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Operating and Troubleshooting

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FReTA

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Clay Stock Preparation, Clay Stock Valve, and Raw Water:

Generally we use a clay stock concentration of 10 g/L for high NTU tests (100 NTU or greater), and concentrations of 3.33 g/L for low NTU tests (less than 100 NTU). It is not necessary for the clay stock concentration to be precise. The clay stock valve which meters in clay to the raw water is controlled by a feedback loop in Process Controller. If the raw water turbidity drops too low, as measured by the influent turbidimeter, the clay stock valve will open every few seconds to allow more clay into the raw water until it has reached the target turbidity.

Common Problems: If you find that the influent turbidity is too high or too low (or has large fluctuations) you may need to adjust the clay stock valve. First verify that it is not clogged and that it is opening only when the turbidity is too low at the correct intervals. If the valve is functioning properly, you may need to adjust the settings in Process Controller. The setpoint 'off time' controls the amount of time after the valve has closed before it will open again when the turbidity is below the target. It can be either increased or decreased. For mid to high NTU experiments, an off time of around 10 seconds generally functions well. For experiments as low as 5 NTU, the off time may need to be as great as 20 or more seconds. Additionally, the flow can be reduced by partially closing the manual valve before the clay stock valve. Currently we are working on putting in new tubing to the influent turbidimeter with a larger diameter; we wish to increase the flow rate to the turbidimeter and therefore reduce residence time in the loop to reduce the response time in the feedback loop.

Alum Stock Bucket and Pumps:

We usually use a stock alum concentration of 2.5 g/L. With this stock concentration, the pumps can handle alum dosages ranging from about 5-160 mg/L (for a plant flowrate of 5 mL/s). For lower alum dosages, we use a stock concentration of 700 mg/L. This allows for approximate dosages ranging from 1.5mg/L to 40 mg/L. Be sure to verify that the dosages you want can actually be delivered to the plant by the alum pump; higher or lower dosages may require adjustments to the stock alum concentration depending on plant flowrate. Additionally, fresh alum should be used at the start of each experiment.

Common Problems: Make sure that the alum pump is functioning; it is possible that the yellow cable connecting it to the stamp box may have come loose. Additionally, it is not uncommon for bubbles to form in the tubing leading to the alum pump when it is first connected to the stock bucket. If you find that there are large bubbles, they could restrict the flow and you may need to flush them out. Additionally, if you find that the pump is not delivering the correct dosage, it may need to be recalibrated in Process Controller.

Connector Joining Alum line to Main raw water line

One significant problem we encountered during our research was the precipitation of aluminum hydroxide in the connector joining the alum line to the main raw water line of the flocculator. The problem occurred because our stock alum is mixed in distilled water. Distilled water has a lower pH than the tap water the raw water is mixed with. Alum is more at lower pH values; when the stock alum hits higher pH and alkalinity of the raw water, it quickly begins to precipitate as aluminum hydroxide(it is the precipitation of aluminum hydroxide that makes flocculation possible). However, as we ran more experiments over the course of the semester, more aluminum hydroxide built up in the connector joining the two lines. At its worst it almost entirely blocked the alum feed and caused a significant back pressure in the raw water line. (Since the precipitation occurred in the T connector joining the alum and raw water lines, it was able to block both). This problem caused significant setbacks for our research until we were able to locate it. Future teams should consider redesigning this section of FReTA to avoid the problem. Additionally, until a solution is implemented it is important to check the connector for the buildup of aluminum hydroxide regularly. A blockage can begin to form in as little as a week or two of experiments.

Tube Flocculator:

The flocculator can be run at one of three lengths, either 2796 cm, 5592 cm, or 8388 cm. Occasionally it may need to be cleaned by running a piece of sponge through it during backwash.

Common Problems: It is important to clear the flocculator of large bubbles before beginning an experiment because they can cause floc breakup and skew results. Bubbles can easily be removed during backwash by opening the release valve on the influent tubing when they reach the beginning of the flocculator. Additionally, tubing that is not in use but has water in it should be plugged to prevent it from dripping on the equipment.

Influent turbidimeter:

The influent turbidimeter is programmed in to a feedback loop monitoring the raw water turbidity.

Common Problems: Generally the influent turbidimeter has few problems. After high turbidity tests, the cuvette may need to be rinsed out as some of the clay may settle out on the sides of the cuvette, interfering with readings. It is good to verify that the cuvette is not dirty, and clean it if needed before beginning an experiment.

Effluent Turbidimeter, Ball Valve, and Settling Column:

The effluent turbidimeter records the turbidity over time during the settling state as flocs settle down into the settling column. While this happens, the ball valve is closed, sealing off the column from the rest of the flocculator. This is when most of the useful data is recorded; the turbidity vs. time plot is used to contruct a turbidity vs. settling velocity plot in the data processor.

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