Resource Recovery and Reuse in Process Design for Fruit Juice Processing Wastewater Treatment

I. Design Background and Core Philosophy

1.1 Wastewater Characteristics and Resource Recovery Potential

Fruit juice processing wastewater mainly originates from fruit washing, crushing, pressing, polishing filtration, sterilization, and equipment/floor washing. Its prominent characteristics are:

• High Organic Load: Rich in sugars, pectin, organic acids, proteins, vitamins, etc. It has a high BOD₅/COD ratio (typically >0.5), indicating excellent biodegradability.

• Abundant Nutrients: Contains plant nutrients like nitrogen, phosphorus, and potassium.

• Variable Suspended Solids Content: Contains fibrous suspended solids like fruit pomace and peels.

• Seasonal Fluctuations in Quality and Quantity: Closely related to the type of fruit processed and the season.

Based on these characteristics, this wastewater is not just a "pollutant" but a potential "resource reservoir" with high recovery value for water resources, energy, and nutrients. The core design philosophy of this plan is: "Graded Treatment, Segmented Reuse Based on Quality, Synergistic Recovery of Energy and Materials", aiming to achieve a balance between environmental and economic benefits.

1.2 Design Objectives

• Treatment Objective: Ensure all discharged water meets the Class I standard of the Integrated Wastewater Discharge Standard(GB8978-1996) or stricter reuse water standards.

• Resource Recovery Objectives:

  1. Water Resource Reuse Rate ≥ 60%: Product water used for plant landscaping, road washing, cooling tower makeup, raw water for boiler softener preparation, etc.

  2. Energy Recovery: Recover biogas through anaerobic processes, converting it into heat or electricity, supplementing 10%-30% of the plant's energy consumption.

  3. Material Recovery: Extract high-value-added products (e.g., pectin) from high-concentration wastewater, or convert it into high-nutrition organic fertilizer/liquid fertilizer.

II. Overall Resource Recovery and Reuse Process Route

This plan adopts a comprehensive route of "Source Segregation, Material Extraction; Mid-stage Anaerobic, Energy Recovery; End-stage Purification, Water Graded Reuse". The specific process design is as follows:

III. Detailed Design of Core Resource Recovery Units

3.1 Source Segregation and High-Value Material Extraction Unit

• High-Concentration Wastewater (Pressing, Polishing Water):

  • Pretreatment: First, recover coarse fibers (usable as feed or fuel) via a rotary screen or screw press, then use Dissolved Air Flotation (DAF) to remove most suspended solids, colloids, and some oils.

  • Pectin Extraction (High-Value-Added Pathway): For wastewater high in pectin (e.g., from apples, citrus, pomelo), a Pectin Extraction and Concentration System can be added. Processes like acid hydrolysis, alcohol precipitation, filtration, and drying are used to extract commercial pectin. This unit significantly reduces the organic load for subsequent treatment and generates direct economic benefits. The residue after extraction still enters the anaerobic system.

• Low-Concentration Wastewater (Washing, Rinsing Water): After removing large debris with a bar screen, it enters a separate equalization tank. It can be considered for direct integration into the subsequent biological system or, after simple physicochemical treatment, used for initial rinsing stages, achieving "segregated flows, multi-purpose water use."

3.2 Energy Recovery Unit (Anaerobic Digestion)

• Core Process: The pre-treated high-concentration wastewater is mixed with low-concentration wastewater and pumped into an Upflow Anaerobic Sludge Blanket (UASB) or Internal Circulation (IC) Anaerobic Reactor.

• Energy Conversion Process: Under the action of anaerobic microorganisms, organic matter (sugars, organic acids, etc.) in the wastewater is converted into biogas (mainly CH₄, 55%-75%).

• Design Key Points:

  • Mesophilic Digestion: Control reactor temperature at 35-38°C (can utilize waste heat from boilers or biogas power generation).

  • Organic Loading Rate: Design volumetric loading rate based on water quality (typically 6-15 kgCOD/(m³·d)), ensuring COD removal >80%.

  • Biogas Utilization: After dehydration and desulfurization, biogas is used for direct combustion in biogas boilers for heating or to drive biogas generator sets. Waste heat from power generation can be recycled for anaerobic tank insulation, forming an internal energy cycle.

3.3 Water Purification and Graded Reuse Unit

The anaerobic effluent, mixed with low-concentration wastewater, still contains a certain amount of organics and nitrogen/phosphorus, requiring aerobic treatment.

• Biological Treatment: Uses processes like "Hydrolysis Acidification + Aerobic (A/O)" or a Membrane Bioreactor (MBR) for deep removal of COD, BOD, and NH₃-N, ensuring stable, compliant effluent.

• Graded Reuse Process:

  • High-Standard Reuse Water (for cooling towers, boiler softener preparation): Aerobic effluent passes through Ultrafiltration (UF) to remove bacteria and colloids, then through Reverse Osmosis (RO) or Nanofiltration (NF) for desalination and softening. Product water quality is close to tap water, with conductivity <200 µS/cm. RO concentrate can be recycled to the front end or treated separately.

  • General Reuse Water (for landscaping, road washing, vehicle cleaning): Aerobic effluent only needs sand filtration and disinfection (sodium hypochlorite or UV) to stably meet requirements at low cost.

3.4 Byproduct Utilization Pathways

• Digested Sludge and Biological Sludge: Digested sludge from anaerobic digestion and excess aerobic sludge are dewatered by a screw press dewaterer or plate and frame filter press and can serve as high-quality raw material for organic fertilizer. On-site aerobic composting can produce nutrient soil or liquid fertilizer for associated orchards or farmland; alternatively, it can be disposed of off-site.

• Recovered Coarse Fibers: Can be used as biomass fuel or an additive for livestock feed.

IV. Technical and Economic Analysis and Resource Recovery Benefits

4.1 Investment and Operating Cost Estimation (Example: 1000 m³/d treatment scale)

4.2 Resource Recovery Benefit Estimation (Annual)

Conclusion: Through resource-oriented design, the system's annual net operating cost can be significantly reduced, potentially even achieving profitability in cases of efficient recovery like pectin extraction. The estimated payback period is 3-8 years.

V. Operation Management and Implementation Recommendations

1. Source Segregation is Key: Must construct a complete segregated piping network to strictly separate high and low-concentration wastewater. This is the foundation for all resource recovery processes.

2. Seasonal Adjustment: Process parameters (e.g., anaerobic loading, reuse ratio) need flexible adjustment according to water quality/quantity changes in different fruit processing seasons.

3. Intelligent Monitoring: Establish an online monitoring system (flow, COD, pH, biogas flow) for precise management of energy and resource recovery.

4. Reuse Network Matching: The reuse water distribution network should be built independently, supplying different zones according to different water quality standards (landscaping, washing, cooling) to ensure safety.

5. Pilot Testing: Before implementing schemes like pectin extraction or high-value biogas utilization, laboratory-scale and pilot-scale testing are strongly recommended to accurately assess economic feasibility.

This plan transforms fruit juice processing wastewater from a "treatment burden" into a "resource treasure trove". Through the synergistic recovery of water, energy, and materials, it not only achieves the environmental goal of "zero" or "negative" growth in pollutants but can also create significant economic benefits for the enterprise. It serves as an exemplary path for promoting the green, circular, and low-carbon development of the fruit juice processing industry.