Process Design Plan for Solvent-Containing Wastewater Treatment in the Printing and Packaging Industry

I. Project Background and Wastewater Characteristics

During the production processes of the printing and packaging industry, such as ink preparation, printing press cleaning, and equipment wiping, significant amounts of complex solvent-containing wastewater are generated. This type of wastewater exhibits the following main characteristics:

• Variety of Organic Solvents: Contains volatile organic compounds (VOCs) and semi-volatile organic compounds such as ethanol, isopropanol, ethyl acetate, acetone, butanone, toluene, and xylene.

• High and Fluctuating Pollutant Concentrations: Chemical Oxygen Demand (COD) typically ranges from 5000 to 50000 mg/L, sometimes even higher, with significant variations in water quality and quantity depending on production batches.

• Variable Biodegradability: Some solvents like alcohols and esters are readily biodegradable, while others such as aromatic hydrocarbons are resistant to biological degradation.

• Possible Presence of Heavy Metals and Pigments: Some inks contain heavy metal ions like lead, chromium, cadmium, as well as suspended solids like carbon black and organic pigments.

• Volatility and Toxicity: If not properly treated, they can easily cause atmospheric pollution and pose risks to human health and the ecological environment.

Given these characteristics, a combination of processes is required for efficient and stable treatment to ensure compliant discharge, while also considering resource recovery and economic viability.

II. Design Basis and Objectives

• Design Scale: Determined based on the actual wastewater discharge volume of the enterprise, with typical treatment capacities ranging from 10 to 200 tons per day.

• Design Influent Water Quality (Reference):

  • pH: 6.0-9.0

  • COD: 3000-20000 mg/L

  • BOD₅: 800-6000 mg/L

  • Total Volatile Organic Compounds (TVOC): 500-5000 mg/L

  • Suspended Solids (SS): 200-1000 mg/L

  • Chroma: 200-1000 times

• Design Effluent Water Quality: Compliant with the Integrated Wastewater Discharge Standard(GB8978-1996) Level III or stricter local standards. Main indicator limits, for example:

  • COD ≤ 500 mg/L (or meeting municipal sewer discharge requirements)

  • BOD₅ ≤ 300 mg/L

  • TVOC meets requirements for coordinated control of air and water pollutants

  • SS ≤ 400 mg/L

• Design Principles:

  • Segregated collection at source, separate pretreatment based on quality.

  • Main technical route: "Physicochemical Pretreatment + Biological Treatment + Advanced Treatment".

  • Prioritize solvent recovery and resource utilization to reduce treatment load and cost.

  • Effective collection and treatment of exhaust gases to avoid secondary pollution.

  • Automated control for stable and reliable operation.

III. Recommended Treatment Process Flow

For solvent-containing wastewater from printing and packaging, this plan recommends a combined process of "Segregated Collection and Pretreatment → Efficient Physicochemical Treatment → Enhanced Biological Treatment → Advanced Polishing and Safeguard". The specific flow is as follows:

Process Description:

1. Segregated Collection and Pretreatment:

   • High-concentration waste solvents: Collected separately at generation points (e.g., ink tank cleaning). Priority should be given to solvent recovery using distillation or rectification units. The recovered solvent can be reused in production, reducing pollutant discharge and generating economic benefits. The residue should be disposed of as hazardous waste or fed into the subsequent wastewater treatment system.

   • Low-concentration rinse wastewater: Collected centrally and directed to an equalization tank for homogenization of quality and quantity.

2. Comprehensive Physicochemical Pretreatment:

   • Coagulation Flotation: Coagulants (e.g., PAC, PFS) and flocculants (PAM) are dosed into the composite wastewater. A flotation unit removes most suspended particles, colloidal substances, some oils and grease, and color. It also removes a portion of high-molecular-weight organic pollutants. This unit effectively reduces the load on subsequent biological treatment.

3. Enhanced Biological Treatment (Core Unit):

   • Hydrolysis Acidification: Utilizes hydrolytic and acidogenic bacteria to break down refractory macromolecular organics in the wastewater into smaller, more biodegradable molecules, improving the wastewater's biodegradability (B/C ratio) and creating favorable conditions for subsequent treatment.

   • Anaerobic Biological Treatment: Utilizes an Upflow Anaerobic Sludge Blanket (UASB) or Internal Circulation (IC) reactor. In the high-concentration organic wastewater stage, anaerobic microorganisms convert organics into methane (CH₄) and carbon dioxide (CO₂), achieving significant COD removal (60%-80% removal rate possible) and producing biogas for energy recovery.

