How to Use Hydrated Lime (Ca(OH)₂) in Water Treatment

Overview

Hydrated lime, or calcium hydroxide (Ca(OH)₂, CAS 1305-62-0), is the most widely used low-cost alkali in large-scale water and wastewater treatment worldwide. It is produced by slaking (hydrating) quicklime (CaO) with water, yielding a fine white powder with ≥ 90% Ca(OH)₂ content, ≥ 68% CaO equivalent, and moisture ≤ 2%. With a solubility of only 1.5 g/L at 25 °C (reducing further at higher temperatures), hydrated lime is always used as a slurry rather than a true solution — this affects every aspect of dosing equipment design, pipe sizing, and operational maintenance.

The principal mechanism is straightforward: Ca(OH)₂ dissociates to provide OH⁻ ions, raising water pH. The pH achievable by lime alone is limited to approximately 12.4 (the solubility limit), which is more than sufficient for all practical water treatment pH targets. At pH 8–9, heavy metal hydroxides — Cu(OH)₂, Ni(OH)₂, Zn(OH)₂, Pb(OH)₂, Cr(OH)₃ — have very low solubility and precipitate quantitatively, enabling effluent metal concentrations < 0.5 mg/L for most metals with proper settling. At pH 9.5–10.5, carbonate hardness (Ca²⁺ with bicarbonate) is reduced through CaCO₃ precipitation (lime-soda softening), and at pH > 11, magnesium precipitates as Mg(OH)₂.

In municipal wastewater and sludge treatment, hydrated lime is a workhorse chemical. It neutralizes acid effluents from plating shops, mine drainage, and acidic industrial condensates at the lowest cost per ton of OH⁻ equivalent of any alkali. It stabilizes biological sludge at doses of 10–15% Ca(OH)₂ on dry solids basis, raising pH above 12 for extended periods that kill pathogens and suppress biological activity — a process called alkaline stabilization or lime stabilization. Its cost advantage over caustic soda (NaOH) is approximately 3–5× on a per-OH⁻ basis, making it the standard choice wherever the slurry handling complexity is acceptable.

Preparation & Dissolution

Hydrated lime is not dissolved — it is prepared as a slurry (milk of lime) for dosing.

Slurry preparation: Mix hydrated lime powder with water to prepare a 5–20% w/v slurry. For small systems (< 500 kg/day), a 5–10% slurry (50–100 g/L) is most manageable. For large municipal or industrial plants, 15–20% slurry is more economical on tank volume but requires robust agitation.
Mixing equipment: Use a stainless steel 316L or PE-lined mixing tank with a slow-speed agitator (40–60 RPM). Lime slurry is abrasive and will rapidly erode rubber impellers or high-speed mixers. Carbon steel tanks corrode and contaminate the slurry; use only PE, FRP, or SS316L.
Continuous agitation: Lime slurry settles rapidly (within minutes of stopping agitation) and forms a hard scale on tank bottoms and pipe walls. The agitator must run continuously whenever slurry is in the tank. Install flush-out connections at tank drain and in pipelines to clear settled lime before planned shutdowns.
Dosing pumps: Use peristaltic (hose) pumps or progressive cavity pumps — both handle abrasive slurry well. Avoid centrifugal or diaphragm pumps for high-concentration slurry as they clog rapidly. Install flush connections at the pump head for maintenance.
Dosing lines: Size all lime slurry lines at minimum 1 m/s velocity to prevent sedimentation. Use schedule 80 PVC, polyethylene, or rubber-lined mild steel pipe; fittings should be full-bore ball valves (never globe valves, which restrict flow and accumulate scale).

Dosing Guide

Application	Dose (Ca(OH)₂)	Target pH	Notes
Acid wastewater neutralization (general)	Depends on acid load; typically 0.5–5 g/L	7.0–9.0	Conduct bench-scale pH titration with site-specific wastewater to establish exact dose before scale-up
Heavy metal precipitation (Zn, Ni, Cu, Pb)	Theoretical + 20–40% excess; 0.5–3 g/L typical	8.5–10.5 (metal-specific optima)	Different metals have different optimum precipitation pH — check solubility diagrams; combined metals may require pH 9.5–10
Cr(III) precipitation (after Cr(VI) reduction)	0.3–1.5 g/L	8.0–9.0	Add after ferrous sulfate reduction step; Cr(OH)₃ precipitates sharply above pH 8
Drinking water softening (lime-soda process)	135–200 mg/L Ca(OH)₂ (with soda ash for non-carbonate hardness)	10.0–11.0 (in softening reactor); re-carbonation to 7.5–8.5 after	Combined with 30–100 mg/L Na₂CO₃ for complete softening; contact clarifier or sludge blanket reactor recommended
Acid mine drainage neutralization	1–10 g/L (site-specific; AMD acidity determines dose)	8.0–9.5	Produces large volumes of calcium sulfate + metal hydroxide sludge; dewatering required
Sludge alkaline stabilization	10–15% Ca(OH)₂ on dry solids weight	Sludge pH > 12.0 for 2 hours	Kills pathogens (Class B biosolids in US practice); produces significant heat and ammonia vapor — ventilation required

Application Procedure

For acid wastewater neutralization (most common application):

