Water Treatment Chemical Information Sheet**
I. PAC (Polyaluminum Chloride): The Neutralization Expert, Prefers Weak Acidity
PAC is our most commonly used inorganic coagulant. It hydrolyzes in water to generate positively charged aluminum hydroxide colloids that specifically capture negatively charged suspended particles.
>Optimal pH Range: 5.5 - 8.0
>Principle Analysis:
>- If pH is too low (<5.5): High H⁺ concentration inhibits the hydrolysis reaction of PAC, resulting in fine, loose flocs with poor sedimentation efficiency and a high residual of aluminum ions in the water.
>- If pH is too high (>8.5): High OH⁻ concentration causes the aluminum hydroxide flocs to dissolve back into soluble aluminate ions (AlO₂⁻), leading to floc disintegration, loss of purification effect, and increased turbidity.
>Operational Recommendation: When using PAC, first adjust the raw water pH to a weakly acidic or neutral environment around 7.0 to maximize its efficiency. After dosing, the pH will slightly decrease, so monitoring is necessary.
II. PAM (Polyacrylamide): The Chain Master, Depends on the Type
PAM is a high-molecular-weight organic flocculant. It does not neutralize charges but instead uses its long molecular chains to "capture" and "bridge" small flocs, forming larger aggregates. Its optimal pH varies depending on its ionic type.
>Optimal pH Range: Varies significantly by type.
>Principle Analysis:
>- Non-ionic (NPAM): Has a broad application range and is less sensitive to pH, performing well within pH 7.0-8.0.
>- Anionic (APAM): Relies on negatively charged groups on its molecular chains for bridging, making it suitable for treating positively charged suspended solids. It performs best in neutral or weakly alkaline environments where its molecular chains can fully extend.
>- Cationic (CPAM): Combines charge neutralization and adsorption bridging, primarily used for treating negatively charged organic sludge and dyes. It performs excellently under acidic or neutral conditions (pH 7.0-8.0).
>Operational Recommendation: Always conduct small-scale tests first! Different water qualities and PAM products from various manufacturers may perform differently. Typically, PAC is added first to neutralize charges, followed by PAM (usually anionic) for bridging and sedimentation.
III. Activated Carbon: The Adsorption King, Prefers Acidity and Fears Alkalinity
Activated carbon relies on its vast surface area and porous structure to physically adsorb impurities in water. However, its ability to adsorb certain specific substances is significantly influenced by pH.
>Optimal pH Range: 5.0-6.0 (for removing organic matter, color, and odors).
>Principle Analysis:
>- Under acidic conditions, the high concentration of H⁺ ions facilitates the adsorption of certain polar organic compounds and chromogenic substances (e.g., humic acid), resulting in the highest removal rates for color and odors.
>- Under alkaline conditions, the surface charge of activated carbon changes, reducing its adsorption capacity. Additionally, some already adsorbed organic compounds may desorb under alkaline conditions, leading to "release."
>Operational Recommendation: To maximize the adsorption efficiency of activated carbon, especially for advanced treatment such as decolorization and deodorization, adjusting the influent pH to a weakly acidic level is crucial.
IV. pH Preferences of Common Water Treatment Chemicals
1.
Oxidizing Agents (e.g., Sodium Hypochlorite, NaClO)
2.
Function: Disinfection, sterilization, and degradation of organic matter.
Optimal pH: <7.5. Under acidic conditions, hypochlorous acid (HClO) content is high, providing strong sterilization效果 (HClO is 80-100 times more effective than hypochlorite ions, ClO⁻). As pH increases, disinfection efficacy drops sharply.
3.
Corrosion and Scale Inhibitors
4.
Function: Prevent scaling and corrosion in pipelines and equipment.
Optimal pH: Typically alkaline (e.g., 8.5-9.5). Raising the pH helps form a protective film on metal surfaces, slowing corrosion. However, the specific range depends on the chemical formulation and water quality (e.g., calcium hardness, alkalinity). Always strictly follow the product instructions.
5.
Common Acids and Bases
6.
Lowering pH: Use sulfuric acid (H₂SO₄) or hydrochloric acid (HCl)—cost-effective options.
Raising pH: Use sodium hydroxide (NaOH) or lime (Ca(OH)₂).
