Controlling wastewater treatment

Jon Lawson

Michael Dooley reports on the best approach to managing the activated sludge process that’s used in water treatment plants that’s used in water treatment plants

The activated sludge process is by far the n most important method for treating both domestic and industrial wastewaters. Improving the control and visibility of this biological process, which consumes over 2% of the world’s produced electricity (of whichenergy consumption for aeration is by far the biggest contributor), is key to meeting every tightening compliance requirements as well as reducing energy costs.

Instrumentation used in advanced real-time control (ARTC) of the treatment process should be self- cleaning, self-calibrating and measure or predict the following:

* Critical measures of load such as BOD and ammonia

* Critical treatment control parameters including sissolved oxygen, pH, MLSS, nitrate, phosphate, F:M ratio, ORP

*  Predictors of performance such as % settlement (SVI/SSVI), supernatant turbidity, % nitrification, toxicity

* Ideally such instrumentation can provide both feed- forward and feed-back data to the treatment works control system.

One such system is the activated sludge plant controller (ASP-Con), which measures all the above parameters, is self-cleaning and self-calibrating and can be fully integrated with site PLC/SCADA control systems, cloud-based reporting and incorporates GSM- based remote access for monitoring and maintenance.

The system also incorporates the capability to control plant equipment without the need for a separate PLC-based control system. The image below shows a typical installation for the ASP-Con system. The customer can program which parameters are measured and also the frequency of sampling, auto-calibration and auto-cleaning routines.

Measurement of influent ammonium load and mixed liquor suspended solids (MLSS) were validated on a UK-based treatment works.

The data was downloaded by remote access from ASP-Con units installed at approximately 75% of the footprint of two wastewater treatment plants in North America.

The data clearly showed coincidental variations in DO, ammonium and oxygen uptake rate (OUR). This improves visibility of the process and increases confidence in the instruments’ assessment of actual real-time loading conditions. Based on this data the ASP-Con system can make an assessment of the required plant operating parameters (such as DO set point, blower operating pressure, valve positioning, etc.) and adjust them within operator adjustable ranges to assure compliance whilst reducing energy consumption levels.

The provision of automatic sampling and measurement of additional predictors of bacterial performance such as % settlement gives an unprecedented level of plant monitoring, allowing operators and engineers to gain confidence in the change processes being managed on site.

The system automatically recalibrates the sensors for DO, pH, ammonium, nitrate and potassium. The advantages for automatic scheduling and computer controlled end-point determination are obvious.

The system continuously measures the DO, ammonium, pH, temperature, conductivity and the interchangeable measures for nitrate and ORP.

The operator can program the frequency of other measurements for load (oxygen uptake rate), settlement, MLSS, toxicity, % nitrification and critical oxygen point determination.

Combining the abovementioned data gives the operator full confidence over the operation of the treatment system and allows the development of advanced controlled systems. It also helps to considerably reduce both aeration and other energy costs on site.

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Michael Dooley is managing director of Strathkelvin Instruments.

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