Optimization of Comprehensive Wastewater Treatment Process Design for Biomass Power Plants

I. Current Situation Analysis and Optimization Background

1.1 Typical Wastewater Quality Characteristics and Problems

The comprehensive wastewater generated during the operation of biomass power plants (using straw, rice husks, wood chips, etc., as fuel) mainly includes water treatment plant drainage, boiler blowdown, circulating cooling water blowdown, desulfurization wastewater, floor and equipment cleaning water, and domestic sewage. According to investigations, most existing treatment systems commonly face the following problems:

• Complex Water Quality Characteristics:

  • High COD, High Suspended Solids: Cleaning wastewater contains large amounts of plant fibers and humus, with COD potentially reaching 500-2000 mg/L and high SS.

  • High Ammonia Nitrogen, High Alkalinity: Desulfurization wastewater and water treatment regeneration wastewater have high ammonia nitrogen concentrations (can be >100 mg/L), with high pH and alkalinity.

  • High Salinity, High Hardness: Circulating water blowdown and boiler blowdown lead to high TDS, hardness, and chloride ion content.

  • Large Water Temperature Fluctuations: High-temperature wastewater like boiler blowdown affects the stability of biological systems.

• Common Issues with Existing Processes:

  1. Insufficient Pretreatment: Incomplete removal of suspended solids leads to clogging and high load on subsequent biological systems.

  2. Low Nitrogen Removal Efficiency: Imbalanced carbon-to-nitrogen ratio, insufficient carbon source for denitrification, making it difficult to stably meet total nitrogen standards.

  3. Weak Shock Load Resistance: The system is prone to failure and produces unstable effluent under fluctuating water quality/quantity conditions.

  4. Large Sludge Production, Difficult Disposal: Traditional processes produce large amounts of sludge, resulting in high disposal costs.

  5. Unrecovered Resources and Energy: The chemical energy and thermal energy contained in the organic matter of the wastewater are not effectively utilized.

1.2 Optimization Objectives and Principles

• Core Optimization Objectives:

  1. Stable Compliance: Effluent water quality stably meets the Class 1A standard of the Integrated Wastewater Discharge Standard(GB 8978-2002) or stricter reuse standards, focusing on overcoming COD, ammonia nitrogen, total nitrogen, and SS.

  2. Energy Saving and Consumption Reduction: Reduce unit wastewater treatment operating costs by more than 20% through process optimization and energy integration.

  3. Resource Recovery: Achieve a water reuse rate ≥ 40%, and explore sludge biogas energy recovery.

  4. Intelligent Operation and Maintenance: Enhance system automation and shock load resistance, achieving refined, low-manpower operation.

• Optimization Design Principles:

  • Enhanced Pretreatment, Graded Treatment: Implement efficient physicochemical pretreatment for high-SS, high-hardness wastewater to reduce the load on biological systems.

  • Optimized Carbon Source, Enhanced Nitrogen Removal: Rationally utilize internal/external carbon sources, optimize the denitrification process to ensure deep total nitrogen removal.

  • Energy and Material Synergy: Recover waste heat from wastewater, utilize sludge anaerobic digestion to produce biogas, reducing the plant's overall energy consumption.

  • Modularity and Intelligence: Employ flexible, efficient modular process combinations and incorporate intelligent control systems.

II. Optimized Process Route Design

Building upon the traditional "physicochemical + biological" process, this optimization plan proposes an integrated optimization route of "Enhanced Pretreatment - Anaerobic Energy Recovery - Coupled Nitrogen Removal Biology - Membrane-based Depth Safeguard". The core lies in coupling wastewater treatment with the power plant's energy and material systems to achieve dual environmental and economic benefits.

2.1 Full Process Flow Diagram After Optimization

III. Key Points for Optimization Design of Critical Units

3.1 Pretreatment System Optimization

• Optimization Point 1: Heat Recovery and Temperature Equalization

  • Add a plate heat exchanger to recover waste heat from high-temperature wastewater like boiler blowdown, using it for plant building heating or biological system insulation, reducing overall plant heat consumption. Simultaneously, ensure stable influent temperature (25-35°C) for the biological system.

• Optimization Point 2: High-Efficiency Suspended Solids Removal

  • Optimize the traditional primary clarifier to a combination of "Vortex Grit Chamber + High-Efficiency Fiber Filter". The fiber filter can deeply remove fine fibers and colloids, reducing the load on subsequent anaerobic and MBR systems and protecting membrane modules.

