Industrial RO System Specifications: 2025 Engineering Data, Standards & Selection Guide

Industrial reverse osmosis (RO) systems deliver ultra-pure water at recovery rates up to 95%, with salt rejection exceeding 98% under standard conditions (2000 PPM feed, 225 PSIG, 77°F). Key specifications include membrane flux (12–18 GFD for TFC elements), pressure vessel arrays (6–7 elements per vessel), and pre-treatment requirements (chlorine <0.1 PPM, SDI <3). Compliance with FDA, USP, and ASTM standards is critical for pharmaceutical and food-grade applications, where permeate conductivity must remain below 10 µS/cm.

Why Industrial RO System Specifications Matter: A Facility Manager’s Story

Inconsistent water quality can halt production, costing a mid-sized dairy plant an estimated $25,000 per hour due to equipment failures and downtime. Imagine a facility manager, Sarah, in a large food processing plant, grappling with frequent boiler shutdowns. Her plant's boiler feed water, sourced from municipal supply, occasionally spiked in Total Dissolved Solids (TDS) and hardness, leading to rapid scaling within the boiler tubes. This not only necessitated costly chemical descaling and manual cleaning but also caused unscheduled production pauses, impacting delivery schedules and profitability. Sarah knew an industrial RO system was the definitive solution to achieve consistent, high-purity water, but she quickly realized that simply "buying an RO system" was insufficient. Without precise engineering data and a clear understanding of system specifications, she risked investing in a system that was either over-designed (unnecessarily expensive) or under-designed (unable to meet her critical water quality demands and leading to similar operational headaches). This article addresses that exact challenge, providing the deep technical specifications, industry compliance standards, and actionable troubleshooting steps necessary to select, design, and maintain an industrial RO system that truly solves water quality challenges and ensures operational continuity.

Core Industrial RO System Specifications: Parameters You Must Define

Defining precise feed water characteristics is the foundational step for any effective industrial RO system design, with parameters like chlorine content needing to be below 0.1 PPM to prevent membrane damage. Before selecting or designing an industrial RO system, a comprehensive understanding of several key parameters is non-negotiable. These specifications dictate the system’s performance, longevity, and operational efficiency. First, **feed water quality** is paramount. Typical industrial RO systems are designed for raw water with TDS ranging from 500 to 10,000 PPM. Specific contaminants like hardness (measured as CaCO₃), chlorine (<0.1 PPM to protect TFC membranes), iron (<0.05 PPM), silica (<20 PPM), and Silt Density Index (SDI <3) are critical. Exceeding these limits without proper pre-treatment will lead to rapid fouling and scaling of membranes (Top 1 and Top 3 pages). Second, **permeate quality targets** directly influence membrane selection and system staging. For pharmaceutical applications, permeate conductivity must be below 10 µS/cm, while general industrial use might accept up to 50 µS/cm. TDS removal typically ranges from 95% to 99%, and microbial reduction can achieve log 3–6 depending on membrane type and system design. Third, **flow rates** determine the system’s capacity. This includes the design system flow (ranging from 50 to 5,000 GPD for commercial to large industrial units), distribution flow requirements, and total daily water usage in gallons per day (Top 3 PDF). Fourth, **operating conditions** such as temperature range (typically 40–100°F), pressure range (150–1,200 PSIG depending on feed water TDS), and electrical requirements (e.g., 220V/440V, 3-phase) must be specified to ensure compatibility and efficient operation. Finally, **recovery rates**, which represent the percentage of feed water converted to permeate, typically range from 50% to 95%. Higher recovery is desirable for water conservation but is limited by feed water quality, particularly the concentration of scaling ions. The recovery rate is calculated as: Recovery % = (Permeate Flow / Feed Flow) × 100.

