Design Scheme for Disc Tube Reverse Osmosis (DTRO) Membrane System in Landfill Leachate Treatment

1.0 Design Basis and Objectives

• Source Water: Landfill leachate generated from municipal solid waste landfills or incineration plants.

• Design Capacity: To be determined based on actual project requirements. Typical single-system treatment capacity ranges from 50-200 m³/d, with modular parallel expansion capability.

• Feed Water Characteristics (Typical Range):

  • CODcr: 5,000 - 80,000 mg/L

  • BOD5: 2,000 - 35,000 mg/L

  • NH₃-N: 500 - 3,000 mg/L

  • Cl⁻: 2,000 - 15,000 mg/L

  • Ca²⁺, Mg²⁺: High concentration, prone to scaling.

  • SS: 500 - 2,000 mg/L

  • Conductivity: 15,000 - 50,000 µS/cm

• Treatment Objectives:

  • Permeate Quality: Meets the discharge limits specified in Table 2 or Table 3 of the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste(GB 16889-2008), or more stringent local discharge standards, suitable for discharge or reuse.

  • System Recovery Rate: ≥ 75% (Adjustable based on feed salinity and scaling potential).

  • System Stability: Possesses excellent fouling and scaling resistance to ensure long-term stable operation.

2.0 Technology Selection: DTRO Process

Given the characteristics of landfill leachate—high turbidity, high organic content, high salinity, and high scaling potential—conventional spiral-wound reverse osmosis membranes are highly susceptible to irreversible fouling. This scheme recommends employing Disc Tube Reverse Osmosis technology, with its core advantages as follows:

• Open Flow Channels: The unique design between the guide discs and membrane sheets creates turbulent flow, significantly reducing pollutant deposition on the membrane surface.

• High Fouling Resistance: Can tolerate feed water with high SDI values (>20), requiring relatively less stringent pretreatment.

• High-Pressure Operation Capability: Maximum operating pressure up to 160 bar, suitable for deep concentration of high-salinity leachate.

• Ease of Maintenance: Modular design allows for individual replacement of damaged membrane discs, reducing maintenance costs.

3.0 Process Flow Design

The system adopts the main process route of "Pretreatment & Softening + DTRO Membrane Concentration + Ultimate Concentrate Treatment". The specific process flow is illustrated in the diagram below:

Process Step-by-Step Explanation:

3.1 Pretreatment Unit

• pH Adjustment: Precisely adjust the feed water pH to 6.0-6.5 by dosing sulfuric acid (H₂SO₄). This operation aims to convert bicarbonate (HCO₃⁻) in the water to carbon dioxide (CO₂), preventing calcium carbonate (CaCO₃) scaling on the membrane surface.

• Filtration: Employ sand filtration or large-pore cartridge filtration primarily to remove fibers, larger suspended solids, etc., from the leachate to protect the subsequent high-pressure pump and membrane stacks.

3.2 DTRO Membrane Treatment Unit (Core)

• High-Pressure Supply: The pretreated and filtered feed water is pressurized to the required system pressure by a high-pressure plunger pump. The system is equipped with an Energy Recovery Device (e.g., PX pressure exchanger) to recover pressure energy from the concentrate, significantly reducing system energy consumption (by approximately 30-40%).

• Primary DTRO: Serves as the main desalination and concentration unit. High-pressure feed water undergoes separation within the membrane stack. The produced permeate proceeds to subsequent treatment, while the concentrate flows to the secondary DTRO or volume reduction unit. The primary stage recovery is typically controlled at 50-60%.

• Secondary DTRO: Provides deep purification of the primary permeate to ensure final product water quality meets standards. The concentrate produced here, being lower in salinity, is typically recycled to the front end of the primary stage for further treatment, enhancing the overall system recovery rate.

• Online Cleaning & Flushing: The system integrates a fully automated cleaning program, performing online cleaning with acids, alkalis, or specialized cleaning agents periodically or as needed to restore membrane flux.

3.3 Post-Treatment and Product Handling

• Permeate Decarbonation and pH Adjustment: Due to acid addition in pretreatment, the permeate contains dissolved CO₂ and is acidic. CO₂ is removed via a decarbonator (or forced aeration), and sodium hydroxide (NaOH) is dosed as needed to adjust the permeate pH back to the compliant range of 6-9.

• Concentrate Disposal:

  • Volume Reduction (Recommended): The primary concentrate is sent to a Mechanical Vapor Recompression (MVR) evaporator for ultimate volume reduction, achieving "Zero Liquid Discharge" (ZLD). Evaporated condensate can be reused, and crystalline mixed salts are handled as hazardous waste.

  • Recirculation: At suitable landfill sites, a portion of the concentrate can be recirculated into the waste mass for further degradation by microorganisms and soil. However, the recirculation rate must be strictly controlled to prevent salt accumulation.

4.0 Key Design Parameters and Equipment Selection

5.0 Techno-Economic Analysis

• Capital Cost: Mainly comprised of DTRO membrane stacks, high-pressure pumps, energy recovery devices, evaporation system, and control systems. The investment cost per ton of water capacity is relatively high.

• Operating Cost:

  • Power Consumption: Primarily from the high-pressure pump and evaporator, with specific energy consumption approximately 15-25 kWh/m³ of treated water.

  • Membrane Replacement: DTRO membrane disc lifespan is generally 3-5 years, constituting a major maintenance cost.

  • Chemical Costs: Includes acids, alkalis, antiscalants, cleaning agents, etc.

  • Concentrate Disposal Cost: Energy cost for evaporation or hazardous waste disposal fees.

  • Comprehensive Treatment Cost per m³: Approximately 60 - 120 RMB/m³, depending on feed concentration, system scale, energy prices, and disposal methods.

6.0 Summary

This scheme employs Disc Tube Reverse Osmosis (DTRO) membrane technology as the core process for treating landfill leachate. It is characterized by strong adaptability, effective treatment performance, and high stability. Through the combination of "Pretreatment & Softening + Multi-stage DTRO Concentration + Evaporation/Crystallization for Ultimate Volume Reduction", deep treatment of leachate, water resource reuse, and final solidification of pollutants can be achieved. It is a reliable technological choice for meeting stringent discharge standards or "Zero Liquid Discharge" objectives.

Recommendations: Before specific project implementation, pilot testing is essential to accurately determine design parameters (such as flux, recovery rate, cleaning frequency) and operating costs. Concurrently, a comprehensive technical, economic, and environmental feasibility assessment should be conducted for the ultimate concentrate disposal method (evaporation or recirculation).