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. The 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.

This method of reducing the amount of dissolved gases in the water involves running the water upward through a bed of sand, suspending the particles. The sand particles provide extra surface area to which the gas molecules can adhere. The dissolved gas molecules accumulate into groups, which merge to form bubbles. When the bubbles grow large enough, they rise to the surface, carrying gases out of the water and thereby reducing the concentration of dissolved gases. The process is illustrated in Figure 1, at the right.

The goal of theFloating Floc team 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 that small grain sizes 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 in actual plants.

Currently, 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. Placing sand in the water would provide a surface area other than the floc particles on which bubbles can form to leave the water. The aim is to remove enough gas from the water in the grit chamber, so that the water entering the sedimentation tank has a lower bubble forming potential. Previous research focused on quantifying the effectiveness of different sand grain sizes at gas removal. Current research focuses on observing and quantifying the effectiveness different bed depths and bed expansions. Since natural bodies of water vary in dissolved gas concentration throughout the year, we also plan to observe the effects of the influent dissolved air concentration on gas removal.

Floating flocs are a problem for current AguaClara plants. Gas molecules dissolved in the incoming water are adhering to sediment particles in the water so that when these particles collect to form flocs, they float to the surface instead of settling to the bottom of the sedimentation tanks. At the surface, the floating flocs are swept out along with the "clean" water.

If the backwash sand filter method proves to be effective, it could be implemented in AguaClara plants as a layer of sand in the grit chamber. Water would enter the grit chamber from below and flow upward through the sand, as modeled in our experiments. As represented in Figure 1, dissolved gases in the incoming water will attach to the sand particles to form bubbles inside the suspended sand layer, which will float out of the water while still inside the grit chamber. This will decrease the content of dissolved gas in water. The now-unsaturated water will flow on to the rest of the plant.

Our experimental setup is used to model a small section of the grit chamber of an AguaClara water treatment plant. Through our research, we will determine the most effective sand layer depth, sieve size of sand, and water flow rate through the sand in order to remove dissolved oxygen.

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

Experimental Methods and Results

• [Preliminary Summer Results] This page details the procedure followed and the results obtained from an experiment performed to ensure Spring 2009 sand bed depth results are replicable with the new system and aerator.

• Fluidized Bed after Super Saturator 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 super saturator 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 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.