Reverse Osmosis Water Purification Unit

The reverse osmosis water purification unit primarily consists of a multi-stage high-pressure pump, reverse osmosis membrane elements, membrane housing (pressure vessel), and supports. Its primary function is to remove impurities from water, ensuring that the output water meets the required specifications. The high-pressure pump boosts the water from the safety filter to the operating pressure of the RO system and then evenly distributes it to the pressure vessel. The water flow is separated by the reverse osmosis membrane, forming two streams within the pressure vessel. A portion of the incoming water permeates the membranes, resulting in purified water, while the remaining inorganic salts and solid residues are retained and concentrated, forming a concentrate. This process effectively separates the inorganic salts from the water.

Pure water flows from each pressure vessel equipped with a reverse osmosis membrane, merges, passes through a flow meter, and then flows out of the system's outlet into the pure water tank. The concentrated water then flows out of the concentrate outlet of the pressure vessel.

Reverse Osmosis Membrane Desalination Mechanism:

The surface of the semipermeable membrane is covered with numerous extremely fine pores. The membrane selectively adsorbs a layer of water molecules, while salt solutes are repelled by the membrane. Ions with higher valence are repelled to a greater extent. Driven by reverse osmosis pressure, the water molecules surrounding the pores flow through the membrane by capillary action, achieving the desalination effect.

When the membrane pores are larger than the pore size of the reverse osmosis membrane, aqueous salt solutions will leak through the membrane. Monovalent salts leak more frequently, followed by divalent salts, and less frequently, trivalent salts. The pore size of RO membranes is <1.0 nm, so RO membranes can filter out Pseudomonas aeruginosa, one of the smallest bacteria (3000 × 10⁻¹⁰m); they can also filter out various viruses, such as the influenza virus (800 × 10⁻¹⁰m) and meningitis virus (200 × 10⁻¹⁰m); and they can even filter out pyrogens (10-500 × 10⁻¹⁰m).

Advantages of Reverse Osmosis (RO):

Reverse osmosis produces pure water with the following advantages: a compact equipment structure, easy maintenance, a small footprint, and high water yield. It produces pure water without a phase change, resulting in low energy consumption. Furthermore, it generates no acidic or alkaline wastewater, making it a new, energy-saving, and environmentally friendly technology.

The reverse osmosis water purification unit uses reverse osmosis membranes, high-efficiency, low-noise high-pressure pumps, precise instruments, and other components to form an independent reverse osmosis system. The system is assembled on an independent bracket, with an automatic controller installed on the bracket. The entire system is controlled by an advanced PLC, which manages the start and stop of the high-pressure pump, provides automatic high and low pressure protection, and enables automatic fast flushing.

The water output range is from 0.5 m³/h to 1000 m³/h. The reverse osmosis pressure vessel is made of stainless steel or fiberglass, and the reverse osmosis membrane uses a low-pressure composite membrane with a desalination rate of 99%. The operating pressure ranges from 1.3 to 2.0 MPa, depending on the water temperature and water quality. The reverse osmosis water treatment equipment can be specially designed with various types of reverse osmosis membranes to meet the specific needs of different projects.

The typical wastewater-to-purified water ratio in a RO system is 3:1 for small household water purifiers, 1:1 for small industrial systems, and 0.3:0.7 for large systems. Further reductions in wastewater discharge are costly and will inevitably shorten the lifespan of the RO membrane.

This advanced membrane separation technology is finding widespread application in various fields.

Scope of use

 

 

Electronic industry

Pharmaceutical industry

Bottled drinking water

Beverage industry

Metallurgical industry

Chemical industry

District water supply

Desalination of seawater and brackish water

Medium and high-pressure boiler feed water

Pure water for food and beverages

Pipeline water supply for bottled pure water, etc.

Equipment performance

 

The low-pressure switch protects the high-pressure pump from damage due to a water supply interruption.

The high-efficiency, low-noise high-pressure pump reduces operating noise and energy consumption.

The rolled composite membrane, with a high desalination rate and low operating pressure, improves water quality, reduces operating costs, and has a long service life.

Flow meters are provided for both produced water and concentrated water to monitor and adjust the operating water output and system recovery rate.

