emde logo 

List of State Officials - Robert Ehrlich, Governor; Michael Steele, Lt. Governor; Kendl Ehrlich, MDE Secretary 

Volume 1, Number 7

November 2005

eMDE is a monthly publication of the Maryland Department of the Environment. It covers articles on current environmental issues and events in the state. Additional monthly features include: MDE public meetings and hearings schedule, enforcement and compliance notes, and permitting activity.

Maryland Goes Beyond Other States in Enhanced Nutrient Removal (ENR)

By Walid Saffouri, P.E.

Click on photo to view larger image 





































Example for Activated Sludge Process for Secondary Treatment Only 













Example for Activated Sludge Process BNR Treatment 




Example for Activated Sludge Process ENR Treatment 

Back to this issue's cover page 

Maryland introduced landmark legislation two sessions ago that requires taking new aggressive steps in removing pollutants from wastewater entering the Bay. Maryland’s initital 1983 commitment under the Chesapeake Bay Agreement with six Bay states, prompted the State to establish the Biological Nutrient Removal Program (BNR) to reduce nutrients in treated sewage by the most stringent control mechanism used by any state up to that time. Governor Ehrlich’s 2004 Bay Restoration legislation sets a new goal for sewage treatment that is the current state-of-the-art for nutrient removal, referred to as Enhanced Nutrient Removal (ENR). Terms like ENR and BNR have been thrown around in the media for the last few years. But what do they mean?

The primary cause of the Chesapeake Bay's poor water quality and aquatic habitat loss, is elevated levels of nitrogen and phosphorus. Excessive amounts of nitrogen and phosphorus create dense algae blooms that deplete oxygen and light, eventually killing grasses and aquatic species.

Nutrients enter the Bay via rivers and streams from point and nonpoint sources. The vast majority of point source discharges of nutrients are from sewage treatment plants, with smaller contributions from industries. Nonpoint sources of nutrients are runoff from farms, feedlots, lawns, parking lots, streets, and forests; and from air deposition, groundwater, and septic systems.

BNR Program
Under the BNR Program, established in 1984, Maryland provided 50 percent of capital costs in grant funding to local governments to upgrade the 66 largest wastewater treatment plants (WWTPs) in the state. The design capacity of these plants is 500,000 gallons per day or more and they represent approximately 95 percent of the municipal wastewater discharge into the Chesapeake Bay from Maryland. The goal of the BNR Program is to reduce nitrogen levels in the treated wastewater (effluent) down to 8 miligrams per liter (mg/l). Without BNR, a typical WWTP discharges nitrogen at a level of about 18 mg/l. To date, Maryland has provided funding for this program to upgrade 45 of the 66-targeted facilities with the BNR process. An additional ~$100 million in State grant funding is needed to complete the remaining BNR upgrades, and the State is committed to providing the funding through annual capital appropriations.

ENR Program
Maryland’s Enhanced Nutrient Removal (ENR) Program takes the next step beyond BNR and controls point source nutrient discharge to the Bay by upgrading wastewater treatment plants to the limit of technology for nutrient removal. ENR reduces nitrogen discharge from BNR treatment level of 8 milligrams per liter to 3 mg/l and phosphorus from 3 to 0.3 mg/l. The Bay Restoration Fund will provide grants to local governments for 100% of the cost of upgrading a BNR plant to ENR.

Our Goal
he goal is to remove the Bay and the tidal portions of its tributaries from the impaired waters list, under the Clean Water Act by 2010. To meet this goal, the six Bay states and Washington DC, will have to limit the amount of nutrient loading to a maximum of 183 million pounds per year of (lbs/yr) nitrogen and 12.8 million lbs/yr phosphorus. Maryland’s numerical limit is a maximum of 37 million lbs/yr nitrogen and 2.9 million lbs/yr phosphorus. Nutrient reduction from both point and nonpoint sources is necessary to accomplish this goal. To achieve this, Maryland still needs to reduce nitrogen loading by an additional 20 million lbs/yr and phosphorus by 1.1 million lbs/yr.

Governor Robert L. Ehrlich's Bay Restoration Fund, which was signed into law in May 2004, is a source of new state funding to upgrade WWTPs from the Biological Nutrient Removal level to the Enhanced Nutrient Removal (ENR) level. The fund provides up to 100 percent in grants. Under ENR, the WWTPs will be upgraded to lower the nutrients in the treated wastewater to 3 milligrams per liter (mg/l) total nitrogen and 0.3 mg/l total phosphorus. These ENR upgrades will allow Maryland to achieve an estimated 7.5 million lbs/yr of additional nitrogen reduction and 0.26 million lbs/yr of phosphorus reduction. This action alone will accomplish about 37 percent of the 20 million lbs/yr nitrogen reduction goal and about 24 percent of the 1.1 million lbs/yr phosphorus reduction goal for Maryland.

Most of the 66-targeted facilities were upgraded from secondary treatment to BNR by retrofitting their existing activated sludge process. The ENR components are being added for additional nutrient removal.

An Example of Activated Sludge Process for Secondary Treatment Only is available by clicking on the icon to the left.

This process is known as activated sludge treatment, which treats wastewater as follows: Wastewater passes through screens, which captures large items. This is followed by a grit removal tank, which slows down the wastewater flow enough to settle relatively heavy particles such as sand. The screens and grit tank represent the preliminary treatment system, also referred to as the “headwork”.

The wastewater flows into a primary clarifier, in which the velocity of wastewater flow is further reduced to allow for lighter particles to settle. After the preliminary and primary treatment systems (physical treatment process), the wastewater is introduced to the biological treatment process in the aeration basin.

In the aeration basin, bacteria take in ammonia and added oxygen to produce nitrates. This biological process removes more biochemical oxygen demand (BOD) and suspended solids. Approximately 90 percent of the sludge is returned to the aeration basin from the final clarifier to allow for more bacteria growth.

Wastewater is once again placed into settling tanks to remove any remaining solids in the final clarifier.

The final process, disinfection, occurs as the wastewater is brought into contact with oxidizing chemicals (such as chlorine, bromine, ozone, hydrogen peroxide, and related compounds). Chlorine has long been the disinfectant of choice for most systems. It offers reliable reduction of pathogenic microorganisms at reasonable operating costs. Due to the elevated threat of terrorism, however, many communities are changing their disinfection system to Ultraviolet (UV) Radiation (an electromagnetic radiation used for disinfection) to avoid the storage of hazardous oxidizing chemicals at their facilities.

An Example of Activated Sludge Process with BNR Treatment is available by clicking on the icon to the left. 

To achieve BNR, one or two anoxic basins are added to the activated sludge process. In the low-oxygen anoxic basin, the bacteria take in nitrates, returning through the internal recycle from the aeration basin, to produce nitrogen gas, which escapes from the water and is emitted into the surrounding atmosphere. Approximately 90 percent of the sludge is returned to the anoxic basin from the final clarifier to allow for more bacteria to grow and assist in the treatment

An Example of Activated Sludge Process with ENR Treatment is available by clicking on the icon to the left.

To achieve ENR, filters can be added to the BNR process for additional nitrogen and phosphorus removal. An external carbon-source, such as methanol, is added to the filter to increase bacteria growth and further improve treatment.

This process allows Maryland to achieve maximum nutrient removal from wastewater treatment plants and do our part to “save the Bay.”


©2005 Copyright MDE

Editorial Board
Maryland Department of the Environment
1800 Washington Boulevard, Baltimore, MD 21230