Proper control of coagulant is fundamental for efficient operation of a water treatment plant. The standard method for determining coagulant dose has been the jar test. Although the procedure has been used for decades, the shortcomings are well known:
- Non-continuous testing;
- Variability between operators;
- Visual interpretation of floc characteristics;
- Low correlation with plant operation;
- Low turbidity of high color negatively impacts interpretation of test results; and
- Time consuming for operators.
Zeta meters and streaming current monitors (SCM) are two widely used methods for “fine tuning” jar test results. SCM provide continuous online monitoring and control of coagulation feed. However, questions often arise with daily operation, such as:
- Calibration of unit;
- Determining the accuracy of results; and
- The meaning of the SCM number.
The same questions are relevant for zeta meters and zeta potential measurement. In fact, the only issue that may matter during water quality excursions is what test can the operator run that is fast, accurate, repeatable and informs the user to what actions will keep their plant running properly. Faced with this situation, many tests are relative and almost none are absolute.
Faster results
The Huntington (W.V.) Wastewater Treatment Plant is a conventional system along the Ohio River, which processes 12-14 mgd. Treatment chemistry consists of ferric sulfate and caustic. Although the Ohio River does not experience large, sudden changes in turbidity, conductivity and alkalinity, local heavy rains or cloudbursts can cause significant increases/decreases in these parameters.
In these instances, jar tests can become somewhat undesirable in helping operators determine the best treatment chemistry. Online SCM were installed at the Huntington Wastewater Treatment Plant a few years ago but operators were reluctant to rely on SCM data for process control.
In 2002, a laboratory SCM, the Chemtrac ECA 2100, was evaluated to determine of a “change titration” using ferric sulfate would give useful information for defining “approximate” dosage similar to doing jar tests.
The new tests concluded that reproducible and accurate results were obtainable quicker than routine jar tests. A unit was purchased by the Huntington Wastewater Treatment Plant and an SOP delivered for daily operations.
Between May and July 2004, the plant experienced an extraordinary flooding event that greatly impacted the water quality in the Ohio River. While normal turbidity is 25 NTU, 50-65 ppm alkalinity, and ferric dosage 60 ppm, during the rain event the turbidity was 650 NTU with 44 ppm of alkalinity.
To keep the ferric dose acceptable, operators ran jar tests and SCM titrations. Comparing the data, operators revealed that charge titration provided the best answer as to what ferric dosage would produce the best-settled water turbidity.
The titration procedure, which takes about two minutes, is advantageous when compared to the routine jar test that requires more than an hour to perform. In many cases, the raw water quality had changed prior to the time it takes to complete jar testing.
Facing water quality excursions, an operator must be able to make fast and accurate decisions based on laboratory tests. The ECA 2100 provided operators at the Huntington Wastewater Treatment Plant the information they needed to make quick and informed decisions concerning incoming water quality and required coagulant dose.
