Wastewater treatment plants located in the Chesapeake Bay Watershed must meet new requirements limiting nutrient discharge by the year 2010. In an Ampersand two-part series, the process, implementation, techniques, deadlines, and costs associated with BNR will be explored.

What is BNR?

Biological nutrient removal (BNR) refers to the process of removing nutrients, both nitrogen and phosphorous, from wastewater through biological means. The nutrients don’t really disappear, but they are changed to become more manageable.

When the correct environments are established and maintained in wastewater treatment tanks, certain types of bacteria found in wastewater will convert nitrogen and phosphorous to manageable forms.

Under conditions where the amount of dissolved oxygen is extremely low and there is a suitable amount of nitrate, denitrifying bacteria will convert nitrate to nitrogen gas that ultimately escapes to the atmosphere. This is the process that is used for biological nitrogen removal.

Biological phosphorous removal is accomplished by establishing the right conditions for phosphorous accumulating organisms. These organisms will convert soluble phosphorous to an insoluble form and store it within their cell. Soluble phosphorous can then be removed from the wastewater when these organisms are separated from the wastewater in the waste sludge. Though biological phosphorous removal is an option, phosphorous removal is usually easier to accomplish by chemical/physical means such as chemical precipitation, sedimentation, and/or filtration. In some cases, chemical/physical means are necessary to meet the limits in a permit.

A site specific study can determine the best way to implement BNR, as every facility is different. Depending on the existing facility, BNR can be implemented by simply adding a few pumps and associated piping, blower controls, and chemical feeds. In other cases, new tanks, clarifiers, and filters might be necessary. There are many innovative technologies available that can be used to upgrade conventional activated sludge plants to BNR without the need for excessive capital upgrades.