Floating Flocs

Abstract

Floating floc in the sedimentation tanks of AguaClara plants in Tamara, Ojojona, and Marcala results in polluted effluent water. The cause of the problem is thought to be bubbles forming on floc particles due to supersaturation of influent water. The Floating Floc team is conducting research with a back-washed sand filter to remove excess gas from the water in the grit chamber before it reaches the sedimentation tank. The sand provides a surface area on which bubbles can form to leave the water. The goal is to remove enough excess gas, so that the water entering the sedimentation tank has a lower bubble forming potential. Currently, research is being conducted with different sand grain sizes at different bed expansions to observe and quantify the effectiveness of the method for removing the excess gas from supersaturated water. Data collected so far indicates that smaller grain sizes increase the effectiveness of gas removal from the water. However, with smaller sand grain sizes a floating sand problem occurs. Similar to the floc situation, smaller sands are more likely to be carried away by the bubbles, which may result in significant sand loss over time.

Introduction and Objectives

The goal of AguaClara is to provide safe drinking water through sustainable, replicable water treatment systems. Currently, Floating flocs caused by air bubbles forming on floc particles and lifting them to the surface are polluting the effluent water. Gas bubbles form in water when the total dissolved gas pressure is greater than the local solution pressure, or rather when the water is supersaturated with gas. Supersaturation of the water at the plants may be caused by high rates of mixing at plant entrances due to turbulence, churning in the grit chamber, and cracks combined with high pressure regions in the transmission lines. The goal of the Floating Floc team is to find an efficient and cost-effective approach to remove excess gas from water and to implement that method in current and future AguaClara plants to improve the sedimentation process.

Researching is currently being performed with a back-washed sand filter column. The sand provides an alternate surface on which gas bubbles can form and leave the solution. In the actual plants, a layer of sand would be suspended in the grit chamber before the sedimentation tank. We postulate that the main factors affecting gas removal via sand filtration are sand size and amount, flow rate, and bed expansion. We postulate that smaller grain sizes result in better removal of dissolved gas; however, smaller grain sizes introduce the risk of sand loss with bubble formation. Our main objective is to find the optimal parameters to maximize gas removal and minimize the amount of sand carried away by bubbles. The results of our research may result in adjustments to the grit chamber dimensions of AguaClara plants.

Floating Floc Team Semester Goals
Floating Floc Team Meeting Minutes

Current Research

[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.

Additional Information

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.

[Theoretical Modeling of Aeration Method]

• This page discusses the research into the floating floc phenomenon that is being conducted.

Floating Floc Team Annotated Bibliography

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