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.