Floating Flocs

Abstract

Rising flocs in the sedimentation tanks have been polluting the effluent water at the plants in Tamara and the Ojojona. While some treatment plants rely on floating floc to remove particles from water, AguaClara plants are designed to have flocs settle out at the bottom of the tanks. 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 at which the flocs rise in the tank suggests that air bubbles are lifting them to the surface.

Gas bubbles form in water when the total dissolved gas pressure is greater than the local solution pressure or when the water is supersaturated with gas. Supersaturation of the water flowing into the plants may be due to very high rates of mixing due to turbulence at the plant entrance and churning in the grit chamber, which would cause pressure to drop below atmospheric. Cracks in the transmission lines would also allow air to enter the water combined with high pressure regions result in gas being infused into the water.

A procedure is needed to either prevent supersaturation of influent water or remove excess gas prior to sedimentation to increase the effectiveness of the water treatment plant. Currently, research is being performed using a back-washed sand filter to remove excess gas from the water. 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.

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

[Floating Floc Sand Filter Method]

• 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.

Results and Discussion

Conclusions

Additional Information

[Theoretical Modeling of Aeration Method]
This page discusses the research into the floating floc phenomenon that is being conducted.

Floating Floc Aeration Method

• This page discusses past research on the aeration approach to dissolved oxygen removal. The aeration approach attempted to use bubbles as a catalyst to increase the rate of dissolved oxygen transfer out of solution by allowing dissolved oxygen to diffuse into the bubbles. This would increase the bubble size and cause the bubble to rise faster.

Floating Floc Team Annotated Bibliography

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