which might lead to feedstock competition.

Forestry leftovers might be better used as a sustainable source of direct heat, while some crop wastes can be utilised directly for animal feed or advanced biofuels.

For example, the anaerobic digester at the Agricultural College in Sparsholt, England, would need 60,000 tonnes of grass and rye cultivated on 3,000 acres of farmland and transported to the site to produce 49,000 MWh annually. This farmland use and the resulting trucking harms the local ecosystem without providing food for locals to eat.

Feedstock competition and the possibility of greater demand for limited food stocks could conceivably be against the public good. That's undeniable.

However, the fact that this is being raised already shows how emotive this subject is. And, yet it is easily legislated against should it occur. The public outcry would force politicians to act.

5. Anaerobic Digesters Need Transport

Anaerobic digestion usually occurs offsite remotely from the food waste producer's premises. Some small-scale digesters are constructed on farms or large campuses with room for biogas facilities, but that's the exception and not common.

On-site biogas reactors also create digestate that must be stored and then transported to the farm where it is to be used as a fertiliser.

Anaerobic digesters require the off-site transfer of food waste. As with regular composting facilities, these are far from the organic waste sources, meaning fossil fuel (diesel and gas) is used.

Not only does this raise GHG emissions, the extra travelling must hurt each organisations' carbon footprint. One farm found that moving cargo to and from the anaerobic digester used 220,000 litres of diesel a year.

A cartoon of a green energy farm tractorAdmittedly, that's not so good. But there is a trend starting in which the farm business buys biomethane-powered farm equipment and trucks to use their own renewable energy.

But, anaerobic digestion is still preferable:

Compared to one aerobic digestion sustainability study, transporting trash to an anaerobic digestion plant increased CO2 equivalents by 67%. See the udel.edu paper.

Anaerobic digestion is still preferable because the research showed that trucking food waste into a compost plant creates even more emissions and doubles the amount of CO2 emitted.

6. Anaerobic Digesters are Costly

Financially Costly

Anaerobic digestion facilities most often cost over a million dollars to build. On top of that is the cost of the necessary connection to the gas grid and/ or electricity supply. Also essential is the need for high-quality staff training in order to operate the digester.

Not to forget the end-of-life cost of demolishing an AD facility. But so is the demolition of alternatives such as natural gas power stations, or even breaking up and re-use of the large paved areas needed for composting.

Costly in Construction Material Use

Unsustainable aspects of biogas plants relate to their high use of the large quantities of construction materials required when built.

But, let's not forget the end of life of these facilities. When an aerobic digestion facility is deactivated, the earth's resources are not restored.

The “Not so Costly” Use of Waste for Biogas Production

Traditional power plants pay for their fuel.

But, in most cases, a biogas power plant operator compensates the waste transporter to provide the biogas plant with food waste. A coal-fired power plant operator pays for coal delivery. A landfill operator would never be able to fund that as their sole income is from landfill site Gate Fees.

Food waste producers or society pay these fees.

But payback periods can be short compared with developing fossil fuel energy and are wildly lower than for nuclear power.

Brunel University showed that the LFC had a payback period of 1.18 years, compared to 8.17 for AD.

Let's face it. For anaerobic digestion plants, the big gain is from renewable energy and fertilizer which means that fossil fuel can be left in the ground.

“Anaerobic Digesters are Not Green”?

Our Verdict: Despite admitted shortcomings of the current AD processes, this is not true

AD is the leading technology for organic waste processing and it is more sustainable than aerobic digestion, and traditional composting as well.

The Aerobic Alternative

Aerobic digesters create CO2 and foul sewage/industrial effluent that can enter the sewage system or in some instances be used to irrigate the land. There is no renewable energy benefit from aerobic digestion of the sort that anaerobic digestion provides.

Yes. Both anaerobic and aerobic processes emit carbon dioxide considered to be carbon neutral.

Cartoon of a compost heap so anaerobic digesters are not green? Nor is aerobic digestion perfect.
Aerobic composting isn't always idyllic!

Both types of digestion's CO2 emissions are part of the natural carbon cycle and are usually justifiably classed as having no environmental GHG impact.

But the uses for proprietary aerobic digestion systems (with liquid-only output) are limited.

Brunel University London revealed that aerobic digestion utilizing LFC biodigesters produced 73% fewer emissions than anaerobic digestion. However, the loading cost on the sewer is an environmental burden on the receiving sewage works, and is costly, as it is payable under the Volumetric Pricing Formula for Trade Effluents.

By contrast, the use of the anaerobic digestion output (digestate), when put on the land, needs no use of fossil fuel for treatment at the sewage works.

The proximity point which relates sustainability to vehicle transport mileage is relevant. In fact where anaerobic and aerobic digestion occurs (on-site or off-site) may be a significant difference in terms of relative sustainability. But, only relevance in terms of sustainability is only for as long as the vehicles are fossil fuel powered.

The transition away from mineral-based non-renewable fuels has started and can only make the use of anaerobic digestion more favourable as time goes on.

Perhaps the best argument for commercial aerobic digester units is that they can be installed anywhere food is made, and vehicle mileage is low.

Why Choose Anaerobic Digestion (AD) in 2022 and 2023?

Russia's invasion of Ukraine has triggered a food crisis and exacerbated the continuing energy crisis, which has triggered CO2 and fertiliser shortages. The result of this and other post-COVID influences is rising inflation. Consumers are now paying the price for disrupted grain supplies and reduced food output.

Anaerobic digestion shows a way out. With government backing, expanding domestic green gas production will improve the country's energy security while achieving Net Zero.

Digestate and other AD-derived sustainable fertilisers can be used to replace carbon-intensive mineral fertilisers, resulting in a more steady supply and soil repair.

Furthermore, growing AD and bioresources businesses represent a massive commercial opportunity, with the potential to create 60,000 new employment opportunities across the UK.

COP26 Global Methane Pledge cartoon
It's time politicans took urgent action. Action is needed now. Not “Maybe in 10 years' time/ sometime/ never…”

Another fantastic possibility is the UK's participation in the Global Methane Pledge. The Pledge allows the Biogas Industry to demonstrate the value of recycling organic wastes that would otherwise emit methane. This is done by applying best practices. When fully realized, AD has the potential to contribute more than 20% of the UK's contribution to the Pledge.

Globally most nations continue to witness rapid growth in the demand for, and production of, AD and its byproducts. With gas and fertiliser costs at an all-time high, rising demand for bio-CO2, and increased pricing for RTFCs and biomethane certificates, now is an excellent moment to invest.

Do you still say that anaerobic digesters are not green?

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    • March 8, 2023

    I have read your article carefully and I agree with you very much. This has provided a great help for my thesis writing about so called “greenwash. However, I don’t know much about a certain place. Can you help me?

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