How do I fix wet waste in the HOTBIN?

6 February 2014  |  HOTBIN Composting

Is your waste too wet? Do you have:

  • Lots of leachate (yellow/murky liquid) leaking from mesh plate
  • Waste that smells putrid, rancid or drain-like (i.e. it is anaerobic)
  • Or a top layer that will not increase above 30-40°C.

Step 1: Fix the top layer

Mix in 8 hands full (half a bucket or half a carrier bag) of corrugated cardboard (or shredded office paper). Check you have added at least 6 hands full of bulking agent (wood chip) to the food waste over past couple of weeks. If not, add now.

Leave 48 hours. Check if temperature has risen above 40°C. If not, move to step (2)

[Need help on bulking agent? See Below]

Step 2: Check the base layer

Take off the door panel and look at the waste in the base. Match how the waste ‘looks’ and ‘smells’ to one of the four options below. Follow the actions for your match.

If you are unsure, send a photo to:

Option 2.1 How It Looks Next Steps To Follow

Base layer Aerobic & active

  • Leaking water but does not smell putrid
  • Yellow/green non composted material with recognisable waste
  • Temperature below 40°C

Note: Your waste is still active. Typically this will be the case when the waste is less than 6 weeks old.

Aerobic and active

If the waste in the top does not rise above 40°C within 2-3 days, the base is probably suffering from restricted airflow.

Mix in 1 to 2 carrier bags of paper into the base layer. Did you add at least a carrier bag of bulking agent during filling? If not, top up now.

You may find it faster and easier to mix by emptying it out onto a plastic sheet, mixing then adding back.


Option 2.2 How It Looks Next Steps To Follow

Base layer Anaerobic & active

  • Leaking water
  • Some yellow/green waste in base, but also brown/black with slimy partially digested waste
  • Smells putrid
  • Temperature < 40°c

Note: Your waste needs action to remove the smell and return to aerobic state.

Anaerobic and active

Take out the base layer and leave on sheet of plastic to aerate and dry. (This may take a few hours).

Mix in 1 to 2 carrier bags of paper with the base layer. Did you add at least a carrier bag of bulking agent during filling? If not, top up now.


Option 2.3 How It Looks Next Steps To Follow

Base layer Aerobic, not active

Is it mainly brown, sticky pieces, cold (below 20°C), very wet and soggy and claggy, no odour or an earthy odour.

(Typically this will be the case with waste over 12 weeks old)

Note: Your waste has composted and is now “stable”. It is unlikely to reheat to 60°C.

Base layer aerobic, not active

This is ready to take out and use. If you leave it longer it will compress further and eventually airflow will be severely restricted which will prevent the upper layer in the HOTBIN getting above 60°C.

If you are worried the compost looks lumpy and too rough – please visit the post on ‘looks can be deceptive’.


Option 2.4             How It Looks Next Steps To Follow

Base layer Anaerobic & inactive

Is it black sludge, leaking water and has a pungent drain like odour.

Note: Your waste has anaerobically composted. It is now “stable” and unlikely to reheat to 60°C.

If you leave this waste in the base it will compress further and eventually airflow will be severely restricted which will prevent the upper layer in the HOTBIN getting above 60°C.

Aerobic and active

There are two options:

a) Take it out and leave it to dry. Occasionally turning with a fork will help aerate the waste and remove residual odour. Once dried, break it up with a fork. Use the smaller bits as mulch, add the larger bits back into the HOTBIN gradually as a substitute bulking agent (it must be dry!)

b) Dig it in to soil. (Before digging in – double check it is not active by looking at step 2.2. (Active waste will attract vermin and ‘rob’ soil of nitrogen due to ongoing bacterial activity).

Additional Points of Note

If you have removed any material from the base, wipe around the door joints, replace door and tighten cam belts. Gently push the waste down into base to form a new base layer. Add more new waste as soon as possible.

Close the lid and ensure valve plate is set to minimum position (ie, just 2mm open). The HOTBIN should lose all putrid odour and rise into hot zone within 48 hours. Once back to 60°C, continue as normal for 3 months then empty.


Bulking Agent FAQ's

What if I have no shredded office paper or cardboard available?

