The Town of Elkton, Maryland, located in Cecil County near the head of the Chesapeake Bay, has a population of approximately 15,000 and an average elevation of 9.1 m (30 ft). Elkton is an economically strong, stable community that is very conscientious about environmental issues. In an effort to protect the Chesapeake Bay aquatic life, new stringent wastewater effluent regulations were implemented requiring very low nutrient levels in wastewater treatment plant discharge.
The City of Elkton was required to meet new stringent wastewater effluent discharge regulations to protect aquatic life in the Bay. The City’s wastewater treatment plant used fine bubble air diffusers, which did not provide consistent nitrification and sufficient denitrification to meet new requirements.
To ensure compliance, the town selected the proven Orbal® biological nutrient removal (BNR) system from Evoqua Water Technologies. The Orbal system is a suspended growth activated sludge process that consists of a series of concentric oxidation ditch channels operating in series. The design of this system provides a higher overall kinetic reaction rate allowing for treatment at higher organic loadings, the reduction in the size of the basins, and lower construction costs.
The basins at the Elkton WWTP were designed with a 16-day solids retention time to ensure nitrification during cold weather. Other design parameters are a mixed liquor suspended solids (MLSS) of 4,000 mg/L and an organic loading rate of 16 pounds of biological oxygen demand (BOD) / day / 1,000 Ft3.
For enhanced denitrification, an internal recycle wall pump with a maximum capacity of 12.8 mgd was included in the design to recycle nitrates in the nitrified MLSS from the inner channel to the anoxic outer channel. A second anoxic zone, followed by a re-aeration zone downstream of the system basin, was included in the design to achieve greater than 95% total nitrogen removal.
The second anoxic zone was designed to have carbon provided by endogenous respiration. This allows for maximum denitrification without the need for supplemental carbon, resulting in additional daily operational cost savings.
Since being commissioned in December 2008, the Orbal system has produced outstanding effluent quality. The operations staff has been able to optimize energy usage, reducing power consumption by approximately 40% over that of the previous treatment system.