2.4 Pretreatment: Presedimentation, Pre-Oxidation & Algae Control
Key Takeaways
- Pretreatment is everything done to raw water ahead of coagulation — screening, presedimentation, aeration, pre-oxidation, and algae control — and its purpose is to deliver a steadier, lower-load feed so the coagulation process can be controlled rather than chased.
- Presedimentation basins remove heavy grit and a large fraction of storm-event turbidity without chemicals, commonly cutting influent turbidity by half or more during runoff and protecting downstream basins from rapid solids accumulation.
- Copper sulfate is the traditional reservoir algaecide, typically applied around 0.5 to 1.0 mg/L, but it lyses algal cells and can release taste-and-odor compounds and cyanotoxins, so it must not be applied close to a bloom that is about to be withdrawn.
- Powdered activated carbon adsorbs geosmin, 2-methylisoborneol, and dissolved organics and is normally fed early with adequate contact time, but it must not be applied at the same point as an oxidant or it will consume the oxidant instead of adsorbing the target compound.
- Moving the chlorine application point from pre-coagulation to post-clarification is a standard disinfection byproduct control because it removes total organic carbon precursors before free chlorine ever contacts them.
Why Pretreatment Exists
Coagulation is a chemistry that works best on a stable feed. Raw water is rarely stable. A summer algal bloom raises pH and clogs filters; a spring storm drives turbidity from 8 NTU to 400 NTU in six hours; a fall turnover releases soluble manganese and ammonia from the hypolimnion. Pretreatment is the set of unit processes placed ahead of the rapid mix whose job is to absorb that variability so the coagulation process receives something it can be dosed against.
The Class II criteria list this as monitor, evaluate, and adjust source water treatment (e.g., algae control, aeration, mixing) and pretreatment. Pretreatment decisions are among the highest-leverage choices an operator makes, because they change the load on every process downstream.
Intake Screening
Screening is the first physical barrier and removes material that would damage pumps or plug basins.
| Screen Type | Typical Opening | Removes | Cleaning |
|---|---|---|---|
| Trash rack / coarse bar screen | 1 – 3 in clear spacing | Logs, branches, large debris, ice | Manual rake or mechanical rake |
| Fine bar screen | 0.25 – 1 in | Leaves, aquatic plants, fish | Mechanical rake |
| Traveling water screen | 3/8 in mesh typical | Fine debris, small fish, filamentous algae | Continuous or intermittent spray wash |
| Passive wedgewire screen | 1 – 3 mm slot | Fine debris; low approach velocity limits fish entrainment | Air burst backwash |
Approach velocity is the operating variable that matters most. Low approach velocity — commonly held to roughly 0.5 ft/s or less at fine screens — reduces debris pinning, reduces headloss, and limits fish impingement. Rising differential head across a screen is the direct indicator that cleaning is overdue; an unattended screen can pull enough vacuum to collapse a suction line or starve a low-lift pump into cavitation.
Presedimentation
Presedimentation is plain settling of raw water, generally without coagulant, upstream of the main plant. It is used where the source carries heavy silt or sand loads — typically alluvial rivers and reservoirs subject to flashy runoff.
What it accomplishes:
- Removes grit and coarse sand that would otherwise abrade pumps and accumulate in flocculation basins.
- Removes a large share of storm turbidity. A presedimentation basin frequently cuts influent turbidity by half or more during a runoff event, which proportionally reduces coagulant demand and downstream sludge production.
- Dampens turbidity swings, converting a sharp spike into a gradual change the operator can dose against.
- Provides equalization volume and, when it is a large basin, useful detention for a pre-oxidant.
Operating considerations include sludge accumulation and removal (a presedimentation basin fills far faster than a clarifier), short-circuiting when inlet energy is not dissipated, and the possibility of anaerobic conditions developing in accumulated sludge — which can release soluble iron, manganese, and taste-and-odor compounds back into the water the basin was installed to improve.
Where a source carries very high but short-lived turbidity, some plants also use off-stream storage: diverting to a raw water reservoir only when river turbidity is low, and drawing from storage through the event.
