The city is considering alternative ways to raise money for sewer upgrades mandated under a recent consent decree with the Environmental Protection Agency.

Recently announced projects to bring Honolulu into compliance with sewage standards are expected to lead to 3 percent to 5 percent sewer fee increases each year for about 25 years, according to the city. That means the average single-family home’s sewer bill could rise from $91 a month today to between $191 and $308 a month, excluding inflation, in 2035.

Among the alternatives to monthly sewage collection fee hikes that are being considered are establishing a fixed monthly sewer fee instead of the current rate that can vary depending on usage, and creating special fees to cover legal requirements and for hauled sewer waste.

The city Department of Environmental Services said the effort is an attempt to find ways to alleviate the need for the 25-year sewer fee increases.

The city also will consider providing rate relief for fixed- and low-income customers during a yearlong study that is expected to begin in October. The city started a search for a consultant to conduct the study earlier this month.

The consultant, which will be chosen in September, will explore those and other alternatives. It will cost the city $4.7 billion to upgrade sewage and wastewater treatment systems to comply with the EPA consent decree.