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Slow Sand Filtration


Slow sand filtration can be used to remove particulate and microbial constituents. In the process, water is treated by percolation through a bed of sand. On the sand surface, a biologically active mat (schmutzdecke) formed by solids form the water being treated, microorganisms and algae is established. It is within this layer that most of the treatment takes place. The presence of dissolved oxygen in the source water is essential for promoting the growth of the schmutzdecke.

Suspended and dissolved matter are removed by physical and biological mechanisms. The filter will not provide acceptable treatment until the schmutzdecke is established. The basic system consists of a filter box filled with a bed of fine sand supported by a layer of gravel or a synthetic. Recent improvement that some systems have adopted to improve filters performance include the inclusion of a granular activated carbon (GAC) layer within the media to absorb organic chemicals and preozonation ahead of the filter as another means of improving the removal of organics.

Typically, slow sand filters are operated in a rising head manner (constant flow enters the system, and headwater increases to a sufficient level that allows the flow to pass through the filter). Head loss develops due to particulate deposition and microbiological growth from a few inches at the start of a filter run to as high as the filter box will permit. Once terminal head loss is achieved, the slow sand filter is drained, and the schmutzdecke is scraped off the top.

Erosion of the sand bed by influent waters can be a problem as surface erosion can cause short-circuiting. To reduce chances of erosion during filter startup, water should be spread evenly over the top of the sand bed prior to initializing flow, dissipating the energy of the water entering the filter box.

The main parameters controlling the efficiency of slow sand filtration include water quality parameters such as turbidity and temperature, and filter design parameters such as filtration rate. Filtration rate typically range from 0.05 gpm/sf to 0.1 gpm/sf although rates as high as 0.1 gpm/sf may be tolerated for short periods during filter scraping or ripening. Filter media characteristics such as effective size and uniformity coefficient are also important.

The duration of slow sand filters runs depends on water quality and loading rates. Ideally, the raw water turbidity should be less than 10 NTU. With low turbidity in the influent water, the filter can be operated for a period between two weeks and three months. However, they can be operated for short periods or at low loading rates with turbidities up to 30 NTU. During the summer, filter runs may be shorter due to algae growth; however, covering the filter can lengthen filter run times.

Depending on water quality and temperature, the frequency of filter scraping may range from 1 to 12 months. Cleaning of the filters is done by skimming off the top layer of the filter to a depth around 10 inches. This can be done by taking the filter off-line and using a mechanized skimming and suction system or manual labor. Once the depth of the media is reduced to less than approximately 150 inches the filter has to be replaced up to the maximum design thickness.

In most cases, slow sand filters are operated without pretreatment (e.g., coagulation, flocculation, sedimentation) and no chemicals are added to aid the filtration process. In other cases a pretreatment (e.g., coagulation, flocculation, sedimentation) of the raw water is implemented to enhance the filter performance. The need for pretreatment depends on raw water turbidity and loading rates applied to the filter.

Besides the simplicity of design, slow sand filters have several advantages, including low cost of installation and operation, sludge handing problems are minimal and close operator supervision is not required. Limitations of slow sand filters include: large surface area and quantities of filter media are required; they are labor intensive for cleaning; high turbidity levels can quickly clog the fine media sand; in waters with very low nutrients concentrations turbidity removal may be impaired by slow growth of microorganisms in the schmutzdecke; they are less effective at removing microorganisms from cold water because biological activity is reduced at lower temperatures; they do not completely remove organic chemicals, dissolved inorganic substances unless a GAC layer is installed; and they do not remove well fine clays unless pretreatment is provided.

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