CHARLESTON, W.Va. -- Nitro residents should not be surprised when their sewer bills go up in the coming weeks.Members of Nitro City Council voted last month for a two-step sewer rate increase.About 4,100 sewer customers in and around Nitro will see their bills go up by 11.5 percent beginning Friday. Nitro Mayor Dave Casebolt said the rate increase is due primarily to a law requiring the city to increase its bond coverage.The rate increase will bring the average sewer bill, based on 4,000 gallons of water usage a month, to $37.83 a month. That's an increase of $3.91 per month.Casebolt said an additional 7 percent rate increase will go into effect Feb. 1, 2014. That rate increase is to pay for $8.35 million in repairs and upgrades to the Nitro sewer system.A good chunk of the money will go to replace a 42-inch main sewer line from 11th Street West to the sewer pump station near the Nitro High School football field."It's a hundred-year-old line," Casebolt said. "They actually laid that line in brick."Part of the sewer line is collapsing. Casebolt said crews will attempt to install a new sewer line inside the existing brick line to try to keep costs down.Some of the money will also go to replace one pump station, repair another, separate storm sewers and sanitary sewers and pay for equipment and needed maintenance.A portion of the project will also extend sewer lines to Blakes Creek, Eastwood Acres, Cochran Lane and Bailes Drive, Casebolt said. The sewer extensions will add about 140 customers to the sewer system.City officials say the second rate increase will bring the average sewer bill to $40.59, an additional $2.66 on top of the first rate increase.Nitro City Council approved the rate increases on Dec. 18. City Council also established a $70 security deposit for new customers.Casebolt said anyone with questions about the new rates should call 304-755-3669.Reach Rusty Marks at email@example.com or 304-348-1215.