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Maryland State Government Maryland Department of the Environment

The Evolution to Enhanced Nutrient Removal Technology

Primary/Physical Treatment

Initially, wastewater treatment plants had to only achieve primary/physical treatment by providing preliminary process (screens and grit removal units) and primary settling tanks (primary clarifiers).  Primary treatment achieves only 45 to 50% in reduction of pollutants by removing settlable and other easily removable materials.


Secondary Treatment

Secondary treatment introduced the biological process such as activated sludge, trickling filter, rotating biological contractor, and other biological treatment technologies.  Biological treatment systems are living systems that rely on mixed biological cultures to break down waste (that could not be removed by the physical treatment) and allow it to settle in the final clarifier achieving 85 to 90% reduction in pollutants.  As early as 1957, communities in Maryland received federal and state grants to upgrade their facilities with secondary treatment systems.  However, secondary treatment was not required for most plants until the inception of the National Pollutant Discharge Elimination (NPDES) permit in 1972.


Biological Nutrient Removal (BNR)

The Chesapeake Bay has experienced a decline in water quality due over enrichment of nutrients (mainly phosphorus and nitrogen).  The Chesapeake Bay Agreement of 1983, signed by Maryland, Virginia, Pennsylvania, and the District of Columbia, specified a nutrient reduction goal of 40% by the year 2000.  The Maryland Department of the Environment, in support of Maryland's commitment to reduce the amount of nutrients being discharged to the Bay, developed a strategy for achieving the desired reduction by the upgrade of the major 66 wastewater treatment plants to remove nitrogen through a process known as biological nutrient removal (BNR).  Using the BNR process, more than 90% of pollutants are removed, while achieving nitrogen concentration below 8 mg/l total nitrogen.


Enhanced Nutrient Removal Program

Recognizing that more needs to be done, the Chesapeake Bay 2000 Agreement requires further reduction in nitrogen and phosphorus entering the Bay by about 20 million pounds and 1 million pounds per year respectively.  The Maryland Department of the Environment is using the Bay Restoration Fund to upgrade the 66 major wastewater treatment plants, which discharge to the Chesapeake Bay, with enhanced nutrient removal (ENR) technologies.  Once upgraded, these plants are expected to reduce nitrogen and phosphorus in the wastewater down to 3 mg/l total nitrogen and 0.3 mg/l total phosphorus, achieving approximately one-third of the needed reduction under the Chesapeake Bay 2000 Agreement.  Other pollutants will continue to be reduced by more than 90%.

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