Multimedia filtrations

Multimedia filtration (also called prefiltration, sand filtration or multilayered filtration) is mainly aimed at removing sediments and suspended matter. Suspended contaminants are trapped in small crevices and, as a result, water turbidity is improved. A number of media are distinctly layered with the coarsest on top so the suspended matter is collected throughout the depth of the filter according to size. 

The filter beds need to be backwashed periodically as the back pressure increases however, backwashing removes the filter from use. To avoid downtime, often a dual filter bed system is installed. 

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In this context it is the separation of solids from water by forcing the water through a porous filter media. The objective is typically to reduce the level of TDS in the water and often to reduce both the size of the particle remaining and the turbidity of the water. Filtration efficiency and quality is a function of many variable factors, although filtration is usually carried out at relatively low velocities, where velocity and pressure drop are directly related to each other. Typically a sand filter will remove a high percentage of particles above a diameter of 20 to 30 pm, whereas dual or multimedia filtration is required to remove particles down to a diameter of 10 to 20... 

Treatment of selected waste streams by activated carbon, ion exchange, electromembranes, chemical coagulation, sand, and dual and multimedia filtration. 

Multimedia Filtration Turbidity, Suspended solids down to 2-10 microns, SDI... 

Media filtration this step includes multimedia filtration or high-efficiency filtration, and reduces the concentrations of suspended solids, turbidity, and SDI. 

One of the possible configurations for the pharmaceutical water purification system is shown in Fig. 3. The first three stages in that system (multimedia filtration, softening, and carbon filtration) are considered pretreatment steps for the RO unit that conducts final purification. The cartridge filter shown before the RO is provided to protect the membranes from the fine carbon particles. There are many variations of the treatment systems sometimes carbon filter is placed before a softener, sometimes it is eliminated completely. 

After the break tank, process water is treated using various equipment and technologies depending on its intended use and the water analysis. Some of the technologies are multimedia filtration, water softening, activated carbon adsorption, UV treatment, deionization, ultrafiltration, reverse osmosis, final filtration and distillation. 

The water used for preparing parenteral solutions needs to meet USP requirements for water for injection. These requirements include bacterial endotoxins no greater than 0.25 eu/mL, total organic content less than 500 ppb, and a conductivity of 1.3 iS/cm. Water for injection is prepared by distillation or a two-stage RO process. A typical process to produce water for injection consists of multimedia filtration, softener, activated carbon, 1-5 qm preflltration, UV treatment, two-stage RO, mixed DI, UV, and sterile filtration. 

UF and MF membranes alone do not remove total organic carbon (TOC) and tri-halomethane (THM) precursors [13]. Hence, pre-treatment entails addition of chemicals such as coagulants (FeCl3 or FeS04), alum or polyaluminium chloride to increase the size of suspended solids and colloidal particles and thus prevent or minimise colloidal, organic, and/or biological fouling. In the case of seawater desalination, membrane filtration has proven to be superior to multimedia filtration [14,15]. 

The process sequence employed to produce WPU grade water in the pharmaceutical industry typically comprises the following steps multimedia filtration, water softening, activated carbon filtration, prefiltration, UV disinfection, reverse osmosis, continuous electrodeionization (CEDI). For WFI, as mentioned earlier, the WPU water is subjected to a distillation process, and may be supplied hot if needed. 

Figure 7.1 shows the two major treatment options to obtain RO-quality water from sewage and seawater. The key in water reclamation is to first treat the sewage biologically and use MF/UF membrane filtration to remove suspended solids. Two membrane filtration alternatives are available for water reclamation tertiary filtration (TF) of the effluent from a conventional activated sludge (CAS) process and an integrated membrane bioreactor (MBR). For seawater desalination, pretreatment must be provided if the source is open seawater. The current practice involves multimedia filtration, but membrane filtration has also been considered. 

Ultraflltration membranes have been considered as an alternative to multimedia filtration for seawater RO pretreatment in recent years. There have been pilot studies at numerous sites. The benefits of UF over media filtration pretreatment, lower fouling rate, has been demonstrated in long-term tests (2 years) by Jamaluddin et al. (1998). UF has been recently selected as pretreatment for a large-scale desahnation plants in Saudi Arabia, Japan, and China (Wolf et al., 2005). 

In the comparison presented above, no credit was taken for the better pretreatment provided by ultrafiltration. However, the potential benefits provided by ultrafillration, when compared to multimedia filtration include ... 

Coagulation Pretreatment FeCls dose mg/L No Membrane filtration 5 Multimedia filtration... 

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