AbstractDisc tube reverse osmosis (DTRO) has emerged as a cutting-edge membrane separation technology, particularly in high-concentration wastewater treatment scenarios. This article focuses on the two-stage DTRO system, analyzing its structural advantages, operational mechanisms, and application cases in industries such as landfill leachate, industrial wastewater, and municipal sewage. By addressing the limitations of single-stage DTRO—including low water recovery and membrane fouling—the two-stage configuration achieves superior performance, making it a pivotal solution for modern water resource recycling.1. IntroductionReverse osmosis (RO) is a cornerstone of advanced wastewater treatment, but traditional spiral-wound RO membranes face challenges in treating high-salinity, high-pollutant-load wastewater (e.g., landfill leachate with COD > 5,000 mg/L and TDS > 30,000 mg/L). DTRO, first commercialized in the 1980s, uses a unique disc-tube module design with turbulence promoters, enabling it to withstand harsh operating conditions. The two-stage DTRO system—combining a high-pressure first stage and a medium-pressure second stage—further optimizes water recovery and contaminant rejection, addressing the growing demand for sustainable wastewater management.2. Technical Principles of Two-Stage DTRO2.1 Single-Stage vs. Two-Stage ConfigurationSingle-stage DTRO: Treats raw wastewater in one pass, achieving ~70–80% water recovery but leaving a high-concentration concentrate (TDS > 60,000 mg/L) that requires further disposal.Two-Stage DTRO:First stage: Operates at 60–80 bar, treating raw wastewater to produce permeate (60–70% recovery) and a primary concentrate.Second stage: Treats the primary concentrate at 40–60 bar, recovering an additional 30–40% of water from the concentrate.Overall recovery: Up to 90%, with the final concentrate volume reduced by 70–80% compared to single-stage systems.2.2 Key ComponentsDisc-Tube Modules: Stacked polypropylene discs with flow channels, preventing membrane compaction and enabling high cross-flow velocity (1–3 m/s) to minimize fouling.High-Pressure Pumps: Stainless steel centrifugal pumps with variable frequency drives (VFD) for precise pressure control.Chemical Cleaning System: CIP (Clean-in-Place) units with acid/alkaline solutions to remove scaling (e.g., CaCO₃, SiO₂) and organic fouling.3. Performance Advantages3.1 Higher Water RecoveryBy reprocessing the concentrate, two-stage DTRO reduces freshwater intake and wastewater discharge. For example, a 1,000 m³/day landfill leachate plant using two-stage DTRO produces ~900 m³/day of permeate (meets GB/T 19923-2005 reuse standards) and only ~100 m³/day of final concentrate.3.2 Enhanced Contaminant RejectionOrganic matter: COD rejection > 99% (from 10,000 mg/L to < 100 mg/L).Inorganics: TDS rejection > 98%, heavy metal (Pb, Cd, Cr⁶⁺) rejection > 99.9%.Micro-pollutants: Effective removal of pharmaceuticals (e.g., ibuprofen) and endocrine-disrupting chemicals (EDCs) via membrane sieving and adsorption.3.3 Mitigated Membrane FoulingThe two-stage design distributes the pollutant load across stages:First stage handles high suspended solids (SS) and colloids, protected by a pre-treatment system (e.g., ultrafiltration, activated carbon).Second stage treats lower-SS concentrate, reducing fouling rate by 40–50% compared to single-stage systems.Membrane lifespan: 3–5 years, 2–3 times longer than spiral-wound RO in similar applications.4. Industrial Applications4.1 Landfill Leachate TreatmentCase Study: A municipal landfill in Shanghai (2023) installed a 500 m³/day two-stage DTRO system. Results:Permeate quality: COD < 80 mg/L, NH₃-N < 10 mg/L, TDS < 2,000 mg/L (compliant with China’s 《GB 16889-2008》 standard).Operational cost: ~¥15/m³ (including energy, chemicals, and maintenance), 20% lower than single-stage DTRO due to reduced concentrate disposal fees.4.2 Industrial WastewaterPetrochemical Industry: Treats produced water with high salinity (TDS > 100,000 mg/L) and hydrocarbons. Two-stage DTRO recovers 85% of water for reuse in cooling towers.Mining Industry: Treats acid mine drainage (AMD) with high heavy metals (Cu²⁺, Zn²⁺). The permeate meets drinking water standards (WHO guidelines) after post-treatment.5. Challenges and Future Perspectives5.1 Current ChallengesEnergy Consumption: Two-stage systems require ~2–3 kWh/m³ of electricity, higher than single-stage (~1.5 kWh/m³).Membrane Cost: DTRO membranes are 2–3 times more expensive than spiral-wound membranes, though longer lifespan offsets this over time.5.2 Future TrendsIntegration with Renewable Energy: Coupling with solar/wind power to reduce carbon footprint (e.g., a 1,000 m³/day plant in Australia uses 500 kW solar panels to power pumps).Smart Monitoring: AI-based systems (e.g., machine learning algorithms) to predict fouling and optimize cleaning cycles, reducing downtime by 30%.Membrane Material Innovation: Development of graphene-modified DTRO membranes with higher flux (up to 40 L/m²·h) and chemical resistance.6. ConclusionThe two-stage DTRO system represents a paradigm shift in wastewater treatment, balancing high efficiency, sustainability, and reliability. Its ability to handle extreme wastewater conditions while maximizing water recovery makes it indispensable for industries facing water scarcity and strict environmental regulations. As membrane technology advances and costs decline, two-stage DTRO will play a central role in the global transition toward a circular water economy.Keywords: Two-stage DTRO; Wastewater treatment; Water recovery; Membrane fouling; Landfill leachateThis article provides a comprehensive overview of two-stage DTRO, from technical principles to real-world applications, highlighting its potential to address pressing water challenges in the 21st century.
