Plant rehabilitation results in nutrient removal

By Paul Wood

The Conroe Southwest Regional Waste­water Treatment Plant (WWTP)  in Conroe, Texas, was originally built in 1974 and had major improvements in 1987 and 1991. The last improvement project was undertaken in 2006 when fine-bubble aeration was implemented, along with the addition of single-stage, high-speed blowers in an effort to reduce energy costs. This last effort was largely unsuccessful due to hydraulic issues in the plant which caused the water surface elevation in the aeration basins to vary by more than a foot. The variation was too much for reliable operation of the installed blowers. The WWTP therefore had not seen any effective major renovation or rehabilitation efforts in more than 25 years.

In 2014, the city of Conroe contracted Lockwood, Andrews and Newnam Inc. (LAN), a planning, engineering and program management firm, to design system rehabilitation and improvements that would allow the plant to effectively and efficiently service its users’ needs for the next 20 years or more.

Texas is slowly implementing nutrient limits based on the water quality of the receiving streams. In general, phosphorus is considered to be the limiting nutrient associated with inland streams and reservoirs, and nitrogen is considered to be the limiting nutrient associated with coastal bays and estuaries. While the Conroe plant does not have a coastal discharge, the most recent permit renewal established a phased implementation for nitrate discharge limits. The permit required that nitrate concentrations be reported for the first three years. After year three, concentration limits of 15.9, 33.7 and 47 mg/L were imposed for daily average, daily maximum and single grab samples, respectively. When designing upgrades to the plant, it was assumed that a future phosphorus limit would be imposed as well; therefore, plant modifications allow for future biological phosphorus removal in addition to nitrification and denitrification.

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