Analysis of several malfunctions in the operation of reverse osmosis systems

I. Reverse Osmosis Failures Caused by Deteriorating Influent Water Quality

In the initial design, the influent water quality was good, and the system operated stably. However, as the influent water quality deteriorated, and without improvements to pretreatment, the reverse osmosis unit experienced serious operational failures. This manifested as a rapid decline in water production, a rapid increase in operating pressure and pressure difference, etc.

II. Reverse Osmosis Failures Caused by Pretreatment Performance Degradation

Due to the deterioration of pretreatment equipment performance, the turbidity, SDI value, COD value, etc., of the effluent seriously exceeded the influent water quality requirements.

Specifically:

CMF or UF membrane fiber breakage;

Severe bacterial and microbial growth in the buffer tank;

Disordered or skewed flow of filter media in the multimedia filter;

Powderization or severe microbial growth of filter media in the activated carbon filter.

III. Reverse Osmosis Failures Caused by Inferior Cartridge Filters

When the output of the security filter increases, the filter cartridge is easily deformed or the filtration accuracy does not meet the requirements, causing pollutants to directly enter the reverse osmosis unit.

Specifically:

The security filter cartridge diameter is too small;

The quality of the filter cartridge is poor, and the filtration accuracy does not meet the requirements;

The filter cartridge is not tightly pressed and is easily deformed.

IV. Reverse Osmosis Failures Caused by Improper Selection and Addition of Scale Inhibitors

Scale inhibitors are the “protectors” for the safe and stable operation of reverse osmosis. Due to their excellent effects and low operating costs, they have become the mainstream form of scale inhibition. However, many problems have arisen in the selection, addition, and mixing of scale inhibitors.

Specifically:

The performance of the scale inhibitor does not match the water quality;

The performance of the scale inhibitor metering pump is unreliable;

Excessive dilution of the scale inhibitor;

Severe contamination of the scale inhibitor metering tank.

V. Reverse Osmosis Failures Caused by Improper Addition of Other Chemicals

For different water qualities, a certain amount and type of chemicals need to be added to enhance the treatment effect of the raw water. However, due to various reasons, the incorrect use and addition of these chemicals can have serious consequences.

Specifically:

Inappropriate flocculants cause serious membrane fouling;

Excessive addition of oxidants causes membrane oxidation;

Excessive addition of reducing agents causes severe membrane clogging.

VI. Reverse Osmosis Failures Caused by Instrument and Meter Failures

Currently, imported digital instruments are generally used on reverse osmosis units. Some instruments, when correctly installed, display flow rates very accurately and with stable readings, such as GF's 9010 flow controller; however, other types of digital instruments have larger fluctuations in numerical values during operation, especially those with parameter setting functions. The displayed water production is controlled by human factors, so the instruments that act as the "eyes" of the reverse osmosis will affect the technicians' judgment of the reverse osmosis.

Specifically:

High concentrate flow display (actually lower) causes excessive reverse osmosis recovery rate and scaling;

Low concentrate flow display (actually higher) causes low reverse osmosis recovery rate and pressure difference;

Large fluctuations in flow meter readings make it impossible to correctly judge according to a ratio of 10-15%.

VII. Various problems have occurred in reverse osmosis units. Some systems have design flaws, while others have deviations during daily operation and maintenance, causing serious operational risks.

Specifically:

In the initial design, the high-pressure pump head selection was too low, causing the water production to not meet the design requirements when the temperature or influent water quality changes;

Membrane element oxidation causes increased water flux and decreased water quality;

Inversion or damage of the brine seal causes excessively high actual recovery rate, resulting in scaling and water quality degradation;

O-ring damage causes a decrease in water quality;

Mixing new and old membrane elements and different types of membrane elements causes a decrease in system performance;

Overlapping or partial overlapping of the pressure vessel concentrate stop ring and concentrate outlet causes excessively high recovery rate and scaling;

Excessive pressure vessel length causes concentrate leakage to the product water side, reducing product water quality;

The interstage pressure gauge cannot reliably analyze and judge the reverse osmosis operation;

Large pressure difference causes the membrane element to experience the telescope effect and damage;

Increased product water backpressure causes a decrease in product water volume;

Unreasonable reverse osmosis arrangement causes increased water flux in local membrane elements, accelerating pollution; unreasonable reverse osmosis recovery rate design, and insufficient number of membrane elements;

Particulate pollution causes serious mechanical clogging of membrane elements, large pressure difference in one section, and deterioration of water production and quality;

Pollutant deposition and bacterial and microbial pollution caused by system shutdown.

VIII. Failures in Reverse Osmosis Membranes

Conditions for long-term stable operation of the reverse osmosis system:

1. Pretreatment must be taken seriously; the pretreatment effluent should meet the reverse osmosis influent requirements;

2. The intrinsic quality of the reverse osmosis unit must be taken seriously, such as the reasonable selection of membrane modules (elements) and quantity, and the reasonable arrangement and combination of membrane modules;

3. Importance of operation, maintenance, and management of reverse osmosis systems. Currently, the overall operation of reverse osmosis systems in our province's power generation companies is not ideal, mainly reflected in the following aspects:

First, the operation and management of the pretreatment system Insufficient attention to work

Insufficient attention to the operation and maintenance of the pretreatment system, mainly reflected in the following aspects:

The pretreatment system does not perform sterilization and algaecide, or the effect is poor;

The amount of chemicals added to the clarifier is not adjusted in time according to the water quality, and the sludge discharge from the clarifier is not timely;

Filters are not backwashed in time or the backwashing effect is poor, and the operation, backwashing, and cleaning of ultrafiltration are not carried out as required, etc.

Second, the decline rate of desalination rate

Desalination rate is an important indicator that comprehensively reflects whether the reverse osmosis operation is normal. Membrane manufacturers generally promise a desalination rate of no less than 96% within the three-year warranty period. From the statistics of the desalination rate of the reverse osmosis systems that have been put into operation, the desalination rate of the reverse osmosis systems can reach more than 98% when they are first put into operation. As the operation time increases, the desalination rate of the reverse osmosis membrane gradually decreases to varying degrees. From the desalination rate of most enterprises' reverse osmosis, the decrease in desalination rate during long-term operation is reasonable (meeting or basically meeting the desalination rate of no less than 96% within the three-year warranty period). The desalination rate of a few enterprises' reverse osmosis membrane elements is low due to oxidation, mechanical damage, etc.

Third, the determination of the membrane cleaning cycle

When the surface of the reverse osmosis membrane is blocked by inorganic salt scale, metal oxides, microorganisms, colloidal particles, and insoluble organic matter, it will lead to a decrease in standardized water production and desalination rate, and an increase in transmembrane pressure difference. When the above phenomena are caused by chemical pollution, the reverse osmosis system needs to be chemically cleaned.

For the determination of the chemical cleaning time of the membrane, the standardized calculation of the operation data of the reverse osmosis system should be carried out regularly. After the operation data is standardized, the system water production is reduced compared to the initial value. More than 15%, or the desalination rate is reduced by more than 10% compared to the initial value, or the transmembrane pressure difference is increased by more than 15% compared to the initial value H, online cleaning of the reverse osmosis membrane should be carried out in time.