Commercial Activated Carbon Filter — Chlorine, Taste & Odour Removal

Activated carbon is a porous adsorbent made from coal, coconut shell, wood or nutshell. Its internal surface area is very large — typically 500–1,500 m² per gram — and dissolved molecules attach to that surface as water passes through the bed. A commercial carbon filter removes free chlorine, chloramine (more slowly), taste and odour compounds, colour, humic substances, some pesticides and solvents, and ozone. It does not remove dissolved salts, hardness, nitrate, fluoride or microorganisms.

Carbon filtration is a polishing and protection stage, not a complete treatment. It is placed after sand filtration, because turbidity blinds the carbon and shortens its life, and before softeners, RO membranes or UV units, because it removes the oxidants that damage them. In drinking-water plants it improves taste and removes the chlorine used upstream; in process water it protects membranes, ion exchange resin and stainless steel surfaces.

Carbon is not designed as a particle filter. Turbidity, rust and precipitated iron coat the grains and block the pores, so a sand filter or a cartridge precedes the bed in most designs. Carbon also supports bacterial growth once it has adsorbed organics, which is why backwashing, disinfection and a downstream barrier are part of a drinking-water design.

Granular activated carbon (GAC) is held in a pressure vessel and treated water flows through it. It is the standard choice for continuous dechlorination, taste and odour control, and RO pretreatment, because the bed can be backwashed and the carbon replaced independently of the rest of the plant. Powdered activated carbon (PAC) is dosed as a slurry into the water, mixed, and removed later by coagulation, sedimentation or filtration.

PAC suits seasonal problems — an algal bloom, a temporary taste event — because it can be added only when needed and does not require a vessel. It is less efficient per kilogram, produces sludge and cannot be regenerated on site. Carbon block cartridges are a third option for small flows, point-of-use units and polishing after a GAC bed; they are not practical for large plants because of replacement frequency and pressure drop.

Polyamide RO membranes are attacked by free chlorine and other oxidants. The damage is not reversible: salt rejection falls and the element must be replaced. Most membrane manufacturers set a maximum of 0.1 ppm free chlorine at the inlet, and many specify zero. A carbon filter ahead of the RO removes the chlorine from municipal water; sodium bisulphite dosing is the alternative where carbon is impractical or where chloramine is present at high concentration.

Carbon also protects the RO from organic fouling. Humic acids, oils and some surfactants adsorb on the membrane surface and raise differential pressure; removing them upstream lowers SDI and extends cleaning intervals. The carbon must be followed by a 5 µm cartridge filter to catch carbon fines. Monitor free chlorine or ORP after the carbon bed, not before it, and replace the carbon on breakthrough rather than on a fixed date alone.

Carbon beds work over a wide pH range but adsorb most effectively at near-neutral pH; very high or low pH changes the charge of the carbon surface and the speciation of the contaminant. Water temperature affects kinetics, and cold water needs a longer contact time. Turbidity should be low — a sand filter or cartridge ahead of the carbon is standard — because particles coat the carbon and create a food source for bacteria. Stainless steel or FRP vessels are used, with a gravel support layer and a distribution system that avoids channelling.

Design the vessel for the backwash flow, not only the service flow, and leave enough freeboard for bed expansion. Provide sample taps before and after the bed, a differential-pressure gauge and a drain sized for backwash. For RO pretreatment, the carbon outlet should be monitored continuously for free chlorine or ORP, with an alarm and an automatic shutdown if the setpoint is exceeded.