   • Aerobic Biological Treatment: Utilizes biological contact oxidation or a Membrane Bioreactor (MBR). Aerobic microorganisms further degrade the remaining organics. The MBR process uses membrane separation instead of a secondary clarifier, yielding better effluent quality and higher sludge concentration with a smaller footprint, though with higher investment and maintenance costs.

4. Advanced Treatment and Safeguard:

   • If the effluent after biological treatment still fails to meet standards stably (especially COD and color), the advanced treatment unit is activated.

   • Advanced Oxidation Processes: Such as Fenton oxidation (H₂O₂/Fe²⁺) or ozone catalytic oxidation. These utilize the strong oxidizing power of generated hydroxyl radicals (·OH) to thoroughly degrade refractory organic compounds, ensuring effluent COD and color compliance.

5. Sludge Treatment:

   • Sludge generated during physicochemical and biological processes is directed to a sludge thickener. After thickening, it is dewatered by a sludge dewatering machine (e.g., plate and frame filter press) to form a sludge cake with ≤60% moisture content, which is disposed of as hazardous waste by a licensed entity.

6. Exhaust Gas Treatment (Ancillary):

   • Enclose and collect exhaust gases from potential VOC and odor sources such as the equalization tank, anaerobic reactor, and sludge treatment areas.

   • The collected exhaust gases are directed to a treatment system, such as "Activated Carbon Adsorption Unit", "UV Photocatalytic Oxidation Unit", or "Regenerative Thermal Oxidizer (RTO)" for treatment before compliant discharge.

IV. Main Structure and Equipment Design Parameters (Example: 50 tons/day scale)

V. Economic Analysis

• Capital Cost Estimate: Total investment approximately 1.5 - 4.0 million RMB (for 50 tons/day scale), with the exhaust gas treatment system accounting for about 15%-25%. Main investments include: civil works, main equipment (flotation unit, anaerobic reactor, aerobic system, advanced oxidation equipment, dewatering machine), piping, valves, electrical, and automation control systems, exhaust gas treatment unit.

• Operating Cost: Approximately 15 - 35 RMB per ton of water, mainly including:

  • Electricity: 5-10 RMB/ton (pumps, blowers, mixers, etc.).

  • Chemicals: 5-15 RMB/ton (coagulants, oxidants, acids/alkalis, etc., higher when concentrations are high).

  • Steam: 2-5 RMB/ton (if distillation for solvent recovery or sludge thermal drying is used).

  • Labor: 2-3 RMB/ton.

  • Sludge Disposal: 3-5 RMB/ton (high cost for hazardous waste disposal).

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

• Economic Benefits: Value of recovered solvents can partially offset operating costs; biogas recovery can save some energy costs; avoids environmental fines, ensuring sustainable production.

VI. Operation Management and Recommendations

1. Source Control: Promote cleaner production, use environmentally friendly water-based inks and cleaners to reduce the use of toxic and harmful solvents at the source.

2. Strict Segregation and Diversion: It is crucial to achieve complete segregation of high and low-concentration wastewaters. This is key to ensuring treatment efficiency and reducing costs.

3. Parameter Monitoring and Optimization: Regularly monitor influent and effluent parameters (COD, pH, VOCs, etc.) for each unit, and promptly adjust operational parameters such as chemical dosing and aeration based on water quality changes.

4. Biological System Maintenance: Ensure the biological system has balanced nutrients (C:N:P ≈ 100:5:1). Regularly monitor sludge characteristics to prevent acidification in the anaerobic system or sludge bulking in the aerobic system.

5. Safety and Compliance: Solvents are flammable and explosive. Storage and recovery areas must meet explosion-proof requirements. Exhaust gases, wastewater, and hazardous waste must be disposed of in accordance with laws and regulations, with complete management records maintained.

6. Startup and Commissioning: During biological system startup, specialized cultures or activated sludge should be added for acclimation. The commissioning period is approximately 1-2 months.

This plan, through the combined process of "Segregated Pretreatment - Physicochemical - Anaerobic - Aerobic - Advanced Oxidation," can effectively address the challenges of high concentration, poor biodegradability, and complex composition of solvent-containing wastewater in the printing and packaging industry. It achieves stable and compliant discharge while balancing resource recovery and operational economy, providing technical assurance for the enterprise's green and sustainable development.