Aeration Method

Abstract

Two different methods for reducing the dissolved oxygen content in the influent water before it leaves the grit chamber are being explored. One of the methods utilizes the negative pressure in the pipe to create a vacuum, sucking air into the pipe through small holes while the influent is in free-fall hence aerating the water. Many small bubbles will be infused into the water which will in turn increase the gas transfer rate so that once the water enters the grit chamber large bubbles should form quickly and rise to the surface at a much higher velocity.

This section of the research seeks to simulate the conditions in the pipe and the entrance into the grit chamber to determine how quickly the dissolved oxygen content in the water will decrease once it hits the tank that is under atmospheric pressure after it has been exposed to different conditions in the entrance pipe. If this is found to be a viable method to solve the floating floc problem the next step will be to determine how big the holes in the pipe should be for different plant flow rates and how long the retention time has to be in the grit chamber to reach the desired DO content.

Introduction and Objectives

Many of the AguaClara water treatment plants are having the problem of flocs rising to the surface of the water in the sedimentation. This is caused by the formation of air pockets on and inside the floc particles. The current hydraulic retention time in the grit chamber is not sufficiently long enough for all of the bubbles to form and rise to the surface. One solution to this problem is to make the bubbles form and rise faster.

Water in laboratories is often aerated to get gases out of the liquid. This process requires a large amount of air to be pumped into the system, causing many little bubbles. The addition of more small bubbles to the system increases the rate of gas transfer and rapidly creates bigger bubbles. The gas in the water will then rise to the surface more rapidly. The contact time between the air and the water required to allow all or most of the gas to rise out of the water would thus be decreased.

The method of aeration for gas removal would require a high flow rate of air to be injected into the water. Pumps for getting air into the water are impractical to use in the Honduran towns that have water treatment plants designed by AguaClara and are not sustainable. Instead, the properties of gases and liquids can be used to infuse the water with small pockets of air without using mechanical energy.

Henry's Law states:
At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Henry's Law can be utilized to pump air into the beginning of the system. A small hole in the pipe headed to the grit chamber at a point where the water is in free fall would create a negative pressure difference between the inside of the pipe and the atmosphere thus causing an influx of air. Henry's Law can then be applied to calculate the flow rate of air into the water. The density and velocity of the water after passing this hole can then be calculated. A time estimate for the amount of contact time between the atmosphere and water that is needed for all or most of the gas to leave the water can be calculated from those values.

A model of this process was derived last semester and this semester we are testing this theory in the lab. We designed and had built an apparatus that can be used to simulate both the conditions in the pipe and the grit chamber.

Procedures

The apparatus for the aeration method is mainly a segment of clear PVC pipe that is about 9.75" long and has an inner diameter of 4". One end of the pipe is connected to a ---- base that has four evenly placed metal rods attached to it. The rods run freely along the length of the clear PVC pipe and penetrate an ---- lid, which is held in place with butterfly screws at each rod. There are five adapters in the contraption. An adapter in the center of the lid joins the apparatus to a pump via a 3/8" tube. Water can be pumped out from this location when the container is completely full to cause a partial vacuum or air can be pumped in to pressurize the container. Two adapters are located near the base of the contraption that function as a water inlet and air inlet. The air inlet also has an air stone connected to it on the interior of the pipe. The pressure in the contraption is measured with a pressure sensor attached to an adapter near the base, and the dissolved oxygen probe is connected at the bottom of the apparatus near a magnetic stir bar to prevent bubble formation on the probe. O-rings are used to seal the contraption at each adapter location and at the interface between the pipe and the lid.

This contraption is used to simulate the interface between the distribution pipe exit and the grit chamber at the AguaClara plants. The container is filled with water and sealed off and water is pumped out of the lid causing a partial vacuum. The environment created is similar to that in segments of the distribution pipes of actual plants. After the water is put under negative pressure, the pump clamp is released to open the container to atmospheric pressure, which simulates the grit chamber conditions.

Two types of experiments have been run, thus far. The first involves creating a partial vacuum in the container without aeration and observing the effects of the vacuum on dissolved oxygen and bubble formation. The procedure for this experiment is relatively simple. While using Easy Data to monitor the pressure, water is pumped out until the pressure reaches -50 to -70 kPA. At that point, the chamber is opened to atmospheric temperature and the dissolved oxygen is monitored and recorded for no more than two minutes. We wish to see a drop of at least 2 mg/L in that period of time. The second involves maintaining a partial vacuum in the container with slight aeration. The flow of air into the container is regulated by a rotameter that takes either lab air or room air. Originally, lab air was being used; however, later experiments involve detaching the air inflow tube into the rotameter and allowing air to be sucked into the apparatus as it would be through the holes in the actual pipe. After the water is aerated under partial vacuum for a period of time, the apparatus is again exposed to atmospheric pressure and data is recorded in the same manner as mentioned before.

While the water at the actual plants have dissolved oxygen in excess of the 8 mg/L saturation level at atmospheric pressure, the experiments performed have usually involved water that is originally around saturation level or slightly below.

Results and Discussion

Conclusion

No conclusions have been made yet. We will be able to asses the situation better once we have test set up working properly. Current results are not promising but problems with the DO probe prevents us from drawing any definitive conclusions yet.