It shouldn’t come as a surprise that the choices surrounding a new sewage treatment plant for the city of Lawrence aren’t very appealing. Nothing about that subject ever is.

But Lawrence city commissioners are facing a particularly tough decision about when to begin designing and building an $80 million to $90 million sewage treatment plant south of the Wakarusa River.

City leaders long have known that Lawrence’s lone sewage treatment plant has a finite capacity — likely reaching its limits when the city grows to around 100,000 to 110,000 residents. City leaders also have known that it will take about four years to design and bring a new sewage treatment plant on line. What city leaders are struggling to determine is how quickly the city will grow.

During this summer’s budget deliberations, commissioners will have to decide whether 2012 is the year to start designing the project. If so, sewer rates will rise significantly, almost certainly by more than 10 percent.

If city commissioners act now, monthly sewer bills will rise dramatically with no guarantee that a new plant will be needed in the near future. If they wait to start the project, the city in the next five years could have to say no to some developments because it doesn’t have adequate sewer capacity.

It is a delicate decision for sure, but commissioners should be cautious about making significant rate increases to start the project in 2012.

Commissioners should take more time with the subject, in part, because they are dealing with incomplete information. The population of the city is a key factor in determining when this plant will be needed. Determining the city’s population has been tricky of late. The U.S. Census Bureau’s official count put the city’s 2010 population at 87,643 people. But the city’s planning department has estimated the population at 91,464 people. The difference of nearly 3,800 people is significant in determining how quickly our sewage plant will reach its capacity.

Also important is how fast the city is expected to grow. In the 1990s, the city’s population grew at an average rate of about 2.2 percent per year. In the 2000s, it grew at about 0.9 percent per year. What does the city expect the growth rate to be in the current decade? It is not clear that the city has a firm answer. It seems that the city would be well-served to have a serious discussion about what are reasonable expectations for growth given the new realities of the economy.

Ultimately, however, the city will be asked to make a bet on the future. Like all bets, there are risks on both sides. Having the best information available, though, should make city commissioners feel better about wherever they choose to place their wager.