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.
“Some of the vital equipment at the City of Conroe wastewater treatment facility were more than two decades old,” said Greg Hall, Jr., City of Conroe’s wastewater superintendent. “Additionally, the state of Texas was starting to implement stricter discharge limits and requirements. Considering these two critical factors, we decided to take a proactive approach to upgrade the plant. With these improvements, the plant will now effectively serve the city for the next 20 years or more and also ensure that the plant meets the state’s discharge permit.”
RAS system modifications to unify the plant
RAS system modifications involved unifying the East RAS pit and allowing all RAS from both the East and West pits to mix with influent prior to entering the aeration basins. The revised design reduced the number of RAS pumps and changed the operation of the system. The original system had been designed to accommodate a return rate of approximately 150 percent of the forward flow. This was considered to be an extremely high number that is common in older plant designs. Previously, the two wet wells of the East RAS lift station utilized three submersible pumps each. The gate separating the two wet wells was opened and the six pumps were replaced with a total of four pumps (three primaries and a spare).
The three horizontal end suction pumps utilized on the West RAS lift station originally required replacement of impellers, allowing them to operate in a range around the new design point. Unfortunately, previous operations had tried to control flow on these pumps by throttling the pump suction valve, causing severe cavitation damage to the pump’s impeller and volute, requiring complete pump end replacement.
The new process design, which would reduce the RAS flow, required replacement of the RAS discharge lines to maintain velocities in the pipes and keep solids suspended. Smaller-sized pipes were slip-lined where possible, utilizing the existing oversized RAS lines as host pipes. The RAS lines were rerouted to a new mix box, where screened and de-gritted raw sewage influent could be combined with all the RAS from each clarifier. The RAS lines from both the East and West pump stations were individually metered to allow proper control of the plant (RAS flow rate is normally controlled as a percentage of influent flow to maintain basin solids).
Aeration basin modifications to improve flow split
The aeration basins were also modified to allow better split of the combined raw influent/RAS flow. The improvements included a new influent distribution channel to allow the well-mixed RAS and raw influent to be evenly distributed among the six basins. Originally designed as a separate structure, a value-added cost savings proposed by the contractor allowed construction of an influent distribution trough within the basin. This solution allowed sections of the trough to be isolated with stop logs and flow to the individual basins to be shut off with slide gates.
Additional basin improvements reduced water level variability. Previously, effluent from the individual basins was routed directly to the associated clarifiers; this was modified to add a level control point so that pairs of basins were routed to pairs of clarifiers. For each of the three sets of two adjacent basins, effluent flow was rerouted to an existing hydraulic structure and combined before overflowing two weirs, allowing a more even split of flow between two clarifiers. The middle of these three hydraulic structures, where flows from the adjoining aeration basins come together, was also hydraulically interconnected to allow equalization among the three structures. Stop logs were placed on top of individual weirs to prevent flow to a specific clarifier.
These modifications to the hydraulic structures also allow any basin and clarifier to be taken off line independently from the other basins and clarifiers. The installed weir system and piping modifications provide a hydraulic control point in front of the clarifiers, greatly reducing the variability in side water depth previously seen in the plant. They also provide better control of the hydraulic split among the basins and clarifiers. These modifications also allow use of the existing single-stage, high-speed blowers for aeration (previously unusable and idle for many years).
Aeration basin modifications to allow biological nutrient removal
The treatment process was changed from step-feed to a modified Anaerobic/Oxic (AO) process. The plant has a phased permit that incorporates a nitrate-nitrogen limit in addition to an ammonia limit. Texas is slowly instituting nutrient limits on plants and it was considered prudent to allow for future biological phosphorus capabilities as well.
The Anaerobic/Anoxic/Oxic (A2O) process that allows biological treatment of both phosphorus and nitrogen was considered initially during design. However, results from a recently completed A2O plant designed by LAN proved the ability of nitrification and denitrification with a well-controlled aeration system when internal recycle pumps flowing to the anoxic zone are shut off. Thus, an A2O plant is essentially turned into an AO plant with nitrogen reduction capabilities. The proven results and simplicity of this design provided the justification for going forward with this modified AO process.
