ABB analyzer controls coagulant chemical additions at water utility | Automation.com

ABB analyzer controls coagulant chemical additions at water utility

ABB analyzer controls coagulant chemical additions at water utility
May 12, 2008 - An analyzer for dissolved organic compounds now monitors raw river influent and automatically controls aluminum sulfate (alum) additions at a water plant in New Jersey. The township estimates that the system paid for itself within six months in terms of chemical savings and reduced labor.

The utility processes 16 MGD of water. Several neighboring townships rely on it for supply. Influent sources for the water utility include a river, deep and shallow wells, an ASR well, and a pumped water storage reservoir. The versatility in source water options and resulting treatment strategies provide the staff with a process train that can be tailored to maximize efficiency in chemical use, while adapting to changing river conditions.

The analyzer for DOC is an AV400 from ABB Instrumentation (Warminster, PA). A sample pump sends the raw water sample to the AV400 detection cell located within the central treatment building. This cell contains a light source that flashes every 2 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 over 200 readings. A second measurement at 405 nm enables the monitor to compensate automatically for fluctuations in turbidity. A dual-wiper system, housed in the cleaner module, cleans the flowcell optical windows to help ensure the sensor's functionality.

The utility supplements this cleaning system with a rigorous maintenance schedule to assure the instrument's proper operation. Maintenance technicians conduct calibration checks once every month, a process that takes about one hour. Also at this time, they use a 25% solution of HCl 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. This prevents clogging and ensures that the instrument continues to see the required flow.

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, such as 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 now 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, along with other variables, to develop a control signal for the pumps that automatically meters alum to the mixers, as shown in the diagram.

The water plant 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 bench top 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. Such efficiencies can help compensate for the multitude of factors tending to drive up the price of water.

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