Depth vs. Surface Sand Filtration

Depth vs. Surface Sand Filtration

The big goal of this research is to understand the difference between surface and depth filtration and the parameters that determine which regime is operative. We suspect that subsurface injection of the water to be filtered shifts the regime to depth filtration. The head loss and effluent turbidity were measured and compared between a control filter, where water is added above the filter in a conventional downflow design, and a subsurface injection filter in which water is injected into the middle of sand bed through a smaller tube modelling a slotted pipe in the Stacked Rapid Sand Filter.

Current Research

Depth filtration likely occurs when the fluid forces on the flocs that bridge across a pore in the filter bed exceed the strength of the flocs.  Thus the dimensionless parameter that determines whether depth or surface filtration occurs must include both floc strength and pressure drop through a thin layer of flocs. Pressure drop through a thin layer of flocs is influenced by the porosity of the flocs which is a function of their fractal dimension. Small flocs are less porous than large flocs and thus small flocs are less likely to produce surface filtration.

In our research, we set up two filters in parallel that both received the same raw water but with one filter operating as a conventional rapid sand filter and the other filter having the raw water injected below the surface of the sand. The coagulant and clay water were both introduced into a clean water source before the influent turbidity of this water was tested. The water was then pumped to the two filter columns. After leaving the columns, the turbidity of the effluent water was measured and then left the system entirely. We measured both head loss and the effluent turbidity of each filter column as a function of time. We ran different tests at varying levels of influent turbidity, filter velocity, and coagulant dosage to see if any of these parameters significantly affected the head loss or effluent turbidity. In each experiment we also noted any differences between the appearances of the two columns which indicated differences in the location of particle capture.

To backwash the columns, we used turned off the coagulant and clay pumps so that only tap water was used to clean the filters. This water was pumped up through the bottom of the filters to fluidize the sand beds. We noticed that the surface filter was difficult to backwash in this way, even at very high pump speeds, because of the large flocs that had settled on the surface (which occurred in most of the experiments but not all as described further below). These large flocs remained close to the surface of the sand column and did not get flushed out of the filter. Surface washing, the method of using a high velocity jet to effectively clean the surface or scraping off this top layer of floc build-up in the filter, would be necessary to thoroughly clean the filter. The subsurface filter had no visible signs of any large flocs or substantial floc build-up and it had no problems with the backwash method used in the experiment, which suggests that surface washing the SRSF should not be necessary.

Our data suggested that there was no significant difference in the measured effluent turbidities of the two filters. Both filters also show linear relationships in measured head loss over time. However, one advantage we found to using the SRSF design is that the head loss increases at a slightly slower rate over time than the normal surface filter. We also noted that at a higher filter velocity, the surface filter did not show hardly any particle build-up on the surface of the sand column, which suggests that velocity is a determining factor in whether or not depth filtration occurs.

Experimental Apparatus Schematic
Experimental Apparatus in Real Life
Example of Results for Effluent Turbidity
Example of Results for Head Loss
Example of the Difference in Particle Capture Location