If Lawrence-Douglas County Planning Commission predictions are correct, Lawrence could see a population growth of as many as 20,000 residents in the next 20 years on the southern edge of the city.

Growth to the south is anticipated because of two factors, said Planning Commissioner Grant Eichhorn. It’s the next big Lawrence area open to development, and the city’s second sewage-treatment plant is tentatively scheduled to open along the Wakarusa River by 2011.

“It’s not a glamorous topic,” Eichhorn said of sewer plants.

Lawrence’s one sewage-treatment facility, 400 E. Eighth St., has existed since 1956 and has been expanded upon multiple times to keep up with Lawrence’s population growth. All of the city’s waste is pumped underground to the one plant, with waste lines from the west pumping slightly uphill, while curving to miss the obstruction of Mount Oread, Eichhorn said.

Underground pump lines to the new plant will flow downhill, he said, which is more efficient.

The anticipated population increase is based on growth trends in Lawrence and anticipated land-use development, said Bill Ahrens, a transportation planner with the Lawrence-Douglas County Planning Department. He described the city’s urban-growth area, which paints a hypothetical zigzagged band around the city based on where population growth is expected. The band on the south stretches the farthest from city lines, he said.

“We predict according to certain trends,” Ahrens said.

Trend forecasts are formed from predictions made by city staff and consultants from Black & Veatch engineering firm, who have worked on projections together for more than a year.

Roger Coffey, the city’s director of utilities, said the wastewater treatment plant on Eighth Street was able to handle its current capacity. Based on population predictions for the coming years, the utilities department knew it needed another plant, he said.

“Everything will be fine as long as we keep moving and get the plant ready and working on time,” Coffey said.

The new wastewater treatment facility will be constructed on the Wakarusa River east of the Clinton Dam, Coffey said, but the exact location has not been determined.