Foam Filtration

Currently, an AguaClara plant can produce effluent water after sedimentation with a turbidity of about 5 NTU. Our goal is to reduce the effluent turbidity to less than 1 NTU. One potential method of accomplishing this is adding a filtration unit to the AguaClara plants.

Numerous techniques of water filtration are in use today, most of which involve the use of sand as the porous media. A preliminary  literature review revealed a dearth of information on foam filtration. Our team has decided to focus on investigating the actual filtering capacity of a polyurethane foam material as opposed to the traditional method of sand filtration. With proper implementation, a foam filter could reduce the amount of water that is wasted during the backwash cycle of a traditional sand filter. A foam filter could also potentially require less area, and be less expensive to build than a traditional sand filter. However, a foam filtration unit will also require a tight seal with all of the edges of the tank, in order to avoid preferential flow paths which will result in unfiltered water. In addition, a foam filter will likely require manual maintenance by the plant operator.

With these advantages and cautions in mind, we have considered a number of different designs which could provide the desired standard of 1 NTU effluent water turbidity.

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Foam Filtration Past Research - Spring 2010

Flat Foam Sheet Experiments with coagulated particles

Currently, we are testing the filtering capacity of foam under the worst case lab conditions, which consists of filtering water containing unflocculated clay particles. This represents the worst case scenario, because the particles are small and uncoagulated. We would like to test the filtering capacity of the foam under conditions that are more reflective of conditions in an AguaClara plant. This involves adding alum and a rapid mix tube to our system, in order to produce larger particles, which will then lead to, hopefully, higher levels of colloid removal.

Flat Foam Sheet Experiments

Prior to considering the actual design of a proposed filtration unit, it is necessary to test the filtering capacity of the foam material we would like to use. Therefore, a number of experimental trials on foam with varying pore sizes and flow rates were conducted. Pore sizes are measured in the units pores per inch, which is a linear measurement. A smaller pore size corresponds with a larger number of pores per inch, i.e. a pore size of 90 ppi will have smaller pores than foam with 60 ppi. Briefly, it was found that the flat foam sheets alone do not provide enough colloid removal, under the conditions tested, to be an implementable method of filtration for the AguaClara plants. However, under more realistic conditions, where particles would be flocculated, rather than the worst case lab conditions as tested, foam filtration may in fact be feasible. Please see the results and discussion of each experiment for further explanation.

Foam Filtration Current Research

High Velocity Foam Filtration

Since laboratory research showed that horizontal flow foam filtration was not a feasible method for achieving the desired standards, it is necessary to manipulate the geometry of the proposed foam filtration unit in another way. With a high velocity foam filter, the depth of the foam will be increased to accommodate the increased velocity. With an increased filtration velocity, the filtration unit will require less planned area, which reduces the associated construction and maintenance costs.

Foam Filtration Future Research

After laboratory tests on the filtering capacity of the polyurethane foam material are concluded, the following areas of research will be explored:

Aluminum Hydroxide Wash

Research has shown that the foam material has a fairly long ripening time. In order to reduce this ripening time, it may be useful to prime the filter with an aluminum hydroxide rinse after cleaning. This may reduce the time delay in the filter becoming coated with enough alum to achieve the desired filtration performance. In this test, we will need to determine whether the aluminum hydroxide will result in excess aluminum being washed into the effluent water from the filter. It will then be necessary to determine a washing and foam reinstallation method to reduce this effect.

Effect of Natural Organic Matter

If we conclude from laboratory tests that the foam material provides adequate filtration, it will be necessary to test the effects of natural organic matter on the foam material.

Foam Filtration Unit Design

This filtration unit design should be optimized according to the required surface area and filtration velocity parameters determined through previous laboratory experiments. In addition, it should minimize the required plan area, as well as maximize the ease of maintenance for the operator.

Cleaning Method

After determining the effects of natural organic matter on the foam material, we will propose a standardized method for cleaning and replacing the foam.

Foam Media Upflow Design

An alternative to using the foam simply as a flat sheet which water is filtered through is to design a hybrid upflow filter, which uses small pieces of the foam as the filtering material, as opposed to the traditional upflow sand filter. This idea is based on the technique outlined in this article, Water Filtration- Advantages of Fabric Media