Modeling Filter Performance and Standardizing Sand

The members of Stacked Rapid Sand Filter Theory aim to develop a mathematical model for the performance of the sand filter. This model will take input parameters like influent turbidity and coagulant dosage and measure head loss, or the amount of energy that will dissipate from the water. Flow rate is kept constant, and in order to best simulate an actual sand filter with their laboratory model, the velocity is also kept constant, at 1.8 millimeters per second.

Historically, SRSF Theory has been as much about empirics as about theory. Previous semesters built a two-column filter in order to compare surface with subsurface filtration. Surface filtration entails water entering through the top of the filtration column, while subsurface filtration refers to water entering through the side.

This semester, the team built a new model that features two 20-centimeter layers of filtration. Despite the difference between the two-layer model used in the lab and the six-layer filter in the field, prior calculations and considerations ensured that research results would translate well. For example, tube sizes for the model were decided upon based on the metrics taken from stacked rapid sand filters in Honduras.

The team also spent much of the semester implementing an algorithm for Process Controller, the software that controls the pumps and measures turbidity. The algorithm, called a proportional-integral-derivative controller, measures error in a process and using data from past experiments, attempts to correct it. In some of the earlier experiments, the influent turbidity fluctuated slightly even when it was supposed to constant. The PID was applied to mitigate any inaccuracies.

This semester’s goal is to collect data from experiments with varying coagulant dosage and constant influent turbidity, measuring for resulting head loss and effluent turbidity.

So far, AguaClara’s stacked rapid sand filters have made appearances in both Honduras and India. However, while the design of the filter remains similar despite geographical distances, the sand used almost surely isn’t. For this reason, the Sand Source and Testing team seeks to develop a set of standards for what sand can be used, and a set of a procedures for finding out whether a sample is viable or not.

In India, they’re currently vetting sand samples based only on size, and not on anything like acid solubility. This can pose a problem for their sand filter, for example, if the sand they’re using contains limestone, which dissolves in contact with water.

Tests for sand are gathered from various sources, from the America Water Works Association to the American Society for Testing and Materials, to AguaClara’s own internal guidelines. The team’s goal is to tailor the myriad tests for sand not just to fit the needs of the stacked rapid sand filter, but also so that operators anywhere in the world can easily conduct them with the resources available in their setting.

For example, a test for silica content was eliminated because it was impractical and inessential. Other tests were removed because the nature of stacked rapid sand filtration rendered them redundant. Ultimately, it was decided that tests for the acid solubility, porosity, and uniformity coefficient and effective size–essentially, how similar grains are to each other in a given sample– were most important and suited for the SRSF’s needs.

The only real issue regarding the sand used in India is solubility; given the current backwash velocity of AguaClara’s sand filters, the margin of error in regards to the grain size of a sand sample are very wide, and so the tests currently being use in Honduras and India are adequate in that regard. Meghan and Rebecca’s research will provide operators with a more precise method of not only measuring the size of sand, but it’s solubility. Their results also indicate that as long as the sand is determined to be insoluble, then it’s fairly likely that that sand is viable for use in AguaClara plants.