Sand filters are equipped with filter beds that are effective in all three dimensions. They are best used for the filtration of liquids that carry a high contaminant load and where the application of two-dimensional filter media is insufficient or even unfeasible. Apart from the purely mechanical filtration action, the solid/liquid separation process is enhanced by absorption on the filter grain surface. The filtration efficiency can be assisted by heterogeneous catalyzed crystal growth.  
 
Silica gravel, or sand, is used in different particle sizes, depending upon the characteristics of the solids to be removed and the required flow velocity. The support layer is made up from silica gravel with a particle size of 3-5 mm; this layer is then covered with a fine gravel layer of 0.7-2 mm particle size. Even if other materials with different particle sizes, such as bentonite or amorphous aluminunosilicates, synthetic or natural zeolites (partially chemically impregnated) or anthracite are used instead of the silica gravel or sand media, these systems are generally still referred to as sand filters. Complex filtration problems can also be solved by employing some specialized filter media, including chemically reactive filter media, in so-called multimedia filters.

Process description

The basic treatment process is the same, irrespective of the size of the system. The flow direction of the contaminated stream in a downflow filter is from top to bottom, where the filter bed acts as a fixed bed reactor. However, self-cleaning sand filters are operated in an up-flow configuration.

When filters are operated in downflow fashion, the filter bed eventually clogs up as a result of the contaminant build-up. The result is an increase in the differential pressure and a drop in the flow throughput. The pore void volume for most filter media amounts to approximately 35-40 % of the total filter bed volume. When the percentage solids reaches 5-10 %, the filtration capacity is exhausted and the filter must be backwashed. In filtration systems that are equipped with automatic backwashing, the automatic backwash cycle can be controlled by timers, as a function of volume throughput, or by way of differential pressure. In single filter systems, filtration must be stopped during the backwashing cycle. In dual train or multi-filter systems, standby filters are hooked up during the backwash cycle, so that operations can continue without interruption. If two or more filtration trains are used in parallel, the operation continues to run, but at a reduced filtration capacity.

The backwash cycle is comprised of several, fully automatic rinse steps in sequence, which are time controlled. The automatic backwash is carried out by the switching of automatic valves, which must be actuated by hand in non-automatic systems.

• Drain the liquid in the pressure vessel until the entire volume is available for backwashing.
• Air sparge in a counterflow direction. This will loosen up the filter bed and produce a mechanical cleaning action to remove the contaminants from the filter particles. In addition, air sparging is a safe method to prevent the formation of sludge pockets.
• Water and air counterflow rinse; this will discharge the retained solids.
• Water counterflow rinse. This step will flush out all of the retained contaminant solids from the filter particles. The flow velocity in multimedia filters is adjusted so that the separation of filter materials is carried out according to their density.
• Water rinse in the normal operational flow direction to reconstitute the fixed bed, and until the water runs clear.

The entire rinse cycle lasts between 25 to 40 minutes; after completion, the filter is ready for use again.