Foam Filtration
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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 requires less surface area, and could potentially 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.
Foam filtration has been ruled out for an AguaClara plant because the foam would be too difficult for an operator to clean. However, previous research has proven that foam is an effective method of filtration, therefore this team decided to focus on designing a point of use foam filtration unit. Due to chemical dosing and usability concerns, the point-of-use application was ruled out as a possibility for foam filtration. The team then focused on the design of an emergency filtration system that can be transported on the back of a pickup truck. After speaking with communities in Honduras, it was decided that foam filtration would be more useful in small communities. Research has been done to optimize the design such that it is as efficient as possible and provides the desired standard of 1 NTU effluent water turbidity.
Note: Turbidity is defined as the cloudiness of a fluid caused by suspended particles. Higher turbidities correlate to more opaque fluids.
Current & Future Research
After visiting Honduras, the apparatus designed in Fall 2013 was implemented with mild success. The design met the desired constraint of 1 NTU however was difficult to use, especially in cleaning. Therefore, Spring 2014 updated the apparatus for easy use and transport. The Fall 2013 design of the foam filtration unit to fits all into one 55gallon drum. The apparatus consists of a roughing filter that utilizes 30 ppi* foam and a finishing filter with 90 ppi foam approximately 23 inches in diameter. The filter unit was designed on the premise of providing for a community of 100 families at approximately 1 L/s. In order to clean this unit, Spring 2014 team chose and implemented user-friendly simple pulley design with a hoist supporting approximately 500 pounds of concrete will be used to compress the foam with a hard plastic plunging disk. Then a side drain was spin-welded to the drum to drain the dirty water that pools on the surface. During the Summer 2014, our biggest goal is to send another prototype to Honduras. Before deploying the filtration system, several modifications on the design will be made, interacting with the engineers working in Honduras. We will install the chemical dose controller and the flow controller to the structure. Also, the team will substitute the side drain with a siphon tube fixed to the wall of drum for better clean out cycle. Still, transportability, easy fabrication, and reasonable cost are our core concerns in design. Also, we will verify any potential risk of using polyurethane as our filter by contacting foam manufacturers and reviewing literature. Simultaneously, more research will be done to investigate the effectiveness of a wholly constructed apparatus including flow control with coagulant and post-filtration chlorination. The effectiveness of the foam and the times until cleaning will probably vary with the presence of coagulant. Also, alternative designs including hydraulic compression will be keep investigated.
Possibly, a user guide will be made after sending the apparatus to Honduras to aid future non-engineer users in construction and operation of the foam filtration system.
*PPI stands for pores per inch, which is a linear measurement of foam pore size. A larger pore size number (i.e.60 ppi) corresponds to a smaller pore size.