Challenges for Fall 2013

Team Organization

The following sections introduce the objectives of each team with some details. If you wish to read in more detail about the teams' challenges, check out the full challenges document.

Small Scale Plant

Your goal is to produce an easily reproduced and updated scale model representation of the plant that makes it easy to explain and show how a full scale water treatment plant works. The production system that you develop must begin with the AutoCAD drawing of a full scale plant and then with minimal processing send that file (or pieces of that file!) to a 3-D printer.

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The objective of this research is to develop a robust, sustainable, low energy, operator friendly, wastewater treatment system. The expectation is that wastewater treatment technologies have failed to evolve and optimize due to the same influences that have caused drinking water treatment technologies to not evolve. Our goal is to bring the insight of the fundamental mechanisms, the limitations of mass transfer, an understanding of the interactions between reactor geometry and the fluid dynamics of multiphase flow to this task so that we can create a much improved anaerobic digester.

Turbulent Tube Floc

We need to do research with a turbulent flow hydraulic flocculator. We need to test our new limited growth flocculation hypothesis under turbulent conditions to see if the model is correct and to be able to revise the model as necessary. We need to learn how much collision potential is really needed in a turbulent flow reactor. We need to learn what the best energy dissipation rate is for a flocculator. Our current design standard of 10 mW/kg is based on "a reasonable number based on conventional designs". We aren't conventional and there is absolutely no expectation that 10 mW/kg is the right number! A higher energy dissipation rate could be useful to reduce the amount of flocs that settle to the bottom of the flocculators. As the energy dissipation rate increases it should be possible to reduce the residence time of the flocculator, but the head loss will increase and that will require the entrance tank to have a higher level.
The turbulent flow hydraulic flocculator will provide us help us understand the performance tradeoffs and hopefully to develop design modifications that substantially improve flocculator performance.
The turbulent tube flocculator must be designed to easily adjust the energy dissipation rate (perhaps by varying the flow rate) and to adjust the residence time by adding or removing flocculator sections. In the turbulent flow flocculator you will be able to vary energy dissipation rates by either varying Q (in which case energy dissipation rate increases with the cube of the velocity) or by changing the dimensions of the constrictions. If you vary Q, then you are holding the collision potential constant (because energy dissipation rate increases with the cube of velocity and the collision potential is proportional to the cube root of energy dissipation rate) and as you increase Q you are decrease the residence time (and collision potential is proportional to the residence time). If you vary the constrictions the energy dissipation rate will vary and the residence time will be almost unchanged.

Foam Filtration

Provision of safe drinking water for communities smaller than 1000 people is particularly challenging because the per capita costs associated with a full AguaClara plant increase as the community size decreases. Thus we are looking for more cost effective surface water treatment systems that could be applied for flow rates that are less than about 1 L/s. In India we are using a combination of the dose controller and SRSF to create a low cost treatment system that can treat low turbidity water. We anticipate that the SRSF is limited to treating water with a turbidity lower than about 5-10 NTU. In Honduras it is common for water sources to be too turbid in the rainy season to be adequately treated by a SRSF. The reticulated foam filtration system invented by the AguaClara team at Cornell could be a solution to the problem of small communities with turbid water.
Reticulated foam filters have a very high porosity and thus they have a high solids loading capacity. The high porosity also makes it possible to use a higher filtration velocity than can be used with sand.

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Review the report from the previous semester for literature review and background on the many hypothesized mechanisms. Our hypothesis is that arsenic removal requires good contact between arsenic and coagulant precipitate (hence a focus on rapid mix) followed by a highly efficient removal of the flocs that are loaded with arsenic. Given the low turbidity of most groundwater and the very low concentrations of arsenic relative to normal coagulant dosages, it is expected that a low coagulant dose should be adequate if there is ample opportunity for mass transport of arsenic to the coagulant precipitate. If a low coagulant dose is sufficient, then a SRSF should be able to capture the precipitate and produce a very low arsenic concentration. Given that arsenic removal efficiency will likely be correlated with coagulant precipitate removal efficiency, it may be beneficial to use 2 SRSF in series to enhance particle removal and provide additional protection against arsenic laden coagulant precipitate making it into the finished water.
 
