Agar is a type of nutrient used in a laboratory to grow cultures of bacteria, fungi, and other various microorganisms. Proper disposal of agar plates is a safety concern for the lab and the environment. Used plates have a different protocol than unused plates. The type of petri dish the agar is in also determines available disposal methods. Take proper care when working with infectious or other dangerous materials. Follow all lab safety guidelines as directed.

Identify the microorganism growing on the agar plate. Categorize most types of bacteria and fungi, including yeast, as hazardous waste. Human and animal cells or tissues are also biohazard materials. For safety reasons, treat most used agar plates as biohazardous waste.

Decide if anything added to the agar is hazardous. Antibiotics, pesticides, blood, or other potentially harmful chemicals have special disposal guidelines. Treat plates with these additives as a biohazard. Sterilize agar with additives before disposing as solid waste.

Choose the proper biohazard bag for your used plates. Put agar plates with human or animal cells and tissues or bodily fluids into a red biohazard bag. All other types of hazardous wastes go in orange biohazard bags. Seal the bags with tape or tie them to keep the materials enclosed and prevent leaks.

Put all hazardous wastes in an autoclave before disposing of them. Check your autoclave’s specifications and choose the proper cycle for biohazard sterilization. After completely sterilizing the plates, dispose of them in the trash. You may also soak used agar plates in a 10% bleach solution to sterilize them if an autoclave is unavailable. However, it is better to use an autoclave if one is available. Bleach does not affect some types of biohazards and chemicals.

Dispose of unused plain agar in plastic plates directly into a trash can. Agar with any additives are biohazards and should be sterilized before disposal. Scrape unused agar out of glass petri dishes and throw it away. Wash and reuse glass plates. Alternatively, melt unused agar over a heat source or in a microwave and pour it into new plates or storage containers for later use.