How Do We Solve RO Membrane System Problems in Water Treatment
Table of Contents
Reverse osmosis membrane elements are widely used in large, medium, and small water treatment systems. They have significant advantages in brackish water treatment, seawater desalination, and pure and ultrapure water production. The industry recognizes it as an economical and practical core component of membrane water treatment due to its excellent desalination performance, higher water production, lower operating pressure, and longer service life. However, many users only know how to use it and need to learn more about the performance of membrane elements and common problems in the field. This article will help you understand this knowledge in detail.
What are the common problems of RO membrane systems?
Before using the reverse osmosis system, the operator must check whether the machine and equipment are standard and confirm the normal before connecting the water and power supply. If the equipment fails, the operator must try to discover the essence of the problem from the failure phenomenon to implement the countermeasures of overhauling and maintenance as soon as possible. In general, reverse osmosis systems have three main types of failures.
- When the working pressure and conductivity are normal, the water production decreases.
- After standardization, the desalination rate decreases, and the conductivity of the product water in the reverse osmosis system increases.
- After standardization, the water production rate decreases, and it is usually necessary to increase the operating pressure to maintain the rated water production rate, i.e., the operating pressure increases.
What are the main causes of RO membrane system problems?
The causes of RO membrane system problems generally include the following aspects.
- Reverse osmosis failure caused by changes in feed water quality. The pretreatment device is not optimized.
Reverse osmosis failure caused by pretreatment, such as a multimedia filter or activated carbon filter fillers damage, buffer tank microbial growth.
- Reverse osmosis failure caused by security filters, mainly due to the poor quality of security filters and small diameter, which can not be effectively filtered.
- Reverse osmosis failure caused by the dosing system, such as inappropriate flocculant leading to membrane element damage, over-addition of the oxidizing agent, or reducing agent leading to membrane element damage.
- Reverse osmosis failure caused by instrumentation errors, such as scaling or pressure differential caused by high or low concentrated water flowmeters.
- Irrational selection of auxiliary components, such as high-pressure pump pressure being too low, O-ring damage, and reverse osmosis membrane arrangement needs to be revised, resulting in a large amount of local water flow and thus rapid blockage.
What is the solution to the decreasing capacity of the RO membrane system?
After a period of operation of the RO membrane system, sometimes the treatment capacity is significantly reduced, and the design capacity cannot be reached. This phenomenon is mainly related to the working environment, selection and design, installation and construction, and operation management of the RO membrane system.
Working Environment
The feed water temperature is lower than the design temperature (typical seasonal changes), which will cause the RO membrane effluent performance to decrease. The changes of pollutant type, concentration and water viscosity in the water body will also cause the RO membrane permeability to be lower than expected. Therefore, at the beginning of the RO membrane system design, it is necessary to consider the fluctuation range of minimum water temperature and water quality throughout the year and provide a certain safety margin when budgeting the membrane area.
Selection and Design
Due to a lack of design experience, too much consideration of cost will lead to an insufficient safety margin of the selected membrane area, and too high estimation of water flux of the membrane will make the membrane fail to reach the design capacity. Therefore, it is necessary to ensure sufficient membrane area, and the selection and design should be made according to the stable membrane flux rather than the initial flux.
RO membrane system pressure is insufficient, uneven fluid distribution, resulting in water or air flow bias. The membrane surface flow rate is too low, resulting in pollutant deposition and fouling, affecting the overall membrane performance. The solution is to improve the pump, water and air circuit design so that multiple sets of membranes can share the load to avoid some membranes overloading water production while some membranes do not play a role. Suppose the design of the cleaning and maintenance function is not enough, especially no self-washing function with product water. In that case, it can be solved by improving the design of air washing, water washing, and chemical washing and setting the appropriate washing frequency and water volume.
There is no pre-filtration protection before the feed water enters the membrane, which will cause the RO membrane system to clog. The solution is to add a 5 μm precision filter. The RO membrane system will also experience microbial slime fouling, which will interfere with the normal water production of the membrane. Non-oxidizing biocides must be added. EcoLan has organic bromine, organic sulfur, and organic aldehyde non-oxidizing biocides, which can meet various service conditions and chemical resistance replacements. In addition, EcoLan non-phosphorus scale inhibitors can also be used to slow microbial growth.
Installation and Construction
In manufacturing and installing the RO membrane system, failure to remove the debris in time will lead to excessive impurity residue, thus affecting the membrane water production performance. In particular, welding slag, sharp or filamentous material, etc., will lead to serious membrane filaments and membrane sheet breakage. Users must flush the front piping system first, operate the pre-filter, and then the feed water can be entered into the system.
