There are private companies that have specially ventilated buildings for burning drugs to destroy them. Once an agency amasses a certain amount of drugs and the cases are adjudicated, they turn them over to these companies for destruction. Someone from the LE agency stays to watch and ensure they are actually destroyed.
The same thing goes for seized firearms except they go to a place that melts down metal. Most SoCal agencies take them to a steel company where they are melted down and turned into rebar.
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