The Dynamic Pacing of Disinfection Processes Using Online Direct Measurement of Bacterial Content
If you are responsible for managing water and wastewater treatment facilities, you will be familiar with direct online measurement and control of various water quality parameters – both physical (e.g. turbidity, conductivity, temperature) and chemical (e.g. pH, dissolved oxygen, free chlorine, fluoride, iron, manganese, THMs).
But, why not microbiological?
The conventional wisdom is that all aspects of microbiological water quality must be measured in a laboratory, requiring manual sampling and lengthy delays before getting a result, thereby eliminating the possibility of controlling the disinfection process based on the actual microbiological content.
For chlorine disinfection (the most common method of chemical disinfection), conventional practice requires setting a target residual chlorine level and a corresponding dosing setpoint designed to achieve the residual disinfectant target. The target residual disinfectant level is chosen to meet log reduction targets for a range of pathogens including bacteria, viruses and protozoa, however in routine daily operation of water and wastewater treatment plants, bacteria, primarily E.coli, are used as the reference microorganism, as E.coli can be measured onsite, relatively easily and quickly (around 24 hours turnaround time), but still at too low a measurement frequency to be used for automatic process control. Prudence requires that you choose a dosing setpoint that provides a high level of certainty that the product water will be both adequately disinfected and compliant with regulations.
As shown in the above graph, this approach can lead to periods when the disinfectant dose will be higher than necessary to adequately disinfect. Overdosing has additional disbenefits including wasting energy, wasting chemicals, increasing the risk of disinfection by-product formation, producing off-tasting drinking water and increasing costs.
Without direct online monitoring of bacterial load, attempts to reduce the disinfectant dosing setpoint, can lead to periods of underdosing with the risk of pathogens being carried forward with the product water.
So much for conventional wisdom….
Recent advances in bacterial monitoring technology, mean that it is now possible and feasible to control microbiological water quality on-line in the same way that you can control physical and chemical parameters, i.e. through the pacing of disinfection dosing in response to the direct measurement of bacterial content.
The development of the VWMS Coliminder®, capable of directly and accurately measuring E.coli, Enterococci and Total Microbial Activity, operating continuously at measurement intervals of little as 30 minutes, means that you can dynamically set your disinfectant residual setpoint – as the bacterial load increases, the setpoint will increase, and as the bacterial load reduces, the setpoint will decrease. Setpoint changes, including minimum default dosing rates can be made automatically or manually, through your SCADA or any other control system.
Furthermore, having the ability to accept multiple sample inputs, the Coliminder® allows you to check the actual bacterial content of the disinfected water, thereby providing an online measurement of actual log reduction.
Ahhh – true wisdom!
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