Control of Dosing Amount of Special Flocculants for Phosphoric Acid Filtration
The optimal dosing amount of special flocculants for phosphoric acid filtration shall be determined through laboratory tests based on water quality characteristics, treatment targets and flocculant types. Dynamic adjustment shall be carried out combined with parameters such as pH value, water temperature and impurity concentration. The detailed control methods are as follows:
I. Core Methods for Determining Optimal Dosing Amount
Jar Test
Test Procedures
Take equal volumes of raw water samples (1000 mL for example) into multiple beakers, then add flocculants at different dosages respectively (5 mg/L, 10 mg/L, 15 mg/L, etc.). Conduct rapid stirring at 300 r/min for 2 minutes to disperse flocculants evenly, followed by slow stirring at 60 r/min for 3 minutes to facilitate floc growth. Finally, let the mixture stand for 20 minutes and observe the settling performance.
Evaluation Indicators
The core evaluation indicators include floc settling velocity, supernatant turbidity and phosphate removal rate. For instance, relevant research shows that when ferric chloride is dosed at 15 mg/L (pH=6.0), the phosphate removal rate can reach 96.3%. Further increasing the dosage brings no obvious improvement in treatment efficiency, so 15 mg/L is determined as the optimal dosage.
Gradient Test
It is recommended to set concentration gradients with an increment of 10%, 20% and 30%. For example, set the initial dosage at 10 mg/L and gradually increase it to 30 mg/L, then determine the optimal value by comparative analysis.
Pilot-Scale Verification On-Site
After obtaining the preliminary optimal dosage through laboratory jar tests, pilot tests shall be conducted on the production site. Simulate actual working conditions including flow rate, stirring intensity and hydraulic retention time, verify flocculation effects and fine-tune the dosage. For example, a phosphorus chemical enterprise found through pilot tests that the laboratory optimal dosage of 15 mg/L needs to be adjusted to 18 mg/L on-site to compensate for pipeline loss and uneven mixing.
II. Key Factors Affecting Flocculant Dosing Amount
Water Quality Parameters
pH Value
The coagulation performance of inorganic flocculants (iron salts, aluminum salts) is significantly affected by pH. Aluminum salts achieve optimal treatment efficiency at pH 6.5–7.5; their performance drops sharply when pH＜4 or pH＞8. Iron salts are applicable within pH 6–11, yet ferric hydroxide precipitates are prone to form under alkaline conditions.
Water Temperature
Low temperature reduces the hydrolysis rate of flocculants and the growth rate of flocs. For example, when water temperature is lower than 5℃ in winter, the treatment efficiency of inorganic flocculants decreases by 30%–50%. Operators shall increase the flocculant dosage or switch to organic polymer flocculants such as polyacrylamide (PAM).
Impurity Concentration
A higher concentration of suspended solids (SS) in water requires a larger flocculant dosage. For example, the aluminum salt dosage for raw water with high turbidity during flood seasons (SS＞500 mg/L) shall be increased to 50–100 mg/L, while only 5–20 mg/L is required for low-turbidity river water (SS＜100 mg/L).
Types of Flocculants
Inorganic Flocculants (Polyaluminum Chloride PAC, Polyferric Sulfate PFS)
The applicable dosage range is relatively wide: 20–50 mg/L for ordinary wastewater, and 50–200 mg/L for high-chroma wastewater such as printing and dyeing wastewater.
Organic Polymer Flocculants (Polyacrylamide PAM)
The required dosage is low: 0.1–1 mg/L for anionic PAM and 1–5 mg/L for cationic PAM. However, the charge type and molecular weight shall be adjusted according to actual water quality.
Composite Flocculants (Combined PAC and PAM)
The total chemical dosage can be reduced. For example, dosing PAC at 30–100 mg/L together with PAM at 0.5–2 mg/L cuts the comprehensive treatment cost by 20%–30% compared with using inorganic flocculants alone.
III. Dynamic Adjustment Strategies
Real-Time Monitoring and Feedback Regulation
Install online turbidity meters, pH meters and other instruments to monitor effluent water quality in real time. For example, when the turbidity of sedimentation tank effluent rises abruptly, the flocculant dosage may need to be increased by 5%–10%.
Conduct regular raw water quality testing (once per day for example), and adjust the flocculant dosage according to variations in SS, COD and phosphate concentration. For instance, when raw water SS rises due to rainwater scouring during flood seasons, the aluminum salt dosage shall be increased synchronously by 20%–30%.
Staged Dosing
Pretreatment Stage
Dose part of the flocculant (50% of the total dosage for example) before the mixing tank to rapidly neutralize colloidal charges and reduce the treatment load of subsequent units.
Main Treatment Stage
Dose the remaining flocculant in the flocculation tank to promote floc enlargement. For example, an enterprise adopts the split dosing mode of "pre-dosing PAC at 5 mg/L + main dosing PAC at 10 mg/L", which improves the phosphate removal rate by 15% compared with one-time dosing.
Emergency Adjustment Measures
Hydraulic Shock Load
When the inflow rate surges suddenly, increase the flocculant dosage proportionally (e.g., if the flow rate rises by 50%, the dosage shall be increased by 50% synchronously), and strengthen stirring intensity to avoid floc fragmentation.
Abrupt Water Quality Deterioration
If raw water is contaminated leading to a sharp rise in organic matter concentration, add oxidants such as chlorine to break the stability of organic substances before adjusting the flocculant dosage. For example, when the COD of raw water in a plant suddenly rises to 300 mg/L, the PAM dosage is increased from 1 mg/L to 3 mg/L, and chlorine is dosed at 5 mg/L simultaneously. As a result, the effluent turbidity drops from 50 NTU to 10 NTU.
IV. Cost Control and Optimization
Economic Efficiency Evaluation
Calculate the unit water treatment cost by the following formula:
Flocculant cost = Daily flocculant consumption (kg/d) × Unit price (CNY/kg) ÷ Daily water treatment capacity (m³/d)
For example, the unit price of PAC is 2000 CNY per ton, the daily water treatment capacity is 1000 m³, and the dosage is 50 mg/L (0.05 kg/m³). The daily flocculant cost is 100 CNY, and the unit treatment cost is 0.1 CNY/m³.
Compare the cost performance of different flocculants through laboratory tests. For instance, an enterprise compared PAC (treatment cost: 0.12 CNY/m³) and PFS (treatment cost: 0.09 CNY/m³), and found that PFS delivers equivalent treatment efficiency at pH 8–10 while cutting the chemical cost by 25%.
Chemical Saving Measures
Optimize Dissolution and Dosing Systems
Adopt all-in-one chemical dosing systems consisting of dissolving tanks, agitators and metering pumps to reduce chemical residue and waste. For example, after equipment retrofitting in a plant, the flocculant utilization rate rose from 85% to 95%, saving 100,000 CNY in annual chemical costs.
Regular Equipment Cleaning
Clean dosing tanks and pipelines periodically to prevent scaling, which would cause inaccurate flocculant dosing. For example, cleaning dosing pipelines once per month can reduce dosage fluctuation by 5%–10%.