Cost Control in Industrial Wastewater Treatment Process Design for Thermal Power Plants

I. Overall Principles and Framework for Cost Control

1.1 Characteristics of Thermal Power Plant Industrial Wastewater and Cost Composition

Industrial wastewater in thermal power plants mainly originates from desulfurization wastewater, coal-bearing wastewater, oily wastewater, chemical acid/alkali wastewater, circulating water blowdown, boiler acid cleaning wastewater, and domestic sewage. The treatment cost is high and difficult to control. To achieve economical and efficient management, the cost composition must first be clarified.

• Capital Costs: Mainly include process equipment procurement, civil engineering, installation engineering, automation and control systems, design and technical service fees. Among these, the selection of the treatment process route is the decisive factor affecting capital cost.

• Operating Costs: Mainly include energy consumption (electricity, steam), chemical consumption, equipment maintenance and depreciation, labor, sludge/crystallized salt disposal, and water quality monitoring. Operating costs constitute the primary expenditure over the project's lifecycle and are the focus of cost control.

1.2 Core Strategies for Cost Control

The cost control in this scheme adheres to the core philosophy of "Source Reduction, Process Optimization, Intelligent Operation, Resource Reuse, and Lifecycle Economy", establishing a multi-level, comprehensive control system.

1. Source Control Strategy: Reduce the scale and load of the end-of-pipe treatment system by optimizing the plant's water balance, strengthening segregated wastewater collection, and implementing graded reuse, thereby lowering capital and operating costs at the source.

2. Process Optimization Strategy: For different water quality characteristics, select the process combination that is "technically feasible and economically optimal", avoiding over-design or technological redundancy. Prioritize mature, efficient, and low-consumption unit technologies.

3. Operation Optimization Strategy: Achieve precise dosing of chemicals and energy, and optimal operation of equipment through refined management and intelligent control, tapping into the potential for energy and chemical savings.

4. Resource Recovery Strategy: Recover water resources and valuable materials (e.g., reused water, gypsum, crystallized salt) from the wastewater. The direct economic benefits generated can partially or fully offset the treatment costs.

II. Graded Treatment and Cost-Oriented Process Route Design

Wastewater represented by desulfurization wastewater—characterized by high salinity, high hardness, high suspended solids, and heavy metals—is the core challenge and cost center of thermal power plant wastewater treatment. This scheme recommends a cost-controlled process route of "Categorized Pretreatment + Concentration/Reduction + Solidification/Disposal", rather than pursuing extremely expensive technologies solely for "zero liquid discharge (ZLD)".

2.1 Cost-Controlled Full-Process Framework

The diagram below illustrates a treatment framework centered on cost-effectiveness, balancing compliance and reuse, highlighting key points for cost control at each stage.

III. Technical Measures for Cost Control in Key Stages

3.1 Cost Control in Source and Pretreatment Stages

• Optimize Plant-Wide Water System: Conduct water balance testing to maximize the concentration ratio of the circulating cooling water system. This significantly reduces circulating water blowdown and makeup water at the source, directly lowering wastewater treatment scale and associated capital/operating costs.

• Strengthen Wastewater Segregation and Graded Utilization:

  • After simple treatment, directly reuse wastewater with better quality (e.g., circulating water blowdown, boiler blowdown) in processes with lower water quality requirements, such as desulfurization, coal conveyor gallery washing, and ash yard dust suppression, achieving "lower quality for lower-grade uses".

  • Strictly implement segregated collection to avoid mixing high-salinity desulfurization wastewater with low-salinity wastewater, which increases total treatment volume and difficulty.

3.2 Cost Control in Core Treatment Stages

• Softening Pretreatment Optimization:

  • Employ the "Lime-Soda Ash Softening" process to efficiently remove calcium, magnesium, and silica, preventing scaling in subsequent membrane systems and evaporators. Determine the optimal dosing ratio through bench-scale testing to avoid chemical waste. Generated sludge (calcium carbonate, magnesium hydroxide) can be considered for co-disposal with fly ash.