Parameter Category	Specific Parameter	Typical Range / Target	Impact on RO System
Feed Water Quality	Total Dissolved Solids (TDS)	500 – 10,000 PPM	Determines operating pressure, membrane type, recovery rate.
	Hardness (as CaCO₃)	< 5 PPM (post-softener)	Requires softening or antiscalant to prevent scaling.
	Chlorine Residual	< 0.1 PPM	Critical for TFC membrane longevity; requires activated carbon pre-treatment.
	Iron (Fe)	< 0.05 PPM	Causes fouling; requires oxidation/filtration.
	Silica (SiO₂)	< 20 PPM	Scaling potential; limits recovery, requires antiscalant/pH adjustment.
	Silt Density Index (SDI)	< 3	Indicates fouling potential; requires adequate pre-filtration.
Permeate Quality	Conductivity	< 10 µS/cm (Pharma), < 50 µS/cm (Industrial)	Target quality; determines membrane rejection, post-treatment needs.
	TDS Removal	95% – 99%	Indicates membrane efficiency.
Flow Rates	Design System Flow	50 – 5,000 GPD (or higher)	System sizing; determines number of membranes/vessels.
	Recovery Rate	50% – 95%	Water efficiency; limited by feed water and scaling potential.
Operating Conditions	Operating Pressure	150 – 1,200 PSIG	Energy consumption; depends on TDS and membrane type.
	Operating Temperature	40 – 100°F	Affects membrane flux and energy consumption.

RO Membrane Specifications: TFC vs. CTA, 4" vs. 8", and Performance Trade-Offs

Thin-Film Composite (TFC) membranes typically achieve 98% salt rejection, outperforming Cellulose Triacetate (CTA) membranes which usually offer around 95% rejection. The choice of RO membrane is arguably the most critical decision in system design, profoundly influencing permeate quality, operational costs, and system longevity. **Membrane Materials:** * **Thin-Film Composite (TFC):** These are the industry standard for most industrial applications due to their superior performance. TFC membranes offer high salt rejection (typically 98-99%) and broad pH tolerance (2–11). However, they have zero tolerance for chlorine, requiring strict pre-treatment to maintain chlorine levels below 0.1 PPM. * **Cellulose Triacetate (CTA):** While less common in modern industrial systems, CTA membranes possess inherent chlorine tolerance (up to 1 PPM), making them suitable for applications where chlorine removal pre-treatment is difficult or costly. Their salt rejection is lower (around 95%), and their pH range is narrower (4–8), making them more susceptible to scaling and biological fouling. **Element Sizes:** RO membranes come in standardized sizes, with 4-inch and 8-inch diameter elements being the most prevalent: * **4" x 40" Elements:** Standard for smaller commercial and light industrial systems (e.g., 50–5,000 GPD). They offer flux rates of 12–18 GFD (gallons per square foot per day) and a pressure drop of 5–10 PSI per element. The cost per GPD is generally higher ($0.20–$0.50). * **8" x 40" Elements:** Dominant in high-flow industrial applications (e.g., 5,000 GPD and above) due to their higher surface area. They provide higher flux rates (15–25 GFD) and lower pressure drop (3–7 PSI per element), resulting in a lower cost per GPD ($0.10–$0.30) and greater energy efficiency. Zhongsheng’s industrial RO systems with PLC-controlled operation often utilize 8" x 40" elements for optimal performance. **Membrane Configurations:** * **Spiral-Wound:** This is the most common configuration, offering high packing density and efficient flow paths. They are designed for ease of cleaning and replacement. * **Hollow-Fiber:** Rarely used for industrial RO due to lower packing density and higher susceptibility to fouling, though they can be found in some ultrafiltration (UF) applications as pre-treatment. **Ancillary Components:** * **Anti-Telescoping Devices (ATD):** These plastic discs are installed at the ends of membrane elements to prevent them from "telescoping" or collapsing under the high differential pressures experienced in industrial RO systems. Regular inspection during membrane replacement is crucial. * **Brine Seals:** These O-rings or U-cups ensure that all feed water passes through the membrane and does not bypass it by flowing directly to the permeate side. Proper installation and integrity are essential for maintaining high salt rejection.

Feature	Thin-Film Composite (TFC) Membranes	Cellulose Triacetate (CTA) Membranes
Salt Rejection	98% - 99%	90% - 95%
Chlorine Tolerance	< 0.1 PPM (requires dechlorination)	Up to 1 PPM (chlorine tolerant)
pH Range (Operating)	2 - 11	4 - 8
Fouling Resistance	Good (requires effective pre-treatment)	Moderate (more susceptible to biofouling)
Typical Applications	Ultra-pure water, pharmaceutical, power, general industrial	Less common; specific applications with high chlorine in feed.
Element Size (Example)	8" x 40"	8" x 40"
Flux Rate (GFD)	15 - 25 GFD	12 - 18 GFD
Pressure Drop / Element	3 - 7 PSI	5 - 10 PSI
Cost per GPD (approx.)	$0.10 - $0.30	$0.20 - $0.50

