Multi-media Filter and Activated Carbon Filter

Media Filter

Multi-media filter is used in water treatment plant to remove suspended solids like silt, clay, organic matter, algae and microorganisms from the process water. The suspended solids particles that can be removed is as low as 10-40 microns. When coagulant is used to the feed, it can reduce until 5-10 microns. As a result, the turbidity level in the treated water can be reduced.

A typical multi-media filter setup contains three layers of filling; anthracite, quartz sand and garnet/gravel. These materials are graded according to its specific density within a pressurized vessel. Depending on the type of water to be treated, the velocity applied usually in the range of 6-15 m3/h.m2.

Over the time as the turbidity removed from the feed water, the filter will experience high pressure drop. At this point, the backwashing through the media filter should be performed.

Application:

  • Treatment of feed water prior to RO to reduce SDI index
  • Drinking water
  • Wastewater treatment
  • Side stream filtration of cooling tower
  • Boiler feed water

Activated Carbon

Activated carbon is a highly porous carbonaceous medium that attracts and holds organic chemicals inside it. The process of making activated is by heating the base materials like coconut shell, wood, peat, lignite, coal to a red heat (<700oC) in a retort to drive off the hydrocarbon but with an insufficient supply of oxygen. This type of combustion process is essentially a pyrolysis process. After that, to activate the carbon, the char particles are exposed to oxidizing gas such as steam or CO2 at superheated temperature (800-900oC). The gasses develop a porous structure in the char, hence created a great internal surface area (700-1300 m2/g). The large surface area of these pores will actively attract organic chemicals when it is used to treat liquid or air.

Granular Activated Carbon

Forms of activated carbon

In general there are 2 forms of activated carbon with different adsorption capacity. Those are Powdered Activated Carbon (PAC) with typically has a diameter of less than 0.074 mm (200 sieve), and Granulated Activated Carbon (GAC), which has a diameter greater than 0.1 mm ( 140 sieve). Lately there also pelletized form of activated carbon.

GAC is most widely used in water treatment plants for removing a variety of organic chemicals, chlorines as well as unpleasant taste and odour that normally cannot be removed using other physical treatments like water softeners or filtration.

Type of organic chemicals that can be adsorbed on activated carbon:

Readily adsorbed organics

Aromatic solventsi.e. benzene, toluene, nitrobenzene
Chlorinated aromatics i.e PCBs, chlorophenols
Polycyclic aromatic hydrocarbonsi.e acenapthene, benzo(α)pyrene
Pesticides and herbicidesi.e. trichloroethene, carbon tetrachloride, bromoform, chloroalkyl ethers
High molecular-weight hydrocarbonsi.e. dyes, gasoline and jet fuel components, amines, some humics

Slowly adsorbed organics

Low-MW ketones, acids, can aldehydes
Sugars and starches
Very-high-MW or colloidal organics
Low-MW aliphatics

In the water treatment application, the carbon media is placed in a column contactor. The contaminated water is pumped through a bed of activated carbon media with certain flow rate so then minimum contact time can be achieved. Sufficient contact time is required to allow the adsorption process for contaminants removal.

Application:

  • Drinking water (water treatment)
  • Treated wastewater for reuse (i.e. removal of VOC’s)