The process of reducing the content of Total Suspended Solids (TSS) in water by coagulation-flocculation-sedimentation is often not sufficient. Especially if the desired outlet TSS content is less than 10 mg/l, while there are still TSS particles that are not deposited in the clarifier so that the TSS is still high. Then further treatment is needed, namely Filtration. There are 2 kinds of filtration, namely media filter which uses a media bed in the vessel and ultrafiltration which uses membrane system.
In this article, we will discuss about Media Filters. Media Filters in general can filter out particles measuring 5-10 microns (visible particle size is > 40 microns). The flow of water in the media filter is from top to bottom (downflow) through the inlet flow distributor so that it is evenly distributed to the surface of the media. At the bottom of the media, there are strainers with certain slot size that are installed evenly so that clean water that has passed through the media can flow out.
Based on the flow driver, some media filters use gravity and some are pressurized. Gravity media filters are usually used for water treatment installations with very large capacities such as in communal WTPs and using civil/concrete buildings. Meanwhile, pressurized media filters are commonly used for industrial and household scales by using pressure vessels made of FRP or steel material.
Based on the type of media, media filters are divided into sand filter, activated carbon filter and iron filter. Sand filter (SF) uses sand silica and gravel media, aiming to capture TSS and turbidity. Activated carbon filter (ACF) uses activated carbon media, aiming to capture organic substances, colors and odors. Iron Filter (IF) uses manganese greensand media to capture iron and/or manganese content.
Then how to determine the size of the Media Filter? In general, the hydraulic loading rate (HLR) or linear velocity (LV) of the filter media is 10-25 m3/(m2.hour) or 10-25 m/hour. For example, for river water flow rate of 100 m3/hour and LV 18 m/hour, the required filter area = 100/18 = 5.55m2, because the filter is cylindrical, the filter diameter = 2700 mm.
But of course HLR or LV is not the only determinant of filter size. For activated carbon filter we have to consider empty bed contact time (EBCT) first. EBCT is the minimum contact time required for water in activated carbon media so that the adsorption process of impurities into the carbon pores can take place optimally. Usually the EBCT range is 6 – 20 minutes. So for a flow rate of 100 m3/hour, EBCT of 6 minutes, then the required volume of carbon = (100 / 60) x 6 = 10m3. If the maximum bed depth of the media is 1.5m, then the ACF diameter = 2900 mm.
Can we use the media filter continuously? When the impurities have accumulated in the filter media, which is marked by a pressure drop of almost 1 bar, then it is time for backwashing, by flowing water from the bottom up and sometimes having to be assisted by scouring with air, especially for sand filters because this media is quite heavy. In order not to backwash too often, try to limit the water entering the filter to a maximum load of 50 mg/l TSS content. More than that, it is certain that the media filter will often ‘block’.
If you have any questions about Media Filters, please contact our engineering team. Hopefully useful, see you in our next article.