Pressure Vessels, Pumps, and System Arrays: How to Scale for Industrial Demand

Industrial RO pressure vessels are typically designed to house 6 to 7 membrane elements, constructed from FRP, and rated for pressures up to 1,200 PSIG. The structural components of an industrial RO system, including pressure vessels, high-pressure pumps, and their arrangement, are critical for achieving desired flow rates, redundancy, and efficiency. **Pressure Vessels:** These robust cylindrical housings contain the RO membrane elements. Standard industrial vessels are made from Fiberglass Reinforced Plastic (FRP) and are designed to withstand high operating pressures, typically rated from 300 to 1,200 PSIG (Top 2 and Top 3 pages). Each vessel commonly holds 6 to 7 membrane elements in series, ensuring maximum water recovery and rejection within that vessel. The number and arrangement of these vessels determine the overall system capacity. **System Arrays:** * **Parallel Configuration:** Used for achieving high permeate flow rates. Multiple pressure vessels operate simultaneously, each receiving a portion of the feed water. This setup is common in large industrial applications requiring significant volumes of treated water. * **Series Configuration:** Employed to achieve higher salt rejection or to manage challenging feed water. The concentrate from a first stage of vessels becomes the feed for a second stage, further reducing TDS. This is often seen in brackish water RO systems or when aiming for ultra-pure water. Some systems combine both parallel and series arrangements for optimized performance. **High-Pressure Pumps:** These are the heart of the RO system, providing the necessary pressure to overcome osmotic pressure and drive water through the membranes. * **Centrifugal Pumps:** Most common for their reliability and efficiency (typically 60–80%). They are suitable for a wide range of flow rates and pressures (150–1,200 PSIG). Energy consumption for these pumps typically ranges from 0.5–2 kWh/m³ of permeate produced (industry benchmarks). * **Positive Displacement Pumps:** Used for very high-pressure applications or when precise flow control is required. While often more expensive, they can be highly efficient. **Redundancy:** For critical 24/7 operations, an N+1 pump configuration is highly recommended. This means having one spare pump ready to automatically switch over in case of a primary pump failure, preventing costly downtime. Fail-safe shutdown protocols are also essential, protecting the system from damage during power outages or critical parameter excursions. **Pre-treatment Stages:** Effective pre-treatment is vital for protecting RO membranes. This includes: * **Multimedia Filters (MMF):** To reduce turbidity to <1 NTU and remove suspended solids. Zhongsheng’s multi-media filters for RO pre-treatment are designed for optimal performance. * **Activated Carbon Filters:** To remove chlorine (<0.1 PPM), organic matter, and other oxidants that can damage TFC membranes. * **Antiscalant Dosing:** Zhongsheng’s chemical dosing systems for RO pre-treatment precisely inject antiscalants to prevent scaling by keeping the Langelier Saturation Index (LSI) below 0 for scale-forming salts. Learn how pressure flotation systems improve RO pre-treatment by removing suspended solids and oil/grease ahead of filtration stages.