• Concentration Process Comparison:

  • Tubular Membrane/Electrodialysis (ED): For wastewater with TDS in the 20,000-50,000 mg/L range, it offers relatively lower capital and operating costs and a high concentration factor, making it a cost-effective choice.

  • Disc Tube Reverse Osmosis (DTRO): Highly fouling-resistant with high recovery rates (up to 80-90%). Suitable for high-hardness, high-COD wastewater. Although the capital cost is slightly higher, it maximizes volume reduction, lowering the load on the final evaporation/solidification stage. From a full-process perspective, it may be more economical.

• End-of-Pipe Solidification Process Selection:

  • Bypass Flue Evaporation: Utilizes low-grade waste heat from flue gas. Concentrated wastewater is atomized and sprayed into the flue duct between the air preheater and the electrostatic precipitator for evaporation. Water vapor exits with the flue gas, and dissolved solids are captured in the fly ash. This process fully utilizes waste heat, resulting in very low operating costs. It is currently one of the most cost-advantageous ZLD routes for thermal power plants.

  • Forced Evaporation Crystallization: When regulations require crystallized salt production or flue evaporation is not feasible, MVR/MED can be chosen. MVR should be prioritized for its high thermal efficiency, though it has high electricity consumption. Energy consumption can be reduced through optimized design (e.g., TVR coupling, high-efficiency compressors). The high disposal cost of crystallized salt is a significant cost factor.

3.3 Cost Control in Operation and Maintenance Stages

• Intelligent Dosing and Control System:

  • Install online instruments for hardness, alkalinity, turbidity, pH, conductivity, etc. Automatically and precisely adjust the dosage of softening agents, antiscalants, acids, and alkalis based on real-time water quality and flow, eliminating "extensive" dosing. This can save chemical costs by 15-30%.

  • Implement variable frequency drive (VFD) control and optimal operating point search for core equipment like membrane systems and evaporators to reduce electricity consumption.

• Energy Cascade and Recovery:

  • Recover waste heat from evaporator condensate and cooling water to preheat influent or for plant heating.

  • When using MVR, optimize steam compressor selection for higher energy efficiency.

• Preventive Maintenance and Spare Parts Management:

  • Establish strict equipment maintenance procedures to extend the service life of critical components like membrane elements, pumps, and instruments.

  • Maintain a rational inventory of spare parts to reduce losses and emergency procurement costs from unplanned downtime.

IV. Lifecycle Cost Analysis and Optimization

Economic Assessment: Taking a ZLD project for desulfurization wastewater from a 2×300MW unit as an example, adopting the "Pretreatment + Tubular Membrane + Bypass Flue Evaporation" route, compared to the "Pretreatment + DTRO + MVR" route, can save approximately 30-40% in initial investment and reduce unit water treatment operating costs by over 50%, significantly shortening the payback period.

V. Conclusion and Implementation Recommendations

Cost control in industrial wastewater treatment for thermal power plants is a systematic project that must be integrated throughout the entire process of planning, design, construction, and operation.

1. Plan First, Calculate Precisely: Detailed plant-wide water balance testing and comprehensive characterization of all wastewater streams are essential before design to avoid over-sizing or inappropriate process selection.

2. Technology Comparison, Emphasize Lifecycle Cost: Process selection should not blindly pursue advanced technology. Instead, conduct a lifecycle cost analysis, prioritizing routes that are technologically mature, have low energy/chemical consumption, and synergize well with the main plant processes.

3. Empower with Intelligence, Manage Meticulously: Building an intelligent water treatment management and control system is a key tool for achieving long-term stable operation and continuous cost control.

4. Adapt to Local Conditions, Implement in Phases: Based on the urgency of environmental regulations and the plant's financial situation, adopt a phased implementation strategy: "First achieve compliant discharge and reuse, then progressively advance towards ZLD". This reduces the pressure of a large one-time investment.

By implementing the aforementioned cost control measures, the environmental management costs of thermal power plants can be significantly reduced while ensuring stable and compliant wastewater treatment. This enhances the plant's economic benefits and environmental competitiveness, achieving a balance between environmental protection and economic performance.