According to the U.S. Environmental Protection Agency (EPA), biological nutrient removal (BNR) removes total nitrogen and total phosphorus from the wastewater by the actions of microorganisms.
What are the BNR processes in a wastewater treatment plant? What are the factors that affect the removal of nitrogen and phosphorus in the BNR process?
These questions are addressed in this article along with other topics including the challenges associated with phosphorus removal in a wastewater treatment plant.
What is a biological nutrient?
Biological nutrients are defined as non-hazardous biodegradable materials and products from the biodegradation process. But what is a nutrient? In simple biological terms, a nutrient could be from one of the six major classes based on their biochemical properties: carbohydrates, proteins, lipids, water, vitamins, and minerals. Minerals include potassium, nitrogen, sodium, and iron.
Biological nutrients, from the wastewater treatment perspective, are nitrogen and phosphorus and the removal of total nitrogen and total phosphorus via the actions of microorganisms is called biological nutrient removal (BNR).
Total nitrogen
Total nitrogen is the sum of all nitrogen forms in the water. The formula for measuring total nitrogen in the wastewater treatment plant is:
Total nitrogen = total Kjeldahl nitrogen + nitrite (NO2) + nitrate (NO3)
The U.S. EPA’s definition of total Kjeldahl nitrogen
The US EPA defined total Kjeldahl nitrogen as the sum of free-ammonia and organic nitrogen compounds. Free ammonia is that portion of ammonia that is not combined with chlorine. It is represented by the formula NH3 or as an [NH4]+ ion depending on the pH and water temperature. Organic nitrogen compounds are also known as organic Kjeldahl nitrogen.
Measuring total Kjeldahl nitrogen
EPA’s method 351.2, using semi-automated colorimetry, measures total Kjeldahl nitrogen. It includes measuring total Kjeldahl nitrogen in industrial wastes and the applicable range is 0.1 to 20 mg/L of total Kjeldahl nitrogen.
Total phosphorus
Total phosphorus is a measure of all the forms of phosphorus, dissolved or particulate, that are found in a sample. More specifically, total phosphorus includes orthophosphate, phosphorus monoester and diester, phosphonate, polyphosphate, including polyphosphate monoester and pyrophosphate.
The U.S. EPA’s definition of total phosphorus
The EPA’s definition of total phosphorus is all the phosphorus, regardless of their forms, that is in a sample and measured by the persulfate digestion procedure. This includes orthophosphates, hydrolysable phosphorus, and organic phosphorus.
Measuring total phosphorus
The methods to determine total phosphorus include high-temperature combustion and are based on transforming chemicals containing phosphorus into orthophosphate.
The standard method for measuring total phosphorus is the US EPA method 365.3 using colorimetry. It includes measuring phosphorus in industrial wastes in the range of 0.01 to 1.2 mg of phosphorus per liter.
What is the biological removal of nutrients?
The biological removal of nutrients, also known as biological nutrient removal (BNR) is a process involving microorganisms for the removal of total nitrogen and total phosphorus from the wastewater.
The difference between biological nutrient removal (BNR) and enhanced biological phosphorus removal (EBPR)
As the names imply, enhanced biological phosphorus removal (EBPR) removes phosphorus from wastewater via the action of microorganisms while biological removal of nutrients (BNR) includes the removal of total phosphorus.
What is the objective of biological nutrient removal?
The objective of biological nutrient removal (BNR) is to remove total nitrogen and total phosphorus from the wastewater. This is done via the actions of microorganisms present in the wastewater.
Processes for biological nutrient removal in wastewater treatment
Nitrogen removal
Nitrogen removal in a wastewater treatment plant involves three processes, namely ammonification, nitrification, and denitrification.
Ammonification: Ammonification occurs as the wastewater is enroute to the wastewater treatment plant. In this process, organic nitrogen (N) is converted into ammonia (NH3).
Nitrification: Nitrification occurs in the aeration tank. In this process, the microorganisms present in the wastewater treatment system converts ammonia (NH3) to nitrite (NO2) and then to nitrate (NO3).
Denitrification: Denitrification occurs in anoxic conditions in the wastewater treatment process. In this process that also involves the microorganisms, nitrate (NO3) is converted to nitrogen gas (N2).
Phosphorus removal
Phosphorus removal from wastewater requires two processes, namely anaerobic and aerobic processes.
The anaerobic process: In this process that occurs in a zone where there is no oxygen and thus the name anaerobic, the microorganisms present in the water consume fatty acids.
The aerobic process: Following the anaerobic process, the microorganisms uptake phosphorus, resulting in the removal of phosphorus from the wastewater treatment system.
Factors affecting nitrogen removal from wastewater treatment
These factors include:
- BOD5/TKN (BOD5, the 5-day biochemical oxygen demand; TKN, total Kjeldahl nitrogen)
- Temperature in the wastewater treatment tanks
- Mixed liquor recycle rate
Factors affecting phosphorus removal from wastewater treatment
These factors include:
- The amount of high phosphorus-content microorganisms: These microorganisms contain high levels of polyphosphate. Polyphosphate is a phosphate polymer containing individual phosphate molecules linked together by high energy bonds.
- Mixed liquor recycle rate
- The available volatile fatty acids in the anaerobic zone. A BOD5 to total phosphorus ratio of 20:1 is required in some wastewater treatment operations for phosphorus removal by the BNR process.
- The release of phosphorus from the waste activated sludge. This is discussed in the next section.
Challenges associated with phosphorus removal from the wastewater treatment plants
The waste activated sludge in the wastewater treatment, if stored for prolonged time, can lead to anaerobic conditions in the wastewater treatment tanks. In this case, the microorganisms can re-release the uptaken phosphorus back into the water. To prevent phosphorus re-release into the water, daily sludge wasting (the process of removing excessive sludge) and sludge thickening are important.
What methods of aeration exist for wastewater treatment?
Aeration adds air into the wastewater. This is beneficial for aerobic degradation which is a part of the BNR process. Aeration is effective and economically viable compared to chemical treatments for the removal of total nitrogen and total phosphorus from the wastewater.
Aerators can be surface aerators or submerged aerators and both can be mechanical aerators. Coarse bubble diffusers are another type of aerators used in a wastewater treatment systems. The large bubbles they produce eliminate clogging in the system.
Besides bubble diffusers, packed-tower aerators and spray aerators are also used in wastewater treatment. The selection of an aeration method depends on the chemical constituents of the wastewater, the oxygen levels, and the amount of waste activated sludge among other conditions in the wastewater treatment process.
A study comparing intermittent versus continuous aeration mode on nutrient removal and carbon footprint (kg of CO2/m3) in a wastewater treatment plant showed that intermittent aeration performed better for the removal of ortho-phosphate over a 24 days application period. However, the intermittent aeration mode had higher indirect emission of carbon footprint due to additional energy consumption from mixers and sludge recirculation pumps.
About the Author
Saleha Kuzniewski
Saleha Kuzniewski, Ph.D. has authored several publications in the fields of scientific research, biotechnology, and environmental regulations. She is the winner of the 2023 Apex award for publication excellence. She is also the founder of Environmental Remediation & Innovations, LLC. Kuzniewski can be reached at [email protected].