The Dynamic Relationship between Alkalinity and pH in Zero Wastewater Discharge

Alkalinity in wastewater is primarily contributed by three species: hydroxide (OH⁻), carbonate (CO₃²⁻), and bicarbonate (HCO₃⁻). These form a dynamically balanced buffer system, with the dominant species evolving with pH:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻

This equilibrium determines the primary form of alkalinity in different pH ranges:

1. pH < 4.5: Carbonic acid (H₂CO₃) dominates, with almost no alkalinity.

2. pH 6.3-10.3: Bicarbonate (HCO₃⁻) is the dominant species, a common range for most wastewater.

3. pH > 10.3: Carbonates (CO₃²⁻) and hydroxides (OH⁻) begin to dominate.

The distribution ratio of total alkalinity in water (primarily HCO₃⁻ and CO₃²⁻) is precisely determined by pH.

Key Influences and Control Logic:

1. Scale Control (Softening): High-alkalinity, high-hardness water is highly susceptible to calcium carbonate (CaCO₃) scale formation during heating and concentration, clogging membranes and evaporators.

Adding acid (such as H₂SO₄) lowers the pH, converting HCO₃⁻ into CO₂ gas (which is removed via a stripping tower): 2HCO₃⁻ + H₂SO₄ → 2CO₂↑ + 2H₂O + SO₄²⁻

This is the conversion between "alkalinity" and "pH." The amount of acid added is directly determined by the alkalinity of the raw water (consumption of alkalinity) and is controlled by the target pH (usually 6.5-7.0).

2. Corrosion Control

Excessively low pH (<6.5) and alkalinity can make water corrosive, damaging metal pipes and equipment. In softened water, it may be necessary to add alkali (such as NaOH) to raise the pH slightly to form a protective film.

3. Evaporation and Crystallization Optimization

Before entering the evaporator, the pH must be adjusted to the appropriate range (usually >9.5). A high pH promotes silicon precipitation and ammonia nitrogen volatilization, but excessively high pH and alkalinity can lead to scaling of other salts (such as Mg(OH)₂). Accurately controlling the amount of NaOH added is essentially trading alkali for achieving a precise pH target.

In engineering, "conversion" primarily refers to two types of calculations:

1. Calculating alkalinity given the pH and total inorganic carbon (CT) (theoretical calculation)

2. Calculating the acid/base dosage required to achieve the target pH given the alkalinity (core engineering practice)

Core formula and calculation steps:

1. Theoretical relationship

The exact expression for total alkalinity (Alk) is: Alk = [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] - [H⁺] (eq/L)

Using the primary and secondary ionization constants of carbonic acid (K₁, K₂) and total inorganic carbon (CT), each term can be expressed as a function of [H⁺]. This is a complex calculation typically performed with the aid of software such as PHREEQC.

2. Engineering practice "conversion": acid/base dosage calculation

This is the most commonly used calculation in ZLD design. The principle is to determine the amount of acid/base required to neutralize the alkalinity (or acidity) in water to the target pH.

Example: Calculate the amount of sulfuric acid (H₂SO₄) required to lower the water's current pH to the target pH.

Given: The source water alkalinity is Alk₁ = 300 mg/L (as CaCO₃), and the target pH is to be lowered to 6.8 (at which point the residual alkalinity is approximately Alk₂ ≈ 50 mg/L).

Formula: Acid dosage (mg/L) = (Alk₁ - Alk₂) × (equivalent weight of acid) / (equivalent weight of CaCO₃)

Calculation: CaCO₃ equivalent = 50 g/eq; H₂SO₄ equivalent = 98/2 = 49 g/eq (because 1 molecule of H₂SO₄ provides 2 H⁺ atoms); Required H₂SO₄ amount = (300 - 50) mg/L × (49/50) = 250 × 0.98 = 245 mg/L

This means that for every ton of water treated, approximately 245 grams of pure sulfuric acid is required. The core of this calculation is the "alkalinity difference," not the direct pH value, perfectly embodying the logic of "calculating using the capacity factor."