A Framework for Real-Time Process Control

Authors:

  • Katya Bilyk, P.E., Joe Rohrbacher, P.E., Ron Taylor, P.E. - Hazen and Sawyer

Many municipalities have invested in calibrated process models and nutrient analyzers to meet stringent nitrogen and phosphorus effluent standards. This paper will discuss how online instrumentation can be integrated with process modeling to provide a framework for real-time process control. The integration of modeling and operations is the next logical step in biological nutrient removal (BNR) operations, and is the subject of an ongoing WEF Plant Operations and Maintenance Committee Task Group.

When nutrient analyzers are operated and maintained properly, they can provide useful information to operators and engineers about how the plant is performing in real-time. Many of the variables used in process modeling can now be monitored continuously and reliably, such and ammonia influent and effluent from the BNR process.

Several BNR plants with online instrumentation (i.e., multiple dissolved oxygen (DO) control zones, online ammonia, nitrate/nitrite, and orthophosphate probes) will be used to illustrate the real-time process control concepts. Concepts that will be discussed include:

• Using influent temperature and suspended solids probes in the mixed liquor and wasting lines to calculate solids retention time (SRT) and compare with critical SRT for nitrification determined from modeling.
• Feeding supplemental carbon for denitrification based on mass of nitrate and DO in anoxic zones.
• Using real-time carbon to nitrogen ratios to the BNR process to maximize the use of influent carbon by determining internal recycle rates, which in turn optimizes biological phosphorus removal; and optimize supplemental carbon feed.
• Ammonia-based DO control.

For plants that have already invested in instrumentation and a process model, many of these concepts can be incorporated at a relatively low cost.

For more information, please contact the author at kbilyk@hazenandsawyer.com.

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