3.4 Pretreatment: Aeration, Presedimentation & Algae Control
Key Takeaways
- Aeration adds oxygen and strips volatile compounds, so it simultaneously oxidizes iron and manganese and removes hydrogen sulfide, carbon dioxide, radon, and volatile organic compounds.
- Stripping carbon dioxide by aeration raises pH, which is often the goal in itself for corrosion control and for improving downstream oxidation.
- Presedimentation removes heavy grit and sand ahead of coagulation, protecting pumps and reducing solids load on the clarifiers during monsoon turbidity events.
- Powdered activated carbon must be applied early enough for contact time but not so early that coagulant competes for adsorption sites or the carbon is wasted.
- Preoxidation with chlorine ahead of coagulation creates disinfection byproducts because raw water organic carbon is at its highest, so permanganate or a delayed application point is often preferred.
3.4 Pretreatment: Aeration, Presedimentation & Algae Control
Pretreatment covers everything between the raw water intake and the rapid mix basin. Its purpose is to remove or convert constituents that would otherwise overload, foul, or defeat the main treatment train. The Need-to-Know Criteria list "source water treatment (e.g., algae control, aeration, mixing)" and "pretreatment" as elements operators must monitor, evaluate, and adjust.
Screening and Intake Protection
The first physical barrier:
- Trash racks: widely spaced bars excluding logs, branches, and large debris
- Bar screens: closer spacing for smaller material
- Traveling water screens: continuously or intermittently cleaned mesh belts, often spray-washed
- Microstrainers: fine mesh drums, historically used for algae removal
Operator tasks include monitoring differential head loss across screens — a rising differential means the screen is blinding and needs cleaning — and inspecting for mussel colonization on screen faces and guides.
Aeration
Aeration does two things simultaneously, and the exam expects you to know both directions of gas transfer:
Adds oxygen to the water. Strips out dissolved gases and volatile compounds.
What Aeration Accomplishes
| Objective | Mechanism | Notes |
|---|---|---|
| Oxidize iron and manganese | Adds O₂; Fe²⁺ → Fe³⁺, Mn²⁺ → Mn⁴⁺ | Iron oxidizes readily; manganese is slow below about pH 9 and usually needs a stronger oxidant |
| Remove hydrogen sulfide | Strips H₂S gas | Most effective at low pH, where sulfide exists as dissolved H₂S gas |
| Remove carbon dioxide | Strips CO₂ | Raises pH, reduces corrosivity, improves downstream oxidation |
| Remove volatile organic compounds | Strips VOCs | Packed tower aeration is the standard for trichloroethylene and similar solvents |
| Remove radon | Strips radon gas | Packed tower aeration is highly effective |
| Reduce taste and odor | Strips some volatile odor compounds | Does not remove geosmin or MIB, which are not sufficiently volatile |
[!IMPORTANT] Aeration raises pH by stripping carbonic acid, and that pH shift is frequently the operational goal, not a side effect. Raising pH improves manganese oxidation, supports corrosion control by moving water toward calcium carbonate saturation, and can reduce the caustic dose needed downstream. But the same pH increase works against hydrogen sulfide removal, since sulfide shifts from the strippable H₂S gas form to the non-volatile bisulfide ion as pH rises. When you must do both, strip sulfide first at low pH, then aerate for CO₂.
Aeration Equipment
| Type | Description | Best for |
|---|---|---|
| Cascade / multiple tray | Water falls over steps or trays, often with coke or ceramic media | Iron oxidation, CO₂ removal; simple and low maintenance |
| Spray aerator | Nozzles throw water into the air | CO₂ and H₂S removal; footprint and freezing concerns |
| Diffused air | Air blown through diffusers in a basin | Flexible; used in tanks and basins |
| Packed tower (air stripping) | Water trickles down packing against a rising air stream, high air-to-water ratio | VOC and radon removal; the highest-efficiency option |
| Mechanical surface aerator | Rotating impeller agitating the surface | Reservoirs, lagoons, destratification |
Packed tower design turns on the air-to-water ratio and the packing depth; higher ratios strip more volatile contaminant. Off-gas treatment may be required if the stripped contaminant is regulated as an air emission.
