Stonington — The Water Pollution Control Authority is proposing a 7.5 percent rate hike for all customers to help offset the increased cost of electricity and running three treatment plants with upgraded equipment.

The authority has scheduled a public hearing on the rate increase at 6:15 p.m. Tuesday at the police station.

The proposal would raise rates to $5.37 per 100 cubic feet of water as of May 1. The minimum charge would be $41.48 per six-month period. The authority bills customers every six months.

WPCA Executive Director Douglas Nettleton pointed out Thursday that for eight consecutive years the authority did not raise its rates. There have been 5 percent hikes in each of the past two years.

Combined with the proposed 7.5 percent increase, there would be a total increase of 17.5 percent over the past 11 years. That averages out to be a 1.6 percent increase per year.

Nettleton said that the authority currently receives a $300,000 annual subsidy from the town, which also pays for some administrative costs and the debt on sewer plant improvements. Without those contributions, he said the sewer use charge would have to be much higher.

Nettleton said the town is the only one in New England with three plants. Most communities have one or are part of a regional facility.

Nettleton said it is not only expensive to run three 40-year-old plants but upgrades made in 2012 to improve treatment processes require more electricity. Last year he said electricity costs were $96,000 more than budgeted.