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Summer 2009 CDC Research

Overview

Abstract

One of the most recent problems AguaClara plants have developed has been the formation of surface foam at the end of rapid mix. In order to figure out what chemical condition is required for this foam development a rapid mix chamber and flocculator were modeled using jar mixers, an aerator and gallon size plastic tanks. The first test trials were conducted with a constant supply of clay and varying amounts of alum but these did not exhibit any form of foam formation. Subsequent trials included organic matter: humic acid but these only produced weak, temporary foam. It was not until liquid soap, a surfactant, was added to the mixing that a persistent, firm foam developed in the first baffle spacing. From these experiments it was concluded that air entrainment, along with dead organic matter in the raw water were the main factors behind the foam formation. As a result, the most current research is focused on retrofitting AguaClara's designs so that no air entrainment occurs in the entrance tank and rapid mix chamber.

Introduction and Objectives

For the summer of 2009, our team has two main goals:

  • We will attempt to recreate the foam in a laboratory setting that has been forming in many of the AguaClara plants in Hoduras, and find a way to fix both the current plants and any future plants.
  • We will update MathCAD code for the new Nonlinear Chemical Dose Controller and hopefully be able to build a fully functional prototype by the end of the summer.

These two goals are very important to the overall goals of AguaClara for a number of reasons. The foam that forms in the current AguaClara Plants both increases the amount of work that plant operators have to spend to keep the water clean, and reduces the overall effectiveness of the plants as the dirty foam flows out of the plant with the clean water. As AguaClara continues to grow and serve larger and larger communities, we will be building plants with much bigger capacities, where the amount of foam would be impossible to clean. Therefore, it is imperative that we find a solution not only for the current plants in operation, but also for the future plants we design. This summer we hope to learn how the foam is created, and what design changes we can make that will reduce or even eliminate foam formation.

Also, as we evolve to build larger plants, the Linear Chemical Dose Controller (LCDC) won't be able to provide a sufficient dose of chemicals to treat the larger flow rates. The CDC team from past semesters has found that the LCDC can only provide chemical flows up to 400mL/min, which is too low for larger plants. Thus, a Nonlinear Chemical Dose Controller (NCDC), which won't be linearly dependent on water height like the LCDC, is needed for bigger plants. This summer we will need to improve upon the initial design of a NCDC in order to make it more dependable and robust.

It is our hope that at the end of the summer, the plant operators in Honduras will have a quick-fix way to eliminate the foam forming in their plants, and that future AguaClara plants will have larger capacities, allowing them to provide clean, foam free drinking water for more and more people.

Summer 2009 CDC Research Team's goals and meeting minutes.

Experimental methods and results

The group in attempt to recreate surface foam in a lab setting, similar to that found in AguaClara Plants in Honduras and to determine the cause of the foam formation applied a rapid mix testing under different parameters. The parameters include Aluminum Sulfate (Alum) dosage, concentration of Natural Organic Matter (NOM) such as Humic Acid, location of alum addition, aeration of water and addition of a surfactant. In the first week of our experiment, we tested for alum dosage in order to determine whether it caused foam formation. Alum dosages of 35 mg/L, 45 mg/L, 55 mg/L and 65 mg/L were added to the rapid mix chamber. It resulted in no foam formation in rapid mix chamber or in the first baffle.

The following week the group used humic acid in order to determine the effects of organic natural matter on surface foam formation. Varying the dosage at 1mg/L, 2mg/L, 5 mg/L and 10 mg/L, humic acid was mixed in the rapid mix chamber with alum. In the mean time the water in the first baffle was being aerated. Soon after, bubbles started forming. The formation of bubbles increased as the concentration of humic acid was increased. However, the bubbles dissipate quickly and were unable to form foam. The group observing the property of the bubbles added soap, a surfactant, to the rapid mix chamber. This resulted in a higher bubble formation, which lasted for a while forming a chain of bubbles over the surface of the water.

Results and Discussion

Our first experiment sought to determine whether or not alum was the sole factor in the formation of surface foam. After dosing the water with varying alum concentrations, it was determined through photographs and observation that no foam was formed. Sample photos are included in Figure 1 below.
 
Although the foam was not formed, this data was crucial to our research. Not only did it minimize the list of potential foam formation factors, it also help build our understanding of foam formation. As a result of foam not forming due to alum addition at the surface of the water, we also were able to rule out alum addition under the water surface as intuitively it was a solution to the form formation.

Our second experiment sought to determine whether or not the addition of Natural Organic Matter (NOM) contributed to the formation of surface foams. At all concentrations of Humic Acid, we again found there was no surface foam formed. We believed this was due to a lack of air bubbles which naturally are found in AguaClara plants so at this point we modified our experiment to include an aerator to provide them.

In our next experiment, we varied the concentration of NOM but included an aerator to provide bubble directly into what would be the (flocculation?) tank. At concentrations below 2 g/mL we found that no foam was formed. However at higher concentrations of Humic Acid, we found that large bubbles would rise and then quickly pop in the center of the tank. In the meantime, smaller bubbles would form around the edges of the tank and were slightly more persistent as seen in Figure 2 below.

Bubbles form when water molecules form bonds around air pockets. A surfactant is generally an organic molecule that has both hydrophobic and hydrophilic. Due to this polarity, surfactants form micelles in water which helps to stabilize air bubbles and prevent them from aggregating. Humic acid is a weak surfactant, so though it did reduce the aggregation of air bubbles it still occurred. This resulted in large, non persistent air bubbles that formed a foam at the surface of the water.

Although, a surface foam was formed at concentrations of Humic Acid greater than 2 g/mL, it was not the foam that is found in AguaClara plants. The foam we created in lab contained large non-persisent bubbles, however the foam we sought to create contained small persistent bubbles. At this point we began searching for a stronger surfactant to further prevent the aggregation of bubbles, thus replicating the foam found in Honduran AguaClara plants.

In order to support our hypothesis, we added soap to our last experiment with Humic Acid simply to see if would create the surface foam we wanted. Indeed the soap caused a persistent foam, similar to that found in AguaClara plants and can be seen in Figure 3 below.

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