By Kevin Wilcox
The City of Conroe, which has nearly doubled in size in the past 15 years, plans an innovative wastewater treatment plant to manage future demand.
The new wastewater treatment plant being developed for fast-growing Conroe, Texas, features an A-B treatment process that significantly reduces operating costs over the life span of the facility. © LAN
January 19, 2016—Conroe, Texas, is one of the fastest-growing cities in America, swept along in part by the rising economic tides of the burgeoning Houston metropolitan area about 40 mi to the south. Increasingly, however, the city is becoming the target of business and commercial developers as well—to the point at which Conroe is actually a net gainer of inhabitants during the day.
As the city's population has nearly doubled in the past 15 years, Conroe's Southwest Regional Wastewater Treatment Plant inched closer and closer to an important benchmark established by the Texas Commission on Environmental Quality (TCEQ). When average flows at a wastewater treatment plant in Texas reach or exceed 75 percent of its permitted capacity for three months in a row, the permittee must initiate engineering and financial planning on a solution to increase capacity.
With ExxonMobil building a large campus 15 mi south of the city and as many as 9,000 more residential units on developers' drawing boards, the benchmark loomed large. "Clearly we knew that in the future we would exceed the capacity of our existing plant," says John Scott Taylor, P.E., M.ASCE, the executive director of infrastructure services for the City of Conroe. So in 2011, city leaders contracted with Lockwood, Andrews & Newnam, Inc. (LAN), an engineering and design firm headquartered in Houston, to develop a wastewater treatment master plan. That master plan identified a two-pronged approach to meet future demand—an extensive rehabilitation of the Southwest Regional Wastewater Treatment Plant and the development of the new South Conroe Wastewater Treatment Plant.
"The overriding factor in building a new plant is that our existing plant cannot be expanded," Taylor says. "We are basically landlocked at that plant site. And so we need to have a new plant in a different location."
The South Conroe Wastewater Treatment Plant is being designed as a 6 mgd facility with the ability to be expanded 12 mgd. It will employ an innovative A-B treatment process that is relatively uncommon in the United States, but yields significant energy savings and reduced lifecycles costs.
"Originally we looked at a conventional activated sludge facility, which is similar to what the city has now," says William E. Schlafer, P.E., M.ASCE, a project manager for LAN. "We identified that the A-B process plant is going to be slightly more expensive to construct—the costs are very similar but there are a few things here and there that may cost a little bit more—but the operating costs are significantly less."
In the A-B process, wastewater is moved from bar screens and grit removal into an aeration basin that is part of stage A. The LAN engineers estimate conservatively that this stage will remove 60 percent of the biochemical oxygen demand (BOD). The wastewater then enters into the stage A clarifiers, which are somewhat smaller than conventional clarifiers.
Following the A stage, the water moves to an aeration basin and clarifiers that more closely resemble the dimensions and processes of a typical activated sludge wastewater treatment facility; this is stage B. After stage B the current design calls for the processed water to flow through through disc filters for polishing and then immediately to ultraviolet disinfection, before being discharged into a seasonal creak that flows for 4,500 ft before reaching a nearby river.
"Energy savings make up a majority of the cost savings that we are talking about from doing this [A-B] process," Schlafer explains. "Overall it is substantially cheaper to operate." Operation and maintenance costs are also projected to be less because the facility requires fewer blowers, which can be expensive to maintain over their life cycle.
Because the A-B process is more common in Europe, LAN hired ARAconsult, headquartered in Innsbruck, Austria, to provide expert advice on the workings of the system and to review the plant's design. The TCEQ has also reviewed the design and approves of the process.
Taylor says the city initially met with some resistance from staffers who were concerned about what the A-B process would mean for plant operations. To assuage those concerns, the team is building a small, pilot-scale facility that will enable operators to gain experience before the new facility comes online.
The new plant will be constructed on a portion of an approximately 2,000 acre tract of land that was formerly a Boy Scouts of America (BSA) campground. As urban development increasingly encroached upon the site, the BSA purchased land in a more remote location. "The site is challenging," Taylor says. "It's in floodplain. There are some wetlands in that area, and the soil is not the greatest." What the site does offer, however, is a large swath of undeveloped land—an increasingly rare commodity in the Houston metropolitan area.
Ninyo and Moore, a geotechnical firm headquartered in San Diego, with a local office in Houston, is providing engineering design support for the site preparation, which will be significant. Approximately 25.5 acres of the site will have to be excavated as much as 2 ft deep to remove poor soil and organic matter and to free perched water tables for drainage installation. The site will then have to be filled approximately will 11 ft of soil to bring the plant to a point that is 2 ft above the 100-year floodplain. The fill material will be sourced from an adjacent property to reduce the cost of transportation.
Another engineering challenge of the project is how best to integrate the two wastewater treatment plants into a single system. The new South Conroe Wastewater Treatment Plant will be more efficient and less expensive to operate but the Southwest Regional Wastewater Treatment Plant includes a 5 million gal capacity impoundment that can be pressed into service to manage infiltration during heavy stormwater events.
"One of the things that we've discussed is that I want the flexibility to divert flow between the two plants and make adjustments on a day-to-day basis," Taylor says. "My preference is to take a large amount of flow to the new plant, but I want to make sure there is enough flow going to [both plants]."
The current design calls for the construction of a diversion structure at the site of an existing 54 in. diameter sewer line and a planned 18 in. diameter force main. The structure will feature a series of adjustable weirs that can be raised or lowered via a supervisory control and data acquisition (SCADA) system at the new facility.
"We feel like the city needs and can benefit from the flexibility to adjust that flow," Schlafer says. "The idea is to set up weirs in the diversion structure and by adjusting the height of those weirs, we can determine the amount of flow that goes to each plan."
Because the plant will generate sludge of different compositions in several stages of the process, the new facility will include a sludge-blending unit to create a homogeneous mix before it is sent to a thickening unit. After the thickening unit, the sludge will be placed in a sealed digester, covered with a large dome. Unlike conventional wastewater plants—in which steps are taken to reduce the release of methane—the new plant will enhance methane release and collect it for use in the plant. The methane will likely be used to power gas generators at least part of the time at the plant, reducing energy costs even more.
Because the approximately 2,000 acres surrounding the site are being developed for residential and commercial applications, the new facility will include state-of-the-art odor-control methods, as well as a buffer zone covered with as many trees as possible.
The team has submitted a permit application to the U.S. Army Corps of Engineers for the project, anticipating that they could receive approval in the first quarter of 2017. The construction process, including the challenging site preparation phase, is expected to take more than two years, with the facility complete and operational by the summer of 2019.