The produced water conductivity meter continuously monitors the water quality of the produced water.

Inlet and outlet pressure gauges continuously monitor the pressure difference across the reverse osmosis membrane, indicating when cleaning is needed.

Calcium Carbonate Scale:
When the scale inhibitor addition system or the acid addition system fails, the pH value of the feed water increases, which can lead to the deposition of calcium carbonate. The occurrence of calcium carbonate scale should be detected as early as possible to prevent the crystals from accumulating on the membrane surface. If calcium carbonate scale is discovered early, it can be removed by lowering the feed water pH to between 3 and 5 and running the system for 1-2 hours.
For calcium carbonate scale that has been precipitated for a longer period, a 2% citric acid cleaning solution (with pH not lower than 4) should be used for cleaning.
Note: Ensure that the pH of any cleaning solution does not drop below 4, as this may damage the RO membrane element, especially at high temperatures. The pH value should also not exceed 10. Sulfuric acid or hydrochloric acid can be used to lower the pH, and sodium hydroxide can be used to raise the pH.

Calcium Sulfate Scale:
Cleaning solution #2 (see Table 2) is an ideal method for removing calcium sulfate scale from the surface of the reverse osmosis membrane.

Metal Oxide Scale:
The method used to remove calcium carbonate scale can effectively remove deposited hydroxides (e.g., iron hydroxide).

Silica Scale:
Silica scale, which is not bound to metal oxides or organic matter, can generally only be removed using specialized cleaning methods.

Accumulated Sediments:
Organic sediments (such as microbial slime or mildew) can be removed using a cleaning solution. To prevent regeneration and reproduction, the system generally requires a long soaking period to be effective. For example, if the reverse osmosis system is out of service for more than three days, detoxification should be performed.

Selection of Cleaning Fluid:
When backwashing the reverse osmosis system, it is recommended to use the cleaning solutions listed in Table 2 for cleaning the reverse osmosis membrane element. It is crucial to perform a chemical analysis of the pollutants before determining the appropriate cleaning fluid. A detailed analysis and comparison of the results can ensure the proper selection of cleaning agents and methods. Each cleaning procedure should be recorded, including the cleaning method and the results obtained, to provide a basis for determining the most effective cleaning method under specific water supply conditions.
For inorganic pollutants, cleaning solution #1 is recommended. For calcium sulfide and organic pollutants, cleaning solution #2 is recommended. For severe organic contamination, cleaning solution #3 is recommended. All cleaning solutions can be used at a temperature of 40°C for 60 minutes. The required amount of cleaning solution is calculated by adding the amount per 379 liters of water. When preparing the cleaning solution, mix the cleaning agents and reverse osmosis fresh water in the appropriate proportions.

During cleaning, the cleaning solution is circulated at low pressure and high flow rate on the high-pressure side of the membrane. At this time, the membrane element is still installed in the pressure vessel, and a special cleaning device is required to complete the process.

General Steps for Cleaning the Reverse Osmosis System:

Use a pump to introduce clean, chlorine-free reverse osmosis fresh water into the pressure vessel and drain it for a few minutes.

Prepare the cleaning solution using reverse osmosis fresh water (see Table 2 for details).

Circulate the cleaning solution in the pressure vessel for 1 hour or the pre-set time. For an 8-inch pressure vessel, the flow rate should be 35-40 gallons per minute (8-9 m³/h).

After the cleaning is completed, drain the remaining solution from the chemical box, clean it, and then refill it with reverse osmosis fresh water.

Use a pump to transfer fresh water from the chemical box into the pressure vessel and drain it for a few minutes.

After flushing the reverse osmosis system, open the fresh water outlet discharge valve of the reverse osmosis system. Return the reverse osmosis equipment to normal operation and check the quality of the fresh water discharge after 15-30 minutes (ensure there is no foam, no cleaning agent residue, and that the conductivity meets the standard).

Open the fresh water outlet valve of the reverse osmosis system, close the fresh water outlet discharge valve, and resume normal operation of the reverse osmosis equipment.

Table 1 Characteristics and treatment methods of reverse osmosis membrane pollution

Table 2 Common cleaning methods recommended for reverse osmosis equipment

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