What are Bulking Agents and why do you Need Them?

These materials are called bulking agents. They're carbon-rich additions that you use in the amount recommended for your system. Some toilets need them after use; others have a different dosing routine. Here's why the choice matters.

More than just a cover over the waste

Covering fresh waste is useful, but the material does more than hide it. It contributes carbon, helps manage moisture and, with a suitable particle structure, keeps spaces open for air to move through the compost.

Think of the difference between coarse shavings and a finely milled, absorbent material. They won't create the same conditions in the chamber. That's why your toilet's recommended mix is worth following, rather than replacing it with whatever happens to be in the shed.

Carbon and nitrogen work together

The microorganisms doing the composting use carbon for energy and nitrogen for growth. Human waste supplies nitrogen; the bulking material helps balance that with carbon.

Too much available nitrogen can contribute to an ammonia smell. A lack of oxygen causes anaerobic conditions. You can have both problems at once, but they're not the same thing. If the pile is wet or compacted, look at airflow and moisture as well as whether you're adding enough of the right material.

Choosing the right material

• Wood shavings: suitable grades can provide useful carbon and open structure. Use untreated material of the grade specified for the system.
• Hemp hurd: a fibrous material used in some bulking mixes. Check whether it suits your system.
• Coco peat: can help manage moisture when used in a specified blend or as the initial material in certain compact toilets.
• Peat moss and prepared blends: some models call for these. Choose the blend recommended for your toilet, rather than one that simply looks similar.

Use clean material of the right grade. Sawmill sweepings, treated or painted wood and an improvised mix of garden waste aren't equivalent to the specified bulking agent. The size of the particles matters too, because it affects how the material holds moisture and allows air through.

How much should you add?

Start with the measure and frequency in your manual. You might need a dose after solids use, a daily or weekly addition, or an initial charge in the chamber. Use that routine rather than guessing from another toilet's instructions.

Too little structure can leave the pile wet and compacted. Too much material can fill the chamber early or dry it out. A consistent measured dose is usually easier to maintain than letting things drift and then making a large correction.

Not sure what to buy? Start with our Bulking Agents & Dosing Guide. If you need a hand, tell us your toilet model, how many people use it and what material you're adding now.