Summer 2009 CDC Research

Overview

Abstract

In some AguaClara plants, a surface foam develops at the end of rapid mix. The first experiment goal was to understand the chemical conditions required for this surface foam to develop at the end of rapid mix and the first baffle. 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 surface foam formation. Subsequent trials included organic matter: humic acid, but these only produced large non persistent bubbles. It was not until a stronger surfactant, liquid soap, was added to the baffle spacing that a surface foam with strong persistent bubbles developed. From these experiments it was concluded that air entrainment along with a surfactant in the raw water are the main factors behind surface foam formation. In Honduras, the raw water may contain decaying matter which decays to fatty acids acting as the surfactant while the waterfall at the LFOM creates- the perfect- condition for air entrainment. The process of air entrainment along with natural surfactants in the water thus allow for the formation of surface foam formation. With this in mind, the research is now focused on retrofitting AguaClara's designs so that no air entrainment occurs in the entrance tank and rapid mix chamber by eliminating waterfalls and or implementing hydraulic jumps.

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. Once that is completed, we will design a way to retrofit the current plants to fix the problem
• We will learn about the current design for a Nonlinear Chemical Dose Controller and then update MathCAD code for the controller and hopefully be able to build a fully functional prototype by the end of the summer.

(You did not update your two semester goals as asked)

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. Although the foam cannot flow very far in the plant itself, it can be blown around by wind to the surface of the sedimentation tank. and also Small bubbles formed by decreased surface tension in rapid mix in the water could flow persist as far as the sedimentation tank, where they would come out, possibly causing the floating floc problem. We suspect that the surfactants and natural organic matter lower the surface tension energy requirements, possibly creating these small bubbles that stay in solution long enough to reach the sedimentation tank.

As AguaClara continues to grow and serve larger and larger communities, we will be building plants with much bigger larger capacities, where the amount of foam produced from our conventional LFOM design would be impossible to clean. Currently, the plant operators simply scoop the foam off the surface of the water with buckets - not the most sanitary or effective solution. 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 learned how the foam is created, and now we are exploring 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. (Perhaps a sentence describing linear dose control and how this applies to plants with link) Thus, a Nonlinear Chemical Dose Controller (NCDC), which won't be linearly dependent where flow will not vary linearly with on water height like the LCDC, is needed for bigger plants. (Is this process orifice driven? If you are not describing this fully, please put in links and describe what the reader should look for in the links in this paragraph) 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

Determining the Cause of Surface Foam Experiments

Retrofitting Plants to Prevent Aeration Experiments

The experimental method for this procedure can be found at the Experimental Method for Retrofit Designs page.

Results and Discussion

Our first experiment sought to determine whether or not alum was the sole factor in the overdosing alum could aid 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.

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. Although the NOM did lower the surface tension better enabling bubbles to form, we believe they did not form due to a lack of air bubbles which naturally are found in AguaClara plants. As a result, we modified our experiment to include an aerator to provide them.

In our next experiment, we again varied the concentration of NOM but included an aerator to provide bubbling directly into what would be the first baffle spacing. At concentrations below 2 mg/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.

Bubbles form when water molecules form bonds around air pockets. A surfactant is generally an organic molecule that has both hydrophobic and hydrophilic ends. While surfactants reduce the surface tension of water they also form micelles in water which helps to stabilize air bubbles and prevent them from aggregating. The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC. 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.

(Explain what you mean by weak and strong surfactant. If humic acid is a surfactant is there a CMC where it will form persistent bubbles?)

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 similar to natural foams found in rivers and lakes. 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. (Explain how soap could be similiar to fatty acid surfactants in nature)

In our bucket without holes experiment, we found that at heights less than 14cm we can prevent a hydraulic jump. A hydraulic jump normally causes a significant loss of energy and the production of turbulence often resulting in the entrainment of air. This experiment helps us determine an ideal height for our cup design. (Again, I would ensure that we test much lower hydraulic jumps for this. We have eliminated the cup design for the most part.)

See the retrofit designs page for illustrations of the hypothetical designs we need to test.