Challenge

The City of Elkton, Maryland, located near the head of Chesapeake Bay, 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 the new requirements.

Solution

The City chose the Orbal® biological nutrient removal (BNR) system from Siemens Water Technologies.  A suspended growth activated sludge process, the Orbal system is designed for BNR and energy efficiency.  To solve the variability of treatment conditions caused by diurnal flow and load changes, the new wastewater treatment system design incorporated the SmartBNR™ electrical control system.

The Orbal basin at Elkton was 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 Orbal 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.

Results

Commissioned in December 2008, the Elkton Orbal system is producing outstanding effluent quality. The operations staff has been able to optimize energy usage to reduce power consumption by approximately 40% over that of the old system.

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