Monitoring nutrient levels
- By Cindy Chen
- Nov 01, 1999
New regulatory developments affecting nutrient discharge criteria
Removal of nitrogenous compounds from wastewater is critical, as excessive ammonia and nitrate levels in discharged effluent can pose a threat to marine and human life.
Nitrates are the most widespread agricultural contaminant and are a human health concern since they can cause methemoglobinemia, or "blue-baby" syndrome. Ammonia exerts an oxygen demand in aquatic environments; 4.7 grams of oxygen are required to oxidize one gram of ammonia, threatening marine life. These factors call for effective removal of these forms of nitrogen from wastewater before they are discharged to natural water systems.
Monitoring nutrients in wastewater effectively will become even more important with new requirements to control nutrient discharges on the horizon (see New regulatory developments).
Long lag times
Current control strategies for optimizing the nitrification and denitrification processes of biological nutrient removal (BNR) plants and other wastewater treatment plants (WWTPs), including industrial processes, are often limited by long lag times in the availability of laboratory data. Laboratory testing for nutrient levels can take several days; tests for other parameters such as biological oxygen demand (BOD) can take up to a week. These delays make it difficult to effectively optimize and control wastewater treatment processes.
Bypassing the lab
A new technology has been developed that bypasses laboratory testing to continuously and accurately monitor ammonia and nitrates directly in wastewater or activated sludge. Isco-STIP Process Buoys PBS 1 and 2 measure ammonia and nitrates, respectively, through direct immersion in aeration basins or final effluent.
Principles of operation
The entire chemical process, including reagent reservoirs, is contained in the submersible, mast-mounted buoy. The buoy is filled by the hydrostatic pressure of the water and emptied by air pressure. The process computer, housed in a weatherproof enclosure, can support two buoys of either type.
In addition to controlling the chemical analysis and parameter measurement, the onboard computer with its graphical user interface handles the operational control of all maintenance and test routines. Multitasking capability allows simultaneous handling of special and routine operations. For example, it is possible to recall and view or print the stored parameter charts and maintenance data of the last 14 days without interrupting the ongoing analysis and data acquisition. The built-in floppy disc drive allows storage of 90 days worth of data on a single diskette. Data can be imported into spreadsheet programs like Excel.
The PBS 1 process buoy, designed for ammonia measurement, is equipped with a purgeable cell, in which wastewater is separated from sludge and solids before it is fed into the reaction cell. During the ammonia measurement, the necessary pH value is regulated to help ensure high measurement accuracy with low reagent consumption. The buoy automatically calibrates itself daily using the standard addition method, and at the same time compensates for variability in the wastewater.
For nitrate measurement using the PBS 2 process buoy, ionic strength is regulated via a conductivity probe. Nitrates are measured with an ion-selective nitrate probe, which is automatically calibrated and conditioned daily in the wastewater by the standards addition method.
Usually, 30 percent to 40 percent of the cost of a conventional on-line cabinet-style wastewater analyzer can be directly related to installation plumbing. These are costs above and beyond the actual equipment itself.
The design of the process buoy eliminates the need for wastewater pumps and online filtration devices. The package is designed for installation outdoors and can be installed within a few hours, which makes it particularly well suited to existing treatment plants and industrial applications. Valves within the buoy contact only air, reagents and calibration standards, helping ensure a high level of reliability.
A typical application for continuous nitrates monitoring is to control chemical additions such as methanol in the denitrifying process. The nitrate buoy's continuous nitrates reading is available via a 4 to 20 milliampere output that is readily interfaced with dosing and metering pumps for chemical injection. This can yield savings by optimizing the chemical injection process and avoiding wasteful overdosing of chemicals.
Continuous monitoring of ammonia may be used to optimize the aeration process. Inadequate aeration can result in poor conversion of ammonia to nitrates and produce ammonia surges at the effluent discharge.
