Ammonia, Nitrate and Phosphate: Not Just for Effluent Monitoring

Infrastructure Security Article January 22, 2001
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Therefore, rapid and continuous monitoring of a plant’s discharge is becoming more common. This is the simplest use of continuous monitoring equipment.

In addition to monitoring plant effluent, this equipment can be used to monitor and ultimately control some plant operations. This can result in a more efficient plant. It also helps ensure problems are seen and remedied before discharge.

TresCon on-line water analysis systems, which are offered by WTW Measurement Systems Inc., in Ft. Myers, Florida, allow for the continuous determination of ammonia, phosphate, nitrate, or nitrite in most water and wastewater processes. TresCon is a modular device that can be configured to monitor one, two, or three of the parameters mentioned simultaneously.

The treatment of wastewater is a microbiological process. Simply put, bacteria are utilized to remove (literally eat) organic contaminants from raw sewage. These bacteria require a source of oxygen to effectively consume these contaminants. This oxygen may be available as dissolved oxygen (DO), nitrate ions, or sulfate ions.

If DO is available, the bacteria will utilize this source of oxygen ahead of any other source. This is aerobic metabolism. In aerobic metabolism bacteria utilize DO to convert ammonia to nitrate in the nitrification process. Once DO is depleted, nitrate becomes the preferred oxygen source, and an anoxic metabolic pathway is followed. Nitrate is converted to nitrogen gas (N2) and the bacteria continue to convert food to energy. This is called denitrification. Finally, if DO and nitrate are not available, anaerobic metabolism dominates and sulfate is the oxygen source, producing hydrogen sulfide (H2S) and other sulfur compounds.

DO measurement is typically used to monitor and to some extent control the treatment process. However, DO measurements alone show only the level of residual DO. So long as there is measureable DO, the availability of oxygen is not limited and aerobic (nitrifying) activity is occurring. DO measurements alone provide little or no information regarding the rate of aerobic biological activity. Adding ammonia, and nitrate measurements will paint a much more complete picture of the state and health of the treatment process.

This is especially true when monitoring the condition of anoxic and anaerobic processes. Since there is no DO present during these processes, other parameters must be monitored to ensure proper operation.

By monitoring ammonia and nitrate in the aeration basin, nitrification and denitrification can be more appropriately balanced. When DO alone is monitored, excessive air must be supplied to provide a "safety stock" of DO. This safety stock is used to compensate for any unpredictable swings in biological activity and oxygen demand. Excess DO is also quite expensive, since operating blowers longer or harder than needed consumes electricity quickly.

The first response to inadequate nitrification is to increase aeration. But by how much and for how long? Again, DO alone will not give any information on the balance of nitrification and denitrification. With ammonia and nitrate values one can look directly at these two processes.

Many plants are exploring methods of biological phosphorous removal. By lowering the nitrate load, anaerobic conditions can be maintained so that biological phosphorous removal is possible. Again, since anaerobic conditions mean the absence of any measurable DO, and very low levels of nitrate, other parameters need to be measured. By combining DO measurement with nitrate measurement, more careful control of an anaerobic zone is possible. Monitoring phosphate also allows operators to ensure phosphorous removal is actually occurring without having to wait until their discharge limits are exceeded.

Not surprisingly, more information on the "health" of the treatment plant allows for smoother plant operation. How can we obtain this information? The first item to consider is response time. The more rapidly an analyzer provides information, the more easily and reliably that information can be used to control processes and divert any upsets that occur.

It is also important that measurements be as close to continuous as possible. Many on-line analyzers are simply cleverly packaged sampling devices. That is, determinations are performed in a batch mode. This means the information the analyzer is providing is only as timely as the last sampling period. Of course, many of a plant’s processes move slowly enough that periodic readings are sufficient. However, when continuous data is available, it allows for tighter control of the process.

On the practical side, equipment should be as self-sufficient as possible. This means automatic self calibration and self cleaning.

To operate the TresCon system, a continuous supply of water to be analyzed is required. A flow rate of 2—3 liters/hour is sufficient. The analyzer continuously "sips" from this sample stream, and feeds sample to the measurement modules installed. In the ammonia module, sample is mixed with a basic reagent to raise the pH. This converts ammonium compounds to gaseous ammonia which is sensed by an ammonia-sensitive electrode. In the nitrate module, a UV light source is used and absorption at two wavelengths determines the nitrate concentration. Phosphate is determined photometrically. Sample is mixed with a molybdate-vanadate reagent. The intensity of the yellow color developed is directly proportional to the concentration of ortho-phosphate. Nitrite is also determined photometrically by mixing the sample with a dye that will turn pink in the presence of nitrite ions.

TresCon provides a continuous reading of all three parameters being measured. These are available on the instrument display, as well as three programmable analog outputs for relay to a plant’s DCS or PLC system. It will also calibrate and clean itself on intervals determined by the operator.

For further information, phone 800-645-5999.

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