01 March 2005
Using nitrate and dissolved organic sensors in wastewater plants
By Robert Lagrange
Some wastewater sensor designs take root from the strong absorption of UV light by nitrate and dissolved organics. These sensors work in drinking water and wastewater. The main draw to the technology is the lower cost of ownership compared to standard wet chemistry analyzers. In drinking water, a nitrate analyzer insures the water stays below the 10 mg/l limit. Total organic carbon (TOC) removal is one use for dissolved organic analyzers. In wastewater, to comply with nutrient removal, you'll need to optimize denitrification using a nitrate measurement. Industrial wastewaters will offer additional challenges as color or biological oxygen demand (BOD) load may limit the use of some technologies.
Measure nitrate using wet chemistry based on colorimetry or ion selective electrodes. Those instruments require maintenance and consume reagents. Plus they have a high cost of ownership. You can usually measure organics with a TOC analyzer based on UV/Persulfate or high temperature combustion. You can also find analyzers based on the absorption of UV light at 214 and 254 nm. They require a sampling line, which increases the installation cost and maintenance. The new technology gives you the ability to measure directly in open channels and tanks, even in the presence of biomass.
REVIEW OF TECHNOLOGIES
Colorimetry is the most accurate in most applications. Add a reagent to the sample being measured to develop a color with intensity proportional to the concentration of nitrate. The reaction takes time depending on the temperature. Most analyzers will measure the temperature and do an internal compensation. Some will maintain the sample temperature providing a faster reaction. At 40ºC, the reaction takes seven minutes. You can't use colorimetry in the pulp and paper industry because the color of the water is the same as the color developed during the reaction. Colorimetry requires a complex sample preparation system. The main disadvantage is the intensity of the light source and the optical path define a limited range.
A sensor designed for in-line installation in open channels and tanks. The light source is a flash light with high emission in the UV spectrum. The beam is directed through the measuring area and split to reach the two detectors.
ION selective electrodes
Useful in some industrial applications, ION selective electrodes use a potentiometric measurement equivalent to a pH measurement. The main advantage is the large measuring range. The presence of chlorides may limit the use to measure nitrate as they create interferences for which you can't compensate. The disadvantage of the method relates to the instability of the electrode, change in zero, and sensitivity. Most analyzers use a method called single known addition (SKA), where a second reading is taken after adding to the sample of a known nitrate quantity. Adding an ionic strength adjustment buffer (ISAB) will insure a better measurement, more so than if you take a pH adjustment at the same time. Both methods require a cumbersome and costly installation. And maintenance requirements are high, mainly in wastewater.
TOC is an indirect measurement in that carbon molecules present in the water oxidize to CO2, and an infrared sensor measures the amount of CO2 released. Oxidation can occur through high temperature combustion, a combination of UV light and Persulfate or an ozone/caustic mix. Combustion provides complete oxidation but requires higher maintenance. With other methods, oxidation may not be complete. This limits their application as the correlation will change with the degree of oxidation. Those analyzers require sampling lines and high maintenance.
Nitrate has a very strong absorption at 214 nm in the UV range. Most dissolved organics have a strong absorption at 254 nm. This property has been around for quite some time in systems designed to measure on sampling lines. You could have the measuring cell mounted inside the analyzer or outside. This type of unit requires a high-maintenance due to the sampling line when applied to wastewater. Limitations in the technology include potential interferences—high and changing concentration of nitrite or organic compounds absorbing UV light in the vicinity of the measuring wavelength. The presence of suspended solids in the water will create a scattering of light. You need to compensate for an accurate reading. Instruments will be blind to organics, such as methanol, with no or little absorption at 254 nm.
A solution is an organic sensor for in-line installation in open channels and tanks. If in-line installation is not possible, you can have a trough chamber mounted at the measuring end of the sensor. In those cases, typically for some applications in drinking water, you'll need a sampling line.
The light source is a flash light with high emission in the UV spectrum. The beam is directed through the measuring area and then split to reach the two detectors. Use solid state detectors to measure the absorption at two wavelengths. The reference wavelength establishes the base signal. The particles' scattering of light in the measuring gap, the aging of the source, and the absorption of UV light by other components such as organic material, determine the base signal. Then subtract this base signal from the actual absorption measurement. The intensity is directly proportional to the nitrate or dissolved organics concentration.
In nitrate application, the limits of this technology are nitrites and organic content. Nitrites have an absorption band very close to 214 nm. The unit is unable to differentiate between nitrate and nitrite. When the nitrite concentration is low and stable, you can calibrate the instrument to the nitrate calibration. In all other applications, the signal will be proportional to NOx. For applications with high COD, the applicability of the sensor will depend on the conditions. If the absorption at 254 nm is higher than the absorption at 214 nm, you won't be able to use the sensor. An excellent application situation is when the COD is below 300 mg/l. You can apply it at higher COD if the COD concentration is stable (variations less than 50 mg/l).
There are also some limitations in dissolved organics applications. You won't be able to see the organics that do not absorb the wavelengths. When using it as an equivalent for BOD or TOC, you'll need to establish a site/application specific correlation. That correlation works well when a single component is present in the water. When multiple components are present, the correlation will remain valid as long as the composition remains stable. Automatic cleaning is available. Air that a locally installed compressor supplies keeps the measuring area clean. As long as the detectors receive enough light to be in their linear range, the measurement will be accurate.
