13.3 Aeration, Air Stripping & Activated Carbon Adsorption (Subclass 10)
Key Takeaways
- Air stripping transfers volatile contaminants from water into air and works only for compounds with a high Henry law constant, such as trichloroethylene, tetrachloroethylene, benzene and radon.
- Packed tower efficiency is controlled by the air-to-water ratio, packing depth and water temperature, and cold Pennsylvania groundwater strips less readily than warm water.
- Granular activated carbon performance is governed by empty bed contact time, typically 10 to 20 minutes for organics removal, and by the strength with which the target compound adsorbs.
- Carbon reaches breakthrough when the adsorption front exits the bed, so exhausted carbon must be replaced or thermally reactivated rather than backwashed clean.
- Powdered activated carbon is fed ahead of clarification for short-duration taste and odor or spill events, must contact the water before chlorine is applied, and is removed with the sludge.
Two Processes, Two Different Physics
Subclassification 10 combines aeration and activated carbon adsorption because together they cover the dissolved contaminants that filtration cannot touch. They work by opposite mechanisms:
- Aeration and air stripping move a contaminant out of the water and into the air. The contaminant must be volatile.
- Activated carbon adsorption holds the contaminant on a solid surface. The contaminant must be adsorbable, which generally means organic, higher molecular weight and poorly soluble.
Choosing the wrong one is the most consequential error in this subclass. Air stripping will not remove geosmin; carbon will not remove radon economically.
Aeration Equipment
| Type | Description | Typical use |
|---|---|---|
| Packed tower (air stripper) | Countercurrent tower filled with plastic packing; water flows down, forced air flows up | Volatile organic compounds, radon; the highest-efficiency option |
| Diffused aeration | Air blown through diffusers in a basin | Oxidation of iron and manganese, carbon dioxide removal, modest stripping |
| Tray aerator | Water cascades over stacked slat or coke trays | Iron oxidation, hydrogen sulfide and carbon dioxide removal at small systems |
| Spray aerator | Nozzles spray water into the air | Simple oxidation and gas exchange; freezes readily in Pennsylvania winters |
| Cascade or step aerator | Water falls over weirs | Carbon dioxide removal, post-treatment oxygenation |
Packed Tower Air Stripping
Removal depends on the air-to-water ratio, the packing depth and the compound's volatility. Practical operating ranges are air-to-water volumetric ratios of roughly 30 to 1 up to 100 to 1 for chlorinated solvents. Three operator realities follow:
- Temperature governs performance. Volatility falls as water cools, so a tower that easily meets an MCL in August may struggle at 48 degree F groundwater in February. Winter is when compliance samples fail.
- Fouling changes everything. Iron, manganese and calcium carbonate precipitate on the packing when aeration raises pH and oxidizes metals, choking airflow and destroying transfer. Sequestration, acid cleaning or metals pretreatment upstream is the control.
- Off-gas may be regulated. Air discharge from a tower stripping volatile organics can require air permitting or vapor-phase carbon treatment, and the tower must be located so the plume does not enter occupied spaces.
Because a tower aerates the water thoroughly, it also strips carbon dioxide and raises pH, and saturates the water with oxygen. Both effects change corrosion behavior downstream, so a stripper installation almost always forces a corrosion control review. Towers also expose finished water to open air, so a post-tower disinfection and covered clearwell arrangement is required.
Granular Activated Carbon
GAC is a bed of activated carbon, usually bituminous coal based, placed in contactors either as a post-filter adsorber or as filter media replacing anthracite in a filter-adsorber arrangement.
- Empty bed contact time (EBCT) is the bed volume divided by the flow rate. Typical values are 10 to 20 minutes for organics and disinfection byproduct precursor control, and shorter for taste and odor polishing. Longer EBCT means longer bed life.
- Adsorption strength varies enormously. Large, poorly soluble organic molecules adsorb strongly; small polar molecules do not. This is why GAC handles solvents, pesticides, taste and odor compounds and many per- and polyfluoroalkyl substances well, and handles nitrate and radon poorly.
- Breakthrough occurs when the adsorption front reaches the bottom of the bed and target compounds appear in the effluent. Carbon is then replaced or thermally reactivated off site. Backwashing a GAC bed removes particulates and reclassifies the media, but it does not restore adsorptive capacity.
- Biological activity. In a bed operated after ozone, bacteria colonize the carbon and consume biodegradable organic matter, which is desirable and is called biologically active carbon. It also means the bed sheds bacteria, so the carbon contactor must sit ahead of final disinfection.
Powdered Activated Carbon
PAC is fed as a slurry for episodic problems: a seasonal taste and odor event driven by geosmin and 2-methylisoborneol from an algae bloom, or a spill upstream of the intake.
- Doses commonly range from 1 to 20 mg/L, occasionally higher during severe events.
- Feed as early as possible to maximize contact time, typically at the intake or rapid mix.
- Feed before chlorine, because chlorine reacts with and consumes the carbon and the carbon exerts a large chlorine demand.
- PAC is removed with the settled sludge and filter backwash, so a high dose increases residuals volume and can carry over into filters if clarification is weak.
- PAC dust is a nuisance and a housekeeping hazard, and wet PAC is extremely slippery.
Choosing Between Them
| Contaminant | Preferred process |
|---|---|
| Trichloroethylene, tetrachloroethylene, benzene | Air stripping, with vapor-phase carbon if off-gas is regulated |
| Radon | Air stripping (or GAC in special cases with radioactivity handling concerns) |
| Geosmin and 2-methylisoborneol | Powdered or granular activated carbon; not strippable |
| Disinfection byproduct precursors | Granular activated carbon, enhanced coagulation, or membranes |
| Per- and polyfluoroalkyl substances | Granular activated carbon, ion exchange or reverse osmosis; not strippable |
| Hydrogen sulfide and carbon dioxide | Aeration |
A groundwater system uses a packed tower to strip trichloroethylene and meets the MCL every summer, but effluent concentrations rise above the limit each January. What explains the seasonal pattern?
A granular activated carbon contactor begins passing the target organic compound after eighteen months of service. What is the correct response?
An algae bloom produces a strong earthy taste and odor from geosmin at a surface water plant. Which treatment is appropriate, and where should it be applied?