An aid in complying with NPDES permits
The process buoy is also suitable for continuous monitoring of industrial effluent quality to ensure permit compliance. For example, to meet National Pollutant Discharge Elimination System (NPDES) permit requirements, fertilizer manufacturer Potash Corporation Saskatchewan (PCS) Nitrogen monitors nitrate concentrations in the effluent from its Augusta, Ga., plant.
The problem the company faced was the delay in getting test results from the lab, typically consisting of a couple of days. The corporate management wanted a continuous monitoring system interfaced with the company's supervisory control and data acquisition (SCADA) system to provide alarms if permit levels were exceeded. This would allow PCS Nitrogen to investigate the source of the problem, such as a leak in the process plumbing, and respond immediately.
The company's solution was to install an Isco-STIP nitrate buoy. Frank Ferraro, environmental specialist at PCS Nitrogen, said, "The process buoy system was simple to install and is easy to maintain. The readings correlate extremely well with our laboratory and have allowed us a reliable way to monitor our nitrate nitrogen levels."
Treatment plant and process performance will improve and costs of operation will decrease when online analyzers such as process buoys are used to both minimize short-term load variability and adjust the plant to actual conditions.
Understanding of the wastewater treatment process by treatment staff will increase as online data is made available. This will result in improved management and lower discharge levels of nutrients to meet NPDES permit requirements, thus improving the quality of our waterways.
New regulatory developments affecting nutrient discharge criteriaBy Ingrid Truemper
In his 1998 State of the Union Address, President Clinton announced a major new clean water initiative, the Clean Water Action Plan, and proposed $568 million in new resources in the fiscal year 1999 budget. A key goal of the plan is the development of water quality criteria for the nutrients nitrogen and phosphorus by the year 2000.
National nutrient strategy
To achieve this goal, the U.S. Environmental Protection Agency (EPA) released a national nutrient strategy in June 1998 outlining the process and approach for the development of numeric criteria for nutrients and adoption of nutrient provisions of state water quality standards. Under the approach described in the strategy, EPA is developing nutrient guidance documents for various types of waterbodies - e.g., rivers, lakes, coastal waters and wetlands - which are scheduled to be completed by 2001. (See www.epa.gov/ost/standards/nutrient.html for updates.) States will be able to use these guidance documents and target ranges as they develop numeric criteria for nutrients as part of state water quality standards.
Regional and waterbody-type approach
There is a great deal of natural variability in nutrient levels and nutrient responses throughout the country, due to differences in geology, climate and waterbody type. For these reasons, EPA decided its custom of developing water quality criteria guidance in the form of single numbers for nationwide application is not appropriate for nutrients. According to EPA, distinct geographic regions and types of aquatic ecosystems need to be evaluated differently, and criteria specific to those regions and ecosystems need to be developed.
Waterbody-type technical guidance
An essential technical element of this strategy is waterbody-type guidance documents describing the techniques for assessing the trophic state of a waterbody and methodologies for developing regional nutrient criteria. In addition, each technical document will provide criteria guidance under section 304(a) of the Clean Water Act in the form of regional numerical target ranges for phosphorus, nitrogen and other nutrient endpoints. EPA expects states and tribes to use these target ranges as the basis for adopting nutrient criteria into water quality standards in the absence of more site-specifically developed water quality criteria and standards. EPA is using state databases to develop these regional target ranges, supplemented with new regional case studies and demonstration projects to provide additional information.
Additionally, EPA has proposed revisions to the total maximum daily load (TMDL) regulations (40 Code of Federal Regulations (CFR) Part 130) under section 303 (d) of the Clean Water Act that include new requirements to control nutrient discharges (see www.epa.gov/fedrgstr/EPA-WATER/1999/August/Day-23/w21416.htm). This action also encompasses revisions to the NPDES permitting program (see www.epa.gov/fedrgstr/EPA-WATER/1999/August/Day-23/w21415.htm ).
Click here to post comments about this topic, and read what others have to say.
This article originally appeared in the November, 1999 issue of Environmental Protection magazine, Vol. 10, Number 11, pp. 40-43.
This article originally appeared in the 11/01/1999 issue of Environmental Protection.