Advanced Treatment in the Process Design Scheme for Food Waste Treatment Wastewater

I. Design Basis and Objectives for Advanced Treatment

1.1 Influent Water Quality Characteristics (Advanced Treatment Stage Influent)

Food waste treatment wastewater, after undergoing front-end "pretreatment (solid-liquid separation, grit and grease removal) + main biological treatment (e.g., anaerobic digestion, A/O, or MBR)", still exhibits the following characteristics, which are the focus and challenges of advanced treatment:

• Residual Refractory Organics: Contains macromolecular and heterocyclic refractory organic compounds such as lignin, cellulose degradation products, and humic acids, making it difficult to stably reduce COD to low levels (typically 150-400 mg/L).

• Prominent Ammonia and Total Nitrogen Issues: Ammonia nitrogen in the biological effluent may already be low, but total nitrogen (mainly nitrate nitrogen) concentration may still be high (30-80 mg/L), requiring further removal.

• Chroma and Turbidity: Appears dark yellow to brown, with high color, affecting aesthetics and reuse.

• Potential Presence of Pathogens and Salts: Need to ensure compliance with hygienic indicators. Salts may be introduced from cleaning, disinfection, etc.

1.2 Design Objectives and Water Quality Standards

The core objective of advanced treatment is to ensure the effluent stably meets the Class A standard of the Water Quality Standard for Discharge to Municipal Wastewater Pipelines(GB/T 31962-2015) or stricter local acceptance/ discharge standards. It should also have the potential for upgrading towards reuse applications like landscaping, irrigation, and flushing. Specific targets (example) are as follows:

1.3 Design Principles

1. Targeted: Select processes targeting core issues: refractory COD, nitrate nitrogen, color, etc.

2. Synergistic: Units should connect effectively, achieving a "1+1>2" synergistic treatment effect.

3. Safeguarding: As the final barrier for compliant discharge, it must have high reliability and shock load resistance.

4. Economical: Under the premise of ensuring effectiveness, choose process combinations with reasonable operating costs and simple management.

II. Design of Core Advanced Treatment Process Route

To achieve the above objectives, a combined process route of "High-Efficiency Clarification Safeguard + Advanced Oxidation for Complex Breaking + Deep Denitrification + Fine Filtration & Disinfection" is recommended. This route aims to progressively reduce pollution factors, ultimately achieving comprehensive water quality compliance and reuse potential.

The specific process flow diagram is as follows:

III. Explanation of Key Advanced Treatment Unit Processes

3.1 High-Efficiency Coagulation-Sedimentation Unit (Pretreatment & Safeguard)

• Function: Removes residual colloids, suspended solids, part of colloidal COD, and color from the biological effluent. Simultaneously ensures TP compliance through chemical phosphorus removal, providing high-quality influent for subsequent advanced oxidation or membrane treatment, reducing their load and fouling risk.

• Process Points: Dosing coagulant (e.g., PAC, PFS) and flocculant (PAM), using mechanical mixing flocculation and tube/plate settler process. Surface loading rate should be 0.6-0.8 m³/(m²·h). Decolorant can be added simultaneously to enhance color removal.

3.2 Advanced Oxidation Unit (Core for Breaking Refractory COD & Color)

• Function: Utilizes hydroxyl radicals (·OH) generated by strong oxidants to non-selectively oxidize and decompose residual refractory biodegradable organics and chromophores in the wastewater, significantly reducing COD and color.

• Process Selection:

  • Ozone Catalytic Oxidation: Preferred process. Ozone, under the action of a catalyst (e.g., supported metal oxides), produces more ·OH, resulting in high oxidation efficiency, no secondary pollution, and high automation. Design contact time 30-40 minutes, ozone dosage 30-80 mg/L.

  • Fenton Fluidized Bed Oxidation: Suitable for situations with large water quality fluctuations and higher concentrations. H₂O₂ reacts with Fe²⁺ in a fluidized bed reactor to produce ·OH. Requires precise pH control (3-4) and supporting neutralization-sedimentation facilities. Higher chemical costs and significant sludge production.