Behind the Byline
Robert Lagrange is business manager of water and wastewater at Endress+Hauser Inc., Greenwood, Ind.
Typical organic sensor applications
While there is little economic justification, you can use the organic sensor for monitoring purposes such as a safety device. Monitoring of the raw water coming into a water treatment plant will tell the operator what to do with the water. In final drinking water, the instrument is not NSF approved, and requires by-pass installation on a sampling line.
Blending of drinking water
Some water plants are mixing water with high nitrate content and cleaner water to maintain the 10 mg/l limit. This is a straight-forward application. At least one sensor should measure the mixed water. Use the signal in a feed-back control to adjust the ratio of the flows. This control will work if the two sources have a stable composition. As in any feed-back only control, the result is a reduction in variations. But because the measurement is after the fact, there may still be some upsets. When you expect changes in one or both sources, you should make additional measurements on the incoming water and use a feed-forward control strategy. Typically the contaminated source is the most likely to show changes. Thus the control should take into account the concentration of nitrate in the contaminated water to adjust its flow.
Use either ion exchange or reverse osmosis to remove nitrate on part or all of the water. These processes are expensive to operate. By monitoring the concentration of nitrate in the influent, you'll get the information to decide if you need to treat the water and how much you need to treat. While you can monitor the performance of each column or membrane based on conductivity, you should also measure the final nitrate concentration. When you only treat part of the water, remixing will take place as above.
Use the dissolved organic sensor to ensure compliance in surface water plants. The enhanced coagulation rule states absorption at 254 nm relates to TOC removal. This is an easy way to eliminate organics before adding chlorine to reduce the formation of total trihalomethanes (TTHM).
Denitrification control and optimization in wastewater plants may well be the best application for a nitrate sensor. To denitrify, the sensor submerses directly in the tank or channel with no need for sampling lines or chemical reagents and with minimum maintenance needs. It does require cleaning, though, as biomass likes to grow and stick. When denitrification takes place before nitrification, the measurement should control the internal recycling. The sensor will mount close to the outlet of the denitrification tank.
You'll need two sensors when denitrification takes place on a tertiary filter. The first sensor on the influent will provide the information you need for proper dosification of the organic compound (usually methanol) the biomass requires. The second sensor will measure the final nitrate concentration and provide the feed-back information to close the loop. You could also use a dissolved organic sensor at the inlet as feed forward information to the dosification of methanol.
Under the right conditions you can measure nitrate and dissolved organics in water and wastewater with an optical sensor. With a capital cost equivalent or one somewhat lower to the one required for a wet chemistry analyzer, the optical sensor significantly reduces cost of ownership. Because of its high reliability, it's easier to use in control and safety applications, with a high return on investment.
Keeping chlorine out of the rivers and oceans has been an age-old problem in the wastewater industry, but Richard Baril, product marketing manager at Emerson Process Management, Rosemount Analytical, in Irvine, Calif., points to security as one of today's major issues. Baril said he'd been working with the government and the EPA and several other groups for a year and a half and has really been focused on the drinking water side—the infrastructure protection. A presidential directive a few years ago assigned the Department of Homeland Security and the Environmental Protection Agency (EPA) to identify certain critical infrastructures they need to protect: power, drinking water, and wastewater. "The guidelines show how someone in the utility could design online contaminate moderating systems," Baril said. Some are continuous. In this publication, there are several manufacturers and different ways of measuring things.
But the common way to moderate the systems right now is through oxidation reduction potential (ORP), chlorine residuals, dissolved oxygen, and turbidity, Baril said. The goal for manufacturers in the future? An American Society of Civil Engineers (ASCE) project is in progress to clarify specifications for instruments in municipalities.
Although the Public Health, Security and Bioterrorism Preparedness and Response Act of June 2002 amended the Safe Drinking Water Act to require all public water suppliers serving populations greater than 3,300 to complete vulnerability assessments and establish emergency response plans, Baril said right now, "there are no specifications for security and protecting yourself for water quality monitoring." In an article on homeland security for drinking water supplies, ENSR International, a global provider of environmental and energy development services, said a vulnerability assessment required manufacturers to "identify potential threats, assess the critical assets of the system, evaluate the likelihood and consequences of an attack, and develop a prioritized set of system upgrades to increase security.
Safe drinking water's top 10
Prepare (or update) an emergency response plan. Make sure all employees help to create it and receive training on the plan.
Post updated emergency 24-hour numbers at your facilities in highly visible areas (pumphouse door, vehicles, office) and give them to key personnel and local response officials.
Get to know your local police, and ask them to add your facilities to their routine rounds, and practice emergency response with public health officials.
Fence and lock your drinking water facilities and vulnerable areas (wellhead, hydrants, manholes, pumphouse, and storage tanks).
Lock all entry gates and doors and set alarms to indicate illegal entry. Do not leave keys in equipment or vehicles at any time.
Install good lighting around your pumphouse, treatment facility, and parking lot.
Identify existing and alternate water supplies, and maximize use of back flow prevention devices and interconnections.
Use your Source Water Assessment information to work with any businesses and homeowners that are listed as potential sources of contamination and lesson their threat to your sources.
Lock monitoring wells to prevent vandals or terrorists from pouring contaminants directly into ground water near your source. Prevent pouring or siphoning contaminants through vent pipes by moving them inside the pumphouse or treatment plants, or if that isn't possible, fencing or screening them.
In case of an emergency, first call 911, then follow your emergency response plan.
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