Nitrogen Reduction Program

Nitrogen is an odorless, tasteless, colorless nonmetallic chemical element that occurs naturally in the earth and in our atmosphere. It is a vital component of life for many organisms, but too much nitrogen can be harmful. In waterways, having too much nitrogen can deplete the oxygen that fish and other aquatic life need to thrive.

For many years, DC Water has been a leader in voluntarily reducing nitrogen and phosphorus discharge from the wastewater treatment process. We have an ongoing and aggressive program to reduce nitrogen levels discharged from our Blue Plains sewage treatment plant into the Potomac River, a tributary of the Chesapeake Bay.

We treat an average of 384 million gallons of wastewater per day, and through research and ongoing improvements, we have consistently removed more nitrogen than the goal established under the voluntary Chesapeake Bay Agreement (an environmental protection covenant)

We continue to work to keep our waters clean and to protect our environment. Efforts are underway for an extensive construction project at our Blue Plains plant to significantly reduce nitrogen in treated water outflows. Our goal is to meet or exceed the new U.S. Environmental Protection Agency (EPA) requirement to reduce nitrogen production to 4.7 million pounds per year.

NPDES Permit August 31, 2010
8 31 10 final blue plains permit.pdf [1208K]
NPDES Permit Fact Sheet
8 31 10 final fact sheet.pdf [671K]

Existing Nitrification/Denitrification Process Description

The nitrification/denitrification process, similar to the secondary treatment process, is a suspended growth biological system consisting of reactors and rectangular sedimentation basins. The nitrification process facilitates the oxidation of ammonia nitrogen to nitrate nitrogen resulting in decreased oxygen demand on the Potomac River from the plant effluent. The denitrification process uses methanol as a food source to support the growth of microorganisms that convert the nitrate nitrogen to nitrogen gas.

Secondary effluent flows into a stilling basin at the head of the nitrification reactors. If necessary, sodium hydroxide (caustic) is added to the secondary effluent stream upstream of the stilling basin to provide alkalinity that is consumed during the nitrification process. If denitrification is occurring, little or no caustic is needed. The flow from the stilling basin is hydraulically split to distribute flow between the odd-numbered reactors and the even-numbered reactors. Note that each reactor has five stages and each stage is equipped with two turbine aerators, or mixers. The turbine aerators maintain dissolved oxygen levels and provide adequate mixing to keep solids uniformly distributed in Stages 1, 2 and 3 for nitrification while denitrification occurs in the last two stages. Methanol is added in stage four to provide a source of carbon for the denitrifying organisms. Stages 4 and 5 are not aerated but are operated in an anoxic mode to allow denitrification to occur. The process now employs the same mixed liquor to support both the nitrifying and denitrifying bacteria. Mixed liquor from the odd and even side reactors flows to their respective sedimentation basins. The settled solids are pumped back to the reactors, as return activated sludge, and excess biological solids are wasted to Secondary.

The equipment installed for methanol consists of four underground and three above-ground storage tanks with transfer pumps, a day tank, 24 methanol metering pumps, monitoring and control equipment, and associated piping, valves and appurtenances. Due to the flammability of the methanol, a fire suppression system is also provided.

The Nitrification/Denitrification process has 12 reactors and 28 sedimentation basins. The plant has the capability to use up to 8 dual purpose sedimentation basins to augment the 28 dedicated basins. The sedimentation basins have plastic chain and flight sludge collection mechanisms. Nitrification aeration system components include five process air blowers and associated piping and valves to distribute air to the reactors and channels.

Alkalinity needs are currently met with the Interim Sodium Hydroxide facilities (completed in 2001). These facilities include 5 storage tanks and 3 feed pumps. The total volume of storage is over 30,000 gallons. Polymer and metal salts can be added to the sedimentation basins from the centralized plant-wide chemical systems.