Aeration as Pretreatment
Aeration transfers gases into or out of water. Applied to raw water it serves several purposes:
| Objective | Mechanism | Typical Application |
|---|---|---|
| Oxidize iron and manganese | Adds dissolved oxygen to convert soluble Fe²⁺ and Mn²⁺ to filterable precipitates | Groundwater and post-turnover surface water |
| Strip carbon dioxide | Removes CO₂, raising pH | Groundwater with high CO₂; reduces caustic demand |
| Strip hydrogen sulfide | Removes H₂S and its rotten-egg odor | Anoxic groundwater and hypolimnetic withdrawals |
| Strip volatile organics and radon | Transfers volatile compounds to the gas phase | Contaminated groundwater |
| Add oxygen to anoxic water | Raises dissolved oxygen | Hypolimnetic withdrawals |
Common equipment includes cascade and multiple-tray aerators (simple, low maintenance, effective for CO₂ and iron), spray aerators, diffused air systems, and packed tower (forced draft) aerators, which give the highest transfer efficiency and are the standard choice for volatile organic removal.
Note the interaction with iron and manganese: aeration alone oxidizes iron readily, but manganese oxidizes very slowly with oxygen below about pH 9.5, which is why manganese usually requires a stronger oxidant such as permanganate or chlorine dioxide rather than aeration alone.
Pre-Oxidation
A pre-oxidant applied to raw water can oxidize iron and manganese, destroy taste-and-odor compounds, control zebra mussels and other biofouling at the intake, improve coagulation of some organics, and suppress algal growth in basins.
| Oxidant | Strengths | Cautions |
|---|---|---|
| Free chlorine | Cheap, effective, provides residual | Forms TTHM and HAA5 on contact with organic precursors; the main reason plants move the application point |
| Potassium permanganate | Excellent for manganese and many taste-and-odor compounds; forms no halogenated byproducts | Overdose leaves pink water and MnO₂ deposits; requires downstream filtration to catch the precipitate |
| Chlorine dioxide | Oxidizes manganese, works over a wide pH range, does not form TTHM | Generates chlorite and chlorate; chlorite MCL is 1.0 mg/L, ClO₂ MRDL is 0.8 mg/L |
| Ozone | Most powerful; excellent for taste and odor, color, and some cyanotoxins | Forms bromate where bromide is present; no residual; high capital cost |
The Application Point Decision
Historically many plants prechlorinated at the intake or the rapid mix. The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules made that expensive: free chlorine in contact with raw-water total organic carbon forms trihalomethanes and haloacetic acids, and formation continues all the way to the customer.
The standard byproduct control is to move the chlorine application point downstream — typically to the clarifier effluent or the filter effluent — so that coagulation, clarification, and filtration remove organic precursors before free chlorine ever meets them. Where a pre-oxidant is still needed for manganese, biofouling, or taste and odor, plants substitute permanganate, chlorine dioxide, or ozone, none of which form trihalomethanes.
Powdered Activated Carbon
Powdered activated carbon (PAC) adsorbs dissolved organic compounds, most importantly the earthy-musty taste-and-odor compounds geosmin and 2-methylisoborneol (MIB), both of which humans detect at nanogram-per-liter concentrations that no conventional process removes.
Operating rules that matter:
- Feed early and allow contact time. Adsorption is a mass-transfer process; PAC needs minutes of contact in a mixed environment. Feeding at the rapid mix with short detention gives poor performance.
- Do not feed PAC at the same point as an oxidant. Chlorine, permanganate, and ozone all react with the carbon surface. Co-feeding wastes both chemicals — the carbon consumes the oxidant and the oxidant occupies the adsorption sites. Separate the feed points.
- Do not feed PAC immediately ahead of membranes without careful evaluation; carbon fines can foul or abrade membrane surfaces.
- Expect it in the sludge. PAC increases solids production and darkens residuals.
- Typical doses range widely with the target compound and background organic load, commonly from a few mg/L for mild episodes to several tens of mg/L during severe blooms.
Algae Control
Why Algae Are an Operating Problem
Algal growth in a raw water reservoir causes, in rough order of operator pain:
- Taste and odor from geosmin and MIB.
