Floating Flocs

Abstract

Rising flocs in the sedimentation tanks have been a problem from the very beginning at the plant in Tamara. The plant in Ojojona is also having this issue but to a lesser degree. Initially the rising of the floc was thought to be caused by alum overdosing but the problem persisted even after the dosage was changed. The speed with which the floc particles rise in the tank suggests that air bubbles are lifting them to the surface. Some water treatment facilities purposely make the flocs rise to the surface as a way to remove particles in the water but since the Tamara and Ojojona plants are designed to have the flocs settle out at the bottom of the sedimentation tanks the layer of particles at the surface of the tank pollutes the effluent.

Gas bubbles can form in a solution of water when the total dissolved gas pressure is greater than the local solution pressure. Very high rates of mixing can cause the pressure to drop below that of the atmosphere, creating a negative local gauge pressure. This decrease in pressure causes more gas to enter as the total dissolved gas pressure is again greater than the local water pressure. There is a lot of water turbulence at the entrance of the Tamara water treatment plant because of influent's high velocity. The churning in the grit chamber could behave like a very high rate rapid mixer and thus significantly increase the amount of gas in the water. Another possible reason for the high oxygen content of the water is the transmission line leading from the mountain stream in Tamara to the treatment plant. There are some breaks in the line that allow air into the transmission line; this air-water combination then goes through regions of high pressure causing the air to be infused into the water. Different methods of reducing this influx of gas into the water or to take it out at the plant are being researched. The hope is that decreasing the amount of gas in the water at the beginning of the water treatment process will solve the problem of rising floc in the sedimentation tanks.

A procedure is needed that will either prevent supersaturation of air in the transmission line or remove dissolved oxygen prior to sedimentation to increase the effectiveness of the water treatment plant.

Introduction and Objectives

The goal of the Floating Floc Team is to devise a solution to the floating floc issue in the Tamara and Marcala water treatment plants. We are currently looking into two different approaches to the problem - an aeration and a sand filter method. Both approaches involve decreasing the dissolved oxygen content of the water in the grit chamber before it enters the flocculation tank. The hypothesis is that the water coming into the plants is supersaturated with oxygen, which causes the formation of bubbles. These bubbles tend to form on dirt particles in the water, causing the flocs to float to the surface instead of settling out in the sedimentation tank. As a result, the floating flocs contaminate the effluent water. We hope our research will lead to a remedy to this serious problem in current and future AguaClara plants.

Our aim is to find the most effective of the two approaches mentioned above. We hope to implement that method in current and future AguaClara plants so that the extra dissolved oxygen is removed from the water before it leaves the grit chamber. This should eliminate the issue of flocs rising to the surface in the sedimentation tank.

Floating Floc Team Semester Goals
Floating Floc Team Meeting Minutes

Experimental Methods and Results

[Floating Floc Phenomenon Research (Fall 2008)]
This page discusses the research into the floating floc phenomenon that is being conducted.

Floating Floc Aeration Method Experiment

• The aeration approach provides bubbles that will act as a substitute for sediment in the water, causing dissolved oxygen to diffuse into the bubbles. This will increase the bubble size, causing the bubble to rise faster.

[Floating Floc Sand Filter Method Experiment]

• The sand filter approach involves forcing water upward through a layer of sand, as if backwashing a sand filter. The sand filter method provides a substance besides the sediment in the water on which the bubbles can form. When the bubbles grow large enough, they will rapidly rise to the surface.

[Quiz]
This quiz checks that you have a basic understanding of the principles behind this research.

Additional Information

Floating Floc Team Annotated Bibliography