Dose by Numbers

Sept. 11, 2008

About the author: Chris Westlake is northeast analytical regional sales manager for ABB Instrumentation. Westlake can be reached at 215.534.8352 or by e-mail at [email protected].

An analyzer for dissolved organic compounds (DOCs) now monitors raw river influent and automatically controls aluminum sulfate (alum) additions at a water utility in New Jersey. The township utilizing the analyzer estimates that the system paid for itself within six months in terms of chemical savings and reduced labor.

The utility processes 16 million gal of water per day, and several neighboring townships rely on it for supply. Influent sources for the water utility include a river, deep and shallow wells, an aquifer storage and recovery well and a pumped water storage reservoir, as indicated in Figure 1. The versatility in source water options and resulting treatment strategies provides the staff with a process train that can be tailored to maximize efficiency in chemical use and adapt to changing river conditions.

UV Calculations

The analyzer for DOCs is an AV400 from ABB Instrumentation, Warminster, Penn. The instrument accurately reports the dissolved organics using an ultraviolet (UV) absorption technique. When properly calibrated and maintained, the measured sample absorbance can be interpreted as an accurate measure of dissolved organics.

UV absorption is a nonspecific aggregate measurement of organic carbon concentration. The instrument uses a broad-spectrum, high-intensity xenon strobe lamp to generate pulses of light that pass through the sample in the flow cell to a filtering and detection system. A measurement wavelength of 254 nm is used against a reference wavelength of 405 nm, at which the sample constituents of interest do not absorb.

The instrument produces readings in units of mg/L of carbon defined against a calibration standard. The utility evaluated instrument performance for a period of time during which the online value was recorded and compared to grab samples analyzed in the operations laboratory to confirm its accuracy.

In operation, a sample pump sends the raw water sample to the AV400 detection cell located within the central treatment building. The cell’s light source flashes every two seconds through the sample. The detected absorption at 254 nm is updated each time the lamp flashes, and during this brief flash duration, the instrument takes more than 200 readings. The second reference measurement at 405 nm enables the monitor to compensate automatically for fluctuations in turbidity.

Maintenance Schedule

A dual-wiper system, housed in the cleaner module, cleans the flow-cell optical windows to help ensure the sensor’s functionality. The New Jersey utility supplements this cleaning system with a rigorous maintenance schedule to assure the instrument’s proper operation. Maintenance technicians conduct calibration checks once a month—a process that takes about one hour—and use a 25% solution of hydrochloric acid to clean the optics.

The instrument is installed downstream of the chlorine dioxide injection, and depending on source water selection, oxidation of material in the raw water can create stains on the optics. The technicians have a weekly scheduled job that requires flushing of the lines and filter maintenance to prevent clogging and ensure that the instrument continues to see the required flow.

Measurements in Action

The measured signal goes to the AV400 transmitter mounted nearby. The transmitter display shows inferred values most useful to the user. A typical level of dissolved organics in the river at the intake is about 0.15 to 0.2 mg/L. The signal output from the transmitter is a 4- to 20-mA current proportional to the instantaneous reading. This kind of analyzer requires no consumables (e.g., reagents), which is a significant economic advantage. The instrument calibration is validated by instrumentation technicians using a pure solution of known carbon content.

A closed-loop system controls alum treatment additions. The plant SCADA sends real-time online measurements of dissolved organics in the raw water influent to the control room computer. Software processes the dissolved organic measurement and other variables to develop a control signal for the pumps that automatically meters alum to the mixers, as shown in Figure 2.

The New Jersey water plant using this technology has experienced substantial savings in chemical use for this process segment. As conditions change, the coagulant dose immediately tracks the online UV254 result. This avoids over- or under-feeding during the period of time necessary for the operator to manually respond. The online instrument also precludes grab sampling and benchtop analytical for UV254, freeing the operations staff to complete other tasks and negating the need for a dedicated UV spectrophotometer in the lab.

The utility views the continued employment of advanced technology as necessary to keep improving water plant efficiencies, which can help compensate for the multitude of factors tending to drive up the price of water.

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About the Author

Chris Westlake

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