(a) A primary treatment unit may be an aerated lagoon, a partially aerated lagoon, a facultative lagoon, an evaporative lagoon, or an Imhoff tank.(b) A secondary treatment unit may be a stabilization lagoon, a constructed wetland, an evaporative lagoon, or an overland flow process. A secondary treatment unit may be used for polishing and tertiary treatment.(c) A treated effluent storage lagoon downstream of the sampling location in the wastewater treatment facility permit is not considered a treatment unit for the purposes of this chapter.(d) A secondary treatment unit must be preceded by a primary treatment unit.