Diagram of upgraded nitrification/denitrification reactor top

Historical Performance of the Nitrification/Denitrification Process

DC Water has operated the full-plant BNR system since 2000 to meet a goal of 7.5 mg/l total nitrogen on an annual average basis, or 8.4 million pounds per year. As shown, performance has been variable; however, the plant has met the goal defined in the permit. The variability of performance is best explained by reviewing effluent TN concentrations, along with flow and temperature data, as shown in Figure 3.

Wastewater Temperature and Flow
Daily temperature of the wastewater is shown in green and the green line shows the 7-day moving average (MA) of temperature. The 30-day MA of plant effluent flow is shown as a red line. As Blue Plains receives wet weather flow, the wastewater temperature (and TN removal performance) is significantly impacted by a combination of temperatures and precipitation during the January to April period. Year 2002 was a very dry year with low precipitation during the January to April period and had the highest wastewater temperatures during this period. Year 2003 and into 2004 was a wet period and had the lowest wastewater temperatures during the corresponding January to April periods. The above average rainfall increased the groundwater table throughout the Blue Plains service area and significantly increased infiltration for an extended period of time.
Effluent TN concentration
Daily effluent TN concentrations are shown in brown and a 30-day moving average (MA) was applied, as shown by the brown line. The data shows that effluent TN performance degrades significantly when temperatures fall below 13° C. It is also noted that during the coldest winter months, the poorest TN performance lags the coldest temperatures. This is the result of switching one stage of the reactors from an anoxic stage (denitrification) to aerated stage (nitrification) so that DC Water can protect the nitrifying organisms and continue to meet the permit requirement for ammonia nitrogen. In addition, it takes weeks to re-establish the growth rate and amount of dentrification organisms after a cold period.
Annual TN Load
The annual TN load, shown by the orange line, is the cumulative sum of the daily TN load values over each year, starting on January 1st of each year. Daily TN load is calculated as follows: [flow (mgd) x TN concentration (mg/l) x 8.34]. As shown, DC Water has met the TN goal of 7.5 mg/l in each of calendar years in the figure; however annual performance is very dependent on temperature and rainfall-induced infiltration during the cold-weather months, which are beyond DC Water's control.
chart showing historical nitrogren removal performance as described above top

Need for Expansion of Existing Nitrification/Denitrification System

For calendar year 2005, the total nitrogen discharge from Outfall 002 at Blue Plains was just over 5 million pounds, or the equivalent concentration of 5.3 mg/l. Although these numbers indicate that perhaps DC Water could meet the 4.7 million pounds per year total nitrogen discharge limit without significant upgrades, the data is deceiving. Specifically, additional upgrades to the process will be necessary to achieve the new permit limit because the conditions that occurred in the year 2005 represented a dry year and the system must meet the permit in all years. As described above, weather-dependent factors such as temperature and flow impact the performance of the system. For the same process performance, additional flow increases the total nitrogen load in the plant effluent. For example, the total nitrogen discharged in the year 2003 was over 7 million pounds which would indicate that the 4.7 million pound limit could not be met in a wet hydrologic year without significant upgrades to the process. Furthermore, future conditions, namely population increases and by-products from DC Water's Biosolids Management Plan (detailed in the "What We Do" section of this website) will contribute additional nitrogen to the wastewater. Blue Plains is expected to reach its rated capacity, annual average flow of 370 mgd, by the year 2030 with an estimated population in the service area of 2.6 million people, which is a 25 percent increase from the estimated service area population for the year 2005 (COG Cooperative Forecast Round 7.1, March 2008).


Required Enhanced Nitrogen Removal Facilities

The projected flows and loads were used in conjunction with the BioWin process model to project the needed new process facilities. Modeling was done for low temperature conditions (12 degrees C) as that is when the microorganisms that remove nitrogen grow most slowly and when the solids settle least efficiently. This modeling, combined with other process engineering tools, has identified the need to provide 39 million gallons of additional denitrification reactor capacity and 5 million gallons of post aeration capacity. This equates to an additional 8 reactors, similar in size and shape to the existing 12 reactors.