OAKLAND -- East Bay Municipal Utility District customers will be hit with one of the agency's largest water rate increases in history -- 20 percent in two steps over the next year, the EBMUD board decided Tuesday.

Thirteen customers objected strongly, saying at Tuesday's public hearing that the district should pare down costs rather than raise water rates so much so fast in an era of slow inflation.

"I question whether any increase of this magnitude can be just and reasonable," said Jim Tuthill of Lafayette, a part-time teacher at UC Berkeley's School of Law. "This is preposterous."

The board disagreed. The elected directors said their water supply agency for 1.3 million people in Contra Costa and Alameda counties needs big increases because it deferred maintenance of pipes and other systems in recent years of drought and recession.

EBMUD officials also said the price of steel, concrete, chemicals and other materials to run a water district have increased faster than the overall inflation rate.

"It's time for us to do better than limp along," said Director Lesa McIntosh of Richmond. "It's painful, but it's necessary."

The vote to approve the increases in two steps was 5-2. Directors John Coleman of Lafayette and Katy Foulkes of Piedmont voted no, saying a large increase was justified but not as much as proposed by district managers.

A 9.75 percent water rate increase goes into effect July 1, and a 9.5 percent increase goes into effect a year later.

The two increases will add a total of $8.05 a month to the average bill of a customer using 246 gallons of water per day.

In a related move, the board approved sewer rate increases of 8 percent this year and 9 percent next year for some 600,000 people in western Alameda and Contra Costa counties.

EBMUD staffers said failing to enact the water rate increases would put the district's credit rating in jeopardy and increase the risk of water service outages caused by aging water pipes breaking more often.

"The district has faced some difficult circumstances, compounded by recession and drought," Eric Sandler, EBMUD's finance chief, told the board.

Several water customers said the board should postpone the rate increase for at least 90 days and ask an auditor to seek ways to avoid such large increases.

"This increase will disproportionately affect the working poor and those with disabilities," said Darrell Johns of Oakland, who works as a volunteer helping low-income residents.

He and others complained that the water district has approved annual increases greater than the Consumer Price Index for several consecutive years.

EBMUD rates overall are lower than most other water suppliers in the Bay Area, officials said.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.