Compost 'accelerators' with my HOTBIN?

Wednesday, 5 February 2014  |  HOTBIN Composting

There many powders and liquids sold as 'compost activators' and 'compost accelerators'. The implied message in the name is they will “speed up” composting.

Do you need to add activators and accelerators when using the HOTBIN?
No, save your money.

To speed up your HOTBIN composting, add 'easy to digest' bacteria food. Examples are grass, food waste, chicken poo, chicken pellets. These are eaten quickly so heat is released. Composting is faster at higher temperatures, so these wastes act as ''natural accelerators'.

Below we outline the theory and practise so you can evaluate commercial accelerators against our simple no cost options.

What can help accelerate the composting process?

For most people, accelerating implies getting the waste to breakdown faster. Accelerators should therefore reduce the time it takes to create compost.

  • The most important compost accelerator is heat. Composting is 32 times faster at 60C than composting at 10C (scientific fact based on the Q10 equation that defines the speed of bacteria reactions.
  • Next is particle size - small things have a higher surface area. if you chop and shred your waste and it will decompose faster.
  • Finally bacteria food - heat is released when bacteria consume waste. The easier the food is to eat, the faster the population grows and the more heat is generated. (Foods high in sugar and carbohydrates are consumed faster than wood).

Are there other things that affect speed?

Composting relies on bacteria to decompose the waste. A starting group of bacteria have to be present in the waste and a number of conditions (heat, food, oxygen, water) – have to be right to allow their growth. The conditions are rarely ideal. One item out of balance can 'limit' or 'restrict' optimal growth. Removing this limit gives the appearance of speed - but this is not really accelerating the speed - it is merely allowing the compost to reach its optimal rate.

What are commercial accelerators made from?

One or more of the following:

  • Enzymes (proteins) needed by bacteria
  • Bacteria cultures (ie a small amount of bacteria)
  • Ground up food

Are they worth adding?

In our opinion there is little or no benefit from buying and adding commercial accelerators. They are costly and are mostly supplied without evidence on how well they work. (Ask for reports that state using X amount reduced time by Y amount). The science papers we have found indicate no significant benefit from accelerators.

How are they supposed to work?

Enzymes – are biological catalysts that help bacteria digest the waste. Bacteria make their own enzymes - so if you feed them waste they make the enzymes they need to break it down. We have found no scientific paper that reports substantial benefits from adding powders containing enzymes to compost.

Bacteria - if the waste is truly sterile, then a culture of bacteria will be needed to get things started (ie it needs inoculating with compost bacteria to start the process). Most waste already has bacteria on, and a handful of compost (or soil) contains millions of bacteria. Rarely do you need to inoculate a compost heap with a commercial powder or liquid containing bacteria. Scientific papers that have looked at the efficacy of adding inoculation always arrive at the same conclusion – bacteria grow so fast that the inoculation has no effect. (For clarity we view inoculation slight differently to ongoing mixing of new and old waste. Mixing old and new will bring bacteria into contact with the new waste).

Food - ground up bacteria food is the main part of most accelerators. Ground up food is easy for bacteria to digest and allow the bacteria population to grow fast. The more bacteria activity, the more heat is generated.This is a great theory but in practise 10g of commercial ground up food (accelerator) is a tiny amount of food - compare with a bucket of grass (10Kg , 1000 times more food), or a caddy of food waste (5Kg, 500 times more food). It really makes little economic sense to add small amounts of expensive food. Sometimes the accelerators will claim a high nitrogen content - but grass, cooked food and chicken pellets have high nitrogen!

Diluted liquid food waste is hopeless in hot composting. You are diluting and adding 99% water with just a few grammes of food. In hot composting, liquid food additions are definitely not recommended – see the science of water in composting here.

What accelerators should you add?

Free accelerators are best! Fresh grass mowing is an excellent natural accelerator followed by food waste, chicken poo, blood/bone meal.

Fresh grass is an excellent natural accelerator because it is easy to digest (high cellulose, low lignin), fast to digest (high surface area, thin), and normally there are many Kgs available. Mixed in, they will "accelerate" stubborn heaps to 70°C within a few hours.