Industry-Specific Compliance Standards for Industrial RO Systems

For pharmaceutical applications, RO systems must consistently produce permeate meeting USP <643> Total Organic Carbon (TOC) limits of less than 500 PPB and USP <645> conductivity requirements of less than 1.3 µS/cm at 25°C. Adhering to industry-specific compliance standards is non-negotiable for industrial RO systems, as these regulations ensure product quality, safety, and operational integrity. * **Pharmaceutical Industry:** * **USP <643> (Total Organic Carbon):** Requires TOC levels in purified water and water for injection (WFI) to be less than 500 PPB. * **USP <645> (Water Conductivity):** Mandates conductivity levels below 1.3 µS/cm at 25°C for purified water and WFI, ensuring minimal ionic impurities. * **FDA 21 CFR Part 110 (Current Good Manufacturing Practice):** Requires sanitary design, materials of construction, and operational procedures to prevent contamination. * **FDA 21 CFR Part 211 (Current Good Manufacturing Practice for Finished Pharmaceuticals):** Further specifies requirements for water systems used in pharmaceutical manufacturing. * **Food & Beverage Industry:** * **NSF/ANSI 61 (Drinking Water System Components – Health Effects):** Certifies that materials used in RO systems do not leach contaminants into the treated water. * **3-A Sanitary Standards:** Dictates hygienic design and fabrication practices for equipment in dairy and food processing to prevent microbial growth and facilitate cleaning. * **EU Regulation 1935/2004:** Specifies requirements for materials and articles intended to come into contact with food, ensuring they do not transfer harmful substances. * **Power Generation:** * **ASTM D5127 (Standard Guide for Ultra-Pure Water Used in the Electronics and Semiconductor Industries):** While primarily for semiconductors, it defines parameters for ultra-pure water, often referenced for boiler feed water due to similar high-purity requirements. * **EPRI Guidelines (Electric Power Research Institute):** Provides comprehensive guidelines for water chemistry in boilers and cooling towers, often requiring permeate conductivity below 0.1 µS/cm for high-pressure boilers to prevent corrosion and scaling. * **Semiconductor Industry:** * **SEMI F63 (Guide for the Use of Gaseous and Liquid Chemicals):** Specifies stringent purity requirements for process water, including TOC levels <1 PPB and particle counts <0.1 µm. * **ASTM D5127 (Type E-1 Water):** Defines the highest grade of ultra-pure water with extremely low conductivity, TOC, and particulate matter. * **Municipal Water Reuse:** * **EPA 40 CFR Part 503 (Standards for the Use or Disposal of Sewage Sludge):** While focused on biosolids, it informs the quality requirements for treated wastewater destined for reuse, particularly concerning microbial limits. * **WHO Guidelines for Drinking-water Quality:** Provides international benchmarks for microbial and chemical limits in water intended for potable reuse applications.

Industry	Key Compliance Standard	Relevant RO System Parameters / Targets
Pharmaceutical	USP <643> (TOC)	Permeate TOC < 500 PPB
	USP <645> (Conductivity)	Permeate Conductivity < 1.3 µS/cm at 25°C
	FDA 21 CFR Part 110/211	Sanitary design, materials, validation (IQ/OQ/PQ)
Food & Beverage	NSF/ANSI 61	Materials in contact with water certified for health effects
	3-A Sanitary Standards	Hygienic design for easy cleaning and preventing microbial growth
	EU Regulation 1935/2004	Food contact materials compliance
Power Generation	ASTM D5127 (Ultra-Pure Water)	TDS < 0.1 PPM, Conductivity < 0.1 µS/cm (for high-pressure boilers)
	EPRI Guidelines	Specific water chemistry for boiler and cooling tower operations
Semiconductor	SEMI F63	TOC < 1 PPB, Particles < 0.1 µm, Resistivity > 18 MΩ·cm
	ASTM D5127 (Type E-1)	Highest purity water, extreme low contaminant levels
Municipal Reuse	EPA 40 CFR Part 503	Microbial removal, pathogen reduction, and specific contaminant limits
	WHO Guidelines	General microbial and chemical limits for potable reuse

