Monthly Archives: July 2012

HOTBIN investigates an interesting composting method

We had an interesting question come in last week!
Was Black Soldier Fly (BSF) composting the most efficient composting system ever?

After many years in composting, we had not come across BSF before, intrigue and a hint of competiveness that any system might be better than the HOTBIN, we did a bit of googling. You learn something new every day!

  • BSF (Hermetia_illucens) is a species of fly with native to America (with a cousin down in Australia). There will be a UK entomologist who will no doubt identify a cousin in UK, but so far not aware – and hence possible reason we have not come across them in UK)
  • It has a niche habitat – rotting food/manure. The fly lays eggs in compost and the larvae (some would say maggots) eat the food waste – and they appear pretty good at it. (This is no different to house flies and or vinegar/fruit flies – BSL are much bigger and eat more!)
  • The larvae are not meant to be allowed to hatch into flies (ie pupate), they are collected and used as chicken feed and/or fishing bait
  • There are a number of specialist bins designed to house, retain and harvest the larvae. As a food waste disposal system, it more closely resembles worm composting.

What about claims to be the “most efficient”? We are always suspicious of ‘fastest and best’ and try and uncover the scientific facts. There was no reference to a specific quote and we did not see the claim on sites listed below. It looks like there is enthusiasm for BSF, as highly efficient and this enthusiasm comes from the visible disappearance of food waste as BSF larvae eat food within 2-4 days. If you compare 4 days to “composting” even hot composting at 30 days – you might think it’s more efficient. But decomposition (eating waste food) is just a set of biochemical reactions involving enzymes – the rules are fixed: the rate at which reactions take place is governed by the Arrenhenius equation – which basically boils down to temperature. BSF Larvae die at 40C, thermophilic bacteria operate at 60C and in many cases 70C.

In simple terms, larvae at 40C work at the same speed as bacteria at 40. If we say composting at 10C is rate X1, then larvae and bacteria at 40C are X8 times faster, but bacteria at 60C are 32 times faster – speed wise, we think no contest!

But is speed what we need to focus on? What are we trying to accomplish? Are we seeking to dispose of food waste fast, make fish bait or make great humus with high nutrient and water holding capacity that dramatically improves soil fertility? (With the added plus of diverting food from landfill).

Nature eventually recycles all plant and animal matter back to carbon dioxide and water. Does it matter if the larvae eat food, then the chickens eat the larvae and then the bacteria eat the chicken poo? Or that the bacteria eat the food (and release carbon dioxide), but leave some residual waste (compost) that gets eaten much later and then finally becomes carbon dioxide. Both routes are carbon neutral and better that sending it to landfill where it will decompose anerobically releasing methane.  BSFL larvae eat the food and produce a small amount of residual compost like material. Composting and hot composting produces a lot of compost for the garden. Compost is beneficial to the soil before it is finally returned to carbon dioxide.

We should perhaps note that for many humans, the reaction (rightly or wrongly) to flies, larvae & maggots are negative. Even though BSF appear as good guy (does not bite, sting or carry diseases problematic to humans), we know from experience (backed up by surveys), that one of the biggest reasons people stop composting is flies and maggots. The prospect of actively promoting lots of maggots in waste food seems at best destined as a specialist area.

http://en.wikipedia.org/wiki/Hermetia_illucens

http://www.thebiopod.com/

http://blacksoldierflyblog.com/about/

http://blacksoldierflyblog.com/bsf-bucket-composter-version-2-1/

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Grass composting in the HOTBIN

What can you expect when you add grass into the HOTBIN

As we explained in our previous grass post there are advantages to adding cardboard or shredded paper to the HOTBIN to help you compost grass most successfully; it helps calm the ammonia smell and creates extra structure.

Here’s what happens to grass in your HOTBIN through a series of pictures.

Grass added to the HOTBIN

Adding Grass into the HOTBIN with shredded paper

One hour later …

HOTBIN 1hr after grass and paper has been added

24 hours later…see how it has shrunk already!

Grass in the HOTBIN 24hrs later

48 hours later…

Grass in the HOTBIN after 48 hrs

4 days later…this picture speaks for itself

Grass in the HOTBIN 4 days later

7 days later…It’s almost gone and is steaming away!

Grass in the HOTBIN after 7 days

Our control experiment!

We put some grass in a bag outside with no insulation… the temperature never gets above 30C

After 24 hours…

Outdoor grass test Outdoor grass test after 24 hours

After 48 hours…We added some insulation on the top to see if this helps?

 Control experiment after 48 hrs

After 4 days…The insulation hasn’t made a difference!

Outdoor grass test on day 4

After 7 days…there is still no real drop in volume

Grass in our control experiment grass day 7

Compost your grass in a HOTBIN

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A text book start for the HOTBIN

The Compost Woman, who knows an awful lot about composting, reviews the HOTBIN! And I’m pleased to announce she has had a text book start as she has already reached 60C!

http://www.the-compostbin.com/2012/07/hotbin-trial-day-1.html

She started with an very full HOTBIN having plenty of material available for her base layer.

The Compost Woman's base layer

After 24 hours there was already a noticeable difference as the the material started to decompose.

After 24 hours the waste has already decreased

She is a freelance Forest School Leader and Environmental Educator who works with both adults and children on all sorts of things. As well as a volunteer Master Composter and Master Gardener, helping people make compost and grow veg at home or at school.

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