Reservoir Aeration and Destratification
Rather than treating the symptom at the plant, some utilities aerate the reservoir itself. Hypolimnetic aeration oxygenates the bottom layer without breaking stratification, keeping iron and manganese insoluble in the sediments and suppressing sulfide. Destratification (bubble plume or mechanical circulation) mixes the whole water column continuously, preventing the anoxic hypolimnion from forming and eliminating the turnover event entirely.
Presedimentation
Presedimentation removes heavy, readily settleable material before coagulation, typically in a basin or a grit chamber ahead of the plant.
Why it matters, especially in Arizona:
- Protects equipment: sand and grit abrade pump impellers, valve seats, and meters
- Reduces solids loading on the coagulation and clarification train during monsoon turbidity events
- Reduces coagulant demand, since settleable material removed by gravity never consumes chemical
- Stabilizes plant operation, damping the shock of a turbidity spike from hundreds to thousands of nephelometric turbidity units
Presedimentation basins may operate plain (gravity only) or chemically assisted with a low coagulant dose to capture finer material. Sludge accumulates and must be removed on a schedule; a basin allowed to fill with sediment loses detention time and stops working, which is a classic finding on plant inspections.
Preoxidation and Powdered Activated Carbon
Preoxidation
Applying an oxidant ahead of coagulation can oxidize iron and manganese, control algae and zebra or quagga mussels in intake piping, destroy taste and odor compounds, and improve coagulation.
| Oxidant | Strengths | Cautions |
|---|---|---|
| Chlorine | Inexpensive, effective, provides residual | Creates disinfection byproducts where raw water total organic carbon is highest; can lyse algal cells |
| Potassium permanganate | Excellent for iron and manganese, no chlorinated byproducts | Overdose produces pink water and manganese dioxide deposits; dose must be matched carefully |
| Ozone | Powerful; destroys geosmin and MIB; no chlorinated byproducts | Forms bromate if bromide is present; capital and energy intensive |
| Chlorine dioxide | Effective on manganese and taste and odor at high pH | Forms chlorite and chlorate; dose limited |
[!WARNING] Prechlorination at the intake was standard practice for decades and is now used cautiously. Raw water carries the highest concentration of natural organic matter in the plant, so chlorinating there maximizes trihalomethane and haloacetic acid formation. Many plants have moved the primary chlorine application point downstream of clarification, after coagulation has already removed much of the precursor material.
Powdered Activated Carbon
PAC is dosed as a slurry for episodic taste and odor and for organic contaminant spills. Application principles:
- Provide adequate contact time before the carbon is removed with the settled solids
- Apply before or at the rapid mix, but recognize that coagulant and PAC compete; adding PAC at the very same point as a heavy coagulant dose reduces adsorption efficiency
- Do not apply PAC downstream of the filters, where it has nowhere to go
- Typical doses range from a few milligrams per liter for mild events to tens of milligrams per liter for severe blooms; jar testing with the actual odor compound is the only reliable way to set the dose
- PAC adsorbs chlorine as well as odor compounds, so simultaneous application of PAC and chlorine wastes both
A groundwater source contains 2.5 mg/L of dissolved hydrogen sulfide and elevated carbon dioxide. The operator plans to aerate. What is the correct sequencing consideration?
An operator applies potassium permanganate ahead of the rapid mix for manganese oxidation, and customers begin reporting pink-tinted water. What has occurred?
Why do many utilities apply the primary chlorine dose after clarification rather than at the raw water intake?
During a severe monsoon runoff event, raw water turbidity rises from 8 NTU to 900 NTU. What is the principal benefit of a properly maintained presedimentation basin in this situation?