Filtration (Water & Wastewater Treatment Systems, WWTS_CAMIX Vietnam)
DESCRIPTION
Filtration is the process of removing solids from a fluid by passing it through a porous medium. Coarse, medium, and fine porous media have been used depending on the requirement. The filter media are artificial membranes, nets, sand filter, and high technological filter systems. The choice of filters depends on the required filtering speed and the cleanness requirement. The flow required for filtration can be achieved using gravity or pressure. In pressure filtration, one side of the filter medium is at higher pressure than that of the other so that the filter plane has a pressure drop. Some portion of this filter type must be enclosed in a container.
The process of removing the clogged portion of the filter bed by reversing the flow through the bed and washing out the solid is called back washing. During this process, the solid must be removed out of the system, but otherwise the filters must be either replaced or taken out of service to be cleaned.
Filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass.
Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size and filter thickness).
The main types of filtration used in wastewater treatment are:
- Fast sand filters
Sand filters are used mainly to decrease the suspended solids concentration. They are quite versatile because they can be used in any treatment plant where a limpid effluent with a low presence of organic substances and suspended matter is required. The filter media is made of anthracite or quartz sand layers.
Water impurities are captured by the filter media through a physical process. Air and water backwash is used in order to clean the filter after fouling due to the accumulation of solids; the filtration cycle depends on the inlet suspended solids concentration.
- Activated carbon filters
In many cases, the industrial wastewater needs advanced treatments in order to remove the pollutants properly. Activated carbon is considered as one of the most versatile adsorbents, thanks to its elevated specific surface and the wide variety of substances it is capable to adsorb.
Commercially, it can be found in granular form (GAC) or in powder (PAC). The most common installation foresees gravity filters and has a very high purification yield of the polluted solution. Activated carbon can be also used to clean gas flows and to remove odors and organic compounds.
Secondary treatment processes are highly effective in reducing the BOD in wastewater. However, the secondary clarifiers used to settle out microorganisms in the secondary treatment process are not totally effective. Some of these microorganisms remain in the wastewater after it leaves the secondary clarifier, and they add BOD since the decay of these microorganisms will exert its own oxygen demand.
Sand filters, similar to those used for producing drinking water, are sometimes used for additional removal of microorganisms and other solids. Unlike water treatment sand filters, however, the filters used in wastewater treatment often use large, lightweight aggregates (such as coal) at the top to improve efficiency and facilitate cleaning. Partially treated wastewater usually contains higher concentrations of solids than does water in a water treatment plant, so these filters must be designed for greater efficiency and for more frequent cleaning.
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