Design and Maintenance: Ultra-Pure Water Treatment System with Reverse Osmosis

Reverse Osmosis (RO) system is the primary driving force for desalination and the core part of a water treatment system. The current market RO desalination rate has reached over 99.6% and the membrane models are roughly divided into 4-inch and 8-inch sizes. The design flux for a 4-inch membrane is 0.20t and for an 8-inch membrane is 0.80t/h. Designs with a flux greater than this can cause serious water production problems, especially in regions with large temperature differences.

Figure 2 RO membrane.

The RO membrane itself is directly affected by changes in the temperature of the incoming water. Using 25°C water temperature as the standard, the production of water from the membrane will decrease by 0.3%-0.5% for every 1°C drop in temperature. The RO membrane system’s recovery rate should be 65-70%.

The design of the RO system should be based on multiple factors such as the water quality and climate of the region, as well as the design concepts of the industry it will be used for. To maximize the advantages and performance of the RO system, comprehensive evaluations should be conducted from various angles and aspects, such as pump selection, membrane quantity design, RO system layout ratio, membrane recovery rate design, and pre-treatment design.

Membrane System Design

The selection of membrane types and recovery rates varies depending on the special treatment requirements (ultrapure water, intermediate water, wastewater) and the source of the raw water (river water, well water, reservoir water, wastewater treatment discharge water).

In the ultrapure water field, for example, there are RO membranes and anti-pollution type RO membranes, each with different designs of membrane models and recovery rates.

Figure 3 RO membrane design.

The best way is to have a raw water quality report or extract customer-provided raw water for professional testing to achieve a good design based on water quality data. RO systems should be equipped with a physical cleaning system that specifically targets RO for physical cleaning and appropriate chemical cleaning based on changes in RO membrane pressure differences and water production (chemical cleaning is recommended once every 3-6 months).

Procedures for System Shutdown

  • When the system is shut down for an extended period of time, it is necessary to flush the entire membrane system with high-quality feed water to replace the high-salinity concentrate from the pressure vessels and membrane elements.
  • Flushing should be done at a low pressure of about 3.0 bar (40 psi), and high flow rate flushing may cause the pressure differential at both ends of the element or pressure vessel to exceed the maximum specified value.
  • The feed water used for low-pressure flushing should not contain any chemical agents used for pretreatment, especially antiscalants.
  • Therefore, the dosing should be stopped before flushing. After the flushing is completed, the feed valve should be completely closed.
  • If the concentrate discharge port is lower than the pressure vessel, air should be introduced into the high-concentration water pipeline above the pressure vessel to destroy the siphoning effect.

If the system has been shut down for more than 48 hours, the following should be noted:

  • Prevent the membrane elements from drying out, as an irreversible decline in water generation may occur after the elements are dried out.
  • Take appropriate protective measures to prevent microbial growth or perform periodic flushing every 24 hours.
  • Avoid extreme temperatures that may affect the system.

The longest shutdown time for a membrane system without any protective measures against microbial growth is 24 hours. If flushing every 24 hours cannot be achieved but the system must be shut down for more than 48 hours, chemical agents must be used for preservation.

Optimal Methods for Membrane Cleaning

Inorganic salts, bacteria, colloidal particles, along with insoluble organic substances are just some of the contaminants that can build up on reverse osmosis membranes over time. These contaminants accumulate on the membrane surface, resulting in a decline in both the uniform water output and the desalination efficiency of the system.

The membrane elements need to be cleaned if:

  • Water output experiences a decline of > 10%.
  • The pressure of the feed water and the concentrated water escalates by 15%.
  • Salt rejection rate > 5%.

After 48 hours of use, the system will be evaluated on the criteria listed above. During daily operation, the pressure differential (OP) between each section of the pressure vessel must be measured and recorded, and as the feed water passage inside the element becomes blocked, AP will increase. 

It should be noted that if the feed water temperature decreases, the water production of the element will also decrease, which is a normal phenomenon and not caused by membrane fouling. Conditions such as pretreatment methods, pressure management concerns, and recovery rates might affect the final water output and salt rejection percentage. When problems with the system are observed, the cause of these problems may not necessarily be due to membrane fouling, but these reasons should be considered first.

Safety Precautions for Cleaning

Figure 4 Safety Precautions for UF System Cleaning.

1. Adhere to approved safety protocols when using any cleaning chemicals. It’s important to consult with the chemical manufacturer to obtain guidance on safely handling, using, and disposing of chemicals.

2. When preparing the cleaning solution, ensure all cleaning chemicals are completely dissolved and mixed before incorporating them into the membrane system’s circulation.

3. After circulating the cleaning solution with the membrane elements, flush the elements using high-quality water devoid of residual chlorine or other oxidants (minimum temperature > 20°C). It’s advisable to use the membrane system’s product water. If there are no pipeline corrosion issues, dechlorinated tap water, and pre-treated feed water may also be used. 

Upon returning to regular operating pressure and flow rate, initiate the flushing of the cleaning solution at a reduced flow rate and pressure. During the cleaning process, the cleaning solution may infiltrate the product water side, so it’s crucial to discharge the product water for at least 10 minutes or until it’s clear after the normal operation has begun.

Figure 5 Operating pressure and flow rate.

4. While circulating the cleaning solution, the temperature should not surpass:

  • 40°C at pH 2 – 10
  • 35°C at pH 1 – 11
  • 30°C at pH 1-12

Figure 6 pH meter.

5. To prevent the “telescoping” effect of the element, it is crucial to ensure that the flow direction of the cleaning solution for elements with diameters larger than 6 inches is consistent with the normal operating direction. This is because the thrust ring within the pressure vessel is solely located at the concentrate end. This consideration should be taken into account when cleaning smaller element systems as well.

6. Standard cleaning agents include acidic and alkaline agents. Acidic agents help eliminate inorganic contaminants like iron pollution, whereas alkaline agents target organic impurities such as microorganisms.

Avoid using sulfuric acid as a cleaning agent since it can lead to calcium sulfate precipitation. Ideally, the cleaning solution should be prepared using the membrane system’s product water.

In many situations, pre-treated and approved feed water can also be utilized for creating the cleaning solution. If the source water has a high buffering capacity, additional acid or alkali may be needed to achieve the desired pH value. The typical pH for acidic cleaning is around 2, while alkaline cleaning has a pH of approximately 12.

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