Composting brewery and kitchen waste on site
Spent grain is the largest waste stream in a brewery. Composting turns it, with kitchen scraps, into something useful. It only works if you manage the mix, the moisture and the heat.
What goes in and what stays out
Spent grain, hop residue, trub and yeast slurry from the brewhouse can be composted. So can raw vegetable scraps, coffee grounds and tea from a pub kitchen. Meat, bones, dairy and cooked oily food attract rats and flies and are best kept out of open piles. Used cooking oil needs its own route, which our guide on kitchen waste and used cooking oil covers.
Keep caustic and acid cleaning chemicals out of the pile. Rinse drain residues away from the compost area.
Why spent grain is difficult
Fresh spent grain leaves the mash tun hot and wet, roughly three quarters water. It is rich in protein, so it is high in nitrogen. In warm weather it sours within a day or two, then turns putrid. On its own, grain packs down, loses air and rots without composting.
The fix is to mix it with dry, carbon-rich material that adds air and soaks up moisture: dry leaves, wood chips, sawdust, chopped straw or shredded cardboard. A rough starting point is one part grain to two or three parts dry material by volume. Adjust by feel.
The recipe: carbon, nitrogen, air, water
Good compost needs a balance of carbon and nitrogen of roughly 25 to 30 parts carbon to one part nitrogen at the start. Grain and food scraps are the nitrogen. Leaves and wood chips are the carbon. It also needs air and moisture, ideally like a wrung out sponge, between 40 and 60% moisture.
If the pile smells of ammonia, add more carbon. If it smells sour or rotten, it is too wet and short of air, so turn it and add dry material. If it is dry and not heating, add water in small amounts.
Heat and turning
A working pile heats to 55 to 65 °C within days. That heat kills most pathogens and weed seeds. Hold it above 55 °C for at least several days, turning when it starts to cool. Check with a long compost thermometer pushed into the middle of the pile. Log the reading and the date at each turn, so you can see the pile heat, peak and cool. On a brewery floor the usual mistake is to dump fresh grain on top of a cold pile every morning and never turn it. Add in layers, mix each load in and turn on a schedule. A pile that stays cold has too little nitrogen, too much water or too little air.
A small pub can use a few enclosed bins or a three bay system: one bay filling, one cooking and one curing. Cure the finished compost for a few weeks before you use it. Compost used while still active can burn plants.
Pests, smell and the monsoon
Rats and flies are the common complaints. Bury fresh material in the centre of the pile and cover it with a layer of dry carbon. Use closed bins with a base where you can. Keep the site away from the kitchen door and neighbours.
In the monsoon, cover the pile so that rain does not leach nutrients and soak the grain. Leachate, the dark liquid that drains from a wet pile, should be collected or kept away from drains and water sources. Never let it run into a stream.
Check the rules and the alternatives
Local rules on composting differ. Solid waste, pollution control and municipal authorities may each have a say, especially above a certain size. Ask them before you build. The rules vary by place and change.
Composting is also not the only use of grain. Fresh spent grain is a popular animal feed and can go to a farmer on the day. Our guide on brewery waste and spent grain covers the options. Compost what the farmer cannot take.
Is it worth it?
Add up the cost of hauling waste, the labour to turn a pile and the value of finished compost for gardens or herb beds. A small brewpub with a kitchen garden may find it easy. A large brewery usually does better with feed or a processor. If you move site, plan the waste route with the building. Our guide on moving a brewery to a larger site lists what else changes.
Common questions
Can I compost spent grain on its own?
Not well. It is too wet and too rich in nitrogen. Mix it with dry carbon material such as leaves, wood chips or straw.
Why does my compost pile smell?
Usually because it is too wet or short of air. Turn it and add dry carbon material.
Do I need permission to compost on site?
Rules differ by place and size. Check with your local municipal and pollution control authorities before you build.