- Filter clogging and short filter runs, especially from diatoms and filamentous forms.
- pH elevation in the epilimnion from photosynthetic CO₂ consumption, which shifts coagulation out of its optimal pH range.
- Increased disinfection byproduct precursors, as algal cells and their extracellular organic matter are TOC.
- Cyanotoxins from cyanobacterial (blue-green) blooms — microcystins, cylindrospermopsin, anatoxin-a and others — which are a public health concern, not merely an aesthetic one.
In-Reservoir Control
| Method | How It Works | Key Limitation |
|---|---|---|
| Copper sulfate | Copper ion is toxic to algae; traditional dose commonly 0.5 – 1.0 mg/L as CuSO₄, adjusted for alkalinity | Lyses cells and releases intracellular taste-and-odor compounds and cyanotoxins; copper accumulates in sediment; toxicity to fish rises in soft, low-alkalinity water; application is regulated |
| Chelated copper formulations | Keeps copper soluble and available longer in hard water | Higher cost; same cell-lysis concern |
| Reservoir destratification / aeration | Mixes the water column, disrupting the stable, warm, sunlit surface layer cyanobacteria exploit, and keeps the hypolimnion aerobic | Capital and power cost; must be sized to the basin |
| Nutrient control in the watershed | Reduces the phosphorus and nitrogen that drive blooms | Slow; requires land-use cooperation; the durable fix |
| Selective withdrawal | Draws from a depth below the surface scum layer | Requires a multi-level intake; deep water may be anoxic |
The Copper Sulfate Timing Trap
This is the classic Class II judgment question. Copper sulfate kills algae by lysing the cells. Everything inside the cell — geosmin, MIB, and, in a cyanobacterial bloom, cyanotoxins — is released into the water in dissolved form at the moment of the kill.
The consequences:
- Applying copper sulfate immediately upstream of, or shortly before, withdrawal converts a manageable particulate problem into a dissolved problem arriving at the plant. Dissolved geosmin and dissolved microcystin are far harder to remove than intact cells, which coagulation and filtration capture physically.
- The correct practice is to treat early, well before the bloom peaks, and to allow adequate time and distance between application and withdrawal.
- During an active cyanobacterial bloom near the intake, the preferred response is usually not to lyse the cells at all. Instead, shift to selective withdrawal below the scum layer, remove intact cells gently through optimized coagulation and filtration, avoid oxidant doses that rupture cells ahead of removal, and back that up with PAC or ozone for any dissolved toxin fraction.
In-Plant Algae Control
- Cover or shade basins where practical; open, sunlit sedimentation basins grow their own algal mats.
- Physically clean launders, weirs, and basin walls on a schedule. Algal mats on weirs release clumps that carry through to filters.
- Maintain a residual through the basins where byproduct rules allow it, or use a non-halogenated pre-oxidant.
- Watch filter run times as the leading indicator; a bloom usually shows up as shortening runs before it shows up as a customer complaint.
Putting Pretreatment Together
A well-run pretreatment train on a difficult surface source in late summer might look like this: traveling screens kept clean at low approach velocity; presedimentation absorbing storm turbidity; permanganate fed at the intake for manganese and biofouling with enough travel time to complete the reaction; PAC fed at a separate point downstream of the permanganate demand, with mixed contact time, for geosmin and MIB; no free chlorine until after the filters; and selective withdrawal set below the surface bloom. Each of those choices reduces load or variability at the rapid mix, which is the entire point.
A reservoir is experiencing an active cyanobacterial (blue-green algae) bloom concentrated near the surface directly over the raw water intake. Which response best protects finished water quality?
A conventional surface water plant has exceeded its TTHM locational running annual average. Raw water total organic carbon is elevated, and the plant currently applies free chlorine at the rapid mix for manganese oxidation and zebra mussel control. Which change most directly addresses the byproduct exceedance while preserving the pre-oxidation function?
Operators feeding powdered activated carbon for a geosmin episode observe almost no improvement in threshold odor number despite a substantial PAC dose. PAC is currently fed into the rapid mix chamber at the same point as the permanganate feed. What is the most likely explanation?