How does the RO system work and how to deal with RO membrane contamination?

How does the RO system work and how to deal with RO membrane contamination?

Reverse osmosis technology, also known as RO (Reverse Osmosis), is mainly the use of the pressure difference between the two sides of the membrane as a driving force so as to achieve the separation and filtration of the membrane, is a very advanced and effective energy-saving membrane separation technology.

RO basic principles and advantages
Reverse osmosis membrane is the core element to achieve reverse osmosis technology, is an artificial semi-permeable membrane with certain characteristics, the use of polymer materials, simulation of biological semi-permeable membrane materials made.

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation that separates solvents from an aqueous solution by using a pressure difference as a driving force, and it is the process of realising water filtration of impurities. It is called reverse osmosis because it goes in the opposite direction to natural osmosis. The technical principle is to apply pressure to one side of the membrane at a higher osmotic pressure than that of the solution, and when the pressure exceeds its osmotic pressure, the solvent permeates in the opposite direction, separating these substances from water. The solvent obtained on the low-pressure side of the membrane is called the permeate; the concentrated solution obtained on the high-pressure side is called the concentrate. If seawater is treated by reverse osmosis, fresh water is obtained on the low-pressure side of the membrane, and brine is obtained on the high-pressure side. It is possible to make reverse osmosis pressure, to achieve the purpose of separation, extraction, purification and concentration. Reverse osmosis is a water treatment technology using membrane separation, belonging to the physical method of cross-flow filtration. Its advantages are as follows:
Under the condition of room temperature, relying on the pressure of water as the driving force, the running cost is low;
No large amount of waste acid and alkali discharge, not polluting the environment;
Simple system, convenient operation, high degree of automation;
There is a large adaptable range for the quality of raw water, and the quality of the effluent water is stable;
The equipment covers a small area, and the workload of overhaul and maintenance is small.

RO water treatment process

First, a one-stage treatment process. After the liquid enters the membrane module, the pure water and concentration of liquid lead. Compared with other reverse osmosis water treatment process, the overall process is more convenient, simple operation, but has high limitations, can not meet the higher water quality requirements.
Second, a multi-stage treatment process. On the basis of a one-stage treatment process, the liquid is concentrated in multiple steps. Compared with the one-stage treatment process, the complexity of this process is higher, and it can meet higher water quality requirements and realise the recycling of water resources. 
Third, two-stage one-stage treatment process. In cases where it is difficult to meet the actual water quality requirements using the one-stage method, the two-stage one-stage process can be used. Compared with the above two primary process, the use of two-stage one-stage treatment process can extend the application life of reverse osmosis membrane, and does not require excessive human operation, the corresponding processing costs have also been reduced.

RO application in water treatment

Deep treatment of urban wastewater
In the deep treatment of urban water pollution, reverse osmosis technology can contribute to the sewage recovery rate increase, the application is more extensive. Different materials of reverse osmosis membrane produced by the depth of water pollution treatment effect there are differences. Generally speaking, in the deep treatment of urban water pollution, after the treatment of urban residential sewage to meet the standard, the higher requirements for the treated water quality (such as water reuse), at this time, the hollow fibre membrane of cellulose triacetate, spiral rolled polyvinyl alcohol composite membranes and so on can play a better effect.
Compared with other materials of reverse osmosis membrane, the above two materials of reverse osmosis membrane on the faecal coliform bacteria retention rate of 100%, the chromaticity is not higher than 1 degree, permeate in 1mg/L ~ 2mg/L. At the same time, these two materials of reverse osmosis membrane has a higher water flux, anti-pollution ability is stronger. 

Industrial wastewater treatment
 (1) treatment of heavy metal ions will reverse osmosis water treatment technology applied to industrial wastewater treatment, the effect is also very good, in line with the overall design principles of industrial economic rationality, can reduce energy consumption and operating costs and operational management difficulties. Reverse osmosis devices used for industrial wastewater treatment are generally internal pressure tubular or rolled components, and the pressure is generally stable at about 2.8 MPa, which is very effective in the recovery of heavy metal ions. Among them, the operating pressure of the reverse osmosis device based on the internal pressure tubular component is stable at 2.7MPa, at which time, the recovery rate of nickel is above 99% and the separation rate of nickel is in the range of 97.12%~97.17%. 
(2) Treatment of oily wastewater Generally speaking, the oil in oily wastewater mainly exists in three forms, including emulsified oil, dispersed oil and floating oil. Comparatively speaking, the treatment method of dispersed oil and floating oil is relatively simple, relying on mechanical separation, precipitation, and activated carbon adsorption, which can greatly reduce the content of the corresponding oil. However, for the emulsified oil, it contains organic matter, can play the role of surfactants, and the oil is generally micron-sized particles, so it has a very high stability, it is difficult to effectively and quickly achieve the separation of water and oil. 
With the support of reverse osmosis water treatment technology, concentration and separation can be achieved without destroying the emulsion, and then the concentrated liquid is incinerated and the permeate is recycled or discharged. At this stage, in the treatment of oily wastewater, for the consideration of the final treatment effect and effluent water quality, it is common to use reverse osmosis water treatment technology in combination with other treatment methods. For example, the use of self-formulated DEMUL - B1 as an emulsifier to emulsify the high concentration of O / W-type spinning oil wastewater, and then OSMONICS SE reverse osmosis membrane for further treatment of emulsified water samples. The results show that the COD removal rate of the water purified by ‘emulsion breaking - reverse osmosis’ treatment reaches 99.96%, and the oil content is almost undetectable.