Characterize the feed: Measure inlet pH, alkalinity, acidity (titrate to pH 8.3), and total metal concentrations daily until the feed is well characterized. Conduct a bench-scale lime addition titration curve for the specific wastewater — lime demand is highly non-linear near precipitation pH thresholds.
Pre-mix and deliver slurry: Prepare lime slurry at 10–15% concentration. Set the dosing pump based on titration results. Install the lime slurry injection point in a turbulent zone (e.g., inlet channel or inline mixer) to ensure immediate dispersion and prevent localized high-pH zones that could re-dissolve precipitated metals.
Reaction tank: Provide a reaction/mixing tank with 20–40 minute HRT and slow agitation (40–60 RPM) to allow precipitation reactions to complete before the clarifier. Inadequate mixing or too short HRT leads to incomplete metal precipitation and carryover of fine, hard-to-settle floc.
pH control: Use a pH controller with proportional dosing — pH overshoot above 11 in metal precipitation applications can re-dissolve amphoteric metals like Zn²⁺ and Pb²⁺ (they form soluble zincate and plumbate at high pH). Set upper alarm at pH 10.5 for most mixed-metal applications.
Clarification: Settle in a lamella clarifier or sludge blanket clarifier for 60–120 minutes. Add 0.5–2 mg/L anionic PAM to improve floc aggregation and settling velocity if needed. Underflow sludge will be 5–15% DS and requires further dewatering.
Effluent check and re-carbonation (if softening): For softening applications, re-carbonate the high-pH clarifier overflow by bubbling CO₂ or adding dilute CO₂ to lower pH from 10–11 to 7.5–8.5 before distribution. This also converts excess Ca(OH)₂ to CaCO₃, eliminating calcium leaching into the distribution system.

Monitoring & Control

Parameter	Frequency	Target
Inlet pH / acidity	Continuous or hourly	Baseline to adjust dosing; alert on sudden pH drops (acid spill) requiring emergency dose increase
Outlet pH	Continuous	Application-specific: 8.5–10.0 for metals precipitation; 7.0–8.5 for general neutralization
Lime slurry density	Daily	Verify slurry concentration (Baumé hydrometer); 10% slurry ≈ 1.07 g/mL; if density drops, lime hopper may be empty or slurry is diluted
Effluent total metals (key target metals)	Every 4 hours or daily composite	Site-specific discharge limit; most permit Zn < 1 mg/L, Ni < 0.5 mg/L, Cu < 0.5 mg/L
Sludge blanket level in clarifier	Every 2 hours	Below weir overflow by ≥ 1 m; rising blanket indicates excessive sludge accumulation — increase sludge withdrawal
Lime storage hopper weight / level	Daily	Ensure ≥ 3-day buffer supply; lime delivery lead times can be 2–5 days

Common Mistakes

Stopping agitation in the lime slurry tank during operation: Even brief agitation stoppages (30–60 minutes) allow Ca(OH)₂ particles to settle and compact into a hard cake on tank bottoms and scale on walls. This solid deposit is extremely difficult to remove without physical cleaning (water jetting, scraping) and can block drain valves completely. The agitator must run continuously, and any planned shutdown must be preceded by complete tank drain and flush.
Using lime to neutralize high-sulfate wastewater without planning for gypsum scaling: When hydrated lime neutralizes sulfuric acid (H₂SO₄) or sulfate-rich wastewater, the product is calcium sulfate (CaSO₄·2H₂O, gypsum). Gypsum has low solubility (approximately 2 g/L) and deposits as hard scale on tank walls, pipe internals, pump impellers, and instrumentation. In high-sulfate applications (> 2,000 mg/L SO₄²⁻), this scaling can block pipes within weeks. Design for easy pipe replacement, use rubber-lined pipe in critical sections, and schedule frequent descaling. Consider switching to NaOH in the highest sulfate-load zones where gypsum scaling is unmanageable.
Over-shooting pH above 11 in mixed-metal precipitation: pH above 10.5–11.0 causes amphoteric metals such as zinc (Zn²⁺) and lead (Pb²⁺) to re-dissolve as zincate (Zn(OH)₄²⁻) and plumbate (Pb(OH)₃⁻) complexes, sharply increasing effluent dissolved metal concentrations. Operators who try to ensure complete precipitation by targeting pH 11+ for mixed-metal wastewater will paradoxically see rising Zn or Pb in the effluent. Maintain pH 9.0–10.5 for mixed Zn/Ni/Cu/Pb applications — jar test results for the specific wastewater are essential.
Neglecting lime dust hazards during bulk deliveries and bag cutting: Hydrated lime powder is caustic (pH 12.4 in suspension) and can cause rapid burns to moist mucous membranes and eyes. It is also a respiratory hazard — CaO/Ca(OH)₂ dust can cause chemical pneumonitis with prolonged inhalation. Many operators receive bulk lime deliveries or open bags without adequate respiratory protection. Full-face respirator (P100), chemical splash goggles, nitrile gloves, and chemical-resistant coveralls are required for all dry lime handling. Provide eyewash stations within 10 seconds of every lime handling area.
Inadequate hydraulic retention time before settling: Lime precipitation of metal hydroxides produces extremely fine, low-density floc in the absence of adequate reaction time. Introducing lime slurry directly into a clarifier inlet (without an upstream mixing/reaction tank with 20–40 min HRT) results in carryover of unsettled precipitate, poor effluent clarity, and high effluent metal concentrations. The reaction tank and clarifier are two distinct steps — do not combine them into one vessel without engineered contacting devices.

Storage & Handling

Shelf life: 6–12 months in sealed bags; lime reacts with atmospheric CO₂ (carbonation) to form CaCO₃, reducing active Ca(OH)₂ content — keep bags sealed until use
Temperature: Store in a cool, dry, well-ventilated structure; excessive heat accelerates carbonation; never store near heat sources or in direct sunlight
Container: Original 25 or 50 kg woven polypropylene bags with PE inner liner, or bulk pneumatic silos; bulk silos must be airtight with moisture-excluding aeration pads
Safety: CAUSTIC — Ca(OH)₂ in contact with moisture (including skin and eye moisture) generates heat and causes chemical burns; RESPIRATORY HAZARD — all dry handling requires P100 dust respirator or half-mask with P100 filters; provide eyewash and safety shower within 10 seconds travel time of all lime handling areas