Above: This biological nutrient removal system can achieve high levels of nutrient removal, when combined with an efficient filtration system. Right: Advanced filtration technology like this system can be used to reach enhanced nutrient removal. Photos: Siemens Water Technologies.

Technical solutions can range from retrofits to construction of a new facility. Because of the broad range of possible solutions, including enhanced nutrient removal (ENR) biological systems and filtration technologies, cities need to find a partner with the appropriate level of technical expertise to help meet the demands of their specific application.

For new treatment plants, cities looking to incorporate ENR into the design phase of a facility have a wide range of options. Continuous flow reactor biological treatment plants, sequencing batch reactor treatment plants, oxidation ditch technology, or membrane bioreactor technology are appropriate for new projects.

For upgrades, the choices are equally broad. Newer, more technically advanced treatment methods can be used to refurbish the equipment at existing wastewater plants. With only a few system upgrades, this allows a facility to continue to use the capital equipment in which it has already invested.

For example, the city of Elkton, Md., is upgrading its wastewater treatment facility with an aerated anoxic process to reach the state's ENR limits. A three-channel oxidation ditch with a post anoxic reactor system and tertiary filtration will help the city meet its enhanced nutrient removal requirement of 3 mg/L total nitrogen and less than 0.3 mg/L total phosphorous.

The King William Wastewater Treatment Plant in Virginia is upgrading its facility with membrane bioreactor (MBR) systems to meet ENR limits. The plant will use two MBR treatment trains to treat 200,000 gallons per day to effluent limits of less than 3 mg/L total nitrogen and less than 0.3mg/L total phosphorus.

For wastewater facilities with existing biological nutrient removal, technology options such as denitrification filters or continuous backwash filters can provide effluent polishing to achieve the regulated limits for ENR. For example, a continuous backwashing filter allowed the city of Aberdeen, Md., to reduce its 1 mgd facility's total nitrogen discharge from 7 mg/L to 3 mg/L.

Another option is a two-stage clarification plus filtration process, which has been demonstrated in pilot scale to obtain effluent phosphorus consistently below 0.03 ppm.