3）Desalination of brackish water
In the process of desalination of brackish water, through the introduction of reverse osmosis water treatment technology, can effectively inhibit the salty water contains magnesium ions, calcium ions and other inorganic salt ions, to achieve the enhancement of pure water quality. At this stage, people's quality requirements for pure water enhancement, the original treatment (in the salty water with scale inhibitors) is difficult to meet the people's realistic requirements, the introduction of reverse osmosis water treatment technology for the inevitable choice. In the use of reverse osmosis device to carry out the operation of brackish water desalination, the need for regular testing of the SDI index, strict control of the recovery rate, pay attention to the pressure difference between the membrane module, and real-time determination of water production and desalination rate changes. In practice, the desalination rate of the reverse osmosis device is stable at more than 96%, and the quality of desalinated water meets China's drinking water standards.

How to deal with RO membrane pollution

Membrane contamination refers to the contact with the membrane fluid particles, colloidal particles or solute macromolecules due to physical and chemical effects with the membrane or due to the concentration of polarisation so that the concentration of certain solutes on the surface of the membrane exceeds its solubility and mechanical effects caused by adsorption on the membrane surface or within the membrane pore, deposition caused by the membrane aperture becomes smaller or clogging, so that the membrane flux and the separation of characteristics of the phenomenon of significant decline in irreversible changes.

Microbiological contamination
 1) Formation causes 
Microbial contamination refers to the accumulation of microorganisms in the membrane - water interface, thus affecting the performance of the system phenomenon. These microorganisms to reverse osmosis membrane as a carrier with the help of reverse osmosis concentrated water section of the nutrient salts and reproduction and growth, the formation of biofilm layer on the surface of the reverse osmosis membrane, resulting in reverse osmosis system in and out of the water between the rapid increase in the pressure difference between the rapid decline in the amount of water produced and the rate of desalination, and at the same time, contaminate the product water. Biofilm composed of microorganisms can degrade membrane polymer or other reverse osmosis unit components directly (through enzyme action) or indirectly (through local pH or reduction potential action), resulting in shortening of the membrane life, so that the integrity of the membrane structure has been damaged, and even cause major system failures. 
2) Control methods 
Biological pollution can be controlled by continuous or intermittent sterilisation and disinfection of the feed water. The raw water from the surface and shallow underground should be set up to sterilise and dosage device, chlorine biocides, dosage is generally based on the residual chlorine content of the feed water > 1mg / L shall prevail. 

Chemical pollution
 (1) the formation of causes
Common chemical contamination is the deposition of carbonate scale within the membrane element, in most cases, misuse, scale inhibitor dosing system is not perfect, scale inhibitor dosing interruptions during operation. If not found in time, in a few days time will appear operation pressure increase, pressure difference increase, water production rate drop phenomenon, if the choice of scale inhibitor and water quality mismatch or dosing amount is insufficient will also occur within the membrane element scaling phenomenon, lighter membrane element scaling can be restored through chemical cleaning its function, serious, will also result in part of the pollution of the serious membrane element scrapped.
2）Control method
Prevent scaling in the membrane element, first of all, select the reverse osmosis scale inhibitor suitable for the water quality of the system, and determine the optimal dosage. Secondly, strengthen the monitoring of the dosing system, pay close attention to the operation parameters of the slightest change, find abnormalities in time to find the cause. In addition, most of the reasons for the high content of Fe3+ in the water are brought by the piping system, therefore, the system piping including water piping as far as possible using steel lined plastic piping to reduce the content of Fe3+.

Suspended particles and colloid pollution 
1) Causes of formation 
Suspended particles and colloids are the main substances that foul the reverse osmosis membrane, and they are also the main reason for the exceeding of the SDI (sludge density index) of the effluent. Due to the different water sources and regions, the composition of suspended particles and colloids also has a big difference. Usually the main components of unpolluted surface water and shallow groundwater are: bacteria, clay, colloidal silica, iron oxides, humic acid products, and flocculants and coagulants (e.g., iron salts, aluminium salts, etc.) artificially over-inputted in the pretreatment system. In addition, the positively charged polymers in the raw water combine with the negatively charged scale inhibitors in the reverse osmosis system to form precipitation, which is also one of the causes of such pollution. 
2) Control methods 
When the content of suspended solids in the raw water is >70mg/L, the pretreatment method of coagulation, clarification and filtration is usually adopted; when the content of suspended solids in the raw water is <70mg/L, the pretreatment method of coagulation and filtration is usually adopted; when the content of suspended solids in the raw water is <10mg/L, the pretreatment method of direct filtration is usually adopted. In addition, microfiltration or ultrafiltration is a recent rise in the membrane treatment of turbidity and non-dissolved organic matter in an effective way, can remove all the suspended solids, bacteria, most of the colloids and non-dissolved organic matter, is the more ideal reverse osmosis system pretreatment process. 

RO application considerations

Reverse osmosis technology in the application of water treatment process, the sewage should be necessary filtration. Filtering work is the basis of reverse osmosis technology to play a role in the filtration process should be strictly controlled to avoid impurities mixed in the water into the reverse osmosis system, in order to protect the osmosis membrane and equipment, improve water output, reduce the possibility of corrosion.
To carry out regular flushing of reverse osmosis device, especially for the scale quality to clean, maintain the good performance of the semi-permeable membrane, prolong the service life of the device.
When not in use, the reverse osmosis unit will be affected by the accumulation of sewage, which will breed microorganisms, therefore, during the downtime of the unit, it is necessary to carry out rinsing and disinfection, and the temperature during the downtime of the good settings, in order to protect the reverse osmosis membrane.
Operators should strictly comply with the operating procedures and operating standards, and constantly improve their professional quality, should be carefully checked before the use of the device, to avoid damage to the device because of operator error, to ensure that the device can be operated normally, and the smooth development of wastewater treatment work.