Plans for a new city sewer treatment plant on the Wakarusa River took a big step forward Tuesday night.

City commissioners approved annexation, rezoning and a special-use permit for the $88 million plant that will sit on 536 acres at North 1175 and East 1600 roads.

Commissioners unanimously approved the project despite objections from neighbors. About a dozen property owners signed a formal protest petition opposing the site of the plant.

Longtime area resident Debra Green came to the meeting and told commissioners that she was worried the plant would increase flooding problems on her site. City engineers have said that the sewer plant site may need to be raised in elevation by about 10 feet, which Green said would make flooding more likely on her property downstream.

But city engineers on Tuesday said the project would meet all city stormwater regulations, which prohibit a project from creating flooding problems on another person’s property.

City commissioners have consistently supported the sewer plant project over the past year, saying it is needed for future growth. The plant is expected to be operational in 2011.