A mechanically operated lagoon is a large, lined excavation that receives sewage, mixes it with oxygen so that biological processes can destroy most of the disease-causing organisms, and discharges the sewage as treated sewage. Correct operation and maintenance is important because a neglected lagoon will produce foul odors, become a breeding place for flies and mosquitoes, and require costly repairs. This technical note describes how to operate and maintain a mechanically aerated lagoon.