Core Design of Water Reuse Process Scheme in Environmental Engineering

I. General Principles and Objectives of Core Design

1.1 Design Philosophy and Positioning

Water reuse is a key engineering technology for achieving sustainable recycling of water resources. This core design aims to construct a water reuse process system characterized by "safety and reliability, economic efficiency, operational flexibility, and intelligent management". Its positioning is not merely the endpoint of wastewater treatment, but an important water supply unit within urban or regional water resource recycling systems, serving as strategic infrastructure to alleviate water scarcity and reduce environmental pollution.

1.2 Core Design Objectives

1. Water Quality and Safety Assurance: Product water quality must stably meet the water quality standards for the intended reuse application, ensuring hygiene safety, ecological safety (for landscape use), and usage safety (e.g., non-corrosive to equipment, no scaling).

2. Efficient Process Integration: Core process unit technologies are mature, stable, and adaptable. Units are well-integrated, with the overall system achieving high recovery rates and low energy/chemical consumption.

3. System Flexibility and Reliability: Capable of handling seasonal fluctuations in source water quality and sudden pollution events. Includes redundancy and emergency safeguards to ensure uninterrupted water supply.

4. Lifecycle Economy: While meeting objectives, optimize design and operational strategies to reduce per-unit product water capital and operating costs, achieving sound economic and social benefits.

1.3 Source Water and Reuse Application Matching

• Primary Sources: Municipal wastewater treatment plant secondary effluent (most common, stable source), domestic miscellaneous drainage from building complexes, compliant industrial discharge wastewater, rainwater, etc.

• Primary Reuse Directions:

  • High-Quality Reuse: As makeup water for circulating cooling water systems, raw water for boiler feedwater, high-end industrial process water. Extremely high requirements for turbidity, hardness, salinity, microorganisms, and corrosive ions.

  • Conventional Reuse: Urban miscellaneous water (landscaping, road washing, vehicle cleaning), landscape and environmental water (river, lake, wetland replenishment), agricultural irrigation. Focus on controlling suspended solids, pathogens, nutrients (nitrogen, phosphorus), and hygienic indicators.

  • Ecological Replenishment: For groundwater recharge, watershed ecological base flow replenishment. Most comprehensive and stringent water quality requirements, needing control of organics, heavy metals, emerging contaminants, etc.

II. Core Technology Routes and Process Combinations

2.1 Design Approach

Adopt a differentiated, modular design philosophy of "determining treatment process based on end-use water quality, and determining pretreatment intensity based on source water quality". The core lies in constructing multiple barriers through the process chain of "Pretreatment Safeguard + Main Desalination/Purification + Disinfection/Stabilization".

2.2 Core Process Route Selection and Flow Diagram

Based on different reuse targets, three representative core technology routes are defined. Among them, the "Dual-Membrane Process (UF+RO)" is the gold standard for producing high-purity reclaimed water. Its process and combination, illustrated in the diagram below, represent the highest level of advanced treatment and resource recovery.

III. Core Treatment Unit Functions and Design Key Points

3.1 Enhanced Pretreatment Unit

• Function: Key to ensuring long-term stable operation of the main process (especially membrane systems). Removes suspended solids, colloids, part of the organics, hardness, and microorganisms, reducing the load on the main process.

• Core Technologies:

  • Chemical Coagulation/Sedimentation/Flotation: Efficiently removes phosphorus, colloidal organics, and color.

  • Multi-Media/Cloth Filters: Serve as precision pretreatment for membrane systems, reducing turbidity and SDI.

  • Biological Pretreatment: For source water with high ammonia nitrogen, biological aerated filters (BAF) can be used to remove ammonia and part of the organics.

3.2 Main Purification Unit

1. Membrane Separation Technology:

   • Ultrafiltration (UF): Serves as pretreatment for RO and as an independent purification unit. Can almost completely remove bacteria, viruses, colloids, and suspended solids. Product water turbidity <0.2 NTU. It is the core for ensuring microbial safety of the effluent.

   • Reverse Osmosis / Nanofiltration:

     • Reverse Osmosis (RO): Core desalination unit. Removes >99% of ions, organics, and microorganisms. Product water is near-pure, suitable for high-quality industrial reuse.

     • Nanofiltration (NF): Selective separation. Removes divalent ions and large organic molecules while retaining some monovalent ions. Suitable for scenarios requiring moderate hardness and organic removal.

2. Advanced Oxidation Technology:

   • Ozone Oxidation: Used for oxidation of refractory organics, decolorization, and disinfection. Can significantly improve sensory water quality indicators and biological safety.

   • UV/Hydrogen Peroxide, Ozone/Hydrogen Peroxide, etc.: Combined processes for degrading trace emerging contaminants, ensuring safety for ecological water replenishment.

3.3 Disinfection and Stabilization Unit

• Function: Kills pathogens, prevents microbial growth in distribution networks, adjusts chemical stability of water to prevent corrosion and scaling.

• Core Technologies:

  • Combined Disinfection: "Ultraviolet (UV) + Chlorine" or "Ozone + Chloramine". UV provides instant, powerful disinfection. Chlorine or chloramine provides residual protection in the network.

  • Chemical Stabilization: Post-mineralization (dosing lime, CO2) or addition of corrosion inhibitors to RO product water (which is corrosive) to adjust pH and hardness, achieving chemical stability.

IV. Key Design Parameters and Economic Analysis

4.1 Core Unit Design Parameters (Example)

4.2 Economic Analysis

• Capital Cost Estimate: Per-unit water production cost varies greatly depending on the process route and treatment scale. The conventional filtration-disinfection route is approximately 800-1500 RMB/ton capacity. The dual-membrane advanced treatment route is approximately 2000-4000 RMB/ton capacity.

• Operating Cost:

  • Conventional Route: 0.8 - 1.8 RMB/ton, mainly electricity, chemicals, and membrane replacement.

  • Dual-Membrane Route: 1.5 - 3.5 RMB/ton, where electricity (high-pressure pumps) and membrane replacement are the main costs.

• Benefits:

  • Direct Economic Benefits: Replaces equivalent tap water, saving water fees and discharge fees. Typical payback period is 3-8 years.

  • Indirect Benefits: Reduces freshwater withdrawal, cuts pollutant discharge, enhances regional water resource resilience. Significant social and environmental benefits.

V. Conclusion and Implementation Recommendations

The core design of water reuse is a systems engineering project involving multi-objective optimization. The "Dual-Membrane Process" is a reliable technological choice for producing high-quality reclaimed water and achieving high-value water resource reuse. The combination of "Filtration + Advanced Oxidation + Disinfection" is a mature and economical solution for meeting urban miscellaneous and landscape water demands.

Keys to Successful Implementation:

1. Comprehensive Source Water Investigation: Long-term, comprehensive water quality monitoring and analysis of the source water is essential, with particular focus on organic characteristics, hardness, and seasonal variation.

2. Rigorous Pretreatment: The reliability of the pretreatment system directly determines the success and lifespan of the main process (especially membrane systems).

3. Full-Process Intelligent Management and Control: Establish an intelligent control system based on online water quality monitoring to achieve precise chemical dosing, fault warning, and energy-efficient operation.

4. Professional Operation and Maintenance: Water reuse systems, especially membrane systems, require professional operation and maintenance teams, along with scientifically developed cleaning and maintenance protocols.

This core design provides the technical framework and decision-making basis for constructing safe, efficient, and economical water reuse systems. It serves as crucial technical support for promoting water resource recycling and building a water-saving society.