Why does our porta potty smell like rotten eggs after a rain?

The rotten egg smell after a rain is hydrogen sulfide gas being released from the holding tank.

Hydrogen sulfide is produced by bacteria that thrive in the low-oxygen environment of the tank. A heavy rain cools the tank and stirs up the contents, which releases the trapped gas. This happens even after a fresh service because the bacteria are always present. The soggy ground itself isn't the cause, but it can make the area around the unit damp and hold the smell longer.

The type of deodorizer matters. Cheaper deodorizers that just mask odors with fragrance won't stop this. You need a deodorizer that contains biocides to control the bacteria producing the gas. Many operators now use formaldehyde-free deodorizer because wastewater plants often restrict formaldehyde. A weekly service visit is standard, but if the unit gets heavy use in the heat, it might need service more often to keep the bacterial load down. Ask your rental company if they can switch to a more effective, bacteria-targeting chemical for your unit.