Meeting Higher Standards

Ayman R. Shawwa

The Aquia Wastewater Treatment Facility (WWTF), located in Stafford County, Va., was originally built in 1980. In 1990, it underwent a major upgrade and expansion to its biological treatment system that included the installation of new 4.2-mgd countercurrent aeration basins, followed by a secondary clarifier, and a pump station for return activated sludge (RAS), nitrified recycle (NR) and waste activated sludge (WAS). The biological treatment system—known as the Schreiber train—was designed to achieve nitrification and denitrification in a single activated sludge system to provide total nitrogen (TN) removal to 8 mg/L. The upgrade also included an alum feed system and a chemical mix tank for phosphorous removal. In 2004, the WWTF was expanded by adding a new 6.5-mgd Schreiber train, and in 2007, the aeration capacity of the original 4.2-mgd train was increased to match the new 6.5-mgd train. In 2010, the facility expanded its capacity to 10 mgd by adding a new 6.5-mgd Schreiber train in order to adhere to new legislation in Virginia that capped the discharge of nutrients to the Chesapeake Bay watershed.

Each Schreiber train consists of two countercurrent aeration basins, a secondary clarifier and tube-mounted screw pumps for RAS and NR, all manufactured by Schreiber LLC of Trussville, Ala. The countercurrent aeration basins are 154 ft in diameter and 16.4 ft deep. They are connected hydraulically by a straight wall passage in the original 4.2-mgd train and by 36-in.-diameter pipe for the two 6.5-mgd trains. The first aeration basin includes an interior concentric concrete wall with overflow weirs, which is designated as the primary anoxic zone. Each aeration basin is equipped with a rotating bridge aeration assembly that provides mixing and aeration using membrane fine-bubble diffuser tubes suspended from the bridge. The mixing velocity of the rotating bridge in both aeration basins is controlled with equally spaced velocity baffles located inside the tank wall.

Prior to the 2010 upgrades, Schreiber trains were operated in a Modified Ludzack-Ettinger (MLE) process configuration to achieve TN removals to less than 8.0 mg/L and total phosphorus (TP) of 0.18 mg/L (with chemical precipitation) on an annual average basis. In this configuration, wastewater influent is conveyed by gravity from the headworks into the center feed well of the anoxic zone in the first aeration basin. The influent is combined with RAS from the underflow of the secondary clarifier, and with NR from the second aeration basin, and mixed with a mixing assembly suspended from the rotating bridge. Mixed liquor overflows from the anoxic zone to the outer race of the first aeration basin, which is aerated with rotating air diffuser assemblies as well as stationary, wall-mounted, fixed aeration assemblies. The rotating and stationary diffuser assemblies can be lifted with a motorized hoist without the need for draining the aeration basin. Mixed liquor flows by gravity from the aerated outer race of the first aeration basin to the second aeration basin, which is aerated with rotating and stationary air diffuser assemblies.

Mixed liquor from the second aeration basin overflows through an effluent weir to a chemical mix tank, where alum is added for chemical phosphorus removal, and then flows by gravity into a 170.5-ft-diameter secondary clarifier. Each Schreiber train is provided with a set of positive displacement rotary lobe air blowers for aeration.  Variable-speed blowers provide air to the rotating bridge diffusers while constant-speed blowers provide air to the stationary diffusers.

Rotating bridge with aeration diffuser assemblies.

Upgrades

The Chesapeake Bay Program laid out a tiered program for incrementally reducing nutrient loads from significant municipal dischargers. Prior to 2010, the Aquia WWTF had achieved Tier 2 reduction for TN (8.0 mg/L), as well as TP of 0.18 mg/L, which corresponds to reductions more stringent than Tier 3 limits. In 2010, a new effluent permit required Aquia WWTF to comply with concentration- and load-based limits for TN and TP. The concentration-based limits required Aquia WWTF to meet annual average effluent limits of 3 mg/L TN and 0.18 mg/L TP; the load-based limit stipulates an annual waste load allocation of 73,093 lb per year TN and 4,386 lb per year TP. The main objective of the upgrades is to meet all new permit requirements at 10-mgd design flow with one Schreiber train temporarily offline for routine maintenance. Stafford County favored expanding and upgrading the WWTF in a manner consistent with equipment and operation philosophy prior to 2010 while specially maintaining the flexibility of inspection and maintenance of the Schreiber trains. The following is a summary of major improvements that were implemented at Aquia WWTF:

• Construction of a new 6.5-mgd Schreiber train, which allowed plant operators to treat the full design flow with one Schreiber train temporarily out of service. This provided flexibility for maintaining equipment and conducting repairs while meeting permit requirements.
• Installation of new denitrification basins with a carbon feed system to provide additional nitrate removal. The new basins were constructed between the countercurrent aeration basins and the secondary clarifier for each Schreiber train. Each denitrification basin consisted of a secondary anoxic zone followed by a small aerobic zone to strip nitrogen gas prior to clarification. The carbon feed system supplies easily degradable carbon on an as-needed or intermittent basis. Following the completion of the upgrades, the Schreiber trains were operated in a four-stage Bardenpho process configuration.
• Upgrade of the aeration control system, which included automatic starting and stopping of all aeration blowers and controlling the speed of the rotating diffuser blowers to maintain an operated entered DO set point.

Countercurrent aeration basins configured as MLE process configuration.

Results

Stafford County achieved compliance with the new nutrient regulations and consistently met all current permit requirements. Based on 2012 annual average operational data, influent CBOD₅ averaged 287 mg/L with removal efficiency of 99.8%, influent NH₃-N averaged 35 mg/L with removal efficiency of 99.6% and influent phosphorus averaged 9.2 mg/L with removal efficiency of 98.8%. In addition, the WWTF consistently produced effluent TN concentrations lower than 3.0 mg/L with an annual average of concentration of 2.1 mg/L. The excellent performance of the upgraded Schreiber trains was also evident in the annual effluent TN and TP mass loading for compliance year 2012. The TN and TP mass loadings were only 35% and 40%, respectively, of the permit load-based limit. The generated TN and TP mass load credit can be sold by Stafford County to other municipal WWTPs on the Nutrient Trading Market.