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.
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 is:
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 siphon tube is designed to fill up when the filters have reached a terminal headloss (a set point we determine). Thus, they only backwash when they are dirty.
3) Since the headloss energy is used to prime the siphon, and eventually trigger backwash, fewer valves are required which reduces cost and labor.
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.
Future Challenges
Siphon-Aided Backwash Sub Team
We will develop a bench-scale model of the Siphon-aided system and test it for effectiveness and feasibility. Be more specific. What do you mean by effectiveness? What sorts of variables are important in determining whether such a system is successful?
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.