2.8 Pretreatment: Presedimentation, Aeration, Algae Control & Taste-and-Odor Management

Everything that happens before coagulation

Pretreatment is the set of unit processes applied to raw water before the main coagulation, flocculation, sedimentation, and filtration train. Its purpose is to reduce the load on the plant, remove constituents that the main train handles badly, and stabilize a variable source so process control downstream can be steady.

Presedimentation and roughing

Presedimentation is plain gravity settling of raw water, with no coagulant, in a basin or reservoir ahead of the plant. It is standard practice on rivers carrying heavy sand and silt, and its value in Colorado shows up during spring runoff and after wildfire when raw turbidity can jump from 5 NTU to several hundred in hours.

Benefits are concrete:

• Removes abrasive sand that would otherwise destroy pump impellers, chemical feed check valves, and meter internals.
• Cuts coagulant demand, because the coagulant no longer has to sweep out the fraction that settles on its own.
• Buffers turbidity spikes, giving the operator time to adjust dose.
• Provides equalization storage during intake interruptions.

Roughing filters — coarse gravel or crushed rock beds operated at low rates — do the same job where basin space is unavailable, and are common ahead of slow sand filters.

Screening ahead of everything removes trash, leaves, and aquatic organisms. Bar racks with 1 to 3 inch openings handle debris; traveling water screens with fine mesh remove leaves, fish, and mussels; microstrainers with 20 to 60 micron fabric remove algae.

Aeration

Aeration transfers gases in both directions across the air and water interface. It adds oxygen and strips volatile gases. That single sentence explains everything it can do:

It cannot remove nonvolatile material: hardness, nitrate, arsenic, fluoride, and total dissolved solids pass straight through.

Three families of equipment appear on the exam:

1. Waterfall or gravity aerators — cascade, tray (often with coke or ceramic media), and spray nozzles. Water is broken into thin films or droplets falling through air. Tray aerators are the classic groundwater iron and manganese unit.
2. Diffused aerators — compressed air released through diffusers at the bottom of a contact basin. Effective and enclosed, so off-gas can be controlled, but energy intensive.
3. Packed tower (air stripping) aerators — water distributed over high-surface-area packing with a countercurrent air stream. This is the workhorse for VOC and radon removal, and the air-to-water ratio is the main control variable.

Aeration side effects an operator must anticipate: stripping CO2 raises pH, which can shift the corrosion balance and change coagulation chemistry; adding oxygen makes the water more corrosive to iron pipe; open aerators are an entry point for airborne contamination, so they need screening and housekeeping; and in cold Colorado weather, exposed cascade aerators ice up.

Oxidation for iron, manganese, and taste and odor

Where aeration is too slow — manganese in particular oxidizes very slowly with oxygen alone below pH 9 — a chemical oxidant is fed at the intake or raw water line so reaction and precipitation are complete before the filters.

Potassium permanganate deserves particular attention because it is heavily tested. It is a purple crystalline solid mixed into solution and fed by metering pump. Correct dosing turns the raw water faintly pink, and the color must disappear before the water reaches the filters. Persistent pink water leaving a filter means the dose is too high — excess permanganate is passing through and will produce pink water complaints and, on manganese greensand, will bleed manganese. Underdosing leaves soluble manganese that oxidizes later in the distribution system and produces black water complaints. Manganese greensand filters are regenerated with permanganate, either continuously or intermittently.

Taste and odor

The two compounds responsible for most earthy and musty complaints are geosmin and 2-methylisoborneol (MIB), produced by cyanobacteria and actinomycetes. Human threshold detection is in the range of 5 to 10 nanograms per liter — parts per trillion — which is why customers can taste something no routine analysis will show. Neither compound is a health hazard, but complaints about them dominate customer contact during late-summer reservoir blooms.

Effective responses, roughly in order of practicality:

1. Change the intake depth to draw below the bloom.
2. Powdered activated carbon (PAC), dosed typically 5 to 30 mg/L.
3. Ozone or chlorine dioxide oxidation.
4. Granular activated carbon as a filter cap or post-filter adsorber for chronic problems.
5. Reservoir management — copper sulfate algaecide (with care for aquatic life and for releasing cell contents), aeration or destratification, and nutrient control in the watershed.

Chlorine is the wrong tool. Chlorinating a bloom lyses algal cells, releasing intracellular geosmin, MIB, and potentially cyanotoxins, and simultaneously reacts with the organic load to raise THM and HAA formation. Prechlorination for taste and odor almost always makes complaints worse.

PAC feed point rules

PAC needs contact time to adsorb, so it is dosed as early as practical — at the intake, in the presedimentation basin, or at the rapid mix. Two constraints govern:

• Never feed PAC and an oxidant at the same point. Free chlorine, permanganate, or ozone will consume the carbon's adsorption capacity, and the carbon will consume the oxidant. Separate the feed points, or feed PAC first with the oxidant well downstream.
• PAC is removed with the floc, so it must be fed upstream of coagulation and settling. Dosing after sedimentation loads the filters with black carbon fines and risks carbon breakthrough into the clearwell.

Housekeeping matters: dry PAC is a combustible dust and a slip hazard when wet, and its handling area needs dust control, bonding and grounding on transfer equipment, and no ignition sources.