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Turbidity is defined as an expression of the optical property that causes light to be scattered and absorbed, rather than transmitted in straight lines through the sample. Simply stated, turbidity is the measure of relative sample clarity. Turbidity instruments measure the average volume of light scattering over a defined angular range. Particle size and the concentration of suspended solids, as well as the level of dissolved solids, can affect the reading. Turbidity instruments can measure not only turbidity but also suspended solids. Turbidity is measured in Nephelometric Turbidity Units or NTU, which represents the average volume scattering over a defined angular range.
The presence of these suspended solids creates visual turbidity. In the case of water treatment for human consumption, it is necessary to remove these suspended solids. The necessity is twofold; first the presence of suspended matter in drinking water is objectionable, so it is removed for aesthetic reasons. Second, the presence of suspended particles interferes with the disinfection process. The suspended solids interfere in two specific manners. First, they create a shield for the microorganisms, thus inhibiting the ability of the disinfectant to destroy the microorganisms. Second, the suspended solids chemically interact with the disinfectant and may reduce its lethality towards the microorganisms. All of these factors dictate the need to remove the suspended solids.
There are numerous technologies available to perform "liquid/solids separation. Some are more sophisticated, complex and costly than others. Fortunately, for small industrial packaged processes designed for treating water for human consumption the technology is relatively simple, straight-forward and not overly expensive.
The process of filtration involves the flow of water through a granular bed of sand or other suitable media. The flow may be either with gravity or pressure. Large water filtration systems typically utilize gravity, while smaller packaged systems typically utilize pressure to control the flow. The type of media used can also vary, depending on the defined intention of the system. Some filtration systems use only one filtration media while others may use two of more. Regardless, the filtration media is granular in shape and varies in two important characteristics - specific gravity and particle size.
The actual filtration process is twofold:
Physical removal via blocking and impeding the flow of suspended solids
Chemical filtration whereby the surface charges of the suspended solids and the filtration media cause coalescence.
Technically, the term "water treatment" refers to any modifications made to raw water. Included under the umbrella term "water treatment" is water softening and water filtration. As such, these two different functions are frequently "confused", "mixed" or used interchangeably. Water filters and softeners do, in fact, serve very different purposes. The purpose of filtration is to provide clean water for drinking and other sanitary purposes. The main goal of water softening is to remove specific minerals, such as calcium and magnesium, to prevent water "hardness". Only removing minerals does not necessarily provide water fit for human consumption. The removal of calcium and magnesium from water is achieved by chemically replacing the calcium and magnesium ions with sodium ions. Sodium does not separate and cause scaling in distribution pipes.
Slow Sand Filters
Slow sand filters have been around for a very long time. They are rarely used anymore because of the space requirements. The sand (~ 3.5 feet in depth) is supported by gravel (~1 foot in depth) and the water, which can have no more than 10 NTU of suspended solids, flows from the top of the filter across the schmutzdecke layer, through the fine sand and gravel to the underdrain system and discharged. There is no pre-treatment prior to the slow sand filter.
The schmutzdecke layer is a sticky mat of suspended matter. Slow sand filters are cleaned by either a manual or mechanical scrapping of the top layer of the sand, thus no backwashing is required. After several scrapings it is necessary to add replacement sand. Once replacement sand has been added, it is necessary to allow the schmutzdecke layer to re-build. This may take as much as two to three days. Thus, a second system is required to maintain uninterrupted filtration.
Rapid Sand Filters
Unlike slow sand filters, rapid sand filters use coarser sand there is pre-treatment in the form of coagulation and flocculation. Thus, the filtration rates are much higher. Sand filters may utilize a doen-flow or an up-flow design. Rapid sand filters do not utilize a schmutzdecke layer. The filtration of suspended solids is achieved by flowing through a 3-4 foot deep bed of sand, also supported by a layer of gravel. The sand is routinely cleaned by backwashing. The backwashing frequency is determined by a specific pressure drop across the bed. During the backwash cycle, it is necessary to activate a second sand filter, capable of handling the entire water flow.
Multimedia filters may use either two or three different filtration media, always supported by a layer of gravel (~0.5-1.5 feet deep). Again, depending of the NTU level, there may or may not be pre-treatment in the form of coagulation and flocculation prior to filtration.
Regardless of whether the filter is dual- or multi-media, the configurations regarding size and specific gravity are the same. The media on top is most course and lightest, with a lower specific gravity. As you go down into the subsequent media layers they become finer and heavier. As such, the top layer removes most of the suspended solids and those particles that pass through the coarse media on top are removed by the finer media below. Backwashing is the method used for cleaning the filter media.
The common materials used as filtration media are course anthracite, silica sand, fine garnet sand and, of course, gravel as a support medium.
Deep-bed, single media filters
As the name implies, these filters are much deeper (~ 4-6 feet) than sand and multi-media filters (~3.5-4.5 feet) and consist of only a single filtration media. The media is typically either course sand or anthracite coal. Again, backwashing is the method of cleaning the media.
Lamella plate separators
Lamella plate separators are also utilized to remove suspended solids. In this case, pre-treatment is always necessary in the form of coagulation and flocculation. Once the water has been pre-treated it flows into the bottom of a vessel fitted with lamella plates. The lamella plates are installed at an angle in a stacked formation. As the pre-treated suspended solids attempt to rise to the surface encounter the tilted lamella plates and settle tothe bottom. Typical clarification rates with lamella separators are in the range of 4gpm/ft2 to 8 gpm/ft2. The solids are periodically removed from the bottom of the vessel, dewatered and disposed.
Micro-sand ballasted separator
This process represents a Veolia Water proprietary technology, ACTIFLO®. The design of this technology greatly enhances the settling rate of suspended solids. The ACTIFLO®technology is based on a ballasted clarification process, as depicted in Table 1.
Under this approach, suspended solids are "literally" attached to a "small rock" (microsand), with a specific gravity of 2.65. Consequently, the suspended solids settle at a velocity rate that is magnitudes faster than that of conventional clarification.