Technical Analysis and Operation Maintenance of RO Reverse Osmosis Units

Technical Analysis and Operation Maintenance of RO Reverse Osmosis Units

I. Detailed Process Principle:

The reverse osmosis unit (RO unit) serves as the core equipment in desalination systems, operating on the principle of membrane separation technology. This unit effectively removes most ions and SiO₂ from water, significantly reducing TDS (Total Dissolved Solids) content, thereby alleviating the burden on subsequent desalination equipment. Within the RO unit, feedwater is divided into two streams: one flows perpendicular to the membrane, causing salts and colloidal substances to concentrate on the membrane surface; the other flows parallel to the membrane, carrying away the concentrated substances. This design enables self-cleaning during operation. Higher water flux through the membrane elements correlates with increased recovery rate and greater concentration on the membrane surface. However, due to concentration effects, the substance concentration at the membrane surface differs from that in the main water flow, leading to concentration polarization. This phenomenon increases salt concentration at the membrane surface, elevates osmotic pressure, and consequently increases salt permeation rates and energy consumption. To address this issue, appropriate cleaning methods are required to restore membrane performance.

II. Structural Overview:

The reverse osmosis unit primarily consists of composite membrane elements, FRP pressure vessels, carbon steel skids, and instrumentation control cabinets. The control cabinet houses multiple instruments, including conductivity, flow rate, and pressure gauges, designed to provide real-time monitoring and support automated operation.

III. Performance Parameter Overview:

The performance parameters of reverse osmosis units encompass multiple aspects, including key indicators such as water production rate, salt rejection rate, and recovery rate. These parameters not only reflect the operational efficiency of the unit but also provide users with essential criteria for evaluating its performance. By understanding these performance parameters, users can make more informed choices and utilize reverse osmosis units effectively to meet their specific requirements.

IV. Performance Parameter Overview

The performance parameters of reverse osmosis units are not only diverse and complex but also interrelated, collectively forming a comprehensive system for evaluating unit performance. To help users gain a clearer understanding of these parameters, we have compiled a dedicated performance parameter overview table. This table allows users to quickly reference the definitions, units, and normal ranges for each parameter, enabling more accurate assessment of reverse osmosis unit performance.

V. Short-Term Storage Methods 2. Feedwater Quality Requirements:

To ensure safe and efficient operation of the reverse osmosis system, raw water entering the system must undergo appropriate pretreatment and adjustment to meet specified water quality parameters.

VI. Installation Considerations:

When installing the reverse osmosis system, the following conditions must be met to ensure proper and efficient operation:

Upon arrival at the site, the reverse osmosis system should be placed indoors, with the ambient temperature maintained between 5°C and 38°C.

If ambient temperature exceeds 35°C, enhanced ventilation measures must be implemented to ensure installation and operation occur within suitable temperature conditions.

Installation must be completed within one month of the reverse osmosis unit's arrival at the site, followed immediately by a water trial run to ensure the unit can be successfully commissioned.

Prior to the water trial run, all valves must be confirmed closed to prevent accidental leakage of the protective solution, which could damage unit components.

After positioning the unit, carefully adjust its support points to ensure components are securely level and maintain reliable contact with the foundation surface.

Before connecting piping and valves linking the unit to the feedwater pump, degreasing must be performed. The pump's wetted parts must undergo the same degreasing process. The maximum discharge height of the unit's product water outlet pipe should be controlled below 8 meters.

VII. Detailed Commissioning Procedure:

Commissioning the RO unit requires adherence to a series of steps to ensure proper operation and production of water meeting specified quality standards. These steps are critical not only for the unit's safety and stability but also directly impact the final water quality outcome. Below is a detailed description of these commissioning steps to help you better understand and master each phase of the process.

Before initiating water commissioning, the feedwater must be analyzed and tested to confirm compliance with feedwater specifications.

Adjust the pressure control of the feedwater pump and the automatic water quality monitoring system to ensure proper unit operation.

Thoroughly inspect all piping connections for integrity, verify the presence of all pressure gauges, and confirm tightness at low-pressure pipe joints to eliminate any deficiencies.

Open the pump pre-drain valve, start the pretreatment equipment, and adjust it to deliver a water flow rate exceeding the unit's inlet flow to ensure thorough pretreatment.

Fully open all pressure gauge valves, inlet valves, concentrate drain valves, and product water discharge valves to prepare the unit for commissioning.

Close the pump pre-drain valve. After the membrane modules are fully filled with water, close the unit inlet valve to ensure internal stability. Start the high-pressure pump. Slowly open the plant inlet valve while closing the concentrate drain valve, maintaining the total plant pressure differential below 0.3 MPa.

