Activated Carbon Filter Sizing Calculator

EBCT (Empty Bed Contact Time) is the carbon bed volume divided by the flow through it — the time water is in contact with the GAC. Design 3–5 minutes for dechlorination and simple taste-and-odour polishing, 5–10 minutes for organics and colour, and up to 12–15 minutes for demanding COD or potable duties.

Divide the design flow by the chosen service velocity (typically 8–15 m/hr) to get the required bed area, then compute the diameter as D = √(4A/π) and round up to the next standard vessel size. A larger diameter lowers velocity and raises contact time; a smaller one does the opposite.

From the bottom up: coarse pebbles, then gravel, then a graded silica-sand support layer, then the granular activated carbon working bed, with freeboard above for backwash expansion. The support layers stop carbon and fines from reaching the under-drain nozzles while distributing flow evenly across the bed.

Allow at least 40% of the GAC bed depth as freeboard, and never less than about 300 mm. Freeboard is the empty space above the carbon that lets the bed expand during backwash without carrying fines and carbon over the outlet. Too little freeboard means carbon loss and turbidity breakthrough after every wash.

Backwash coconut-shell GAC at roughly 10–12 m/hr for 8–12 minutes — enough to expand the bed 20–40% and lift out trapped solids and fines without washing the carbon out. Follow with a short settle and rinse before returning to service. Warm water expands the bed more, so trim the rate seasonally.

GAC is a consumable that exhausts as its pores fill. Typical replacement is every 12–24 months, but it depends entirely on duty: dechlorination beds last longest, while heavy organics or colour loads can exhaust carbon in months. Track outlet quality, pressure drop or periodic iodine-value checks rather than a fixed calendar.

Yes — choose the Potable water polishing duty, which applies a conservative velocity and longer EBCT, and specify a high-iodine coconut-shell GAC. The vessel, media schedule and BOQ are a sound preliminary basis, but potable systems also need certified media, disinfection and regulatory sign-off that sit outside a sizing tool.

After. A pressure sand or multigrade filter goes first to strip suspended solids and turbidity; the activated carbon filter then polishes the clarified water, removing chlorine, colour, odour and dissolved organics. Feeding turbid water straight onto carbon blinds the bed, raises pressure drop and wastes expensive GAC on a duty sand does cheaply.