3.3 Deep Biological Denitrification Unit (Targeting Total Nitrogen Compliance)

• Function: Primarily removes total nitrogen present in the form of nitrate nitrogen (NO₃⁻-N).

• Process Selection: Denitrifying Deep Bed Filter.

  • Principle: Fills the filter with media like quartz sand to form a biofilm. An external carbon source (e.g., sodium acetate) is dosed into the influent. Under anoxic conditions, denitrifying bacteria on the media reduce NO₃⁻-N to N₂ gas.

  • Advantages: Integrates filtration and biological denitrification, high denitrification efficiency (effluent TN can be stable <10 mg/L), strong shock load resistance, no need for secondary clarifier.

  • Design Points: Filtration rate 4-6 m/h, regular combined air-water backwashing to prevent clogging.

3.4 Membrane Separation Polishing & Disinfection Unit (Ultimate Purification & Safety Assurance)

• Ultrafiltration (UF) System: Serves as a precision safety filter for nanofiltration or reuse. Can 100% retain bacteria, viruses, and macromolecules. Product water turbidity <0.2 NTU, SDI₁₅ <3, providing core assurance for subsequent processes or direct reuse.

• Nanofiltration (NF) System (Optional): When further requirements exist for effluent hardness, salinity, or small organics (e.g., high-quality reuse), an NF system can be added. It removes divalent ions, some monovalent ions, and small organic molecules.

• Disinfection System: Employs combined "Ultraviolet (UV) + Sodium Hypochlorite (NaClO)" disinfection. UV provides instant microbial kill, NaClO provides sustained disinfection capacity, maintains residual chlorine in the distribution network, ensuring absolute hygienic safety.

IV. Main Design Parameters and Equipment Selection Points

V. Techno-Economic Analysis (Example Scale: 500 m³/d)

• Capital Cost Estimate: Total investment for the advanced treatment system is approximately 2.5 - 4.5 million RMB. The ozone system, membrane system, and deep bed filter constitute the main portions.

• Operating Cost: 1.5 - 3.0 RMB per ton of water (advanced treatment stage only), mainly including:

  • Electricity: 0.5-1.2 RMB/ton (ozone generator, blowers, membrane pumps, UV lamps).

  • Chemicals: 0.5-1.5 RMB/ton (coagulant, carbon source, disinfectant, membrane cleaning agents).

  • Membrane Replacement: 0.3-0.6 RMB/ton (UF membrane life 3-5 years).

  • Maintenance & Others: 0.2-0.5 RMB/ton.

• Benefits: Achieves stable wastewater compliance, mitigating environmental risk; Product water can be used for plant landscaping, road washing, vehicle cleaning, etc., saving freshwater costs; Lays a solid foundation for future reclaimed water reuse.

VI. Operation Management and Implementation Recommendations

1. Influent Stability is Prerequisite: Must ensure stable operation of the front-end biological treatment system, keeping effluent quality fluctuations within the design tolerance of the advanced treatment stage.

2. Intelligent Control: The advanced treatment units should integrate PLC automatic control, enabling online linkage between chemical dosing and water quality parameters (e.g., ORP, NO₃⁻-N, turbidity) to optimize operation and reduce chemical and energy consumption.

3. Ozone System Safety: The ozone generation, dosing, and off-gas treatment areas require enhanced ventilation and monitoring. Install ozone leak detectors to ensure safe operation.

4. Membrane System Maintenance: Strictly implement the backwash and chemical cleaning procedures for the UF system. Monitoring the change in Transmembrane Pressure (TMP) is key to ensuring long-term stable membrane operation.

5. Modularity and Flexibility: Consider designing the advanced oxidation and deep denitrification units as modules that can be operated in parallel or bypassed, enabling flexible activation based on actual influent characteristics to optimize operating costs.

This advanced treatment scheme, through a multi-stage, progressive combination of processes, effectively addresses the challenges of biological effluent from food waste treatment wastewater. It provides reliable technical assurance for achieving comprehensive compliance and resource-oriented reuse. Its technical route is clear, unit functions are well-defined, offering good engineering applicability and economic viability.