Comprehensive Wastewater Treatment Process Design Plan for the Tannery Industry (Decolorization and Desalination Special Section)

I. Design Basis and Objectives

1.1 Water Quality Characteristics and Challenge Analysis

Tannery composite wastewater has an extremely complex composition. Even after conventional pretreatment (screening, equalization, coagulation sedimentation) and biological treatment (anaerobic, aerobic), residual color and salinity in the effluent remain the primary bottlenecks for advanced treatment and reuse, characterized as follows:

• Complex Sources of Color: Primarily from incompletely degraded dyes, tanning agents (e.g., chrome tanning, vegetable tanning agents) and their hydrolysis/oxidation by-products, forming refractory chromophoric organics that result in persistent color.

• High Salinity Concentration: Mainly originates from the extensive use of NaCl, Na₂SO₄, Ca(OH)₂, (NH₄)₂SO₄, etc., in processes like pickling, liming, deliming, bating, and tanning. This leads to effluent TDS (Total Dissolved Solids) typically as high as 2000-8000 mg/L, with Cl⁻ concentration potentially reaching 1000-4000 mg/L. High salinity inhibits and corrodes microorganisms and metal equipment and hinders reuse.

• Poor Biodegradability: The wastewater at the advanced treatment stage typically has a low B/C ratio, with residual pollutants mostly being refractory to biodegradation.

• Objective: This plan aims to design an efficient and stable advanced treatment system focusing on resolving color and salinity issues, enabling the effluent to meet stringent local discharge standards (especially for total dissolved solids and chloride ion indicators) or achieve specific process water quality requirements for reuse.

1.2 Design Influent and Effluent Targets (Advanced Treatment Stage)

II. Selection of Decolorization and Desalination Process Route

Addressing the characteristics of "refractory color and ionic salinity," this plan recommends a combined process route of "Chemically Enhanced Decolorization + Membrane-based Salt Fractionation/Concentration + Ultimate Volume Reduction." The core principles of this route are:

1. Fractionated Removal: Firstly, destroy chromophores and remove color/partial COD through chemical or advanced oxidation methods, providing protection for subsequent desalination membrane systems.

2. Graded Desalination: Utilize membrane processes with different selectivity for mono- and divalent ions to achieve staged separation and concentration of salts, reducing the load on ultimate treatment.

3. Resource Recovery and Volume Reduction: Subject the high-concentration brine to fractional crystallization or evaporation/solidification to reduce waste volume and explore potential for salt resource recovery.

III. Detailed Design of the Decolorization and Desalination Process System

3.1 Process Flow Diagram

3.2 Explanation of Core Unit Processes

(1) Chemically Enhanced Decolorization Unit

• Function: Serves as the first stage of advanced treatment, further removing residual color and colloidal organics, reducing the risk of organic fouling for subsequent membrane systems.

• Process: Employs a "High-efficiency Decolorant/Coagulant Composite Dosing" process. After rapid mixing via a pipeline mixer, the water enters a decolorization reaction tank for slow flocculation, followed by solid-liquid separation in a clarifier. The high-efficiency decolorant can specifically destroy complex chromophores in tannery wastewater. Used in conjunction with PAC and PAM, it forms dense flocs, achieving 70-90% color removal. The effluent then passes through a multi-media filter to further ensure turbidity compliance.

(2) Core Membrane-based Salt Fractionation and Desalination Unit

• Function: This is the core unit for desalination, utilizing the combined application of Nanofiltration (NF) and Reverse Osmosis (RO) to achieve efficient salt separation and maximize freshwater production.

• Nanofiltration (NF) Salt Fractionation System:

  • Principle: Utilizes the high rejection rate of NF membranes for divalent ions (e.g., SO₄²⁻, >95%) and partial permeability for monovalent ions (e.g., Cl⁻, Na⁺) to preliminarily separate sulfate salts from chlorides in the wastewater.

