The 5Rs of Zero Waste - No 5. To Rot

The 5Rs of Zero Waste  - No 5. To Rot

This year our #Regenuary campaign is focussing on food service and how the foods we eat outside of the home can be prepared with a lower impact, be it fine dining or a sandwich from a petrol station, small changes adopted on mass scale can make a huge difference.

The 5Rs.

In sustainability and zero waste systems there is an often used guide called the 5rs, there are a few variants of this but for our purpose we will define them as:





5.Rot (compost or ferment)

To learn more about the origin of the 5 Rs please see the first post in the series.



What do we mean by rot? Rotting simply is the decay of matter by bacterial or fungal action, it is nature's way of returning nutrients back to the earth to complete the circle of nutrients.

In the context of zero waste the term rot can mean a few different things, it can mean to compost or to ferment. In very general terms and for our purpose we make the distinction that fermentation is controlled decomposition to create a new food or drink product whereas composting produces food for plants not humans.

We fairly extensively covered fermentation in the kitchen in the previous Rs so for this post we'll be thinking more about composting food waste that will not be upcycled into new foods.

To compost

Almost anything from a cutting board that doesn't go into a pan or plate can go in the compost bin. Such as:

Any fruit and vegetable waste: peels, skins, seeds, leaves, stalks
Coffee grounds (including paper filters)
Nut shells
Tea bags
Paper napkins
Bones, skin, feathers, fur, scales, meat scraps etc

Pretty much anything organic can be composted and that is because it is the carbon based compounds in these materials that is being broken.

The stages of composting

From Cornell

Different communities of microorganisms predominate during the various composting phases. Initial decomposition is carried out by mesophilic microorganisms, which rapidly break down the soluble, readily degradable compounds. The heat they produce causes the compost temperature to rapidly rise.

As the temperature rises above about 40°C, the mesophilic microorganisms become less competitive and are replaced by others that are thermophilic, or heat-loving. At temperatures of 55°C and above, many microorganisms that are human or plant pathogens are destroyed. Because temperatures over about 65°C kill many forms of microbes and limit the rate of decomposition, compost managers use aeration and mixing to keep the temperature below this point.

During the thermophilic phase, high temperatures accelerate the breakdown of proteins, fats, and complex carbohydrates like cellulose and hemicellulose, the major structural molecules in plants. As the supply of these high-energy compounds becomes exhausted, the compost temperature gradually decreases and mesophilic microorganisms once again take over for the final phase of "curing" or maturation of the remaining organic matter.

When a compost is fully matured it is simply fertiliser for plants. Compost can be added to fields or gardens, used in pots or beds, in green houses, polytunnels or outside but the process of composting can create a very useful byproduct of biogas.


In a biogas fermenter, sometimes also called a digester, the composting is anaerobic, meaning without oxygen being present, this causes a slightly different reaction and results in biogas which can be as much as 75% methane (CH4) but also contains some Carbon Dioxide (C02) and Hydrogen Sulphide (H2S) which is another flammable gas that has the characteristic rotten egg smell. 

These flammable gasses are part of a very different carbon cycle than using fossil fuels as they originated in the atmosphere before being absorbed by plants, this is what's known as a closed loop and does not contribute to a net atmospheric gain in these gasses whereas extracting methane from bedrock means it was last an atmospheric gas 120,000,000 years ago.

Uses for biogas

Like natural gas, the energy contained in biogas can be used to generate heat and electricity. Also, biogas can also be purified by removing the inert or low-value components (CO2, water, H2S, etc.) to produce renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or further processed to produce alternative fuels, energy products, or other advanced biochemicals and bioproducts.


The remains of a fermentation in a biogas reactor is called 'digestate' and this is also a very useful material. With appropriate treatment, the digestate can be used in many useful applications, as a nutrient-rich fertilizer, as an organically enriched compost, or simply as a soil improvement, in which case the farm can use it as a fertilizer on the fields.  

Composting for a commercial kitchen.

