Oahu residents will pay more for sewer and water service come Tuesday as a result of rate hikes approved in previous years.

On the sewage side, both the monthly base fee and usage charge now collected by the city Department of Environmental Services are going up by 4 percent. Residents of single-family homes or duplexes will pay $68.39 in a monthly base fee and $4.08 for every 1,000 gallons. That’s up from the current $65.76 base fee and $3.93 per 1,000 gallons. Consumers will continue to get a 20 percent irrigation credit for yard-watering, car-washing and other water uses that do not enter or affect the city’s sewer system.

In 2012 the City Council adopted a six-year plan that increases rates by 4 percent during each of the first four years, 5 percent in the fifth year and then 8 percent in the plan’s final year.

Lori Kahikina, city Environmental Services director, said the increases will allow the city to meet the costs of capital improvements, debt service repayment and upgrades tied to personnel and infrastructure agreed to in a December 2010 consent decree with the federal Environmental Protection Agency and state Department of Health. The city agreed to make more than $3.5 billion in wastewater collection and treatment system upgrades over 25 years.

The settlement came when the EPA was poised to deny the city’s request to extend waivers that exempt Oahu’s two largest treatment plants — at Sand Island and Hono­uli­uli — from needing to perform secondary treatment.

Meanwhile, both the monthly flat rate charge and usage rate assessed by the Hono­lulu Board of Water Supply will also go up Tuesday.

The monthly billing charge will be $8.44 a month, while the water quality charge will rise to $4.03 per 1,000 gallons of water for the average residential customer who uses up to 13,000 gallons a month.

The increases are the fourth in a series of five increases that began in January 2012 with each billing charge hike an increase of about 9.6 percent and each usage rate hike about the same.

Board of Water Supply officials said the hikes are necessary to ensure adequate infrastructure and treatment system operations, and keep up with increases in cost for everything from fuel and materials to labor.