Troubleshooting Industrial RO Systems: Common Issues and Diagnostic Steps

A 10-15% drop in permeate flow or an equivalent increase in differential pressure across a membrane array often signals the onset of scaling or fouling in an industrial RO system. Proactive troubleshooting is essential for maintaining optimal performance and extending the lifespan of RO membranes. * **Scaling:** * **Symptoms:** Gradual decrease in permeate flow, increase in feed pressure, increase in differential pressure across the membrane vessels, and a slight increase in permeate TDS. * **Causes:** Precipitation of mineral salts (e.g., calcium carbonate, sulfate, silica) due to exceeding solubility limits. Common causes include high Langelier Saturation Index (LSI), high silica levels (>20 PPM), insufficient antiscalant dosing, or incorrect pH. * **Solutions:** Implement acid cleaning (e.g., citric acid, hydrochloric acid) to dissolve mineral scales. Adjust antiscalant dosage based on feed water analysis and recovery rate. Consider pH adjustment of feed water. * **Fouling:** * **Symptoms:** Similar to scaling – reduced permeate flow, increased feed pressure, increased differential pressure, and sometimes a significant decline in salt rejection (especially for organic or biological fouling). * **Causes:** Accumulation of suspended solids, colloidal particles, organic matter, or biological growth (biofouling) on the membrane surface. Often indicated by an SDI >3 in the feed water. * **Solutions:** Implement alkaline cleaning (e.g., caustic soda with a detergent) for organic and biological foulants. Upgrade pre-treatment stages, such as installing more effective multimedia filters or UF systems, and ensure proper chlorine removal and bio-control. * **Pressure Drop:** * **Normal Range:** A typical pressure drop across a single 8" membrane element is 3–7 PSI, and across an entire pressure vessel (6-7 elements) is 20-40 PSI. * **Causes:** Excessive pressure drop can be due to severe fouling (biofouling, organic build-up) or physical damage/compaction of the membrane elements. * **Solutions:** Conduct chemical cleaning. If cleaning is ineffective, individual element replacement may be necessary. Ensure proper pre-treatment to prevent future fouling. * **Permeate Quality Decline (Increased TDS/Conductivity):** * **Symptoms:** Permeate conductivity consistently above target, increasing salt passage. * **Causes:** Membrane degradation (e.g., chemical attack from chlorine), physical damage (tears, abrasions), O-ring leaks within the pressure vessel, or membrane fouling. * **Diagnostic Steps:** Perform conductivity profiling (testing permeate from individual elements or vessels to pinpoint the faulty element). Conduct a dye test to visually identify damaged membranes or leaking seals. * **Fixes:** Replace damaged membrane elements or O-rings. Ensure proper pre-treatment to prevent chemical degradation. * **Automatic Shutdowns:** * **Causes:** Safety interlocks triggered by critical parameter excursions, such as low feed pressure, high permeate conductivity, high differential pressure, high temperature, or pump motor overload. * **Troubleshooting:** Check all sensors for calibration and functionality. Verify pump operation, motor current, and power supply. Review system alarms and historical data logs for patterns. * **Prevention:** Implement regular sensor calibration, proactive maintenance on pumps and motors, and ensure adequate pre-treatment to prevent conditions that trigger shutdowns.

RO System Cost Analysis: CAPEX, OPEX, and ROI Calculation Framework

The Capital Expenditure (CAPEX) for an industrial RO system can range from $5,000 for smaller units to over $500,000 for large-scale, customized installations. Understanding both CAPEX (Capital Expenditure) and OPEX (Operational Expenditure) is crucial for justifying an industrial RO investment and predicting its long-term financial impact. **CAPEX Breakdown:** * **System Cost:** The core RO unit itself, ranging from $5,000 for small commercial systems to $500,000+ for high-capacity industrial systems (e.g., 50,000+ GPD). This includes membranes, pressure vessels, pumps, controls, and frame. * **Installation:** Costs typically range from $2,000 to $50,000, depending on system size, site complexity, and existing infrastructure. * **Pre-treatment:** Essential components like multimedia filters, activated carbon filters, and chemical dosing systems can add $1,000 to $20,000+, varying with feed water quality. **OPEX Breakdown (per cubic meter of permeate, m³):** * **Energy Consumption:** A significant OPEX component, typically $0.10–$0.50/m³ for standard systems, depending on feed water TDS and electricity rates. * **Membrane Replacement:** Membranes have a lifespan of 3–5 years. Replacement costs average $0.05–$0.20/m³, influenced by membrane type, feed water quality, and maintenance. * **Chemicals:** Antiscalants, cleaning chemicals, and pre-treatment chemicals (e.g., chlorine neutralizers) typically cost $0.02–$0.10/m³. * **Labor & Maintenance:** Routine checks, cleaning, and preventative maintenance labor can be $0.05–$0.20/m³. **ROI Calculation Framework:** The Return on Investment (ROI) helps quantify the financial benefits of an RO system. ROI (%) = ((Annual Savings – Annual OPEX) / CAPEX) × 100 **Example:** A $100,000 RO system (CAPEX) that saves a facility $50,000 annually in reduced water purchases, wastewater discharge fees, and boiler chemical costs, with an annual OPEX of $15,000, would have: Annual Savings – Annual OPEX = $50,000 – $15,000 = $35,000 ROI = ($35,000 / $100,000) × 100 = 35% This indicates a payback period of approximately 2.86 years ($100,000 / $35,000). **Cost-Saving Strategies:** * **Energy Recovery Devices (ERDs):** Can reduce energy OPEX by 30–50% in high-pressure applications (e.g., seawater RO). * **Membrane Cleaning Optimization:** Regular, effective cleaning protocols can extend membrane life by 20–30%, reducing replacement frequency. * **Pre-treatment Upgrades:** Investing in robust pre-treatment (e.g., ultrafiltration, advanced media filtration) can significantly reduce membrane fouling, extending lifespan and lowering cleaning chemical costs.

