What is Leachate (Brown Liquid)? Can I Use it?

Thursday, 15 August 2019  |  HOTBIN Composting

Water is released during the composting process. In the HOTBIN, much of this water is removed as steam through the valve in the lid. However, when there is excess water it will drain down towards the ground, as it does, it dissolves humic compounds from the compost to form a dark brown liquid. This liquid is commonly called leachate. (it is sometimes called compost tea - but this term is more accurately used to describe a digester process where all waste is converted to liquid - see compost tea section at end of post).

Do I get leachate with the HotBin?

Collecting the HOTBIN LeachateYes.

You normally only get a small amount - a tiny dribble totalling a few mm a day (maybe a espresso cup in a week).

Be aware things can change dramatically when high water content food waste is added. Each 5 litres (small caddy) of food waste will have 3-4kgs of waste which can contain up to 2 litres of water (i.e. a full large mixing jug).
 

If the HOTBIN is not operating at hot composting temperatures 40-60°c, this excess water is not evaporated as steam and will drain to the base of the HOTBIN and out through the mesh plate (Mk.1) or collect under the bas plate of the bin ready for draining (Mk.2 and Mini).

Excess water can lead to the compost/waste becoming saturated (water logged) and this prevents effective aeration. An excess of water is not a good thing in a HOTBIN and will need remedial action.

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

Yes - you can collect the leachate and pour onto your plants and soil.

Leachate is not a high value NKP (Nitogen, Potassium, Phosphorus) fertiliser. The brown odourless liquid will be mainly Fulvic Acid and Humic Acid - both improve the uptake of minerals and can stimulate plant growth. Ensure you have collected an odourless, dark brown liquid. take care if you have collected a fruity / acidic yellowish liquid, or a black pungent drain like thick slimy goo – see ‘why does the leachate smell’ below.

How do I collect the leachate?

With the HOTBIN Mk.2 & HOTBIN Mini the leachate will collect in the bottom of the HOTBIN under the aeration plate, this can then be drained off by unscrewing the cap at the bottom of the bin on the right-hand side. You can put a shallow container (a takeaway tub is ideal) under the tap and collect the leachate.

Do not worry if you have a HOTBIN Mk.1 you can still collect the leachate it will just require more of a DIY solution. Using a shallow container put this if front of the mesh plate at the bottom of the HOTBIN in the middle. Set the tray on the ground and push tight to the mesh plate. Alternatively get a sponge on a tray and set this against the mesh plate (be careful not to fully block the mesh), this will absorb the drips.

You could also place the HOTBIN on a slab and raise this up using some bricks, this allows you to place the container slightly under the HOTBIN ensuring all the liquid is collected including drips.  

You can also let the leachate drain away to soil I this is where the HOTBIN is situated.

Use a small tray to collect leachate Leachate collected using a sponge

 

 

 

 

 

 

Why does my compost leachate smell? What do I do if it smells?

If your leachate smells fruity, acidic, or is pale yellow rather than dark brown these are good indicators you have excess water in your HOTBIN and the water is leaching intermediary organic acids out as it drains to the base. These acids are produced in all forms of composting as waste is broken down into ever smaller compounds that bacteria can utilise.  

We do not recommend you apply such liquid to plants. Many of these acids can be phytotoxic (bad for plants). If you have collected such liquid, always leave it for several weeks until the bacteria (present in the liquid) consume the acids - it will turn odourless and is then safe to use on your garden.

Leachate that smells is also an indication that the contents of your HOTBIN may be too wet and requires attention to stop the contents from turning anaerobic! You can remedy this by adding in extra shredded paper/cardboard to absorb the moisture and extra woodchip to provide additional aeration in the HOTBIN.

If you have a black liquid that has a pungent drain like odour - your heap has gone anaerobic. See how to fix an anaerobic HOTBIN here.

Should I dilute HOTBIN leachate, what dilution?
 

No. You do not need to dilute leachate.

As the leachate from the HOTBIN does not have a high value of NKP it will not burn the plants and therefore can be used neat. However, if you wish to dilute this to make if go further you can do so.

If your leachate has an odour this an indication that it is anaerobic due to depletion of oxygen, you can leave this it to stand so the bacteria can complete the digestion of the intermediary organic chemicals. Once the liquid is odourless then you can use this.

Will the leachate stain my patio?

Whilst the liquid may leave a brown mark on a patio, this will wash off in the rain or with a stiff brush (and washing up liquid if necessary) as it is water soluble. Please note you will often see small amounts of water and wetness around the base of the bin - especially in the morning - this is just the steam condensing and running down the unit and should not be confused with liquid seeping from behind the mesh plate.

What is Compost Tea?

The terms leachate and compost tea are often mixed up. Compost tea is used to describe the liquid produced during anaerobic digestion where the goal is to convert all the waste to liquid. This liquid contains all the original plant nutrients but very little humic compounds. Whilst there are some aerated compost tea processes, most compost teas ocur as a result of anaerobic digestion (e.g. Bokashi liquid).