Used coffee grounds

Lore & Background

Used coffee grounds are generated in vast quantities—over 15 million tonnes annually worldwide—with roughly 650 kg of spent grounds produced from every tonne of green coffee. In coffeehouses, a device called a knockbox (also known as a bash bin, coffee column, slam piece, or bang bang) is used to store and release the spent espresso puck; these are typically made of stainless steel, rubber, or plastic to withstand heat and repeated impact.

The chemical composition of used coffee grounds is broadly consistent, though grounds from instant coffee production contain fewer extractable compounds. They are rich in sugars (about half their weight), proteins, lipids, and lignins, and contain significant levels of potassium, nitrogen, magnesium, and phosphorus. Caffeine content is roughly 48% of that in fresh grounds, while tannin levels are notably lower.

Various traditional and modern uses have been documented. In Finland and Karelia, grounds have been added to pancake batter. In divination, patterns in coffee grounds are read for fortune-telling. Dried grounds have been used to fill pincushions, and homemade applications include wood staining, air fresheners, and body soap scrubs.

Reader's Guide

Used coffee grounds represent a substantial waste stream with considerable potential for valorization. A 2026 review of their chemical and functional properties surveyed applications as food additives, agricultural amendments, adsorbents for wastewater treatment, biofuel feedstocks, and ingredients in pharmaceutical, nutraceutical, biotechnological, cosmetic, personal-care, and sustainable material products, including biorefining.

In agriculture, grounds are used as a soil amendment and mulch, slowly releasing nitrogen. They are said to be appreciated by worms and acid-loving plants, though the acids leached typically yield a neutral pH, and experimental use as primary worm feedstock has been shown to reduce pH. Fresh grounds have phytotoxic properties that can be reduced through composting. Gardeners have reported their use as a repellent for borers, slugs, and snails. Commercial initiatives such as Starbucks' 'Grounds for your Garden' and community schemes like 'Ground to Ground' aim to divert grounds from landfills.

Industrial applications include biogas production, wastewater treatment, bioethanol production (after defatting and acid hydrolysis), and biodiesel production via solvent extraction, methane/catalyst mixing, or algae feeding. Hydrothermal liquefaction can produce a high-energy char. In concrete, 10–15% of sand can be replaced with used grounds. Caffeine recovery for commercial use has been suggested. In 2021, Forest Green Rovers trialed a kit made from 35% used coffee grounds combined with recycled plastic. Café noise management has been tested using sound-absorbing panels made from coffee waste and urea resin, reducing reverberation time from 1.2 s to 0.7 s and sound pressure by 7 decibels.

Precautions include avoiding burning dried grounds, as they release hazardous nitrogen oxides. Feeding grounds to ruminants, pigs, chickens, and rabbits is considered undesirable due to high lignin content.

Did You Know?

• Used coffee grounds contain about 48% of the caffeine found in fresh coffee grounds.
• A knockbox used to store spent espresso pucks is also called a bash bin, coffee column, slam piece, or bang bang.
• In concrete making, 10–15% of sand can be replaced with used coffee grounds.
• Burning dried used coffee grounds is not recommended because they give off hazardous nitrogen oxides.