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One of the biggest costs for water-water treatment is energy consumption; the cost of which is likely to escalate with increased demand. Ineffective process control may not only be costly but environmentally impacts though the inefficient use of an un-renewable resource. Furthermore, poor process control in one area may also impinge on neighbouring processes ultimately affecting effluent quality.
Aerobic digestion is still one of the primary methods of waste-water treatment and used to remove harmful ammonia through the process of nitrification. Surface or diffused aeration is often widely deployed and traditionally, the concentration of dissolved oxygen (required by the nitrifying bacteria) has been monitored using DO sensors. Concentrations have traditionally been maintained within a specified concentration range typically 1.8 to 2.2mg/L.
Water Companies have recently investigated alternative control philosophy to optimise the treatment process and save on energy costs. The realisation that the aeration should be controlled dependant upon the load entering the aeration ditch rather than simply maintaining a relatively high DO concentration has prompted scientist and engineers to further automate using additional measurements.
The direct measurement of the ammonia/ammonium concentration entering the treatment basin to provide forward control of the level of aeration required for treatment has proved extremely successful. In some instances an additional sensor has been integrated to also provide feed-back control, from a sensor located at the outlet. The DO instrumentation then is used to provide a minimum set point but this can often be reduced to half the concentration previously set. The resultant cost saving is estimated to be typically 25%, which for even a medium sized plant is significant.
PPM can offer two alternative monitoring approaches. The IQ Sensor Net system uses a digital interfacing platform and the connection of robust submersible sensors for the direct measurement of ammonium concentrations. The probe technology is simple to install, simple to maintain and is installed directly into the treatment basin to give best possible speed of response without sample preparation. Operators are required to perform a matrix calibration, using a photometer (typically monthly) to validate performance. The sensor is automatically cleaned using compressed air and digitally transmits data to the controller thus allowing for significant sensor to instrument separations. The controller-display unit may then be installed at the most convenient location. Additional parameters such as DO, MLSS, pH, conductivity and nitrate may also be incorporated allowing for complete process control from a single instrument.
Alternatively PPM also manufactures the Proam ammonia monitor which has also been used to monitor the load entering and leaving the treatment basin. Since the analyser automatically calibrates daily against two known standards, all measurements are validated by the instrument giving additional confidence in the measurement. The analyser technique requires sample preparation to remove the suspended solids component before analysis and a frost protected enclosure to contain the instrumentation, so the site installation must allow for this.
The Protoc TOC analyser has also been installed at the inlet to provide continuous measurement of organic load entering a treatment facility. The ability to predict load and control the addition of coagulant is preferential to regulating chemical dosing on flow alone. Similarly, the measurement of phosphate may also be used where chemical dosing is required for P-removal.
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