There are three options:

  1. Do not add excessively wet items (eg, juice pulp, salads) to the HOTBIN.
  2. Drain / dry excess water – eg, squash and then strain water out before adding.
  3. Continue to add wet items and accept it will not get above 40°C and will generate lots of leachate. (You will need to stop adding meat, fish and cooked food).

We can’t change the laws of chemistry and physics. The HOTBIN will only get hot and drive off water as steam when you have waste with more calories than the calories needed to vaporise the amount of water in the waste.

Why do I use corrugated cardboard and shredded paper in the HOTBIN?

Dry corrugated cardboard and shredded office paper is easy for composting bacteria to digest and they are also very ‘dry’ (ie have very low water content, typically less than 5% water). They balance wet food waste to ensure there is enough heat to drive off excess water as steam. If you want to learn why we specify the paper type (office paper rather than newspaper, please read our post on woody and lignified materials).

How do I create lots of chopped up cardboard quickly?

Everyone tends to have corrugated cardboard boxes, but tearing them up can be tiresome. You can quickly cut into strips using a craft / Stanley knife – but you need to take care to avoid taking your fingers off! A safer way is use a ‘multi sheet’ cross cut paper shredder. Most 8-sheet units will shred cardboard boxes. (Note if you put too much strain on a low sheet feeder it will just overheat and conk out!).


Anaerobic and Leachate Explained

Why does the bin turn anaerobic and leak water?

The HOTBIN simplifies the composting science to allow fast hot composting, this aerobic composting breaks down the waste to release water and carbon dioxide. Food waste can be 80% water so a 5 litre caddy of food waste has 1.5 litres (a measuring jug) of water in. This water has to go somewhere, so in hot composting (HOTBIN) most of the water is removed out of the top (through the valve) as hot steam. If this does not happen, water drains down and seeps out of the base as ‘leachate', a wet base layer which restricts aeration and leads to a smelly compost bin.

Removing the water relies on a piece of science known as the energy / water balance.

What is the composting Energy-Water balance?

A scientific equation to balance the amount of water that needs to be removed from the waste with the amount of energy (calories, heat) needed to drive the water off as steam. We can determine exactly how much dry waste (energy) needs to be added to balance any wet mixture. The tables above are a simplified summary that will suit most needs. If you have a special mix we can assess and advise.

What is the water leaking from my HOTBIN?

Water is released during composting. When the waste is ‘too wet’ for hot composting, this water drains down to the base and eventually leaks out as leachate. Leachate cannot be completely avoided, the HOTBIN will occasionally seep water as some water is released in all composting.

You will find there is more water seepage in winter and also on first using the HOTBIN because there is no compost in the base to absorb water.

Is the leachate just water?

No, as water drains down two types of leachate can be formed:

  • A brown odourless liquid
  • A light yellowish liquid, often with a pungent smell.

Can I collect the leachate and use it as a fertiliser on the garden?

Yes if it is a brown odourless liquid you can collect it and pour it on to your soil. It will have nutrients in and humic acids.

If the leachate has a pungent odour and is light yellow, it is still active and potentially phytotoxic (bad for plants). Leave the liquid in a container to rest for a few weeks until the odour goes – ie bacteria have completed decomposition.

How can I collect the leachate?

You can collect the leachate if you take a small plastic tray, set it on the ground and push tight to the mesh plate. Connect the two using putty (or another waterproof flexible membrane), ensure the joint is at the bottom of the mesh plate (see this post).

The mesh grill appears blocked and no water is coming out: It is very rare for the plate to be fully blocked. This would require a huge amount of silt to build up in the reservoir behind the aeration mesh plate. The design of the base reservoir and mesh grill means any small pieces of silt sink to the bottom and water flows out the mesh plate above the silt. If you find the plate is fully gunked up and the water line very close to the top (20-25mm), then dab the plate with a cloth/sponge loaded with neat bleach. This will kill the bio film (removing the gunk) and the water will flow out.

Normally there is about 5mm of wetness below and about 15mm of dry mesh above - creating a clear water line. The air flows in above the water line.

If the mesh plate is constantly fully blocking, please check your HOTBIN is level. When tipped forward, the water and silt will collect behind the grill not in the reservoir.

Do not worry if the mesh plate looks rusty – it is a galvanised mesh. It often looks a rusty colour as the brown leachate dries on grill a rusty colour. If in doubt – clean with bleach as above.