The city has begun hauling raw sewage sludge from its Sand Island Wastewater Treatment Plant to its Waipahu site as part of a feasibility study to determine whether the program can be expanded to help ease congestion at the Honolulu facility.

Test runs began Tuesday and are scheduled to continue through Sept. 9 as the city attempts to determine the impact of the sewage sludge hauling on both wastewater treatment plants.

Hauling by one 5,000-gallon tanker is being done once a day on weekdays only, with the exception of holidays.

The city ultimately plans to haul the sludge to plants in Kailua and Waianae — in addition to Honouliuli — as it deals with delays in the expansion of facilities at Sand Island, which takes in sewage within the urban core from Aina Haina to Halawa. Test runs are not being made to Kailua and Waianae at this time because of mechanical issues at those plants, officials said.

Mayor Peter Carlisle also has ordered a formal environmental assessment be conducted on the plan to haul sewage to all three waste-water treatment plants.

Hauling sewage sludge to the other plants is seen as a temporary means of relieving the over-capacity “digester” at Sand Island while the administration and City Council work out plans for the safest, most cost-effective way to expand the facility.