During winter or where you have no grass, we suggest you buy a 10Kg bucket of chicken pellets (dried poop with high nitrogen). These have the benefit of being dry, easy to handle, and have a mix of both nitrogen and cellulose. Adding one or two hands full is usually enough to raise the temperature of stubborn "brown" materials (eg leaves).

We have discussed heat as the main “accelerator”. Bacteria do not reproduce much when it is cold (below 5C). You may have the right waste mix, bacteria, oxygen etc, but when it is cold bacteria grow so slowly heat is lost from the heap faster than it is produced. Hence one of the fastest ways to 'kick start' and accelerate a compost heap (in the cold) is to use a hot waster bottle This generates enough heat to allow the bacteria to rapidly develop (and release heat).

Evaluating commercial accelerators:

Most commercial accelerators do not list the contents or provide ANY evidence of what effective they have and how well they work. In our opinion this is wrong – all advertising should be supported by evidence. If you use a powder or liquid, ask the supplier to explain what it contains and how it works. Do not accept a fob off that it has sold for years, lots of people use it. Some of the larger suppliers will have MDSD (material data safety sheet) – you will have to search hard for these online, but ask and you will get them. By law they have to state what is in the container. (We have done some of this work below for you).

Those containing a food source, tend to have ground up plant, seaweed or dried blood. Tiny particles of food are easy for bacteria to consume. How much 'ground up' food are you adding and at what cost? Packets typically contain 0.5-3Kg of product and cost £5-15. You normally add a few grammes (say 10g). Compare this to sugar £1.00/Kg and £0/Kg for food waste or grass. By the time you have added a handful of powder (or worse still diluted it in a gallon of cold water) the food/energy input is small and relatively costly. The table below compares accelerators to add to your heap is struggling to get to a high temperature.

KEY: C=Cost  E=Effect  U=Ease of use  A=Availability  V=Value in use

Name Type Price/Kg C E U A V Notes
Grass food £0 5 4 5 5 19 Nitrogen and cellulose, high surface area, fast
Food Waste food £0 5 4 5 5 19 Sugars, CH, proteins. Add paper balance water
Chicken pellets food £1-2/kg 4 4 5 5 18 Nitrogen/cellulose. Easy handling, dry
Sugar (from kitchen) food £1/kg 4 4 5 5 18 Sugar - fastest, high surface area. No nitrogen
Blood / bone meal food £1-2/kg 4 4 4 4 16 proteins, nitrogen
chicken poo (raw) food £0 5 4 3 3 15 Nitrogen/cellulose - Hard to handle
Poo (any!) food £0 5 4 2 3 14 Nitrogen/cellulose. Hard to use
Garrota, J Arthur food £1.7/kg 3 2 4 4 13 Amm sulphate, limestone, lignite - see MDSD
Westland food/bac £1.7/kg 3 2 4 4 13 Granules (similar to Grotta?) + bacteria
Compost or soil bacteria £0 5 1 3 4 13 adds the start population to any sterile waste
Nitra (Bayer) food/bac £1.7/kg 3 2 4 3 12 Granules (similar to Grotta?) + bacteria
Westland Make your own food/bac £1.7/kg 3 2 4 3 12 Granules (similar to Grotta?) + bacteria
Activ8 enzyme/bac £2/kg 3 1 3 4 11 Liquid, claims microbes and enzymes
Urine Nitrogen £0 5 1 2 2 10 98% water. Nitrogen. Don't add to HOTBIN
Just Green Liquid enzyme 12/kg 1 1 4 3 9 Claiming biocatalyst (enzyme)
Ammonium Sulphate Nitrogen £5/kg 2 2 3 2 9 Fertiliser - Nitrogen, no food. Help "all browns"
Biotal enzyme £20/kg 1 1 3 3 8 Liquid, suggest 10g/fill
QR powder enzyme £200/kg 1 1 3 3 8 10g sachets - Claim is homoeopathic enzyme
Lime none £1/kg 4 0 0 0 4 No benefit adding to compost to help start

The above table represents the personal view of the author. We are keen to provide factual information on composting. If any supplier disagrees with what we have said, we welcome feedback on what your accelerator contains and the scientific ref / papers that support the benefits. We will happily update the table if evidence supports it.

We are not recommending it - but we did a quick calculation - adding a bag of sugar looks to be more cost effective! Now think about how wasteful you think that idea is!