After 15 minutes of operation, inspect all high- and low-pressure pipelines and instruments for proper function. Adjust the inlet valve and concentrate outlet valve to achieve a product water to concentrate ratio of 3:1, meeting plant operational requirements.

Test the product water conductivity. Once it meets specifications, first open the product water outlet valve, then close the product water discharge valve to ensure product quality.

During commissioning, observe the following precautions: Maintain inlet water pressure below 1.5 MPa; adjust product water flow based on water temperature; inspect desalination rates of all components after 4 hours of continuous operation; immediately relieve pressure if high-pressure piping leaks are detected.

Next, we will further explore the operational management of the unit.

Strictly control feedwater quality to ensure the unit operates under water conditions meeting specified parameters. Select the lowest feasible operating pressure while maintaining required product water flow and quality to minimize membrane water flux decline and reduce membrane replacement frequency. Determine appropriate feedwater temperature based on actual water demand and boundary pressure to mitigate membrane water flux decline. Adjust the rejection rate based on variations in water temperature and operating pressure, maintaining a rejection-to-product water ratio of 1:2.5 to 1:3 to ensure the unit's desalination rate. During summer when water temperatures are elevated, measures such as reducing operating pressure or rotating shutdowns of reverse osmosis units may be implemented. However, shutdown duration should not exceed 24 hours to prevent bacterial growth on the membrane surface. Avoid prolonged shutdowns; the unit should operate for at least 2 hours daily. If shutdown exceeds 72 hours, chemically clean the unit to protect components. Before each shutdown or startup, flush the unit for 15 minutes with feed pressure below 0.4 MPa. Operators must record operational parameters—including feed, product water, and concentrate water metrics—every 2 hours. Ensure all operators receive professional training before assignment.

VIII. Fault Analysis and Troubleshooting Methods:

Response Strategies for Abnormal Plant Operation:

Next, we will explore response strategies for abnormal plant operation. When encountering such issues, first identify the primary symptoms—specifically, what abnormal conditions have arisen. These symptoms may include, but are not limited to: decreased operational efficiency, unstable performance, or equipment malfunctions. Corresponding measures must be taken to analyze and resolve these issues.

Response Strategies for Reverse Osmosis Component Abnormalities:

When reverse osmosis components exhibit abnormalities, we must first identify the specific abnormal phenomena. These may include reverse osmosis membrane fouling or component leakage. Different abnormal phenomena require distinct countermeasures. For instance, membrane fouling may be addressed through chemical or physical cleaning to restore performance, while component leakage may necessitate seal replacement or repair. When addressing partial abnormalities in RO components, we must monitor a series of critical indicators. These indicators not only aid in accurately diagnosing component issues but also inform targeted countermeasures. For instance, membrane clogging and component leakage are two common phenomena requiring special attention. By carefully observing these indicators, we can promptly implement measures to ensure stable operation of the RO system.

Symptoms and Countermeasures for RO Component Contamination:

When RO components become contaminated, a series of noticeable symptoms emerge. These symptoms not only degrade component performance but may also adversely affect the entire reverse osmosis system. Therefore, timely detection and resolution of these symptoms are critical. Common symptoms of RO component contamination include membrane blockage, reduced water production, and deteriorating water quality. To address these symptoms, corresponding countermeasures can be implemented, such as cleaning components or replacing filter cartridges, to restore normal operation.

Cleaning Formulas for RO Components:

Cleaning is a vital step in addressing RO component contamination. To ensure effective cleaning, understanding appropriate cleaning formulas is essential. The selection of cleaning formulas should be based on the component material and contamination level to remove contaminants without causing damage. Typically, specialized cleaning agents or homemade cleaning solutions can be used. Specific formulas can be referenced from relevant product instructions or professional recommendations.

5. Necessary Conditions and Precautions for Reverse Osmosis System Cleaning: Under standard operating conditions—specifically, when pressure is maintained at 1.5 MPa, water temperature is 25°C, and recovery rate reaches 75%—the reverse osmosis system must undergo immediate chemical cleaning if any of the following conditions occur:

(a) When the total pressure drop across the unit increases by 0.15 to 0.2 MPa compared to initial operation.

(b) The unit's salt rejection rate has decreased by three percentage points since the last cleaning.

(c) The unit's total water production has decreased by more than 10% since the last cleaning.

(d) Even if none of the above three conditions occur, it is recommended to clean the unit every 3 to 6 months.

