Reverse osmosis system troubleshooting and solutions

There are three main types of malfunctions in reverse osmosis systems: reduced water permeability, increased salt passage rate (decreased desalination rate), and increased pressure drop. However, there are many reasons for these malfunctions, and it is important to find the root cause of the problem based on these malfunction phenomena so that maintenance and other countermeasures can be implemented as soon as possible.

External factors causing malfunctions

I. Reverse osmosis failures caused by changes in feed water quality

1. Changes in feed water quality;

2. The pretreatment system cannot be optimized.

II. Reverse osmosis failures caused by pretreatment

1. Multi-media filter media disorder or bypass;

2. Severe bacterial and microbial reproduction in the buffer tank;

3. Severe pulverization or microbial reproduction of activated carbon filter media.

III. Reverse osmosis failures caused by security filters

1. The security filter diameter is too small;

2. The filter element quality is poor, and the filtration accuracy does not meet the requirements;

3. The filter element is not tightly pressed and is easily deformed.

IV. Reverse osmosis failures caused by the antiscalant dosing system

1. The performance of the antiscalant does not match the water quality;

2. The performance of the antiscalant metering pump is unreliable;

3. Excessive dilution of the antiscalant and serious pollution of the chemical tank;

4. Bypass caused by antiscalant dosing.

V. Reverse osmosis failures caused by other chemical dosing systems

1. Inappropriate flocculants cause membrane element pollution;

2. Excessive addition of oxidants causes oxidation of membrane elements;

3. Excessive addition of reducing agents causes serious blockage of membrane elements.

VI. Reverse osmosis failures caused by instruments and meters

1. The high concentration water flow display is too large (actually smaller), causing the reverse osmosis recovery rate to be too high and causing scaling;

2. The high concentration water flow display is too small (actually larger), causing the reverse osmosis recovery rate to be too low and causing a large pressure difference;

3. Flow reading fluctuations cause system misjudgment.

Common malfunctions of reverse osmosis devices

I. When the initial design selects a high-pressure pump with a low head, the water production may not meet the design requirements when the temperature or feed water quality changes;

II. Oxidation of membrane elements causes increased water flux and decreased product water quality;

III. The brine seal is reversed, causing the actual recovery rate to be too high, resulting in scaling and decreased water quality;

IV. Damage to the brine seal causes the actual recovery rate to be too high, resulting in scaling and decreased water quality;

V. O-ring damage causes a decrease in product water quality;

VI. The mixed use of new and old membrane elements and different types of membrane elements causes a decrease in system performance;

VII. The pressure vessel concentrate water stop ring overlaps or partially overlaps with the concentrate water outlet, causing the recovery rate to be too high and causing scaling;

VIII. The pressure vessel length is too large, causing concentrate water to leak to the product water side, causing a decrease in product water quality;

IX. The stepless pressure gauge cannot reliably analyze and judge the reverse osmosis operation;

X. A large pressure difference causes the membrane element to produce a telescope effect and damage;

XI. An increase in product water backpressure causes a decrease in product water volume;

XII. Unreasonable reverse osmosis arrangement causes increased water flux in local membrane elements and faster pollution;

XIII. Unreasonable reverse osmosis recovery rate design and insufficient number of membrane elements;

XIV. Particulate pollution causes serious mechanical fouling of the membrane elements, a large pressure difference in one section, and deterioration of water production and quality;

XV. System shutdown causes pollutant deposition and bacterial and microbial pollution;

XVI. Cast iron base high-pressure pump connected in series in the chemical cleaning system pipeline.

Analysis of common reverse osmosis failures

I. Antiscalant dosing system failure

1. There are three key points in the selection of antiscalant agents:

（ 1) Detailed raw water quality analysis—Detailed water quality analysis is a prerequisite;

（ 2) Reverse osmosis system conditions—temperature, recovery rate, arrangement method, water production, etc.;

（ 3) Using specialized computer simulation dosing software, the system operating conditions and feed water quality can be specifically analyzed, and combined with the performance of the agent, the most cost-effective agent selection can be provided.

2. Failure to grasp the three key points will lead to serious consequences:

（ 1) Mismatched agent type causes reverse osmosis scaling;

（ 2) Insufficient dosage causes reverse osmosis system scaling;

（ 3) Excessive dosage increases costs.