For a commercial kitchen this valuable resource of nutrient rich food waste can be collected by energy companies, sometimes for free as they can use it to generate power and fertilizer as detailed above.

Composting at home?

Most councils in the UK offer residents food scrap bins to be collected from homes but you might wonder, what happens to this stuff? It's used in much the same way as commercial waste, after all it's made from the same stuff!

Here in London for example the food waste collected from homes by the councils is sent to an anaerobic digester in Hertfordshire. This plant uses food waste to create biogas, a renewable energy source used to generate electricity to power the national grid, and heat homes. This plant will also transform the food waste into a biofertilizer, which will be used on farmland to grow more food.

Biodegradable packaging

Food isn't the only category of waste that can be composted, certain materials that are made from natural ingredients can also be digested by bacteria in a fermenter or a humble compost heap.

The obvious materials that can go straight into the compost are uncoated paper, paper towels, cardboard and egg cartons, all will happily rot but there is a new wave of packaging materials that claim to be biodegradable or compostable, but are they?

These new plastics can be categorised as:


Degradable – All plastic is degradable, even traditional plastic, but just because it can be broken down into tiny fragments or powder does not mean the materials will ever return to nature. Some additives to traditional plastics make them degrade more quickly. Photodegradable plastic breaks down more readily in sunlight; oxo-degradable plastic disintegrates more quickly when exposed to heat and light.

Biodegradable – Biodegradable plastic can be broken down completely into water, carbon dioxide and compost by microorganisms under the right conditions. “Biodegradable” implies that the decomposition happens in weeks to months. Bioplastics that don’t biodegrade that quickly are called “durable,” and some bioplastics made from biomass that cannot easily be broken down by microorganisms are considered non-biodegradable.

Compostable – Compostable plastic will biodegrade in a compost site. Microorganisms break it down into carbon dioxide, water, inorganic compounds and biomass at the same rate as other organic materials in the compost pile, leaving no toxic residue

There are two main types of bioplastics.

PLA (polylactic acid) is typically made from the sugars in corn starch, cassava or sugarcane. It is biodegradable, carbon-neutral and edible. To transform corn into plastic, corn kernels are immersed in sulfur dioxide and hot water, where its components break down into starch, protein, and fiber. The kernels are then ground and the corn oil is separated from the starch. The starch is comprised of long chains of carbon molecules, similar to the carbon chains in plastic from fossil fuels. Some citric acids are mixed in to form a long-chain polymer (a large molecule consisting of repeating smaller units) that is the building block for plastic. PLA can look and behave like polyethylene (used in plastic films, packing and bottles), polystyrene (Styrofoam and plastic cutlery) or polypropylene (packaging, auto parts, textiles). Minnesota-based NatureWorks is one of the largest companies producing PLA under the brand name Ingeo.

PHA (polyhydroxyalkanoate) is made by microorganisms, sometimes genetically engineered, that produce plastic from organic materials. The microbes are deprived of nutrients like nitrogen, oxygen and phosphorus, but given high levels of carbon. They produce PHA as carbon reserves, which they store in granules until they have more of the other nutrients they need to grow and reproduce. Companies can then harvest the microbe-made PHA, which has a chemical structure similar to that of traditional plastics. Because it is biodegradable and will not harm living tissue, PHA is often used for medical applications such as sutures, slings, bone plates and skin substitutes; it is also used for single-use food packaging.

The unfortunate truth about bioplastics:

Right now, it’s hard to claim that bioplastics are more environmentally friendly than traditional plastics when all aspects of their life cycle are considered: land use, pesticides and herbicides, energy consumption, water use, greenhouse gas and methane emissions, biodegradability, recyclability and more. But as researchers around the world work to develop greener varieties and more efficient production processes, bioplastics do hold promise to help lessen plastic pollution and reduce our carbon footprint.

To conclude.

Plastics of any kind are best avoided wherever possible but, if unavoidable plastics should either be fully recycled such as PET or fully compostable and dealt with accordingly, the in between options cause more problems than they solve.