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2010 Spring AguaClara Filtration Team

Abstract

The design challenge for the 2010 Spring AguaClara Filtration Team is to design a filtration system for the AguaClara water treatment plant. The filtration system must meet the following requirements: reliably treat the current AguaClara effluent water with a turbidity ranging from 5 to 10 NTU producing effluent turbidity less than 1 NTU, not require any electricity, make minimal use of specialized components that would be difficult to acquire in remote communities, and the system should be easy to construct and to maintain.

Currently, the filtration team has made progress in three fields: clear well backwash, foam filtration, and siphon-aided backwash. Once divided up into those three sub teams, we each conducted a literature review of existing technology and research. We then developed initial designs based on our research and proceeded to test the fundamental theories behind our design. For example, the foam filtration sub team tested the effectiveness of foam in reducing the turbidity of the water while the clear well backwash team developed a bench scale model to test the empirical equations behind the design.

Conventional Filtration

There are many problems with conventional filtration methods, and AguaClara seeks to solve them in a sustainable way.

Method & Result and Discussions for each sub team

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 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 downflow 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.

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. After preliminary research revealed a lack 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.

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 distribute. Backwash requires approximately ten times the flow rate as normal operation. Backwash is accomplished by closing the valve leading to the distribution lines whereby water accumulates in the filter beds until the head difference between the clear well is sufficient and filtered water accumulates in the clear well.

We based our initial design on empirical granular filtration equations from existing literature 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. Further experiments are needed before we can conclude whether or not our design is feasible.

Figure 1: Clear Well Filtration Design

These are the guidelines that the filtration team is following:

  • The operator needs to be able to observe the backwash process, so the filter media must be visible. This is likely to be impossible with pressure filters.
  • The improvement in water quality from filtration is perceived by the community to be small compared to the improvement from the flocculation and sedimentation processes, so the filters need to be less expensive than the flocculation and sedimentation equipment.
  • A large tank to hold water for backwashing is not economically feasible.
  • The distribution tank is not a viable source of water for backwashing because it is impossible to guarantee that there will be enough water in the distribution tank.
  • Many of the communities that will be using our water treatment plants do not have enough water to meet their needs, so the quantity of water that is wasted to clean the filters must be minimized. Stacked filtration and foam filtration may be good options for achieving this.
  • If we can't use stored water for backwash, then the maximum flow of water required for backwash must be at most 50% of the design plant flow rate. This will make it possible to run the water treatment plant at a lower than design flow rate and still be able to operate the filters. This requirement means that if we use 7 filters to backwash 1 filter, that we would need to have another set of 8 filters that could potentially be turned off so that plant could function at 50% of design flow capacity. The requirement of being able to backwash at 50% of design flow rate makes the option of using multiple filters to backwash a filter impractical.
  • All of the basic principles of AguaClara apply to the filtration research. All of the basic principles of AguaClara apply to the filtration research. These include not using electricity, using inexpensive and easily obtainable materials, and teaching the local people how to build, operate, and maintain the plants.

Future Challenges

Siphon-Aided Backwash Sub Team

We will develop a bench-scale model of the Siphon-aided system and test how extending the siphon tube to a lower elevation will change the backwash velocity inside the sand bed. We will also need to test siphon-priming as well as switching states between downflow and backwash.

Clear Well Backwash Sub Team

Additional experiments will be conducted to test the feasibility of our clear well design. We will first increase the scale of our model and see if the difference between the calculated and actual fluidization velocity decreases. If that is the case, we can conclude that wall friction was the major source of error in our experimentation and the actual plant size design is feasible. We will also test multi-layered media and test whether or not the same empirical equations are applicable.

Foam Filtration sub team

Preliminary experimental results suggest that 90 ppi (pores per inch) outperforms the larger pore size foam of 60 ppi. Our team will further test the filtration performance of the 90 ppi foam with aid of alum coagulant and rapid mix. To accomplish this, an alum dosing and rapid mix unit will be added to our experimental setup. Additionally, we would like to further test the effects of higher flow rates on the percentage of colloid removal, and mechanisms of failure of the filter foam.

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