  • Effect: The NF permeate is primarily sodium chloride, with reduced TDS and very low hardness and sulfate, significantly mitigating scaling risk for the subsequent RO system. The NF concentrate is enriched with sodium sulfate, creating conditions for subsequent resource recovery or separate treatment.

• Reverse Osmosis (RO) Desalination System:

  • Principle: Performs deep desalination on the NF permeate. RO membranes have very high rejection rates (>98%) for almost all ions.

  • Effect: The RO permeate TDS can be reduced to <100 mg/L, meeting high-standard reuse requirements (e.g., cooling tower makeup, some process water). The RO concentrate is a high-concentration sodium chloride solution directed to the ultimate treatment unit.

(3) Brine Ultimate Treatment and Resource Recovery Unit

• Function: Treats the NF and RO concentrates to achieve "Zero Liquid Discharge" (ZLD) or "Near ZLD" and explores salt resource recovery.

• NF Concentrate (Sodium Sulfate type) Treatment: Can employ "MVR Evaporation/Crystallization" to directly produce anhydrous sodium sulfate (salt cake) crystalline salt. If purity meets standards, it can be sold as a by-product for resource recovery; otherwise, it is disposed of as hazardous waste.

• RO Concentrate (Sodium Chloride type) Treatment: Due to the presence of refractory organics, it requires prior treatment via "Ozone Catalytic Oxidation" or "Electrochemical Oxidation" for deep mineralization and complex breaking, before entering a "MVC (Mechanical Vapor Compression)" or "MED (Multi-Effect Distillation)" system for evaporation/crystallization, producing industrial sodium chloride salt (quality requires verification) or disposed of as mixed salt.

IV. Key Design Parameters and Equipment Selection Points

V. Economic Analysis

• Capital Cost Estimate: Investment for the decolorization/desalination and ZLD system is relatively high, accounting for approximately 40-60% of the total plant investment. For a treatment capacity of 1000 m³/d, the advanced treatment stage investment is roughly 8-15 million RMB, primarily concentrated in the membrane systems and evaporation/crystallization units.

• Operating Cost: 8 - 15 RMB/m³ of water (for the advanced treatment stage only), mainly including:

  • Power Consumption: 3-6 RMB/m³ (significant consumption by high-pressure pumps in membrane systems and compressors in evaporation/crystallization).

  • Chemical Costs: 1-2 RMB/m³ (decolorant, antiscalant, cleaning agents).

  • Membrane Replacement: 1-2 RMB/m³.

  • Steam/Heat Energy: 1-3 RMB/m³ (if MVR heat is insufficient and requires supplementation).

  • Maintenance & Depreciation: 2-3 RMB/m³.

• Benefits: Production of high-quality reuse water saves on freshwater and discharge fees; successful salt resource recovery can partially offset operating costs.

VI. Conclusion and Recommendations

The combined process of "Chemical Decolorization - NF Salt Fractionation - RO Desalination - Evaporation/Crystallization" proposed in this plan is an effective advanced treatment and resource recovery route for tannery wastewater with high color and salinity characteristics. NF salt fractionation is the technical key of this process, enabling salt classification, reducing ultimate treatment difficulty and cost, and offering potential for resource recovery.

Implementation Recommendations:

1. Strengthen Pretreatment: Ensuring stable, compliant water quality (SS, hardness, organics) entering the advanced treatment system is a prerequisite for the long-term stable operation of membrane systems.

2. Pilot Testing: Long-term pilot testing on actual wastewater is essential to determine the actual separation efficiency of NF membranes for sulfate/chloride ions, optimal recovery rates, and membrane fouling characteristics, and to optimize chemical dosing.

3. Material Corrosion Resistance: The high chloride ion environment imposes very high requirements on the corrosion resistance grade of all equipment, piping, and instrumentation materials. Corrosion-resistant materials must be selected.

4. Intelligent Operation & Maintenance: Establish a comprehensive online monitoring and automated control system, especially for real-time monitoring and alarm functions for membrane system pressure, flow, and conductivity, to achieve precise chemical dosing and cleaning.