A wastewater treatment system treating excess wastewater from a sewer system includes a generally vertically oriented first shaft structure adapted to receive excess wastewater from the sewer system, a generally vertically oriented second shaft structure spaced apart from the first shaft structure, a connector passage extending between the shaft structures proximate lower ends of the shaft structures, and an effluent passage in fluid communication with the second shaft structure. The treatment system further includes a treating agent dispenser for introducing a treating agent into the wastewater to disinfect at least a portion of the wastewater. During a sufficient rain event, the wastewater flows from the sewer system into the first shaft structure, then through the connector passage and the second shaft structure, and into the effluent passage, such that flow time through the shaft structures is greater than flow time through the connector passage.