Grab Sample Turbidity Testing

Grab Sample Turbidity Testing

In order to test the accuracy of the turbidimeters, a grab sample method has been developed. The results of this method will be compared to the results from the turbidimeters.

The sample container will be a 500 mL glass beaker. It is very important that the water undergo minimal shear when entering the beaker so that flocs are not broken in the process. Therefore, the following procedure will be followed to collect a sample. First, the beaker will be gently emerged while angled downward (at approximately 30 degrees), so that the area that the incoming water can pass through to enter the beaker is as large as possible. This can be seen in Grab Sample Procedure #Picture 1.

Then, the beaker will be rotated downward until it is horizontal as seen in Grab Sample Procedure #Picture 2. It will be further rotated until fully submerged as seen in Grab Sample Procedure #Picture 3. The purpose of steps 2 and 3 are to let the water enter the beaker without allowing air to be trapped in the beaker. If air bubbles are trapped in the beaker, then they may break flocks when the beaker is righted and they escape. Then it will be fully submerged into the water and rotated so that the beaker is upright as seen in Grab Sample Procedure #Picture 4. Then, finally, the beaker will be lifted vertically from the water, so that it remains full.

The water will be allowed to settle for 20 minutes to approximate the settling that should occur in the tube settlers.
The length of time that the water should be allowed to settle was determined in the following way.

First, I back-calculated from the flow that we had been running through the tube settlers in the past (40mL/min) to find the critical velocity, Vc, associated with that flow.

This gives a Velocity through the settler tube, or Valpha, to be 0.1358 cm/sec, or 117.3 m/day.
Then Vup was determined using the following equation:

Vup, for this case is 0.1176 cm/s or 101.6 m/day.

Vc is the critical velocity that a floc must have to drop to the bottom of the tube settler and settle out. The following equation from Weber-Shirk was used to determine Vc.

Vc = 0.01032 cm/s = 8.92 m/day
This value is acceptable because it is below the threshold of 10m/day.

Then, the time that the water in the beaker would need to be allowed to settle was calculated as follows:

T = 12cm/(0.01032cm/s) = 19 minutes 23 seconds , or about 20 minutes.

Finally, the settled water turbidity of the grab samples is tested using the bench turbidimeter in the CUWTP lab. This turbidimeter is calibrated regularly by CUWTP staff.

Results of grab sample testing

The above procedure was performed for two of the turbidimeters on April 2, 2008 to test if the tube settler set-up was functioning properly. The following were the results from this test:

As can be seen from this data, the grab test had a settled turbidity that was, in both tests, 52% higher than the turbidity readings from the tube settler/turbidimeter combination. This suggests that the tube settler/ turbidimeter set-up was functioning as it should be. To verify this in another way, we changed the incoming lines between the turbidimeters, and observed that the values from the different turbidimeters agreed.