5-2 Cleaning Precautions:

(a) Select an appropriate cleaning formulation based on the contamination type. Acid-based formulations are recommended for CaCO₃ scaling and heavy metal contamination, while alkaline formulations are preferred for removing organic contaminants. (b) Strictly control the temperature and pressure of the cleaning solution during the process. Solution temperature must not exceed 30°C, and pressure should be maintained below 3.5 kg/cm². Set each cleaning segment to 45 minutes. Ensure the solution is prepared using reverse osmosis product water or deionized water, and mix thoroughly. (c) After cleaning, flush the system with filtered water (passed through a 5-micron security filter) for the specified duration. Flushing typically lasts 30–60 minutes, ensuring complete drainage of reverse osmosis outlet water. (d) Immediately resume normal system operation after flushing. The interval between chemical cleaning completion and restart must not exceed one day to ensure cleaning efficacy and stable system operation.

6. Detailed Cleaning Procedure:

6-1 Preliminary Cleaning: Remove the compression sleeve plugs from all cleaning return ports, replace with compression sleeve fittings, and close the concentrate outlet valve.

6-2 Start Cleaning Pump: Open the cleaning pump outlet valve, start the cleaning pump, and adjust the cleaning pump return valve to control the output flow rate to 80 m³/h (for the component being cleaned). Simultaneously ensure the gauge pressure on the pressure gauge does not exceed 0.4 MPa. Circulate the cleaning solution for 45 minutes, maintaining the cleaning temperature below 30°C.

6-3 Switch to Second-Stage Cleaning: After cleaning, close the main return port for the first-stage cleaning solution and prepare for second-stage cleaning.

6-4 Repeat Cleaning Procedure: Perform second-stage cleaning following the steps in 6-2.

6-5 Post-Cleaning Operations: Upon completion, adjust all valves back to normal operating positions. Open the concentrate and product water discharge valves, then close all valves on the cleaning apparatus.

6-6 Perform low-pressure flushing: Initiate the low-pressure flushing program for further system cleaning.

7. Membrane preservation method:

7-1 Prepare preservation solution: Use the cleaning device to prepare the membrane preservation solution as required.

7-2 Close the concentrate outlet valve and open relevant valves: Close the concentrate outlet valve, then open the cleaning solution inlet valve, cleaning solution outlet valve, and permeate outlet valve.

7-3 Perform low-pressure preservation cleaning: Follow the steps in 6-2, ensuring the total pressure difference across the cleaning pump flow is maintained below 0.4 MPa.

IX. Precautions During Module Assembly/Disassembly:

Handle components with care during assembly/disassembly to ensure proper installation and secure connections, guaranteeing stable system operation.

1. When removing elements, pull them parallel to the module inlet direction to avoid lateral shaking.

2. When installing elements, push them in along the inlet direction with even force; never use hard objects to force them in.

3. Immediately immerse disassembled elements vertically in the preservation solution. Do not allow them to dry in air.

X. Membrane Storage Methods:

Under specific storage conditions, membrane elements can effectively extend their service life. Note the following points during storage: First, ensure membrane elements are immediately immersed vertically in the storage solution after removal. Second, regularly replace the storage solution to maintain its cleanliness and efficacy. Finally, prevent physical damage or chemical contamination to the membrane elements during storage. Following these steps ensures proper preservation, thereby extending the overall lifespan of the membrane elements.

XI. Short-Term Storage Method:

The short-term storage plan primarily applies to reverse osmosis systems that are temporarily shut down for periods exceeding 5 days but less than 30 days. During this time, the reverse osmosis membrane elements remain within the RO system's pressure vessel. Below, we detail the specific operational steps for short-term storage.

XII. Long-Term Shutdown Protection

Long-term shutdown protection measures apply to reverse osmosis systems that remain inactive for over 30 days with membrane elements still housed in pressure vessels. To ensure system integrity, follow these protective steps:

1. Thoroughly flush the membrane elements within the system;

2. Prepare a biocide solution containing 1% sodium bisulfite based on reverse osmosis permeate water, ensuring the system is completely filled;

3. If the system temperature is below 27°C (81°F), replace the biocide solution every 30 days; if above 27°C (81°F), replace the protective solution every 15 days;

4. Before restarting the reverse osmosis system, flush it with low-pressure water for one hour. Confirm that no disinfectant residues remain in the product water before resuming normal operation. Additionally, prior to system installation, membrane elements should be stored in a dry, well-ventilated environment at temperatures between 20-35°C, avoiding direct sunlight and exposure to oxidizing gases.