Floating Floc Sand Filter Method

Abstract

This method of reducing the dissolved oxygen content of the water involves running the water upward through a bed of sand, suspending the particles. The sand particles provide extra surface area to which the oxygen molecules can adhere and accumulate in groups that will merge to form bubbles. When the bubbles grow large enough, they rise to the surface, carrying oxygen out of the water and thereby reducing the dissolved oxygen concentration.

We need to determine the effects of water flow rate, sand depth, and sand particle size on the dissolved oxygen content in order to find the optimal conditions for oxygen removal.

Introduction and Objectives

Floating flocs are a problem for current AguaClara plants. Oxygen molecules dissolved in the incoming water are adhering to sediment particles in the water so that when these particles collect to form flocs, they float to the surface instead of settling to the bottom of the sedimentation tanks. At the surface, the floating flocs are swept out along with the "clean" water.

If the backwash sand filter method proves to be effective, it could be implemented in AguaClara plants as a layer of sand in the grit chamber. Water would enter the grit chamber from below and flow upward through the sand, as in our experimental setup. Bubbles will form inside the suspended sand layer and rise to the surface, and the remaining water will flow on to enter the rest of the plant.

Our experimental setup represents a small section of the grit chamber of an AguaClara water treatment plant. Through our research, we will determine the most effective sand layer depth, sieve size of sand, and water flow rate through the sand in order to remove dissolved oxygen.

Procedures

The experimental setup consists of a vertical glass tube that is 63 cm long with a 2.5 cm diameter. The tube is partially filled with sand, and tap water is sent upward through this tube. The tap water cannot be guaranteed to be super-saturated with oxygen already, so the aeration apparatus previously used by the aeration method has been implemented to supersaturate the cold water before it is fed through the column. The aeration chamber is kept under pressure while the water is bubbled by an aeration stone. Water level and air pressure in the chamber are controlled by pressure sensors and the program ProcessController.

From the bubble chamber, the super-saturated cold water joins hot tap water and flows through the flow accumulator, which uses a pressure sensor, temperature probe, and two valves controlled by ProcessController to regulate the flow rate of water and its temperature. The flow rate is altered to achieve the desired level of suspension of the filter media in the glass column.

After flowing through the suspended filter media, the water and any bubbles that formed in the process pass into the bubble collector.

The bubble collector is made of a 1.5"- diameter PVC pipe that is closed at both ends. Water and bubbles from the glass filter column enter the chamber through the bottom. Inside the chamber (which is initially filled with water before each experiment), bubbles float to the surface while the water flows out through another tube in the bottom. Another tube entering at the top can allow air to enter and leave. This tube as well as the water outflow tube at the bottom are both controlled by valves that are opened and closed by ProcessController, which uses a pressure sensor to monitor the water level inside the chamber.

At the start of an experimental run, the tube is nearly filled with water, the air valve at the top is shut, and the water outflow valve at the bottom is open. As bubbles enter the bubble chamber and gather at the top, the water level slowly sinks. When enough bubbles have entered the chamber to lower the water level as far as possible, the outflowing water valve is shut and the air valve is opened, allowing the chamber to refill. When the water level reaches the maximum level again, the air valve shuts and the water valve opens, and the process begins again. The chamber can be drained by opening both valves at the same time. The system continues running during all of these processes in order to keep conditions as constant as possible. The water level inside the chamber can be visually monitored through an additional clear plastic tube that is attached at the top and bottom.

The rate at which the water depth changes during a run is the same as the rate that air is being added to the collector, and so this is proportional to the rate that oxygen is being removed from the super-saturated water. Our data can be used to find the volume of oxygen removed per liter of water that flows through the system, allowing us to compare the relative effectiveness of each sand size, flow rate, and bed depth combination.

Results and Discussion

• [Results of initial experiments]. These are the results gained from our initial experimental setup, which consisted of the flow accumulator with a DO probe, the glass filter column, and a collection beaker containing another DO probe.
• [Results of secondary experiments]. These are the results gained from the second stage of our experimental setup, which included no DO probes and instead collected the volume of the bubbles formed in the filter in order to monitor oxygen removal rates.