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.
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.