• Optimization Point 3: Hardness and Scale Control

  • For high-hardness circulating water blowdown, install a chemical softening clarifier (dosing lime, soda ash) to pre-remove calcium and magnesium hardness, preventing scaling in subsequent membrane systems and heat exchange equipment.

3.2 Core Biological and Energy Recovery System Optimization

• Optimization Point 4: Anaerobic Energy Recovery

  • Technology Upgrade: Add an Upflow Anaerobic Sludge Blanket (UASB) or Internal Circulation (IC) Reactor to produce biogas from high-concentration organic wastewater (e.g., mixed cleaning water and domestic sewage). The produced biogas, after desulfurization and purification, is used for boiler co-firing or power generation, directly generating economic benefits.

• Optimization Point 5: A/O-MBR Coupled Process for Enhanced Nitrogen Removal

  • Process Coupling: Employ a coupled Anoxic/Oxic process with a Membrane Bioreactor. MBR replaces the secondary clarifier, offering high sludge concentration, good nitrogen removal, and excellent effluent quality.

  • Carbon Source Optimization:

    1. Internal Carbon Source Utilization: Partially recycle anaerobic effluent to the anoxic tank, utilizing residual volatile fatty acids as a denitrification carbon source.

    2. Intelligent External Carbon Source Dosing: Precisely dose external carbon sources (e.g., sodium acetate) based on influent C/N ratio and online nitrate monitoring to avoid waste.

3.3 Advanced Treatment and Reuse System Optimization

• Optimization Point 6: Advanced Treatment Process Upgrade

  • Optimize the traditional "Fenton Oxidation" to "Ozone Catalytic Oxidation". Ozone oxidation produces no secondary sludge, effectively removes refractory COD, and operates more cleanly. Follow with a Biological Aerated Filter (BAF) to further degrade intermediates and safeguard nitrogen removal.

• Optimization Point 7: Graded Reuse and Membrane System Optimization

  • Adopt a "Ultrafiltration + Reverse Osmosis" dual-membrane process. UF serves as a precise barrier for RO. RO permeate is of high quality for reuse as boiler makeup or cooling tower water. Implement graded water supply based on end-use requirements. Optimize RO operating parameters and recovery rate to reduce energy consumption.

3.4 Sludge Treatment System Optimization

• Optimization Point 8: Sludge Anaerobic Digestion Synergy

  • Direct excess MBR sludge to a sludge anaerobic digester, which can operate synergistically with the UASB to increase total biogas production. Digested sludge is more stable, facilitating subsequent dewatering.

• Optimization Point 9: Deep Dewatering and Resource Recovery

  • Use a high-pressure plate and frame filter press to reduce sludge moisture content to below 60%. The dewatered sludge cake has relatively high calorific value and can be considered for safe co-firing in the plant's boiler (requires assessment of heavy metals and chlorine content) to achieve energy recovery and zero off-site sludge disposal, or used as a substrate for landscaping soil.

IV. Intelligent Control System and Operation Optimization

• Establish an Intelligent Water Management Platform: Integrate online monitoring instruments throughout the plant (flow, pH, DO, ORP, ammonia nitrogen, nitrate, sludge concentration, etc.).

• Implement Precise Control Strategies:

  • Intelligent Chemical Dosing: Adjust the dosage of coagulants, carbon sources, and alkalis in real-time based on water quality and quantity.

  • Intelligent Aeration: Use variable frequency drives to control blowers based on online DO and ammonia nitrogen values, achieving on-demand aeration for energy savings.

  • Warning and Emergency Response: Set parameter alarm thresholds; automatically initiate emergency procedures (e.g., adjusting recirculation, switching pipelines) upon abnormality.

V. Technical and Economic Analysis After Optimization

Conclusion: Through multi-dimensional improvements in process reconfiguration, energy recovery, and intelligent control, this optimization scheme transforms the biomass power plant's wastewater treatment system from a mere "cost center" into a "resource factory" that combines pollution control, energy recovery, and water resource regeneration. It achieves synergistic optimization of environmental and economic benefits, aligning with the green, circular, and low-carbon development direction of power plants. Optimization projects should be implemented in phases, prioritizing bottleneck units.