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Filtration Processes

One of the last processes in treating wastewater is filtration. Occurring in tertiary treatment, all kinds of filtration methods are employed to remove suspended solids that remain after primary and secondary treatment, and also bring the water to the desired quality of effluent. Not all wastewater treatment facilities require tertiary treatment, but for those that do have strict water quality standards, filtration is imperative. 


Sand Filtration requires no equipment other than a bed of coarse sand and a layer of gravel at the bottom. As water trickles through the sand, particles become trapped in the sand and the water comes out cleaner than when it entered. This method uses gravity, and sometimes added pressure to cause the water to move through the bed of sand. 



Activated Carbon Filtration uses activated carbon filters to remove fine particles, chemicals, and odors from the water running through it. These filters use adsorption to attract chemicals and particles which latch onto the activated carbon. They typically use multiple layers to ensure the removal of contaminants meets regulations. Every once in a while, the filters must be replaced to maintain efficiency. 


Membrane Filtration takes one more step up using porous materials with tiny permeable membranes of different sizes to filter the water in order to perform high level filtration. The sizes of these pores depend on one of these processes: microfiltration, ultrafiltration, and nanofiltration.


Micro uses membranes with pores of 0.1-10 microns (μm) in size

Ultra uses membranes with pores of 0.01 - 0.1 μm in size

Nano uses membranes with pores of 0.001 - 0.01 μm in size. 


Each type is utilized for different applications such as dealing with suspended solids in microfiltration to dissolved solids and ions in nanofiltration. 


Another process used in membrane filtration is reverse osmosis. Rather than water naturally diffusing from high to low concentration to achieve equilibrium, the application of osmotic pressure is used to force water molecules through a semipermeable membrane while blocking other particles to ensure water of high quality that can be reused or discharged safely. Membrane filtration provides some of the best solutions for filtering wastewater as the resulting effluent is of the highest quality of treated wastewater. 


Finally, Multimedia Filtration uses a combination of media to achieve exceptional filtration. This filtration system uses many different types of media such as sand, activated carbon, and anthracite (hard coal) in a specific sequence to filter wastewater. As the water percolates through each different media, the unwanted particles and qualities of the wastewater are removed and the result is a treated water, free of contaminants and discoloration. This specific filtration process is able to handle a diverse range of wastewater making it a desirable choice for many treatment facilities. 


By understanding the specific mechanisms, designs, operations, and applications of these filtration processes, wastewater treatment facilities can effectively implement tertiary treatment strategies to achieve desired effluent quality objectives. 

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