Common methods and solutions for cleaning reverse osmosis membrane (polyamide membrane)

When pure water and brine are separated by an ideal semi-permeable membrane, the ideal semi-permeable membrane allows only water to pass through and prevents salt from passing through. At this time, the pure water side of the membrane water spontaneously flows into the saline side through the semi-permeable membrane. This phenomenon is called infiltration. If pressure is applied to the saline side of the membrane, the spontaneous flow of water will be suppressed and slowed down. When the applied pressure reaches a certain value, when the net flow rate of water through the membrane is equal to zero, the pressure is called osmotic pressure. When the pressure applied to the saline side of the membrane is greater than the osmotic pressure, the direction of water flow will be reversed. At this time, the water in the brine will flow into the pure water side. The above phenomenon is the basic principle of reverse osmosis (RO) treatment of water.

As an advanced water treatment technology, reverse osmosis technology has been widely used in various industries. In the process of using this process, users have exposed some problems in operational technology management to varying degrees. If these problems can be effectively solved, it will have far-reaching significance for maximizing the economic benefits of the water treatment application market.

Types of common problems in the operation of reverse osmosis systems

 1. Pretreatment and dosing problems
2.Water inlet SDI exceeds standard
3. Low organic matter removal rate
4. Sand filter design issues
5. Carbon powder-containing activated carbon filter outlet water
6. Backwash system design issues
7. Short life of safety filter element
8. Choose scale inhibitor
9. Rapid reduction in production of reverse osmosis water

Pretreatment feed problem

 1. Feeding of flocculant and coagulant:

Turbidity, pH, iron ion content of the source water, and the type of dosage of the subsequent system determine the type and dosage of flocculant and coagulant. And dose and dose optimization testing must be adjusted. Many reverse osmosis users do not work well in the area, resulting in poor quality effluent water.

2. Dosage of fungicide

For the source of organic matter and bacterial microorganisms in raw water, users usually use oxidizing media to kill, such as: sodium hypochlorite, liquid chlorine, chlorine dioxide. For some surface water sources, the medium was added for a long time, and its biological resistance was found to make the bactericidal effect very unsatisfactory. Currently, non-oxidizing fungicides, such as ZDH-881 and MDC-881, have a longer lasting killing effect, and can save the cost of adding subsequent reducing agents.

3. Dosage of reducing agent

The dosage of the reducing agent should take into account the thoroughness of the reduction and should also consider the bacterial contamination after excessive use. Generally, when the user is found to be using too much, bacterial slime will be generated on the filter element.

 RO SDI overflow problem

  1.  Factors of wrong process selection during engineering design:
    Occasionally find and solve the pollution index failure caused by the wrong selection of the engineering design system from time to time. The time solution is to transform the pretreatment system.
  2. Discomfort in pretreatment dose
    For water sources whose raw water pH exceeds 8, the excessive addition of flocculants can easily lead to the formation of inorganic colloids, resulting in SDI exceeding the standard.
  3. Poor filtering effect
    There are many reasons for the poor filtering effect, such as: the choice of the cut-off filter material, the unreasonable filling amount, and the unreasonable control of the regular backwashing time of the filter. Will result in higher SDI.

Low organic removal rate

  1. Improve the pretreatment process and improve the COD reduction measures of the system
  2. Try replacing other non-oxidizing fungicides to remove COD.
  3. When the COD is full, replace or regenerate the filter material of the activated carbon filter.

summary

The above situation usually occurs in reverse osmosis systems with surface water as the source. The particularity of the type of water source also makes the system prone to problems in design, manufacture and operation. For reverse osmosis water treatment projects using surface water as the source, it is recommended to pay attention to and resolve the following Questions in section:

Design:

  1. Comprehensive research and master the basic knowledge of users’ raw water. Conditions and patterns of water changes.Select a reasonable water treatment process and consider sufficient safety margins for equipment operation.
  2. Detailed information about the process, such as: filter type, flow rate; filter material specifications and filling height; the number of filter elements of the safety filter, the parameters of the backwash pump and the cleaning parameters, the control system.
  3. The rationality of grouping control, parameters, arrangement of reverse osmosis membrane, and strict selection and calculation of pretreatment device.

Equipment manufacturing, installation and commissioning process:

  1. Users should conduct comprehensive quality and process monitoring according to the design requirements of process equipment manufacturing, and master the quality assurance of selected materials and components.
  2. During the installation of the equipment, the installation location and materials of the instrument and the filling materials (filter materials, resin, hollow balls, etc.) are strictly positioned and tested.
  3. During the commissioning process, qualitative and quantitative experiments need to be performed on the additives and consumables used in the operation. In particular, please pay attention to the compatibility of PAC, PAM, and scale inhibitors, and choose the operational consumables suitable for the system through experiments.

Equipment operation process:

Follow the operating procedures in strict accordance with the operating procedures.
During the first year of operation of the pretreatment system, according to the characteristics of the water source conditions, at least 4 experiments and adjustments should be performed, and the operating indicators and the amount of added chemicals to be accumulated should be adjusted accordingly.
During the operation, a “water quality analysis report” must be produced at least once a month to analyze changes in water source conditions and correct operating parameters and doses in a timely manner.
Reverse osmosis data needs to be standardized every half month, which is necessary to grasp the changing trend of reverse osmosis operation. Many currently operating water treatment systems lack a corresponding job, which leads to the expansion of minor problems in plant operation.
Water treatment system instruments must be calibrated every 3-6 months to ensure the accuracy of the equipment parameters.

Due to differences in surface water sources, the types of operational problems with reverse osmosis equipment vary from place to place. Therefore, users should carefully analyze the operating conditions of their own systems. Especially in terms of membrane fouling characteristics and system cleaning, it should be explored according to the actual situation to find a solution suitable for the system.
For the shutdown protection process of the reverse osmosis system, if necessary, it is recommended to take appropriate measures for long-term protection. Avoid physical properties of reverse osmosis membranes caused by short-term protection or frequent start-stops under low-pressure flushing.

Summary:

The operating conditions of a reverse osmosis system using surface water as a source of water are more complicated than the operating conditions of groundwater. In order to obtain good operation and use results, the design, manufacture and operation of the equipment must be checked. In particular, optimizing operating conditions and operating system management after the equipment is put into production is the key to equipment health!

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