II. Dilution and dosing failures of antiscalants

1. Excessive dilution can easily cause the antiscalant to be contaminated by bacteria and microorganisms, causing reverse osmosis system scaling;

2. Incorrect selection of scale inhibitor metering pump, with outlet pressure lower than the pretreatment water pressure, resulting in insufficient chemical addition and reverse osmosis scaling;

3. Incorrect installation of the scale inhibitor metering pump, resulting in insufficient scale inhibitor dosage and reverse osmosis scaling;

III. Scale Inhibitor Mixing Failure

Uneven mixing of scale inhibitors can cause slight /severe scaling in reverse osmosis.

Reverse Osmosis Equipment Failure

Reverse osmosis equipment failures can generally be analyzed from three aspects

First, system design joints

Second, installation and commissioning stage

Third, operation and maintenance stage

1. System design stage

（ 1) Raw water quality and special ions - complete water quality analysis and special ions such as iron, manganese, and silicon;

（ 2) Water temperature - design calculation based on the actual operating water temperature;

（ 3) Recovery rate - determine the optimal recovery rate according to the membrane element arrangement to prevent exceeding the water flux of individual membrane elements;

（ 4) Number of membrane elements - ensure that the average water production of each membrane element is less than 1 ton/hour;

（ 5) Product water backpressure - appropriately calculate the product water backpressure according to the product water transportation conditions;

（ 6) Operating years - simulate at least 3 years of operation to ensure the reliability and redundancy of the high-pressure pump selection, so that the operating life of the reverse osmosis can be extended.

Ignoring the above 6 key points can easily lead to serious failures and adverse effects:

A. As the operating years of reverse osmosis increase and the water temperature changes, when the high-pressure pump reaches its full output, the water production still cannot reach the initial design value;

B. The higher pressure on the product water side causes the high-pressure pump to reach its full output, but the water production still cannot reach the initial design value;

C. The number of membrane elements configured in the reverse osmosis is small, so with the extension of the operating years, higher inlet pressure is required to maintain a stable water production;

D. The reverse osmosis recovery rate exceeds the normal value, and the pollution rate accelerates.

2. Installation and commissioning stage

（ 1) Security filter - strictly control the tightness and compaction of the security filter cartridge installation;

（ 2) Instruments and meters - the flow probe should be kept 1.5 meters from the inlet and 1 meter from the outlet, and equipped with a saddle-shaped probe base;

（ 3) Flushing pipeline system - when flushing the pipeline system, install the security filter cartridge to prevent large particles from depositing on the reverse osmosis equipment and its related pipelines;

（ 4) Membrane installation - medical glycerin should be used when installing membrane elements, and the use of detergents and other lubricants should be avoided as much as possible;

（ 5) Scale inhibitor addition - during initial commissioning, ensure the normal addition of scale inhibitors and other chemicals to prevent membrane element pollution and scaling after prolonged commissioning time;

（ 6) Brine seal - check the installation direction of the brine seal when installing membrane elements;

（ 7) Pollution index SDI - keep the system inlet SDI value below 3.

Ignoring the above 7 key points can easily lead to serious failures and adverse effects:

A. Severe mechanical fouling, especially membrane elements are easily scratched by sharp impurities;

B. Unstable flow meter readings, unable to play a monitoring role;

C. Wear and tear of O-rings and brine seals, resulting in decreased product water quality and high recovery rate;

D. Large pressure difference causes the membrane elements to produce a telescope effect, including abnormal pressure difference caused by scaling and fouling.

3. Operation and maintenance stage

（ 1) Instruments and meters - regular calibration of flow meters and regular cleaning of probes;

（ 2) Pressure gauge - regular calibration of pressure gauges;

（ 3) Pressure vessel - correct disassembly and installation of pressure vessel end plates;

（ 4) Concentrate water thrust ring - correct placement of concentrate water thrust ring;

（ 5) Upper limit of operating data - determine the upper limit of operating parameters such as inter-stage pressure difference, and take timely measures when the upper limit is reached;

（ 6) Manual cleaning - for severe mechanical fouling, avoid using excessively strong water jets for flushing.

Ignoring the above 6 key points can easily lead to serious failures and adverse effects

A. The brine seal is installed in reverse, resulting in a high recovery rate;

B. The pressure vessel thrust ring overlaps or partially overlaps with the concentrate outlet, resulting in high recovery rate operation;

C. The mixed use of new and old membrane elements or different types of membrane elements accelerates the pollution rate;

D. The flow meter shows a large or small deviation, affecting the adjustment of the system recovery rate;

E. Excessive pressure difference causes the membrane elements to be mechanically fractured, resulting in irreversible losses;

F. Inaccurate operating pressure can easily cause the system to operate under overload, accelerating the pollution rate.