Columbia’s new system for determining monthly sewer bills has run into a second glitch in three months and, this time, all 72,000 customers are likely to be affected.

The rub is that sewer customers were supposed to be charged a higher rate on the first 100 cubic feet of water they use, called a base rate, after July 1. Base rates jumped dramatically this summer for the first time in years. After the first 100 cubic feet, the rate was to increase, but not as much. Sewer bills are calculated from water usage.

Instead, customers since July 1 apparently have been charged the base rate increase for their entire usage. That would mean they are being overcharged.

“We’ve received a question about the calculation for sewer rates, and it would impact all of our customers,” assistant city manager Missy Gentry said last week. “We’re taking it seriously.”

For individual residential customers, it’s not much money. But it could be the second billing error since the rates were changed this fiscal year.

The latest complaint came from an Irmo resident who noticed the problem on his August bill.

George Glassmeyer, a former police officer, prosecutor and instructor at the state police academy, examined the city’s published rate structure and compared it to his bill.

“I knew there was a new rate schedule, and I saw that it was calculated wrong,” said Glassmeyer, who has been a Columbia customer for the 25 years he has lived in his Irmo home. “The base rate is supposed to include the first 100 cubic feet. They didn’t give me credit for the first 100 cubic feet.”

Glassmeyer’s August sewer bill rose by $5.93, compared with the previous bill, which did not include the rate changes. But his usage also increased, so it remains unclear how much of the higher bill is attributable to the problem with the base rate calculation.

If his experience proves to apply to all 71,974 water and sewer customers inside and outside Columbia’s city limits, the city will face an administrative challenge to fix the problem and then to make adjustments to all accounts.

Gentry said the utilities department and the city’s technology department are looking into Glasssmeyer’s complaint and hope to have an answer this week.

Further confusing to some customers is that City Council also authorized a $2 increase in the stormwater fee that all city residents pay as well as a $8 fee on fire hydrants for all out-of-city customers. All those charges are listed on monthly water and sewer bills, something customers can check if they look closely at their bills.

Columbia started the new rate structure this summer as part of council’s five-year plan to upgrade the crumbling water and sewer systems. Overall, sewer rates rose by 6.2 percent for in-city residents and by less than one-half of 1 percent for customers outside city limits, according to figures the city provided to The State newspaper.

But base rates – the first 100 cubic feet – skyrocketed to as much as 28 times higher for the largest users, mostly businesses and industries.

The change is jarring because for at least two decades, the city charged one base rate monthly for all in-city customers ($5.17) regardless of how much water they used, Joey Jaco, the city’s director of utilities and engineering, said.

Out-of-city customers paid the same monthly base rate ($9.28).

But council decided to require customers who use more water to pay higher base rates.

For in-city users, the fee starts at $6 monthly for most residential customers and rises to $150 monthly for major users. That’s 28 times more. Outside of the city limits, the fee starts at $10.20 and climbs to $255. That’s 261/2 times more.

In August, within weeks of the new rates, an estimated 7,500 customers were billed a $6 fee that should not have been imposed.

Gentry said customers are being credited that fee, though on Friday she said she could not readily determine how many were affected or how much money was involved. Some customers were charged that fee on two consecutive bills.

“It should not happen again,” Gentry said of the next round of bills.

Before deciding on a fix to Glasssmeyer’s complaint, Gentry said that city staffers must examine the language of the law that changed the rates and compare that to council’s intent.

It’s conceivable council might have to change the law.

Glassmeyer said Gentry offered to credit his bill. He chose to wait.

“It’s only five or six bucks,” Glassmeyer said he told Gentry. “I’m willing to wait until you fix this globally.”