The short answer
Aeration
Aeration in water treatment brings water and air into contact to remove dissolved gases and to oxidize dissolved iron and manganese into particles that filtration can remove. Cascade, tray, and spray aerators are passive and need only flow and level control; packed tower air strippers and diffused aeration use blowers and need air flow control, an interlock that stops water when air is lost, and monitoring of the pressure drop across the packing. Stripping carbon dioxide raises pH, which changes corrosion control and disinfection downstream, and oxidizing iron and manganese needs enough dissolved oxygen and contact time, so dissolved oxygen, pH, and the air-to-water ratio are the process variables that matter.
Key points
- Two jobs: strip gases out, put oxygen in. Both need contact between air and water; the equipment sets how much control is involved.
- Passive aerators need flow and level; blowers need air flow control, an air-loss interlock, and packing pressure drop monitoring.
- Air-to-water ratio is the design variable for stripping; hold it as the water flow changes.
- Stripping carbon dioxide raises pH; plan the corrosion control and disinfection chemistry around the aerated water.
- Iron and manganese oxidation needs dissolved oxygen, time, and often pH; the filter downstream does the removal.
- Fouling of packing and trays by iron and biology shows up as rising pressure drop and falling performance.
Why water is aerated
- Carbon dioxide
- Groundwater often carries dissolved carbon dioxide that lowers pH and makes the water corrosive and hungry for lime or caustic. Stripping it raises pH and cuts chemical use.
- Hydrogen sulfide
- The rotten egg odor in some groundwater; stripped by aeration, though oxidation and chlorination may be needed to finish it and the off-gas needs to go somewhere.
- Radon and volatile organics
- Regulated contaminants removed by air stripping in packed towers with high air-to-water ratios; the off-gas may need treatment.
- Methane
- In some wells; stripped to prevent accumulation in structures.
- Iron and manganese
- Dissolved as reduced ions; oxygen from aeration oxidizes iron readily and manganese slowly, forming particles that filters remove. Manganese usually needs a chemical oxidant or a catalytic filter media as well.
- Taste and odor
- Some volatile taste and odor compounds are reduced by aeration; many are not.
Equipment and what it needs
| Type | How it works | Control and instrumentation |
|---|---|---|
| Cascade or step aerator | Water falls over steps or weirs in open air | Flow only; level in the receiving basin; freeze and algae considerations |
| Tray aerator | Water drips through stacked perforated trays, often with coke or media | Flow; tray fouling by iron and biology; blower where forced draft |
| Spray aerator | Nozzles spray water into the air over a basin | Flow and nozzle pressure; nozzle plugging; basin level |
| Packed tower air stripper | Water flows down over packing, air is blown up countercurrent | Blower control, air flow, tower pressure drop, sump level, water-air interlock, off-gas |
| Diffused aeration | Air bubbled through diffusers in a tank | Blower control, air flow, dissolved oxygen, diffuser back pressure |
| Induced draft aerator | A fan draws air through a cascade or tray unit in a housing | Fan status and interlock; freeze protection |
Air-to-water ratio
For stripping, the design quantity is the volume of air per volume of water, and it can range from a few to one for carbon dioxide to tens to one for volatile organics and radon. The blower is sized for the maximum water flow at the design ratio, and the control holds the ratio as water flow changes: a drive on the blower, or an inlet damper, controlled from an air flow measurement with a setpoint computed from the water flow. Running the full air flow at low water flow wastes energy and can flood or channel the packing; running too little air fails the treatment. The tower pressure drop across the packing, measured with a differential pressure transmitter, rises as the packing fouls with iron deposits or biological growth, and it is the maintenance indicator.
Interlocks
- No water without air on a stripper: if the blower stops, the feed pump stops or the water is diverted, because water leaving a stripper that is not stripping is untreated water going to the clearwell.
- No air without water where the blower could overheat, on some designs.
- Sump or basin level: high stops the feed, low stops the transfer pump.
- Tower differential pressure high: alarm, then a reduced flow limit, then shutdown at the value the manufacturer gives.
- Blower discharge temperature and vibration on larger blowers.
- Off-gas treatment status where the stripper has one; a permit condition on some volatile organic installations.
What happens downstream
Aerated water is different water. Stripping carbon dioxide raises pH, sometimes by a full unit, which changes the dose of any pH adjustment chemical, the effectiveness of chlorine, and the corrosion behavior in the distribution system. Oxygen added to groundwater that had none makes it more corrosive to iron mains and turns dissolved iron into particles that stain everything they touch until the filters remove them. Design the chemistry downstream for the aerated water, measure pH and dissolved oxygen after the aerator, and give the filters the detention time the oxidation reactions need. Manganese in particular is slow to oxidize with air alone and is usually handled with a chemical oxidant, a raised pH, or a catalytic media after the aerator.
Frequently asked questions
- Is aeration enough to remove iron and manganese?
- For iron in most groundwater, aeration followed by detention and filtration works. Manganese oxidizes slowly with air unless the pH is high, and most plants use permanganate, chlorine, or a catalytic filter media to finish it. Aeration is the first step, not the whole process.
- How much does aeration raise pH?
- It depends on how much carbon dioxide the water carried; groundwater with a pH of 6.5 from dissolved carbon dioxide can come out of an aerator near 7.5 or higher. Measure it after the aerator and treat the aerated pH as the starting point for corrosion control.
- What is the off-gas concern on a stripper?
- Whatever was stripped is now in the air: hydrogen sulfide is an odor and a corrosion problem near the tower, radon and volatile organics may be regulated in the discharge, and methane is a safety issue in enclosed spaces. The air permit and the site layout decide whether the off-gas needs treatment or just a stack.
- Can a stripper run at reduced water flow?
- Yes, within the turndown of the packing and the distributor, with the air flow reduced to hold the ratio. Below the minimum wetting rate the packing channels and treatment falls off; the manufacturer gives the minimum, and the flow controller should not go below it.
Related topics
- FiltrationControlling gravity and pressure filters: effluent rate and level control, head loss and turbidity on every filter, the backwash triggers and sequence, filter-to-waste and ripening, the turbidity rules that filter monitoring answers to, and the operator view.
- WellsOperating a wellfield as a water source: selecting and rotating wells, pump-to-waste on start, drawdown and specific capacity, wellhead chemical feed, blending for nitrate or arsenic, the telemetry a well needs, and the interlocks that protect the pump.
- Chemical FeedChemical feed from the control side: the chemicals and where they go, metering pumps and dry feeders, flow pacing with residual trim, the dose arithmetic, day tanks and drawdown tests, loss-of-feed detection, interlocks, and overfeed protection.
- Dissolved Oxygen MeasurementOptical and membrane DO sensors for aeration control: how each measures, why optical has taken over, air calibration with pressure and salinity corrections, sensor placement in an aeration basin, fouling and cleaning, and the response time that limits the control loop.
- pH MeasurementGlass electrodes, reference junctions, and why pH is the measurement that drifts: how the sensor works, temperature compensation, two-point buffer calibration, mounting rules, the failure modes of the reference, and using the signal for chemical feed control.
- Differential PressureThe differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.