Plant rehabilitation results in nutrient removal
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.
“The city of Conroe’s wastewater treatment facility has operated some of its most vital equipment for 20-plus years. Due to the age of this equipment, we started to experience more frequent and longer downtimes for repairs,” said Greg Hall Jr., superintendent of the Conroe WWTP. “With the state of Texas starting to implement stricter limits and requirements, the city of Conroe wanted to take a proactive approach, knowing that improvements were going to be needed to ensure the stricter limits were obtainable in the future. These steps would ensure our compliance with the state’s discharge permit.”
Modifications Facilitate Nutrient Removal
The current Conroe WWTP originally was designed using a strategy that utilized three independent trains. The 10-mgd plant has six aeration basins and six clarifiers. Flow from each pair of aeration basins was routed to two dedicated clarifiers. Recycle flow from each pair of clarifiers was routed directly back to the associated aeration basins. The aeration process used a step-feed approach, but influent flow was neither controlled among the basins nor within each basin’s step-feed points. Recycled sludge, likewise, was unmetered and uncontrolled.
One of the major issues that had to be resolved when evaluating the plant hydraulics and operational configuration was how to achieve plant unification. This was in addition to the absolute requirement for biological nutrient removal (BNR) capabilities. In order to better allow plant unification, the LAN design team decided to reroute all of the return activated sludge (RAS) flow to a single location where it could be mixed with the raw influent flow after being screened and processed for grit removal. A new distribution system was then built into the head of the aeration basins, incorporating sluice gates on wall sleeves allowing isolation of each basin. The level in each basin was better controlled by installing matching overflow weirs. This modification reduced the variability in basin depth from more than a foot to a few inches. The RAS piping also was modified. Originally sized to allow return rates of 150% or more of the forward flow, the design team determined that the velocities in these lines would be excessively slow for the expected typical rates of a well-operated activated sludge plant. The RAS lines were slip lined where possible, reducing them to reasonable sizes. Flow metering and variable frequency drive motor control were added to the new RAS pumps to allow both RAS and waste activated sludge (WAS) control.
Each basin was modified with the addition of a baffle wall and a top entry mixer in the first quarter of the BNR basins. Diffusers were removed from this mixed anaerobic zone. This anaerobic selector zone would serve three purposes: as a selector to inhibit the growth of filamentous bacteria; as an anaerobic zone conducive to the growth of phosphorus accumulating organisms; and as a zone to allow floc to sorb and utilize readily available substrates that would otherwise accumulate on the bubble interfaces in the subsequent aeration zones, thus increasing oxygen transfer efficiency. Diffuser densities in the aeration zones were reduced to allow the minimum published air rates where mixing could be maintained—0.05 scfm/sq ft.
The modifications of the aeration basin hydraulics and diffuser configuration allowed the use of the previously installed but never utilized single-stage, high-speed blowers. The blowers are controlled by a new controller that employs a most open valve control philosophy to control the dissolved oxygen (DO) in each aeration basin. In each basin, three individual drops with manual valves allow aeration in each aerobic zone to be manually adjusted.
The process changes designed by LAN have enabled the WWTP to exceed its new discharge permit with regards to nitrates. The average nitrate value for the three months since the modifications and new controls have been functioning in the aeration basin has averaged 4.84 mg/L. The maximum nitrate value recorded in this time was 8.37 mg/L. The effluent ammonia nitrogen has averaged 0.26 mg/L with a maximum of 2.5 mg/L compared to an average daily limit of 2 mg/L.
Biological oxygen demand (BOD) and total suspended solids (TSS) average 3.44 mg/L and 3.75 mg/L compared to daily average limits of 10 mg/L and 15 mg/L, respectively. Effluent phosphorus, while not presently permitted or optimized for removal, is averaging 0.8 mg/L. The expected typical effluent phosphorus limit in Texas is expected to be 0.5 mg/L and should be achievable by better controlling solids wasting and aerobic digestion modifications.
An added benefit to the process changes is a reduction in power consumption. Power consumption has dropped by 25% in the first three months since the plant has been fully operational with all the changes, resulting in savings of almost $10,000 per month. The improved discharge quality also has resulted in a reduction in chlorine usage required for disinfection. Chlorine use has decreased by an average of 81 lb per day to achieve the same residual concentration.
“City of Conroe has one mission, and that is to protect and serve the citizens of Conroe and exceed their expectations. That’s what our goal was and that’s what has happened. The improvements made were a huge success with operations and operation efficiency. With the improvements completed, we have been able to streamline the operations of the plant to a point we never thought possible. This has allowed us to produce a high-quality effluent and cut operational cost at the same time. This is a huge win for the citizens of Conroe,” Hall Jr. said.