By carefully controlling the dissolved oxygen (D.O.) in the aeration basin, an environment that allows both nitrification and denitrification, as well as Biochemical Oxygen Demand (BOD) oxidation, is achieved. Provisions for ammonia and ammonia versus NOx control have been provided but not yet initiated. D.O. control is currently providing excellent results for BOD, nitrate, and phosphorus by maintaining a D.O. in the final zones of approximately 0.5 mg/L. Additional modifications for ammonia-based aeration control are being considered to further reduce energy costs. Aeration control modifications may be required to maintain mixing within the aerated zones.
The final two zones of the aeration basin are currently running at what is considered the minimum mixing level for air, approximately 0.05 standard cubic feet per minute/square foot. Recent studies have shown that if airflow is intermittently increased (for a short period of time) during an otherwise low aeration period, solids settling in the basin can be avoided.
The process modifications that allow Biological Nutrient Removal operation have had a side benefit of altering the biomass in the plant. The selection, which occurs in the anaerobic zone, has significantly reduced filamentous bacteria and allowed an increase in phosphate-accumulating bacteria. Together, these changes have improved the biomass and reduced the sludge volume index. The sludge settling rate has increased and clarification has significantly improved.
A well-functioning plant
- Numerous other modifications and renovations were made to the plant. These included:constructing a new onsite lift station to replace existing non-enclosed Archimedes screw pumps that were at the end of their service life;
- replacing the course screen climber rake system with a new multi-rake fine screen system and washer compactor;
- replacing an aerated grit system with new hydraulically induced vortex grit system that utilizes multiple trays within the same footprint of the existing units;
- installing a gravity thickener in the abandoned clarifier structure, with installation of associated thickened sludge pumps to allow pre-thickening prior to aerobic digestion;
- abandoning a second-stage aerobic digestion unit and shut down of associated 200-HP blowers, made possible by pre-thickening of sludge;
- installing new digested sludge pumping equipment;
- replacing a non-functioning belt press with a new belt press, and a complete rebuild of an existing belt press;
- installing a new polymer make-up system;
- abandoning a nearby lift station by installing a new 54-inch sewer by tunneling;
- relining an existing peak flow diversion basin;
- repairing the plant berm to ensure protection against floodwaters;
- adding emergency generators to allow the plant to operate should primary electrical power from the grid be interrupted for significant periods of time; and
- installing a plant-wide SCADA system to allow monitoring and partial control of process units.
All the modifications to the plant have been functioning since the beginning of 2016. The plant easily meets discharge limits for carbonaceous biological oxygen demand (CBOD), total suspended solids (TSS), ammonia nitrogen, and nitrate nitrogen. In addition, the plant removes a substantial portion of the influent phosphorus without optimizing operations for this removal. Based on current results, it is believed that the Southwest Regional Plant should be able to meet the typical average phosphorus discharge limit seen in Texas, 0.5 mg/L, by better controlling the sludge wastage rate.
One major benefit that has been realized by the process changes has been a 25 percent reduction in energy use, mainly due to better aeration control allowing denitrification. After a few months, the staff also became more comfortable with the new systems, further improving the energy reductions.
Other benefits that have been realized are better clarification associated with inclusion of an anaerobic selector zone and an increased population of phosphorus accumulating organisms. The better treatment has also resulted in a reduction of chlorine use to achieve the desired residual for disinfection. The reduction in TSS concentration in the effluent combined with the reduction in effluent CBOD and ammonia concentrations have resulted in a lower oxidant demand.
“These improvements have allowed us to produce a high-quality effluent and also reduce operational costs significantly,” said Hall. “Overall, the project has been a huge success.”
Paul Wood, P.E., is an engineer at Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering, and program management firm. He can be reached at firstname.lastname@example.org.