It is possible that rapid mix flocculation, floc blanket, plate settler sedimentation, and SRSF would provide a better system for arsenic removal. This disadvantage of using this treatment train is that flocculation and sedimentation require much larger (and more expensive) reactors than an SRSF. Thus it would be better if an efficient arsenic removal system could be created that doesn't doesn’t require a sedimentation tank. Some flocculation time might be necessary to provide opportunity for more contact between arsenic and the coagulant precipitate.
The removal of arsenic by precipitation is expected to be limited by the transport of arsenic to the solid surface of the coagulating agent (either iron or aluminum salts). The flocculation process for groundwater containing arsenic is expected to be inefficient due to the low floc volume fraction.  To compensate for the low floc volume fraction it may be necessary to use a longer residence time. Loss of coagulant to the walls of the reactor will also likely be a major problem for small scale reactors given the low solid surface area in suspension. It may be advantageous to use a contact chamber for rapid mix and initial precipitation to reduce losses to the reactor walls.
Devise methods to conduct research safely and to ensure safe disposal of arsenic contaminated waste. Determine the best way to prepare and to measure very low concentrations of arsenic. Design and fabricate a reactor system and data collection system that will make it possible for us to begin to optimize treatment processes for efficient and reliable arsenic removal. Determine how to create a raw water for testing. Should the raw water be created from distilled water or from tap water? What should be added to the raw water to set the ionic composition? How should pH be controlled?

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The filter bed stratification after backwash may not be very significant if the bed is transitioned quickly from fluidized at 30% to settled. It is possible that the quick transition will not allow much time for smaller diameter sand to migrate to the top of the filter. This could be a lab research project or it is possible that a literature review will reveal that someone has already conducted these tests. In any case, the amount of stratification allowed should be based on a target distribution of flow between the 6 filter layers.
The challenge for this team is to develop a set of quick tests that can be conducted in the field to determine if a sand is suitable. If a sand isn't isn’t suitable because of the wrong sand size, then this team should provide recommendations for sieve sizes to use to select an appropriate size sand. The team could practice preparing a suitable sand from a poorly sieved source by purchasing a river sand and then sieving it to get it within the specifications.

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One of the big goals of the AguaClara program is to design plants that are easy to operate and that provide appropriate feedback to the operator so that the operator can make appropriate adjustments. One of the critical activities for the plant operator is to prepare and mix the stock solutions of chlorine and coagulant. Unfortunately the operator can't can’t see if the solution is well mixed and there is no way to measure the solution concentration. Thus if an operator makes a mistake while dumping the granular sodium hypochlorite or PACl into the stock tank, there isn't isn’t any way for the operator to know that the resulting stock concentration is incorrect. The goal is to provide the operators with the tools so they can check to see if the stock tank is well mixed and if the concentration is correct. The proposal is to use a simple hydrometer that is labeled to read the concentration of PACl or sodium hypochlorite. The other option is to provide a hydrometer and a table that converts the density to concentration.

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AguaClara needs a ram pump to move filtered water up to the chemical stock tanks and also for use in the water treatment plant bathroom. This need is especially important at larger facilities. For example, at San Nicolas the chemical stock tanks will be 750 L and would require the operator to fill and carry approximately 40 buckets of water. The pumping requirement for San Nicolas is 70 mL/s (750 L stock tank in 3 hr). The best efficiency that we obtained during the summer suggests that we might need 1 L/s of drive pipe water to deliver that flow. A goal for this semester is to improve the efficiency of the pump by tuning the waste valve and by using a 1" 1” drive pipe that is the same diameter as we will be using in San Nicolas.

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