That 5,000-foot-long sewer pipe that's been floating on the Ala Wai Canal since last month was sunk into the Waikiki waterway yesterday as a key part of the city's plan to prevent future sewage spills.

In March, a pressurized main sewer line in Waikiki ruptured, and city officials responded by pumping 48 million gallons of raw sewage into the canal rather than risking backup into homes, hotels and businesses.

The city has been working since then on expediting sewer improvements to prevent such a major spill from happening again. The spill fouled the waters of the Ala Wai and closed some of Waikiki's most famous beaches.

The big, black, high-density polyethylene pipe — 48 inches in diameter and fused together in 50-foot sections — has been growing in length since June 13 and going into the water behind Ala Wai Elementary School.

City engineer Eldon Franklin, chief of the Department of Design and Construction's wastewater division, said the city pumped 400,000 gallons of canal water into the pipe to submerge it.

Franklin expects this first phase of the $8 million project to meet a key milestone at the end of this month when the pipe is connected to the existing sewer system near the diamondhead entrance to Ala Moana Beach Park. Those sewer lines then take the sewage to the Sand Island Wastewater Treatment Plant.

He said the pipe will remain submerged in the Ala Wai from 3 to 10 feet below the surface while a permanent fix is being built.

Reach Robbie Dingeman at rdingeman@honoluluadvertiser.com.