Clearing the Waters

In response to my piece yesterday, frequent reader W. Pyle wrote a comment that I think other readers may find useful when thinking about the most recent problems with Trenton’s water quality. Rapid increases in water flow and turbidity may have overwhelmed the ability of Trenton Water Works to cope. However, “the plant, which is relatively new, should have redundant or parallel delivery systems to allow continued operations should there be a failure of one of the units.”

Thank you as always, Mr. Pyle, for your informed comments on water systems and supply!

I always appreciate feedback from readers who know what they are talking about, much more than I! Thank you, all.

Oh, and in case you were wondering, I’ve had no reply from anyone in City Hall. Surprise, surprise!

One of the operational challenges presented to a treatment facility by a surface water supply is maintaining adequate treatment when faced with rapidly changing water quality as measured by temperature, pH, alkalinity, turbidity and other water quality parameters. When those changes occur, on site personnel must adjust the treatment process to achieve and maintain delivered water that meets or exceeds the regulatory water quality requirements. To have a full understanding of the possible scenarios that could have resulted in no output as a result of a plant shutdown or a reduction in output, some knowledge of the treatment process would be helpful. However, to provide a possible scenario and to keep this relatively brief, a mention of the steps of the process includes coagulation, flocculation, sedimentation, and filtration with disinfection occurring before filtration and prior to delivery to the distribution system.

So, when the river water enters the plant, a coagulant (chemical) is added to achieve flocculation (bringing together suspended particles to create heavier masses). The amount of coagulant is determined by the river water qualities mentioned above and manual tests are conducted on site to help determine the proper amount. Other chemicals may be added to adjust the pH of the incoming water. After the coagulant does it job, the water is allowed to flow through the plant at a rate that allows the particles to settle out during the sedimentation process. At this point, the settled water would typically have a turbidity of about 1 NTU and could be somewhat higher if the plant influent has a turbidity greater than 10 NTU. The settled water then flows to the top of the filters and is filtered. The filtered water should have a turbidity of 0.3 NTU or less.

Given the rapidly changing water quality as shown on the turbidity graph, on site operators would have had to make effective adjustments in the treatment process to maintain water quality. If the required adjustments were not done or attempted but found to be ineffective, settled water may have had a turbidity well in excess of 1 NTU and may have also created a settled water that was not able to be adequately filtered. If that happened, the filtered water may have exceeded the allowable filtered water NTU level which would have required the plant to stop delivering water. Such an event would have required the plant to continue to not deliver water until it was able to produce filtered water that met the filtered water turbidity requirement. It may be that after such an event it takes quite a while before the plant is able to produce water that meets all water quality requirements.

This also could have occurred if the equipment used to deliver the treatment chemicals experienced complete failure. That is possible but the plant, which is relatively new, should have redundant or parallel delivery systems to allow continued operations should there be a failure of one of the units.

The turbidity graph shows a maximum recorded turbidity level of about 70. As a way to gauge the severity of this “spike” a review of the historical turbidity data will probably reveal that this was not that sever when compared to turbidity levels spiking to over 1,000. Nevertheless, the spike shown in the graph may have indeed been enough of a change over a short period of time to have cause a treatment upset as mentioned above.

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