Aerobic digestion and anaerobic digestion: The top differences

Aerobic and anaerobic digestion are the two major techniques used for processing biodegradable materials, including organics, discarded as food waste. While both strive to achieve maximum waste degradation, the two are entirely opposite techniques, and each has its own special merits. In this blog post, we will take a closer look at them and highlight the main differences:

What is aerobic digestion?

Aerobic digestion is typically used to treat organic waste dissolved in water. It occurs naturally in any place where organic matter is exposed to circulating air, as well as in the first stages of sludge treatment at a wastewater plant.

How it works

In aerobic wastewater treatment, bacteria make use of oxygen to break down organic matter and other pollutants. The two most common kinds of aerobic wastewater treatment systems are aerated stabilisation basins (ASBs) and activated sludge systems. In the aerobic digestion process, circulating air in the treatment tank is used by bacteria to break down waste. The material that cannot be broken down by the bacteria settles out as sludge. The water is then collected for further treatment, often by means of anaerobic digestion.

Aerobic digestion systems for for food waste management

Aerobic digestion is a cleaner and much faster alternative to traditional food waste disposal methods. By digesting food waste on the site, it is diverted away from the landfill, which is obviously an environmentally friendly choice.

An aerobic digestion system minimises greenhouse gases, especially methane, created when the food waste is taken away by heavy trucks or when it breaks down anaerobically in a landfill. In addition, toxic pollutants from food waste in landfill trickle down into waterways, polluting our natural environment.

Aerobic digestion and circular economy go hand-in-hand

Did you know aerobic digestion supports recycling too? The food waste that breaks down into component resources is recoverable via subsequent treatment processes. In fact, via aerobic digestion, it is possible to harness biomethane and generate electricity, harvest fertiliser for farmers and reuse recyclable water. This process is also called the circular economy.

What is anaerobic digestion?

Anaerobic digestion takes place, as the name suggests, in the absence of oxygen. The waste material is placed in sealed tanks that are deprived of air so that microorganisms can break down the organic waste matter into aldehydes and alcohols as ‘intermediate’ products before ultimately converting them into methane.

How it works

The four key stages of anaerobic digestion are:

  • Hydrolysis
  • Acidogenesis
  • Acetogenesis
  • Methanogenesis

Anaerobic digestion can be the second stage of water treatment after aerobic digestion or as a pre-treatment process before aerobic municipal wastewater treatment (although this is less common). The final product, methane, is commonly converted into electricity.

The history of anaerobic digestion

This concept has been around for centuries. For instance, in 900 BC, rotten organic waste produced biogas that heated the bathwater in many communities, including the Mesopotamians in Assyria.

But it was in the 17th century when the founder of pneumatic chemistry, Jan Baptist van Helmont, discovered that there were specific gases different from the atmospheric air and that the decaying organic matter was capable of generating flammable gases.

Sir Humphry Davy, a Cornish chemist and inventor, officially established that methane could be produced from cattle manure in 1808. It was only after the OPEC oil crisis in the 1970s that anaerobic digestion systems gained in popularity as the world sought energy options beyond fossil fuels and looks for more efficient food waste management practices.

How it works

Anaerobic digestion occurs in a sealed vessel, also known as a reactor, constructed in various sizes and shapes specific to the site and feedstock conditions.

The reactor comprises complicated microbial communities breaking down (or digesting) resultant biogas and digestate, a nutrient-rich substance that can be used as a fertiliser. One can combine multiple organic materials in one digester – this practice is called co-digestion.

Co-digested materials include energy crops, fats, oils and greases (FOGs), crop residues, and manure. Co-digestion can boost biogas production from difficult-to-digest or low-yielding organic waste.

Some critical differences between aerobic and anaerobic digestion

As we have seen, aerobic and anaerobic digestion are biological organic treatment processes involving the breakdown of complex waste matter with the help of living microorganisms. These fundamental differences can be outlined as follows:

  • Aerobic digestion relies on the gaseous oxygen in freely circulating air, while anaerobic digestion calls for the absence of air.
  • The end products of aerobic digestion are carbon dioxide and water. The end products of anaerobic digestion are methane and carbon dioxide.
  • One can often detect the occurrence of aerobic digestion through the ‘earthy’ smell of compost. Anaerobic digestion, however, tends to give off a foul smell due to methane release which is normally managed and contained in anaerobic digesters.
  • In nature, aerobic digestion (in the form of composting) is the preferred method for dry feedstocks. Anaerobic digestion, on the other hand, is preferred for wet waste.
  • At the industrial level, aerobic wastewater treatment involves higher capital investment and relatively high energy consumption. The costs are relatively lower for anaerobic digestion.

The benefits of opting for either aerobic or anaerobic digestion for food waste management

Handling organic waste onsite puts your business in control of either process and eliminates the need for you to take the help of third-party service providers. Onsite aerobic digestion, for instance, minimises the costs of traditional food waste removal, such as bin collection.

It also prevents the buildup of excess food waste in landfill and keeps scavenger creatures such as rats from accessing those zones. This works in favour of the environment.

On the other hand, the anaerobic digestion system generates biogas, which can be used as a fuel (renewable energy such as cooling, electricity or heat) – removing the dependency on fossil fuel energy whilst saving on energy costs.

Whatever process you choose for your business, you know you will only sustainably take better control of your food waste and potentially generate renewable green energy. That is truly commendable, in our opinion!

Waste2ES is on a mission to offer a comprehensive suite of innovative technologies to reduce the broader adverse environmental impact and carbon emissions. Wisely manage your food waste wisely, treat wastewater or FOG, protect your workforce from injury with smart bin lifting – the choice is yours. Check out our website for more info.

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