In certain regions, well water tends to be acidic. In eastern Massachusetts, for example, the pH of precipitation can be as low as 4 or even lower, and in the absence of enough neutralization by soils the resulting groundwater will also be acidic. It is also possible for local geologic conditions to contribute acidity to groundwater, as when organic acids are produced in wetlands.
One result is that water drawn from wells and springs can be acidic enough to dissolve metals from distribution pipes. If the distribution pipes are of lead, or-as is much more common nowadays-are of copper with connections soldered together with lead containing solder, significant amounts of lead can be dissolved into the drinking water that passes through the pipes. Although quantitative measures of the dissolution of lead require chemical analysis of the water, evidence for the corrosion of piping may be revealed by the presence of blue-green stains on plumbing fixtures beneath water taps or shower heads. The color is due to copper in the water; where copper is being dissolved, it is a good bet that lead from lead-based solder is also.
One way to limit the dissolution of metals from the piping is eliminate the metals from the piping system. In fact, in Massachusetts it is now required that copper pipes in new houses be soldered with lead-free solder. This, however, is of no help to the vast majority of older homes that have lead-based solder. In these cases, raising the pH of the water to make it less corrosive can be a way to mitigate the problem.
Specific Objectives of the Project
The task of this design exercise is to create and test a device that neutralizes the acidity and hence much of the associated corrosivity of water in a residential water system. Assume that the pH of water as pumped from a water well for domestic use is 5.0. Assume that the pH must be raised to pH 7.0 or higher to decrease its corrosivity sufficiently.
The device that does this job must function over a range of flows typical of household water use. The uses of most concern are, of course, drinking and cooking. You will have to decide what flow rates are acceptable for this purpose. Keep in mind that, while a user might consider the device unsatisfactory if it takes too long to fill a teapot, a device that can neutralize the acidity from an excessively large flow might take up so much room as to be inconvenient, or be too expensive to be competitive in the marketplace.
Other factors that must be considered include ability to withstand water pressure (domestic water supplies commonly operate at around 40 psi), and to be able to pass the required water flows without excessive pressure drop (assume that a loss of up to 4 psi is acceptable). The device should not leak. Cost and maintainability are, of course, essential to consider as well.
Exercises and Design Activities
First Lab (Week #2):
To verify that a water source is acidic, and to verify that a neutralization scheme is actually working, it is necessary to have a sensor that measures the pH of water. Building and demonstrating such a sensor is a task of the first lab. You will have access to pH electrodes, Proto-Boards, and various parts, as well as directions to assemble a circuit that can display the electrode's output voltage. We will have standard calibration solutions (pH buffers) at pH 4, 7, and 10 available to calibrate your sensor. (To verify pressure behavior, you will ultimately need to install pressure sensors as well-these will be mechanical gages and they will serve to measure the pressure drop (head loss) incurred by your device.) There will also be a series of exercises in pH control, with which to both demonstrate the sensor and renew your acquaintance with acid-base chemistry. (PDF)
Circuit Design (PDF)
Second Lab (Week #3):
The focus of this week will be dissolution rates. Specifically, if dissolution of solid limestone is chosen as the means of water neutralization, it will be necessary to assure that the rate of dissolution will sufficient to neutralize all water even under worst-case conditions. If you are designing a treatment column, this corresponds to fastest water flow.
To arrive at a detailed design for a treatment column (most people will opt for a column, though this is not required) it is necessary to relate dissolution rates to the other parameters of your design. Such parameters include column length, area, and packing material. Thus, we will conduct a series of exercises leading to the production of design curves for limestone sands of various grain sizes. At this time you and your team should also be determining how to make sure that your device meets requirements for withstanding water pressure, not leaking, and being straightforward to build. (PDF)
Third Lab (Week #4):
This set of laboratory exercises will include measurement of the hydraulic properties of sand columns, providing data that will permit you to determine the pressure loss as a function of flow in any given treatment column. (PDF)
Fourth and Fifth Labs (Weeks #5 and #6):
These lab periods are for detailed design, prototyping, and testing of your projects. 'Open shop' hours, outside of regular class time, will supplement in-class time for the construction and testing of prototypes.
Although it is likely that most or all of the class will choose to build treatment columns for this project, other solutions that are conceptually quite different are also possible. A variety of materials will be available-these include limestone sand in various grain sizes, and pipe from which columns of various sizes can be made. Instructors should be consulted if a team wishes to follow an approach that requires materials not available in the lab.
The project will be evaluated both on the process by which it is created, and on its performance. The former will be assessed through instructor observations of participation and reading of notebook. The latter will be assessed by in-class demonstrations of your project.
Design Tips (PDF)
Report Guidelines (PDF)