How should PAC coagulant be dosed for optimal results?

To achieve optimal PAC dosing performance, the core objective is to attain the fastest flocculation rate, largest floc particles, and clearest effluent water quality at the lowest possible dosing cost. Achieving this goal requires systematic attention to the following key aspects: I. Precisely determining the optimum dosage Blind dosing is the primary cause of poor efficacy and high costs. Determining the optimum dosage requires confirmation through beaker testing. Specific steps: 1. Sampling: Collect several representative samples (typically six) of the raw water source (e.g., in 1-litre beakers). 2. Quantitative dosing: Using a pipette or syringe, sequentially add different concentrations of PAC solution to each beaker (e.g., 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 60 mg/L). 3. Rapid Mixing: Activate the mixer and agitate at a high intensity of 200–300 revolutions per minute for 1–2 minutes. This simulates the rapid mixing process of a pipeline mixer, ensuring instantaneous and uniform dispersion of the PAC. 4. Slow Flocculation: Reduce the rotation speed to 40–60 revolutions per minute and stir slowly for 10–15 minutes. This simulates the conditions within a flocculation tank, allowing microflocs to collide and coalesce fully into large, dense flocs. 5. Settling: Cease stirring and allow to stand for 10–15 minutes, observing the sedimentation process. 6. Observation and Judgement: Observe the alum flocs: Note which beaker produces the largest, densest flocs that settle most rapidly. Water quality testing: Take the supernatant from each beaker and measure parameters including turbidity (NTU), pH value, and COD. Determining the optimum dosage: The dosage yielding the lowest effluent turbidity and the most favourable floc morphology constitutes the optimum dosage. Occasionally, excessive dosage may induce colloidal restabilisation (where particles regain positive charge and stabilise), resulting in increased effluent turbidity. This inflection point is clearly observable in beaker tests. Note: Beaker tests are not a one-off procedure. Water quality parameters (such as flow rate, turbidity, pH, and temperature) fluctuate, particularly during the rainy and dry seasons, and between day and night. Beaker tests should be repeated regularly (e.g., at each shift change) or whenever significant changes in water quality occur. II. Optimisation of Configuration and Dosage Process 1. The concentration should be appropriately adjusted. The typical preparation concentration for solid PAC is 5–10% (i.e., adding 50–100 kg of solid PAC to 1 tonne of water). Excessively high concentrations may result in incomplete dissolution and clogging of dosing equipment; conversely, excessively low concentrations lead to excessive delivery volumes and overly dilute effects. When dissolving, adhere to the principle of ‘slow addition with vigorous stirring’: sprinkle the solid PAC slowly and evenly into the agitated water. Avoid adding it all at once, as this may result in the formation of insoluble lumps. 2. The choice of dosing point is crucial. The dosing point should be selected at locations where the water flow is turbulent and thoroughly mixed, such as the suction pipe of the pump, the discharge pipeline, or the inlet to a dedicated mixing tank. This ensures the PAC disperses rapidly and uniformly throughout the entire water body, achieving full contact with colloidal particles. 3. Adopt a phased addition strategy For water with complex composition and difficult treatment, the total dosage may be divided into two or more points for addition. For instance, 70% of the dosage is added at the inlet of the mixing tank for electrical neutralisation and destabilisation; 30% is added midway through the flocculation tank to enhance the entrapment and flocculation process. This strategy frequently reduces overall chemical consumption while improving flocculation efficiency. III. Strictly control environmental factors 1. pH value The optimal pH range for PAC is relatively broad, spanning 6.5 to 8.5. However, within this range, it still exerts a significant influence on the distribution of aluminium salt hydrolysis forms and the coagulation efficacy. pH too low (<6.5): PAC hydrolysis is incomplete, resulting in poor flocculation and potentially yellowish effluent. pH excessively high (>8.5): This leads to the formation of negatively charged aluminate ions (Al(OH)₄⁻), resulting in diminished flocculation efficiency and potentially causing previously formed flocs to redissolve. The pH of the raw water must be monitored in real time. Should the pH prove unsuitable, it should be adjusted using acid (such as H₂SO₄) or alkali (such as NaOH). This constitutes the most critical step in ensuring the efficacy of PAC. 2. The Effect of Water Temperature At low water temperatures, the viscosity of water increases, Brownian motion diminishes, and the frequency of collisions between colloidal particles decreases. Consequently, flocculation proceeds more slowly, resulting in fine, loose flocs that are difficult to settle. At low temperatures, the dosage of PAC may be slightly increased as appropriate, or a flocculant such as polyacrylamide (PAM) may be added. The adsorption bridging effect of PAM can be utilised to expand the flocs and improve their settling properties. 3. Mixing intensity and duration Rapid mixing (high G-value): Sufficient intensity must be applied for a brief duration to ensure instantaneous blending. Slow agitation (low G-force): Sufficient intensity must be maintained over an extended period to ensure floc growth and maturation without disruption. The control of the agitation process directly impacts floc quality. The main products of flocculant manufacturer Xinhuan Water Treatment are: White polyaluminium chloride (food grade), drinking grade polyaluminium chloride, industrial grade polyaluminium chloride, anionic polyacrylamide, cationic polyacrylamide, nonionic polyacrylamide, petroleum recycling polyacrylamide, oil recovery polyacrylamide, polyacrylamide, polymer flocculant polymer flocculation coagulant, polymeric ferrous sulphate, polymerized iron and aluminium chloride, and other water treatment chemicals. Welcome customers to write to us to negotiate business! Tel/WhatsApp：86 19139972558 Email：Sunny@xhwtm.com Contact:Sunny