Reverse Osmosis Water Treatment Equipment: A Comprehensive Guide

I. Preparation Principle

Reverse osmosis water treatment equipment typically comprises three components: a raw water pretreatment system, a reverse osmosis purification system, and an ultrapure post-treatment system. The primary purpose of pretreatment is to condition the raw water to meet the feedwater requirements of the reverse osmosis membrane separation components, thereby ensuring stable operation of the reverse osmosis purification system. The reverse osmosis membrane system represents the most cost-effective purification method for removing over 98% of ions and organic compounds, along with 100% of microorganisms (theoretically), from raw water in a single pass. The ultrapure post-treatment system employs multiple integrated technologies to further eliminate residual trace ions, organic compounds, and other impurities from the reverse osmosis purified water, thereby meeting the final water quality specifications required for diverse applications.

II. Operating Principle

Reverse osmosis represents the most sophisticated membrane-based liquid separation technology. By applying an operating pressure to the feed (concentrate) side that overcomes natural osmotic pressure, the direction of natural water molecule permeation reverses when this pressure exceeds osmotic pressure. Water molecules from the feed (concentrate) partially pass through the reverse osmosis membrane to become purified product water on the dilute side. Reverse osmosis equipment blocks all dissolved salts and organic compounds with molecular weights exceeding 100, while permitting water molecules to pass through. Reverse osmosis composite membranes typically achieve a desalination rate exceeding 98%. They are extensively employed in industrial pure water and electronic-grade ultrapure water production, potable purified water manufacturing, boiler feedwater processes, and similar applications. Utilising reverse osmosis prior to ion exchange significantly reduces operational water consumption and wastewater discharge volumes.

III. Introduction to the Pre-treatment System A reverse osmosis water treatment system generally comprises a pre-treatment system, reverse osmosis unit, post-treatment system, cleaning system, and electrical control system. The pre-treatment system typically includes quartz sand filters, activated carbon filters, and precision filters. Its primary function is to reduce the pollution index and residual chlorine, among other impurities, in the raw water to meet the feedwater requirements for reverse osmosis. Detailed classification of the pre-treatment system:

1. Quartz Sand Filter

The quartz sand filter primarily removes suspended solids, colloids, silt, clay, humic substances, particulate matter, and other impurities from water. It reduces turbidity to clarify water quality, thereby protecting the reverse osmosis membrane.

2. Activated Carbon Filter

This filter utilises the abundant hydroxyl groups and other functional groups on activated carbon's surface to chemically adsorb various substances. It removes odours, organic matter, colloids, iron, and residual chlorine from water while reducing colour and turbidity, thereby minimising contamination of the reverse osmosis system.

3. Water Softener

This unit employs ion exchange resin to replace calcium and magnesium ions in water with sodium ions, thereby reducing water hardness. The resin is subsequently regenerated using water softening salt, also known as ion exchange resin regenerant.

4. Precision Filter

Employing a 5-micron PP melt-blown filter cartridge, this primarily removes particles larger than 5 microns not fully captured by the pre-treatment system. It also retains filter media lost during the preceding three filtration stages, thereby protecting the RO membrane.

The filter cartridge's ability to function under normal operating conditions directly impacts the membrane's performance. Cartridges should generally be replaced every 3-4 months.

IV. Reverse Osmosis System Overview

The reverse osmosis system comprises multi-stage high-pressure pumps, reverse osmosis membrane elements, membrane housings (pressure vessels), and support structures. Its primary function is to remove impurities from water, ensuring the output meets specified requirements. The high-pressure pump elevates the pressure of water from the security filter to the operational pressure of the RO system, distributing it evenly to the pressure vessels. Within these vessels, the water stream is separated by the reverse osmosis membrane, forming two distinct flows. A portion of the feedwater permeates the membrane to form purified water, whilst the remaining inorganic salts and solid residues are retained and concentrated to form concentrate, thereby achieving separation of inorganic salts from water.

Purified water flows out from each pressure vessel housing reverse osmosis membrane elements, converges, passes through a flow meter, and exits the equipment outlet into the purified water tank. Concentrate flows out from the concentrate outlet of the pressure vessels.

Mechanism of Reverse Osmosis Membrane Desalination: The semi-permeable membrane surface is densely covered with minute pores. Water molecules selectively adsorb onto the membrane surface, while salt solutes are repelled. Ions with higher valence states are repelled more strongly. Water molecules surrounding the pores, driven by reverse osmosis pressure, flow through capillary action within the membrane, producing purified water and achieving desalination.