Cost Category	Sub-Category	Typical Range / Comment
CAPEX (Capital Expenditure)	RO System Unit	$5,000 - $500,000 (depending on capacity, customization)
	Installation	$2,000 - $50,000 (site-specific, complexity)
	Pre-treatment Equipment	$1,000 - $20,000 (MMF, activated carbon, softener)
OPEX (Operational Expenditure) per m³	Energy Consumption	$0.10 - $0.50/m³ (depends on feed TDS, electricity rates)
	Membrane Replacement	$0.05 - $0.20/m³ (3-5 year lifespan, varies by membrane type and conditions)
	Chemicals (Antiscalant, Cleaners)	$0.02 - $0.10/m³ (depends on feed water, cleaning frequency)
	Labor & Maintenance	$0.05 - $0.20/m³ (routine checks, preventative maintenance)
ROI Example	Annual Savings (Water/Wastewater/Chemicals)	$50,000
	Annual OPEX	$15,000
	CAPEX	$100,000
	Calculated ROI	35% (approx. 2.86 year payback)

Frequently Asked Questions

Industrial RO membranes, particularly Thin-Film Composite (TFC) elements, typically have a lifespan of 3 to 5 years when operated under optimal conditions. Here are answers to common questions about industrial RO systems:

What is the typical lifespan of an industrial RO membrane?
Industrial RO membranes, particularly Thin-Film Composite (TFC) elements, typically have a lifespan of 3 to 5 years when operated under optimal conditions. This includes maintaining feed water pH between 2-11, chlorine levels below 0.1 PPM, and an SDI (Silt Density Index) below 3. Proper pre-treatment and regular cleaning are crucial for achieving this lifespan.

How do I calculate the required RO system capacity for my facility?
To calculate the required RO system capacity, use the formula: Required Capacity (GPD) = Daily Water Usage (gallons) / Recovery Rate (%). For example, if your facility requires 10,000 gallons of treated water per day and your chosen RO system has a 75% recovery rate, the required system capacity would be 10,000 GPD / 0.75 = 13,333 GPD. This accounts for the concentrate stream.

What are the signs that my RO system needs cleaning?
Key indicators that your RO system requires cleaning include a 10–15% drop in permeate flow, a 10–15% increase in pressure drop across the membrane vessels (differential pressure), or a 10% decline in salt rejection, all normalized for temperature and pressure, as per ASTM D4516 guidelines. These changes typically signal scaling or fouling on the membrane surface.

Can industrial RO systems handle brackish water with high silica?
Yes, industrial RO systems can handle brackish water with high silica content, but specific design considerations are necessary. To prevent silica scaling, recovery rates must be limited, typically to 50–75%. Effective pre-treatment, including antiscalant dosing specifically formulated for silica, and careful pH adjustment (maintaining pH between 6.5–7.5) are critical. For more detailed information, explore our article on brackish water RO system specifications.

What are the energy consumption benchmarks for industrial RO systems?
Standard industrial RO systems typically consume between 0.5–2 kWh/m³ of permeate produced, depending on feed water TDS and system design. For systems incorporating energy recovery devices (ERDs), energy consumption can be significantly reduced to 0.3–1 kWh/m³ (per DOE 2024 data), making them more economical for high-volume or high-TDS applications. To understand how to choose the best system for your needs, refer to our guide on best RO water purification for industrial use.

Recommended Equipment for This Application

The following Zhongsheng Environmental products are engineered for the wastewater challenges discussed above:

• Zhongsheng’s industrial RO systems with PLC-controlled operation — view specifications, capacity range, and technical data
• Zhongsheng’s chemical dosing systems for RO pre-treatment — view specifications, capacity range, and technical data

Need a customized solution? Request a free quote with your specific flow rate and pollutant parameters.

Related Guides and Technical Resources

Explore these in-depth articles on related wastewater treatment topics:

• Learn how pressure flotation systems improve RO pre-treatment
• Zhongsheng’s multi-media filters for RO pre-treatment