10.2 Mixers, Flash Mixing & Chemical Feed Systems
Key Takeaways
- Submersible and vertical mixers keep solids suspended and blend chemicals without adding oxygen, which is why anoxic and anaerobic zones in a nutrient removal plant use mixers instead of air.
- Rapid mix requires high energy input for seconds to disperse a coagulant, while flocculation requires gentle tapered mixing for many minutes to grow settleable floc — mixing too vigorously in the flocculator tears the floc apart.
- Dry polymer must be wetted, dispersed, and then aged or hydrated in a make-down system, typically for 30 to 60 minutes, before it develops full activity; feeding unaged polymer wastes chemical and degrades dewatering performance.
- Sodium hypochlorite and acid must never be stored where a spill or a misconnected fill line could combine them, because the reaction liberates chlorine gas; incompatible chemicals require separate containment areas.
- Chemical feed materials must match the chemical — hypochlorite and ferric chloride attack ordinary metals and require PVC, CPVC, or lined and FRP components, while lime slurry scales and plugs conventional piping.
10.2 Mixers, Flash Mixing & Chemical Feed Systems
Exam Focus: "Mixers" and "Chemical dosing equipment" are two separate line items in the Equipment content area of the 2025 Need-to-Know Criteria, and "Chemical dosing — coagulation/flocculation, nutrient removal/enhancement, pH adjustment" appears again in the Treatment Process area.
1. Why Plants Mix
Mixing serves three distinct purposes, and equipment is selected for whichever one dominates:
- Keep solids in suspension. Without mixing energy, mixed liquor, sludge, and slurries stratify and settle where they are not wanted.
- Blend a chemical uniformly and quickly. A coagulant that contacts only part of the flow treats only part of the flow.
- Prevent stratification. Temperature and concentration layering in tanks and digesters destroys process performance.
Mixer Types
| Mixer | Where Used | Notes |
|---|---|---|
| Submersible mixer | Anoxic and anaerobic zones, equalization basins, sludge holding | A propeller on a sealed submersible motor. Mixes vigorously without transferring oxygen — the entire reason it exists in nutrient removal service. |
| Vertical turbine / top-entry mixer | Rapid mix basins, chemical day tanks, flocculators | Shaft-mounted impeller driven by a gear reducer above the liquid; impeller style sets whether energy goes into flow or shear. |
| Static (in-line) mixer | Chemical injection into a pipeline | Fixed vanes inside the pipe create turbulence. No moving parts, no power, no maintenance — but the mixing energy comes from headloss. |
| Flocculation paddle mixer | Flocculation basins | Slow horizontal or vertical paddles, often in tapered stages. |
| Digester mixing | Anaerobic digesters | Gas recirculation through lances, draft-tube mixers, or external pumped recirculation. Poor digester mixing causes grit accumulation, scum blanket formation, and dead volume. |
| Coarse-bubble air mixing | Aerobic digesters, sludge holding, channels | Uses air both to mix and to hold aerobic conditions. |
The anoxic zone rule. A denitrifying (anoxic) zone must have nitrate but essentially no dissolved oxygen. Air-based mixing would defeat the process entirely, so anoxic and anaerobic zones are mixed mechanically by submersible mixers. If an exam item asks why a nutrient removal basin uses mixers instead of diffusers, this is the answer.
2. Rapid Mix vs. Flocculation — Opposite Energy Requirements
These two steps are adjacent in the process train and are opposite in what they need. Getting them backwards is a classic exam distractor.
| Rapid (Flash) Mix | Flocculation | |
|---|---|---|
| Purpose | Disperse coagulant throughout the flow before it hydrolyzes | Promote gentle particle collisions so destabilized particles grow into settleable floc |
| Mixing energy | Very high | Low, and usually tapered — strongest at the inlet, gentlest at the outlet |
| Detention time | Seconds (commonly 10 to 60 seconds) | Minutes (commonly 20 to 30 minutes) |
| Failure if wrong | Too little energy leaves coagulant undispersed and wastes chemical | Too much energy shears the floc apart, and the sheared fragments will not re-form |
3. Chemical Feed System Components
A complete chemical feed system, from delivery to point of application:
- Bulk storage — tank with level indication, vent, fill connection with a unique fitting, and secondary containment sized to hold the largest tank plus freeboard.