Operation Management
Correctly adding chemicals, especially scale inhibitors, is very important for the RO membrane system. In addition, the membrane performance should be restored by timely flushing or chemical washing according to the operating conditions. Insufficient design experience leads to a high recovery rate, or manual operation exceeds the design recovery rate, decreasing membrane treatment capacity. Therefore, the RO membrane should increase the discharge of concentrated water, reduce the recovery rate, reduce the number of membrane rows, or increase the return flow.
What is the solution to the shortening life or frequent cleaning of the RO membrane?
The membrane life cycle is short in two aspects: one is the membrane itself is permanently damaged, can no longer produce sufficient quantity and quality of water, and needs to be replaced to solve the problem; the second is the membrane is temporarily damaged after severe pollution, need to be cleaned frequently to restore the ability to produce water, the actual use of the time and cleaning cycle is shorter, it is difficult to adapt to the production requirements. We can solve this problem from the process design, such as by improving the quality of the membrane and, simultaneously, strengthening the environment and operation and maintenance management.
Process Design
Inadequate protective design can cause irreversible damage to the membrane. The solution varies depending on the type of damage. For example, to avoid scratches on the membrane filament and membrane sheet, can add a pre-filter; to prevent RO membrane oxidization by oxidizing substances, install ORP instrumentation and reducing agent automatic dosing device; to prevent overpressure physical damage, set up pressure protection devices; to prevent the membrane filament and membrane sheet rupture by water hammer impact, need to design the details of water hammer mitigation. In addition, the reasonable design of the number of RO membranes in series, when using a two-stage RO design, a segmented cleaning is designed.
Membrane Performance
Suppose the RO membrane has a small flow channel and poor anti-pollution performance. In that case, the wide-channel RO membrane should be selected as much as possible, and the anti-pollution membrane should be selected according to water quality.
Operation and Maintenance Management
Failure to properly or timely dosing, especially the RO membrane system scale inhibitor dosing is not timely, will lead to severe clogging of the membrane. Under correct design, proper operation and maintenance are required according to the operating procedures. A membrane system operating under a high-pressure difference will damage the RO membrane sheet. Therefore, it is necessary to check the transmembrane pressure difference and pressure drop of the membrane group and conduct restorative cleaning according to the operating conditions to prevent overpressure differential operation. During chemical cleaning, strong acids, strong alkalis, and strong oxidizing agents will cause irreversible chemical damage to the membrane. Therefore, the operator must master the chemical cleaning process to avoid strong acid and strong alkali environments, and it is strictly prohibited to use strong oxidizing agents to clean the RO membrane.
Working Environment
Water temperature higher than 45 ℃ will shorten the service life of the membrane. Generally, it is necessary to control the water temperature below 40 ℃, and take cooling measures if needed. For the RO membrane system, wastewater containing strong oxidizing or easy-to-precipitate substances will shorten the membrane life circle. Therefore, from the point of view of its safety, increasing the pretreatment safety margin, and adding protective shutdown devices, can avoid such problems.
How to reduce the high operating costs of RO membrane systems?
The RO system has a high-pressure pump and needs to add a special scale inhibitor, so the operating cost is high. From the perspective of optimization and design, we can reduce the operating cost of the RO system from the following aspects.
Select Ultra-Low Pressure Reverse Osmosis Membrane
The operating pressure of a conventional reverse osmosis membrane is 1.3 ~ 1.5MPa, and the operating pressure of an ultra-low pressure reverse osmosis membrane is about 0.8MPa or even lower (closely related to water temperature), which can save more than 30% of the power consumption. For large-scale RO systems, it can basically offset more than 70% of the annual dismantling cost of the membrane, and the saving is even more significant.
High-Pressure Pump with Frequency Converter
High-pressure pumps with frequency converters can slow down the water hammer impact when the pump starts and set a reasonable operating pressure, reduce the valve throttling energy consumption, and save more than 15 % of energy at least quarterly throughout the year.
Reasonable Calculation of Scale Inhibitor Dosage
We can often save up to 20% or more in dosing costs by analyzing water quality data and optimizing the dosing rate. EcoLan, which has its dosage calculation software, can respond more quickly to changes in water quality. EcoLan concentrated scale inhibitor reduces the cost of packaging and transportation of the agent and the labor cost of dosing, which can effectively reduce the operating cost for the customer.
Appropriately Increasing the Number of Membranes to Reduce Operating Pressure
By appropriately increasing the membrane area, the operating pressure of the membrane can be reduced to a certain extent, thus reducing the power consumption.
Reverse osmosis system design is mainly based on feed water quality conditions, product water quality, and water flow requirements. Numerous operational experience has proved that to make the reverse osmosis system run successfully. One is to improve the level of pretreatment as much as possible to ensure that the water quality indicators of the pretreatment system can reach the feed water requirements of the reverse osmosis system; the second is to install enough membrane elements, and reasonably arrange to ensure that the reverse osmosis system of product water quality and flow can meet the requirements. A deep understanding and mastery of this knowledge will be more and more useful in the practical application of reverse osmosis systems.