When pore sizes exceed the reverse osmosis membrane's range, salt solutions permeate through the membrane, with monovalent salts permeating most readily, followed by divalent salts, and trivalent salts least readily. RO membranes possess pore sizes <1.0nm, enabling filtration of bacteria including Pseudomonas aeruginosa (3000×10⁻¹⁰m), one of the smallest bacteria; It also filters various viruses, such as influenza virus (800 × 10⁻¹⁰ m) and meningitis virus (200 × 10⁻¹⁰ m); it can even remove pyrogens (10–500 × 10⁻¹⁰ m). The characteristics of producing pure water using reverse osmosis are as follows: compact equipment structure with straightforward maintenance, minimal footprint, and high production capacity.

When the pore size of the membrane exceeds the range of reverse osmosis membrane pores, aqueous salt solutions will permeate through the membrane, with monovalent salts leaking most readily, followed by divalent salts, and trivalent salts to a lesser extent. With a pore size <1.0 nm, RO membranes can filter out Pseudomonas aeruginosa (3000 × 10⁻¹⁰ m), one of the smallest bacteria; they also remove various viruses such as influenza virus (800 × 10⁻¹⁰ m) and meningitis virus (200 × 10⁻¹⁰ m); and even pyrogens (10⁻⁵ to 10⁻⁵ m). The characteristics of producing pure water via reverse osmosis are as follows: compact equipment structure with straightforward maintenance, minimal footprint, and high water yield; low energy consumption due to pure water production without phase change; no discharge of acidic or alkaline wastewater, making it a new energy-efficient and environmentally friendly system;

The wastewater-to-pure-water ratio in reverse osmosis systems typically stands at 3:1 for domestic compact units. Small-scale industrial systems may achieve ratios as favourable as 1:1, while large-scale installations operate at 0.3:0.7. Further reduction of wastewater discharge incurs higher costs and inevitably shortens membrane service life. This advanced membrane separation technology finds extensive application across diverse sectors.

V. Reverse Osmosis System Maintenance

1. Conduct routine water quality testing and maintain detailed operational parameter logs, addressing anomalies promptly. Daily monitoring and recording of critical metrics such as purified water conductivity, pressure points, and inlet/outlet flow rates is essential.

2. Regularly replenish the salt tank with regenerant, scheduling regeneration cycles based on site conditions and production volume. Typically, this occurs every 3-4 days.

3. Replace security filter cartridges promptly. (Typically every 3-6 months)

4. Regularly wipe down the machine to maintain equipment cleanliness.

5. Frequently inspect pipework for leaks and rectify promptly.

6. Ensure uninterrupted water and power supply to the equipment. In the event of power or water outages, recalibrate equipment timing, particularly resetting regeneration schedules post-power failure. Unauthorised personnel must not operate electrical control buttons or pipework valves to prevent malfunctions.

VI. Emergency Measures Should the equipment cease producing pure water due to power failure or damage to components such as the main pump or membranes, activate the softened water valve to maintain normal operation of water-using equipment. This approach must align with the specific requirements of the water-using equipment.

1. Should significant leakage occur, immediately shut off the equipment's inlet water valve and disconnect the power supply. Notify the service provider for resolution.

2. During water production, promptly replace consumables such as resin, membranes, and activated carbon when the post-membrane pressure gauge exceeds 1.5 MPa, conductivity surpasses 15 μS/cm, or product water quality falls outside specified parameters.

VII. Consumable Replacement Guidelines

As consumables—quartz sand, activated carbon, and softening resin—are granular materials, accumulation may form porous structures conducive to prolonged microbial proliferation. Additionally, activated carbon and softening resin exhibit adsorption saturation limits; prolonged use diminishes filtration efficacy. Replacement should occur when backwashing/regeneration fails to restore pre-treatment performance, thereby safeguarding reverse osmosis membrane inlet water quality.

Under optimal conditions, service lifespans are as follows: quartz sand (10–24 months), activated carbon (10–12 months), resin (10–12 months), precision filter cartridges (3–6 months), and reverse osmosis membranes (approximately 12 months).

Resin regeneration cycle: Under normal conditions, resin regeneration requires 1–3 days.

VIII. Application Fields

1. Power Industry: Boiler feedwater, cooling water reservoirs;

2. Electronics Industry: Ultra-pure water for semiconductor manufacturing, integrated circuit cleaning water, formulation water;

3. Food Industry: Formulation water, production water;

4. Pharmaceutical Industry: Process water, formulation water, washing water, water for injection, sterile water preparation;

5. Beverage Industry: Formulation water, production water, washing water;

6. Chemical Industry: Production water, wastewater treatment;

7. Drinking water projects: Ultra-pure water preparation, drinking water purification;

8. Petrochemicals: Oilfield injection water, advanced treatment of petrochemical wastewater;

9. Desalination: Production and domestic water for island regions, coastal water-scarce areas, vessels, and seawater oilfields;

10. Environmental protection: Recovery of precious metals from electroplating rinse water and water recycling, achieving zero or minimal discharge.