Floating Flocs

Overview

Floating flocs in the sedimentation tanks of AguaClara plants in Tamara, Ojojona, and Marcala result in polluted effluent water. While some treatment plants use floating floc to treat water, AguaClara plants rely on flocs settling out at the bottom. One cause of the problem is thought to be bubbles forming on floc particles due to supersaturation of influent water, which occurs when the total dissolved gas pressure is greater than the local solution pressure. 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.

Currently, the Floating Floc team is conducting research with a backwash sand filter to remove excess gas from the water in the grit chamber before it reaches the sedimentation tank. This method of dissolved gas removal involves running supersaturated water upward through a suspended bed of sand. The sand particles provide an alternate surface area to which the gas molecules can adhere and accumulate to form bubbles that are large enough to rise to the surface, thereby reducing the dissolved gas concentration in the water. The process is illustrated in Figure 1 below. To test this method, we constructed an experimental setup that would simulate a sand filter in a small portion of the grit chamber.

Objective

The main objective of current floating floc research is to quantify the effects of sand grain size, bed depth, bed expansion, and influent dissolved gas concentration on gas removal and to find the optimal sand filter conditions for gas removal. While the influent dissolved gas concentration is not a parameter directly related to the sand filter itself, natural bodies of water will vary in dissolved gas concentration throughout the year, so we would like to observe the effects of varying dissolved gas concentration in the influent water through the filter. Concerning the actual filter parameters, we have found that smaller sand grain sizes are more effective at gas removal but are also susceptible to lift by the buoyancy force of the bubbles. While sand grains that are lifted up in the experimental setup are prevented from leaving the column, this lift problem may cause significant sand loss in actual plants. Thus far, we have found that sand grains between 0.59 mm - 0.84 mm seem to work best. Tentative results from a few bed depth experiments have shown that greater bed depth increases the rate of gas removal by the sand filter; however, further research will be performed to validate these results. The other parameters mentioned have yet to be tested. Please see the Floating Floc Summer 2009 Goals for a timeline of experiments to be performed this summer and the Floating Floc Summer 2009 Research Proposal for a description of experiments to be run.

Floating Flocs Team Semester Goals and Meeting Minutes
Floating Flocs Team Research Proposal
Floating Flocs Team [Future Challenges]

Current Research

[Evaluation of Previous Fluidized Bed Experiments]

• The aerator was replaced with a taller one to increase the residence time of the bubbles in an attempt to achieve more consistent supersaturation of the influent water into the sand filter. A description of changes made to the system along with a diagram of the current setup can be viewed here. This page details the procedure followed and the results obtained from experiments performed to ensure previous sand bed depth results are replicable with the new experimental system.

Previous Research

Previous Fluidized Bed Method Research: Links are separated by adaptations made to the system

• Fluidized Bed after Super Saturator
  This page contains experimental results for gas removal as a function of grain size and preliminary results for gas removal as a function of sand bed depth. These are the results obtained using a pressurized aerator to super saturate the incoming water. The performance of the fluidized bed was monitored with a bubble collector. Although this technique shows great promise, the extent of supersaturation of the raw water was not necessarily held constant for the various experiments especially since the flow rates through the aerator varied.

• Bubble Volume Measurement Method Development.
  These are the results gained from the second stage of our experimental setup, which included no DO probes and instead collected the volume of the bubbles formed in the filter in order to monitor oxygen removal rates.

• Fluidized Bed and Dissolved Oxygen Measurements.
  These are the results gained from our initial experimental setup, which consisted of the flow accumulator with a DO probe, the glass filter column, and a collection beaker containing another DO probe.

Previous Aeration Method Research:

• Floating Flocs 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 theory behind the Aeration method and contains mathematical models predicting air flow through orifices of different sizes and variable length pipes.

Additional Information

Floating Flocs Team Annotated Bibliography

Aeration Method Quiz
This quiz checks that you have a basic understanding of the principles behind the aeration method.