Spring 2010 Terminated Filtration Methods

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.