Calcium Hydroxide (Ca(OH)₂, Commonly Known as Hydrated Lime) in Wastewater Treatment

With its strong alkalinity, low cost, versatility, and environmental friendliness, calcium hydroxide (Ca(OH)₂, commonly known as hydrated lime) is widely used in the field of wastewater treatment, including industrial wastewater, municipal sewage, and aquaculture wastewater. It can simultaneously achieve multiple goals such as pH adjustment, pollutant removal, and sludge improvement. Its core advantages are reflected in the following aspects:

1. Efficient pH Adjustment, Suitable for Multiple Water Quality Scenarios

In wastewater treatment, most processes (such as coagulation, biochemical treatment, and advanced oxidation) have strict requirements for the pH of incoming water. For example, the optimal pH range for microorganisms in biochemical tanks is 6.5-8.5, and the best pH for coagulation reactions is 7-9. Calcium hydroxide is an excellent agent for pH adjustment:

Strong yet Mild Alkalinity: When dissolved in water, calcium hydroxide releases OH⁻ ions, which can quickly neutralize acidic wastewater (e.g., electroplating wastewater and chemical wastewater containing HCl, H₂SO₄, etc.) and stabilize the pH within the target range. Compared with sodium hydroxide (NaOH, which is highly corrosive), its alkalinity is released more gently, and it is less likely to cause a sudden rise in pH due to excessive dosage—thus avoiding damage to subsequent processes (e.g., killing microorganisms in biochemical tanks).

Wide Adaptability to Acidic Wastewater Types: Whether it is low-concentration acidic wastewater (such as weak acids produced during the nitrification of municipal sewage) or high-concentration strong acidic wastewater (such as steel pickling wastewater and printing and dyeing wastewater), precise pH control can be achieved by adjusting the dosage of calcium hydroxide. Moreover, the cost per unit pH adjustment is only 1/2 to 1/3 of that of sodium hydroxide.

2. Enhancing Coagulation and Precipitation, Efficiently Removing Suspended Solids and Colloids

Suspended solids (SS) and colloidal particles (e.g., dye colloids in printing and dyeing wastewater, organic colloids in chemical wastewater) in wastewater are prone to stable dispersion due to their negative charge and need to be destabilized by coagulants (such as PAC, PFS). However, calcium hydroxide can enhance coagulation efficiency through the dual effects of "pH adjustment + calcium ion supply":

Optimizing the Coagulation Environment: Most coagulants (such as polyaluminum chloride) achieve the highest coagulation efficiency under neutral or weakly alkaline conditions. While adjusting the pH, calcium hydroxide provides a suitable environment for coagulation reactions and reduces the dosage of coagulants (which can lower coagulant costs by 10%-20%).

Providing Auxiliary Coagulation Ions: The Ca²⁺ ions dissociated from calcium hydroxide can combine with CO₃²⁻ and PO₄³⁻ in wastewater to form CaCO₃ and Ca₃(PO₄)₂ precipitates. At the same time, Ca²⁺ ions can adsorb the negative charges on the surface of colloidal particles, promoting the "destabilization-aggregation" of colloids to form larger flocs and accelerating precipitation and separation. This effect is particularly significant for phosphorus-containing and carbonate-containing wastewater.

3. Efficient Phosphorus Removal, Meeting Discharge Standards

Phosphorus is a key pollutant causing water eutrophication (e.g., cyanobacteria blooms in lakes and black-odor phenomena in rivers). National standards for total phosphorus (TP) discharge are strict—for instance, the first-class A standard for municipal sewage requires TP ≤ 0.5 mg/L, and industrial wastewater often has a more stringent requirement of TP ≤ 0.1 mg/L. Calcium hydroxide is a low-cost phosphorus removal agent:

Reaction Principle: The Ca²⁺ ions dissociated from Ca(OH)₂ combine with PO₄³⁻ ions in wastewater (including orthophosphate and polyphosphate) under alkaline conditions (with the best effect when pH ≥ 10.5) to form calcium phosphate (Ca₃(PO₄)₂) precipitates. These precipitates have extremely low solubility (solubility product Ksp = 2.07×10⁻³³) and can be completely removed through precipitation and separation.

Advantage Comparison: Compared with dedicated phosphorus removal agents (such as polyferric sulfate and hypophosphite removers), the cost of phosphorus removal using calcium hydroxide is only 1/3 to 1/4. Additionally, no additional agents need to be added. Meanwhile, the generated calcium phosphate precipitates can be treated together with sludge without causing secondary pollution.

4. Assisting in Heavy Metal Ion Removal, Reducing Toxicity

Industrial wastewater (e.g., electroplating, electronics, and mining wastewater) often contains heavy metal ions such as Cu²⁺, Ni²⁺, Zn²⁺, and Pb²⁺. Calcium hydroxide can remove heavy metals through "alkaline precipitation":

Reaction Mechanism: Calcium hydroxide increases the pH of wastewater to a specific range—for example, when pH = 8-9, Cu²⁺ forms Cu(OH)₂ precipitates; when pH = 9-10, Ni²⁺ forms Ni(OH)₂ precipitates. This causes heavy metal ions to form hydroxide precipitates, which are then separated through precipitation or filtration. At the same time, Ca²⁺ ions can combine with some heavy metal ions (e.g., CrO₄²⁻) to form more stable calcium salt precipitates (e.g., CaCrO₄), further improving the removal efficiency.

Advantages: Compared with chelating agents (such as EDTA-based agents, which are costly and may leave residues), calcium hydroxide removes heavy metals without the risk of agent residues. Moreover, the precipitated sludge has high stability (hydroxide precipitates are not easily dissolved in the natural environment) and meets the Hazardous Waste Disposal Standards (GB 5085.3).

5. Low Cost, Environmental Friendliness, and Suitability for Large-Scale Applications

The "economic efficiency" and "environmental friendliness" of calcium hydroxide are the core reasons for its widespread application in wastewater treatment:

Cost Advantages: Its raw material is limestone (CaCO₃), which has abundant reserves and low mining and processing costs. The price of calcium hydroxide is only 1/5 to 1/3 that of sodium hydroxide and 1/2 that of polyferric sulfate. Meanwhile, its versatility (capable of simultaneously achieving pH adjustment, phosphorus removal, and sludge conditioning) can reduce the cost of "adding multiple agents in combination" (for example, there is no need to add a separate pH adjuster, phosphorus remover, and sludge conditioner).

Environmental Friendliness with No Residues: Calcium hydroxide itself is an inorganic base, and its reaction products (CaCO₃, Ca₃(PO₄)₂, and heavy metal hydroxides) are all stable solids, leaving no toxic or harmful residues. In contrast, organic bases like ammonia water are volatile and may cause secondary pollution. Additionally, the calcium salts in the sludge can enhance the potential for resource utilization of the sludge (e.g., using it as a construction filler or soil amendment).

Core Value of Calcium Hydroxide

In wastewater treatment, the core competitiveness of calcium hydroxide lies in its "integration of multiple functions + low cost + high environmental friendliness". Without the need for matching multiple agents, it can simultaneously solve multiple problems such as abnormal pH, excessive suspended solids, excessive phosphorus, and difficult sludge dewatering. Moreover, its cost is much lower than that of dedicated agents, and there is no risk of secondary pollution. It is particularly suitable for large-scale applications in industrial wastewater treatment (e.g., electroplating, printing and dyeing, and steel industries) and municipal wastewater treatment plants, making it a preferred agent that balances both "treatment efficiency" and "economic cost".