- Day tank — a smaller working tank that limits the volume that can be released by a feed system failure, and that lets operators measure daily usage directly.
- Transfer pump and metering pump (see Section 9.2).
- Calibration cylinder for verifying actual feed rate.
- Rotameter or flow indication on the feed line.
- Back-pressure and anti-siphon valves, and a pulsation dampener where the pump output is pulsating.
- Injection quill or diffuser delivering the chemical into well-mixed flow, not against a pipe wall.
- Carrier water where required to dilute and convey the chemical.
Dry chemical feeders come in two accuracies: volumetric feeders meter by volume (a screw or belt) and are simpler but less accurate, while gravimetric feeders meter by weight and are accurate enough for the most demanding duty. Dry feeders need a hopper agitator or vibrator to prevent bridging and arching, and a dissolving/slurry tank downstream.
Polymer Make-Down and Aging
Polymer is the chemical most often fed incorrectly.
- Dry (powder) polymer must be wetted evenly — usually with an eductor or wetting cone that disperses each particle in water. Dumping powder into water forms "fish eyes," gelled lumps with a dry core that never dissolve and that plug lines.
- After wetting, the solution must age (hydrate) — typically 30 to 60 minutes — so the polymer chains uncoil and develop full charge and bridging activity.
- Aged solution is then diluted at the point of use to improve dispersion into the sludge.
- Emulsion polymer requires activation with vigorous initial mixing and also benefits from aging.
- Over-mixing aged polymer shears the long chains and destroys activity — mix enough to blend and no more.
- Polymer solution is extremely slippery when spilled; treat any polymer spill as an immediate slip hazard and use absorbent, not water, to clean it.
4. Materials Compatibility and Chemical Segregation
| Chemical | Compatible Materials | Hazard/Handling Notes |
|---|---|---|
| Sodium hypochlorite (NaOCl) | PVC, CPVC, FRP, HDPE | Degrades with heat, light, and time; off-gasses and vapor-locks pumps; corrosive to most metals |
| Sulfur dioxide / sodium bisulfite | PVC, CPVC, stainless in specific grades | Bisulfite solution attacks carbon steel; SO2 gas is an acute respiratory hazard |
| Ferric chloride | FRP, lined steel, PVC | Highly corrosive and staining; releases HCl fumes |
| Lime (calcium hydroxide) slurry | Rubber-lined or abrasion-resistant piping, hose pumps | Scales and plugs lines; requires continuous agitation; strongly caustic and a serious eye hazard |
| Caustic soda (sodium hydroxide) | Steel, PVC (temperature limited) | Freezes at moderate temperatures at high concentration; severe burns |
| Alum | PVC, FRP, stainless | Mildly acidic; consumes alkalinity |
The segregation rule that prevents fatalities. Never store or plumb sodium hypochlorite where it can contact an acid. Mixing hypochlorite with acid liberates chlorine gas immediately. Incompatible chemicals need separate containment areas with separate drains, and every bulk fill connection must have a unique, chemical-specific fitting so a delivery driver cannot physically connect an acid hose to a hypochlorite tank. Label every tank, line, and fill point, and never accept a delivery without verifying the product against the shipping paperwork and the safety data sheet.
In a biological nutrient removal plant, why is the anoxic zone equipped with submersible mechanical mixers rather than coarse-bubble diffused air for mixing?
An operator increases the paddle speed in a flocculation basin, expecting improved chemical contact, and observes that settled effluent turbidity gets worse. What happened?
A polymer make-down system is producing solution that dewaters sludge poorly, and operators find gelled lumps with dry centers in the mix tank. What are the two most likely errors?