Spring 2010
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Foam Filtration Research
Memebers:
- Sarah Stodter
- Rachel Philipson
- Kara Malhotra
- Dylan Guelig
Research
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
Research conducted during the Spring 2010 semester has concluded that the following methods of filtration are not feasible for use by AguaClara, and research has been terminated.
Siphon-Aided Backwash
The goal of our research is to evaluate the feasibility of a siphon-aided backwash design to be used for a sand or mixed-media down-flow filter. The benefits of siphon-aided backwash are:
1) The height of the clear-well does not need to be equal to the head pressure required to attain 30%-50% expansion.
2) The filter bed can be right next to the clear-well, connected via an open pipe without a valve. Flow of water into and out of the clear-well will be controlled by valves on the influent and backwash manifolds.
3) Using 55 gallon HDPE barrels, we will create a system of parallel filters surrounding a shared clear-well. By backwashing the filters out of phase, we will reduce the demand for a large clear-well. Additionally, the down flow filters will be filling the clear-well while the other filters being backwashed draw from it. This allows faster clear-well recharge rates and shortens the time between consecutive backwash cycles.
We are discontinuing research of the Siphon-Aided Backwash for the following reasons: it is a pressurized system which prevents the operator from properly observing the operation of the system, it is not economically feasible because it requires either the construction of one large clear well or multiple smaller clear wells, and the filtration system itself would require the construction of one large or multiple small filtration systems.
Clear Well Backwash Filtration System
The elevation of the clear well is between that of the filter beds and the effluent source of the sedimentation tank. During normal operation, this elevation difference allows effluent from the sedimentation tank to flow through the filter and subsequently to the clear well. Backwash requires approximately ten times the flow rate as normal operation. Filtered water can be sent to the clear by closing the valve leading to the distribution lines whereby water accumulates in the filter beds until the head difference between the filter and the clear well is sufficient and filtered water is diverted to the clear well.
We based our initial design on empirical granular filtration equations from existing literature (reference) on granular filtration. A bench-scale model of our system was built and tested to observe the discrepancy between the actual and the calculated fluidization velocity required to achieve a target bed expansion. We noted an increasing difference between those two sets of values that prompted us to hypothesize that, if we were to base our design on these empirical equations, we need to implement a significant safety factor.
Figure 1: Clear Well Filtration Design
We are discontinuing research of the Clear Well Filtration System because it is not economically feasible due to the large size of the clear well needed to back wash the filters.