Polyferric sulfate (PFS), a commonly used flocculant in water treatment, works by adsorbing, bridging, and coagulating pollutants in water through polynuclear hydroxyl complexes generated during hydrolysis. Low temperatures in winter weaken its effectiveness in three ways: first, low temperatures slow down the PFS hydrolysis rate, resulting in insufficient hydroxyl complex formation and slow, loosely structured flocs; second, low temperatures reduce the activity of water molecules, weakening the Brownian motion of pollutant particles and decreasing the probability of collisions, which is detrimental to coagulation; and third, low temperatures may lead to incomplete dissolution of PFS, with undissolved particles not only failing to function but also potentially increasing water turbidity and even clogging equipment. Therefore, winter use requires targeted solutions to address the three major problems of slow hydrolysis, difficult coagulation, and poor dissolution. 1. Optimize the dissolution process to improve the utilization rate of active ingredients. The solubility of PFS decreases at low temperatures, and incomplete dissolution is the primary reason for reduced efficacy. It is recommended to use a “heating dissolution + segmented stirring” mode: control the temperature of the dissolving water at 20-30℃ (waste heat from production wastewater or low-pressure steam can be used for heating; avoid temperatures exceeding 40℃, which can lead to PFS decomposition). Prepare a stock solution at a mass concentration of 10%-15% (the recommended concentration at room temperature is 5%-10%; at low temperatures, appropriately increasing the concentration can reduce the amount of dissolving water and increase the local reaction temperature). When dissolving, first add half the water volume, stir for 3-5 minutes, then slowly add the PFS solid while stirring continuously. After adding the solid, add the remaining water and continue stirring for 15-20 minutes to ensure complete dissolution of the solid, with no visible particles remaining. If conditions are limited and heating is not possible, the stirring time can be extended to more than 30 minutes, and the maturation time of the mother liquor can be appropriately increased (1-2 hours at room temperature, and 3-4 hours at low temperature) to allow the hydrolysis reaction to proceed in advance and reduce the reaction lag in low-temperature water after addition. 2. Adjust Dosing Parameters to Adapt to Low-Temperature Reaction Characteristics. Flocculation efficiency decreases at low temperatures, requiring adjustments to the dosage and method to compensate. Regarding dosage, it is recommended to increase the dosage by 10%-30% from the room temperature level (the specific dosage needs to be determined through small-scale testing: take water samples to be treated, add different doses of PFS solution, stir, and observe the settling velocity of the flocs and the turbidity of the supernatant to select the optimal dosage). This avoids poor flocculation due to insufficient dosage. For the dosage method, adopt a “segmented addition + uniform stirring” approach: first add 60%-70% of the PFS solution, stir rapidly for 1-2 minutes (stirring speed controlled at 100-150 r/min to promote full contact between pollutants and flocculant), then add the remaining dosage, switching to slow stirring for 3-5 minutes (stirring speed 50-80 r/min to facilitate floc aggregation and growth). This avoids excessively high local concentrations caused by a single addition, which could affect hydrolysis equilibrium.
3. Improve the reaction environment and enhance flocculation and sedimentation effects. Water pH and temperature are important factors affecting PFS hydrolysis, and these need to be carefully controlled in winter. Regarding pH, the optimal operating range for PFS is 6.0-9.0. If the pH of the water to be treated is below 6.0, a small amount of lime milk or sodium hydroxide solution can be added in advance to adjust the pH to 6.5-7.5 to avoid acidic conditions inhibiting the formation of hydroxyl complexes. If the pH is above 9.0, dilute sulfuric acid can be added to adjust it to 7.0-8.0 to prevent PFS hydrolysis from generating ferric hydroxide precipitate, which would reduce flocculation activity. Regarding temperature, if conditions permit, the temperature of the water to be treated can be raised to above 15℃ using a heat exchanger (the PFS hydrolysis rate can increase by 2-3 times for every 10℃ increase in temperature). If the temperature cannot be raised, the reaction time and settling time can be appropriately extended (30-60 minutes at room temperature, and 60-90 minutes at low temperatures) to ensure sufficient settling of the flocs and reduce effluent turbidity. In addition, a coagulant aid can be used: when the turbidity of the water is low or the pollutant concentration is low, add 0.1-0.3 mg/L of
polyacrylamide (PAM). Utilizing the adsorption and bridging effect of PAM, the flocs settle faster, improving treatment efficiency while reducing the amount of PFS added. 4. Standardize storage and transportation to avoid product performance degradation. Low temperatures in winter not only affect the usage process but may also cause PFS to crystallize and separate during storage, reducing product activity. During storage, solid or liquid PFS should be sealed and stored in a cool, dry indoor place, avoiding open-air stacking and freezing (the freezing point of liquid PFS is approximately -5℃; if the temperature is below the freezing point, it may freeze and crystallize; after thawing, it must be thoroughly stirred and the concentration tested before use). Solid PFS must be protected from moisture and water to prevent moisture absorption and clumping, which would affect dissolution. During transportation, refrigerated vehicles should be selected to avoid changes in product performance due to excessively low temperatures during long-distance transport. Liquid PFS can be appropriately stirred before transportation to prevent separation. Upon arrival at the destination, unload and store promptly to avoid prolonged exposure to low temperatures.
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