You have heard of air pollution, water pollution, and environmental pollution, but have you heard of nutrient pollution? You heard it right. Nutrients can also pollute our environment. Due to human activities, large amounts of nitrogen and phosphorus are dumped into our environment. For ground water, nutrient contamination can cause blue baby syndrome. Nutrients can also pollute surface water and lead to excessive algae growth (algal bloom). The algae can’t sustain their growth and eventually are eaten by bacteria, which use up the dissolved oxygen in water, generating toxic ammonia that kills desirable flora and fauna. This eutrophication has caused an annual revenue loss of over $1 billion in the U.S. tourist business due to the poor water quality.
Even worse, overgrowth of algae in surface water produces microcystins (toxins produced by cyanobacteria) that cause health problems when this water is used as the source for drinking water. The U.S. EPA takes this problem seriously and will require public water systems to monitor ten common microcystins in UCMR (Unregulated Contaminants Monitoring Rules) 4 program that will collect data from 2018 to 2020.
What are the sources of nutrient contamination?
The nutrients come from different sources. The single largest source is agricultural fertilizers. Storm water run-off is another big contributor, carrying nutrients into surface water and wastewater. Wastewater collected from residential areas and industry accumulates the nutrients that may not get removed properly from the treatment process. The great news is more and more wastewater treatment facilities are implementing new technologies to reduce nutrient pollution. For example, the wastewater facilities near Chesapeake Bay recently upgraded their nutrient removal capabilities and, as a result, nutrition pollutants entering the Chesapeake Bay have been drastically reduced to meet the 2025 goal a decade early.
What nutrients are in wastewater?
Wastewater sources of nitrogen and phosphorus are typically from human waste, household detergents (phosphate), and other waste. Many states have already banned the use of detergents containing phosphate to reduce phosphorus pollution.
Nitrogen and phosphorus appears in wastewater in different forms. Ammonia (in equilibration with ammonium), organic nitrogen, nitrite, and nitrate are four convertible forms of nitrogen. Phosphorus exists in the form of phosphate, including orthophosphate, organic phosphate, and condensed phosphate (metaphosphate, polyphosphate, pyrophosphate).
How can we remove the nutrients from wastewater?
In wastewater treatment plants, primary, secondary, and often tertiary treatments are applied. Nutrient removal is a required process implemented either at the secondary treatment or tertiary treatment steps. However, the traditional secondary treatment is sometimes insufficient to remove enough of the nutrients to prevent algal bloom in the receiving surface water. Therefore, wastewater facilities are required to upgrade their facilities to meet regulatory compliance by improving the nutrient removal from wastewater before discharging it into surface water. The process usually involves biological nutrient removal (BNR) to remove phosphorus to 1 ̶ 3mg/L and then nitrogen (through nitrification and denitrification) to the level of 8–10 mg/L.
Wastewater Nutrient Analysis
Accurate methods are required to monitor the nutrient removal process. Total nitrogen, nitrite, nitrate, and total phosphorus are measured in wastewater analysis, along with total suspending solids, pH, dissolved oxygen, chemical oxygen demand, biological oxygen demand, total chlorine as well as coliform monitoring.
Traditionally, U.S. EPA method 365.2 and 365.4 is used for phosphorus analysis, where orthophosphate is measured colorimetrically as a blue-colored complex that is stronger with concentration. EPA 300.0 and 300.1 also measures orthophosphate along with nitrite and nitrate in a single run by ion chromatography. Nitrite and nitrate can also be measured by colorimetric methods.
The traditional method for measuring total nitrogen is by adding nitrate and nitrate amounts (EPA 353.2) to Total Kjeldahl Nitrogen (TKN) (EPA 351.2 and 351.4), which is the sum of organic nitrogen and ammonia. Currently, there is no EPA-approved method for total nitrogen measurement as a single result. However, Kjeldahl method for TKN determinations uses toxic mercury at high temperature. In addition, nitrate at high concentrations, which are common in wastewater samples, interfere with this method. Finally, multiple methods are required to analyze nitrate and nitrite for total nitrogen determinations, adding further complexity to the traditional method.
A New Method for Total Nitrogen and Total Phosphorus Determinations
Recently, a new ion chromatography method has been developed for the determination of total nitrogen (in the form of nitrate) and total phosphate (in the form of orthophosphate) in a single run with comparable results to the colorimetric procedures. The method also simplifies the sample preparation step. By using alkaline persulfate to digest the wastewater samples in strong base (over pH 12), all forms of nitrogen are converted to nitrate; then over time, the pH of the solution drops to around 2, all the forms of phosphorus (dissolved and suspended) are converted to orthophosphate. TKN can be calculated by subtracting nitrate-N in a digested sample from the sum of nitrite and nitrate in the undigested sample. To avoid the interference from chloride, chlorate, and sulfate in the wastewater samples, one can either dilute the samples or use a high capacity analytical column. Since wastewater treatment facilities already have IC systems for EPA 300.0 or 300.1, there is no additional capital investment.