Wastewater treatment plant rehabilitation improves operations, provides nutrient removal, and reduces power use

By Paul Wood, P.E.

Conroe, Texas, is a rapidly growing metropolitan area approximately 20 miles north of Houston. Conroe currently has one wastewater treatment plant — the Southwest Regional Plant — which is permitted for an average flow of 10 million gallons per day (mgd), and currently treats approximately 8 mgd. The Southwest Regional Plant had not seen any effective rehabilitation efforts in more than 25 years. Originally built in 1974, the plant had major improvements in 1987 and 1991. An attempted energy improvement project undertaken in 2006, involving the installation of fine bubble diffusers and single-stage, high-speed blowers, was mostly unsuccessful.

In 2014, the City of Conroe contracted Lockwood Andrews and Newnam, Inc. (LAN) to design system rehabilitation and improvements. The major emphasis of improvements involved process modifications to allow nutrient removal, improve energy efficiency, and improve operations.

Operational improvements centered on unifying the plant. The original plant configuration utilized a step-feed configuration for biological treatment. However, the influent flow was neither metered (nor controlled) among aeration basins, nor within zones of individual basins. The biological aeration unit consisted of six basins, divided into three independent trains. The three trains consisted of two aeration basins each directly connected to two associated clarifiers. The Return Activated Sludge (RAS) pumps were divided between an East RAS pit and a West RAS pit. The East RAS pit had two sections that could be interconnected, but this configuration was not utilized. The three sets of RAS pumps recycled to their respective trains, ensuring that three separate plants existed. This situation, combined with the ineffective raw influent flow split between and within the basins, created substantial operational variability that made easy control of the plant impossible.

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