Process Design Scheme for Canned Food Processing Wastewater Treatment

I. Design Basis and Water Quality Characteristics

1.1 Wastewater Sources and Characteristics

Wastewater from canned food processing mainly originates from raw material washing, peeling/pitting, blanching, sterilization, container washing, and floor cleaning. While water quality varies significantly with product type (fruits/vegetables, meat, seafood, etc.), it generally shares the following commonalities:

• High Organic Load: Rich in carbohydrates, starch, pectin, protein, fats, etc. COD typically ranges from 2000-8000 mg/L. The high BOD₅/COD ratio (>0.5) indicates good biodegradability.

• High Suspended Solids Content: Contains fruit/vegetable peels, pulp fragments, and vegetable residues, resulting in high SS concentration, with some in colloidal state.

• Significant Fluctuations in Water Quality and Quantity: Exhibits distinct seasonality and periodicity, with high volume and concentration during peak production seasons, leading to strong shock loads.

• Potential Presence of Oils and Salts: Meat and seafood canning wastewater contains animal/vegetable oils; washing water from some vegetable canning may have elevated salinity.

• Prone to Putrefaction and Odor: High organic content makes it susceptible to fermentation and odor generation if retained.

1.2 Design Objectives and Principles

• Design Influent Quality (Reference): COD 3000 mg/L, BOD₅ 1500 mg/L, SS 800 mg/L, NH₃-N 30 mg/L, Animal/Veg Oil 50 mg/L, pH 5-10.

• Design Effluent Quality: Meets Class I of the Integrated Wastewater Discharge Standard(GB 8978-1996) or stricter local standards. Key targets: COD ≤ 100 mg/L, BOD₅ ≤ 20 mg/L, SS ≤ 70 mg/L, NH₃-N ≤ 15 mg/L.

• Design Principles:

  1. Mature and Reliable Technology: Select efficient, stable processes resilient to shock loads.

  2. Enhanced Pretreatment: Prioritize removal of SS and oils to protect subsequent biological systems.

  3. Resource and Energy Recovery: Consider biogas recovery and sludge agricultural use to reduce operating costs.

  4. Economically Sound: Optimize capital and operating expenses while ensuring compliance.

II. Core Treatment Process Route

Addressing the characteristics of canning wastewater, this scheme recommends the main process route of "Enhanced Pretreatment + Anaerobic Energy Recovery + Aerobic Main Treatment + Advanced Safeguard". This route is technologically mature, stable, and has strong shock load resistance.

2.1 Full-Process Flow Diagram

III. Key Design Points for Treatment Units

3.1 Pretreatment Unit

• Screening: Install coarse and fine screens (gap 5-10mm and 1-5mm) to intercept raw material residues, preventing pump and pipe blockages.

• Equalization Tank: Designed for 8-16 hours of average flow, equipped with aeration or mixing to homogenize quality/quantity and mitigate shock loads.

• Flotation/Oil Separation Unit: Use Dissolved Air Flotation (DAF) with coagulant dosing to efficiently remove emulsified oils, colloidal organics, and suspended solids. COD removal can reach 30-50%. This unit is crucial for stabilizing subsequent anaerobic and aerobic systems.

3.2 Biological Treatment Unit (Core)

• Hydrolysis Acidification Tank: Breaks down macromolecular organics into smaller molecules, increasing the B/C ratio and improving biodegradability. HRT is designed for 6-10 hours.

• Anaerobic Treatment Unit:

  • Recommended Process: UASB (Upflow Anaerobic Sludge Blanket) or IC (Internal Circulation) Reactor. Anaerobic microorganisms achieve COD removal >80% and produce biogas. UASB suits medium-concentration wastewater with lower investment; IC handles high concentration/shock loads better.

  • Design Key Points: Control temperature in the mesophilic range (35±2°C) and maintain optimal pH (6.8-7.5). Produced biogas requires desulfurization and dehydration before use.

• Aerobic Treatment Unit:

  • Process Options: A/O (Anoxic/Oxic) Process or Biological Contact Oxidation.

  • A/O Process: Utilizes influent carbon source for denitrification in the anoxic zone; completes nitrification and carbon oxidation in the oxic zone, providing good nitrogen removal.

  • Biological Contact Oxidation: Features high attached biomass, no sludge return needed, good shock load resistance, and simple management.

  • COD removal in the aerobic unit can reach 85-90%.

3.3 Advanced Treatment and Sludge Unit

• Advanced Treatment: Serves as a safeguard. Fenton Oxidation (H₂O₂/Fe²⁺) is employed when biological effluent is unstable, effectively degrading refractory organics to ensure COD and color compliance.

• Sludge Treatment: Combine physicochemical sludge (flotation scum) with excess biological sludge. After thickening, dewater using a belt filter press or screw press dewaterer. As the sludge is rich in organics and low in toxicity, the dewatered cake (moisture content <80%) can be used as organic fertilizer raw material or landscaping soil.

IV. Main Design Parameters and Equipment

4.1 Key Design Parameters (Example: 1000 m³/d Treatment Scale)

4.2 Main Equipment Selection

• Screen (Rotary/Step type), Submersible sewage pump (corrosion-resistant), DAF package unit, UASB influent distribution system & three-phase separator, Aeration equipment (fine bubble diffuser/jet aeration), Sedimentation tank scraper/suction machine, Sludge dewatering machine (belt press/screw press).

V. Economic Analysis

• Capital Cost Estimate: Approximately 3 - 6 million RMB, mainly including civil works, equipment, piping, electrical, and automation systems.

• Operating Cost: 1.0 - 2.0 RMB per ton of water (excluding depreciation).

  • Electricity: 0.5-1.0 RMB/ton (pumps, blowers, mixers).

  • Chemicals: 0.3-0.6 RMB/ton (PAC, PAM, nutrients).

  • Labor: 0.2-0.4 RMB/ton.

• Benefits: Biogas recovery can be used for heating, saving energy; compliant effluent avoids environmental penalties; sludge for agriculture reduces disposal costs. The system runs stably and is easy to manage.

VI. Operation Management and Recommendations

1. Production Coordination: Strengthen communication with the production department to balance drainage and avoid shock loads of high-concentration wastewater.

2. Nutrient Adjustment: Canning wastewater is often deficient in nitrogen and phosphorus. Dose urea and phosphates according to the ratio BOD₅:N:P ≈ 100:5:1 to support microbial growth.

3. Seasonal Adjustment: Reduce aeration during off-season to maintain sludge activity; acclimate sludge in advance before the peak season.

4. Safety Management: Anaerobic and sludge treatment areas may accumulate gases like H₂S. Ensure proper ventilation and adhere to fire/explosion prevention measures.

This scheme, through a mature combination of processes, effectively addresses the high concentration and variability of canned food processing wastewater, achieving stable discharge compliance while demonstrating sound techno-economic performance.