The following is a detailed description of the "primary fluoride removal reaction + primary fluoride removal sedimentation + secondary fluoride removal reaction + secondary fluoride removal sedimentation + two-stage A/O + advanced treatment" combined process for treating 165 tons of photovoltaic wastewater per day:
1. Process flow overview
Complete chain: Primary fluoride removal reaction tank → Primary fluoride removal sedimentation tank → Secondary fluoride removal reaction tank → Secondary fluoride removal sedimentation tank → Primary A/O tank → Secondary A/O tank → Advanced treatment (filtration/adsorption/higher oxidation) → Discharge water
2. Detailed explanations of the principles and functions of each unit
(1)Two-stage chemical fluoride removal system
Objective: To reduce the fluoride concentration from high levels to less than 10 mg/L using the chemical precipitation method.
Primary fluoride removal reaction tank:
Reagent addition: Lime (Ca(OH)₂) or calcium chloride (CaCl₂) are added to form calcium fluoride (CaF₂) precipitate.
Reaction conditions: Adjust the pH to 10-11, and stir the reaction for 30-60 minutes.
Feature: The first-stage fluoride removal can remove over 80% of the fluoride, but the residual fluoride may still exceed the standard.
Primary fluoride removal sedimentation tank:
Objective: Separate the sludge through inclined plate sedimentation or radial flow sedimentation, and the supernatant liquid will enter the secondary fluoride removal process.
Secondary fluoride removal reaction tank:
Packing addition: Polyaluminium chloride (PAC) or aluminium sulfate is added to form a complex.
Reaction conditions: Adjust the pH to 6-7 and add PAM for coagulation.
Feature: The secondary fluoride removal process can reduce the fluoride content to below 5mg/L.
Secondary fluoride removal sedimentation tank:
Objective: To further separate the aluminum fluoride sludge and let the effluent enter the biochemical system.
(2) Two-stage A/O biological system
Objective: To degrade COD (which is often present in photovoltaic wastewater as difficult-to-degrade organic substances such as ethylene glycol and isopropanol) and remove nitrogen.
Primary A/O (Anoxic-Aerobic):
Function of the anoxic tank: Denitrification for nitrogen removal, and simultaneous hydrolysis and acidification for the degradation of large-molecule organic substances.
Aerobic tank function: Nitrification (NH₄⁺ → NO₃⁻) and COD degradation.
Secondary A/O:
Objective: To enhance the treatment of refractory organic matter and further remove nitrogen. Special bacterial strains can be added.
(3) Deep Processing Unit
Multimedia filtration: Removal of suspended solids (SS).
Activated carbon adsorption: Removes residual organic substances and color.
Advanced oxidation: Decomposition of refractory COD.
Membrane treatment (UF/RO): Further desalination (if the wastewater contains high TDS)
3. Sludge Treatment
Chemical sludge (CaF₂, AlF₃): After plate and frame filtration, it is disposed of as hazardous waste.
Biochemical sludge: Part of it is returned to the A/O tank, while the remaining sludge is dehydrated and then transported away.
