Design Manual › Tertiary & Reuse › Activated Carbon
FREE ENGINEERING TOOL · TERTIARY
Granular Activated Carbon Adsorber
GAC is the simplest way to take the last slice of dissolved organic matter out of a
secondary effluent — and the easiest unit to get wrong, because the two numbers that matter are not the
ones on the vessel drawing. This tool sizes the bed from empty bed contact time, derives the loading
rate the geometry actually produces, then turns the COD you remove into a carbon usage rate, a bed life
and an annual tonnage.
1Duty — what has to come out
Carbon is consumed by the mass it adsorbs. The more you ask it to remove, the more it
costs — so the influent and target concentrations are the whole economic question.
2Contact time & vessel geometry
Empty bed contact time is the design intent; the loading rate is what the geometry
then gives you. Choose the EBCT, choose the bed depth, and the vessel diameter follows — along with
the velocity, which is the number that actually has a limit.
3Carbon & backwash
Adsorption capacity is the number most often taken from a datasheet and applied to a
completely different water. Measure it on your own effluent if the tonnage matters.
Results — bed, carbon & run time
Carbon bed volume
—m³
Vessel diameter
—m
Carbon usage rate
—g/m³
| Item | Value | Basis |
|---|
4Vessel & hydraulics
The geometry check. EBCT is what you asked for; the loading rate is what you got.
| Item | Value | Basis |
|---|
5Carbon consumption & bed life
This is the operating cost line. Bed life is what tells you whether the plant is
maintainable or whether you will be changing carbon every fortnight.
| Item | Value | Basis |
|---|
6Backwash & head loss
GAC beds collect biomass and solids. Clean-bed head loss is small; the fouled bed is
what the structure and the backwash system have to survive.
| Item | Value | Basis |
|---|
GAC vs PAC vs BAC
| Item | GAC | PAC |
|---|
7Instrument schedule
The one measurement that matters is the one that tells you the bed is exhausted before
the effluent does. Differential pressure and effluent turbidity between them catch most of it.
| Point | Instrument | Why here |
|---|
Quote the adsorber instruments at factory price
Flow, differential pressure, level and effluent turbidity — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
Request a quote for this duty ← Ozone Disinfection Reverse Osmosis for Water Reuse →Response within 24 hours · Inputs stay in your browser — nothing is uploaded.
Sources & parameter basis
| [1] | METCALF & EDDY / AECOM — tertiary adsorption: secondary effluent COD typically 20–60 mg/L; granular carbon can polish to 10–20 mg/L. The mass removed sets the carbon consumption. |
| [2] | Empty bed contact time: 20–40 min for COD and colour polishing, 10–20 min for trace organic (micropollutant) control, 10–20 min ahead of RO as a membrane protection step. More contact time is not free — it buys vessel volume. |
| [3] | Loading rate 5–15 m/h (2–6 gpm/ft²); bed depth 1.5–3.5 m; two or three vessels in series in lead-lag operation so the lead vessel is fully exhausted before it is taken out of service. EBCT = n·H/v. |
| [4] | Adsorption capacity for wastewater dissolved organics at low concentration is typically 0.05–0.40 kg COD per kg carbon — an order of magnitude below the values quoted for high-strength industrial wastes. Measure it on your own water. |
| [5] | Regeneration: thermal reactivation loses 5–12 % of the carbon per cycle as fines and burn-off. On-site regeneration is normally justified above roughly 50–100 t/yr of spent carbon. |
| [6] | Backwash 20–40 m/h for 10–20 min to release trapped biomass and solids. Clean-bed head loss ≈0.35 m per metre of bed at 10 m/h, scaling with velocity; design allowance 2–3 m before backwash or changeout. |
Full derivation and worked example: Activated-Carbon-Calculation-Sheet.md. Drop the
capacity from 0.20 to 0.05 kg/kg and watch the annual tonnage quadruple — that single input is worth
more measurement effort than every other number on the page.
Questions about a duty, a specification, or transparent sourcing? Contact our application engineers →