Definition:

The reverse osmosis system is a membrane separation technology that uses pressure differences to drive solutions through high molecular permeable membranes, achieving selective solvent separation. This technology is widely used in seawater desalination, wastewater reuse, medical dialysis, food processing, and industrial pure water preparation. The concept originated from bionic studies of the gular mucosa of the American seagull in the 1950s, and was later validated by the water recycling systems of the Apollo program. It has since been expanded to military vessels and civilian water purification equipment.

The system comprises a pretreatment unit, a high-pressure pump, a membrane module, and a cleaning module. The core reverse osmosis membrane has a pore size of 0.0001 microns, achieving a desalination rate of up to 99.5%. To ensure optimal performance, the system must maintain an influent pH value between 3 and 10, residual chlorine levels below 0.1 mg/L, and water temperatures below 45℃. Multiple filtration stages are employed to reduce impurity loads. A composite spiral-wound membrane module is used to achieve a water utilization rate of 70%, with a working pressure ranging from 6 to 20 kg. An automatic control valve regulates the recovery rate. Membrane fouling can lead to a decrease in water production or a reduction in desalination efficiency, requiring chemical cleaning and optimized pretreatment for maintenance. The separation mechanism involves theories of dissolution diffusion, capillary flow, and hydrogen bonding, which explain the adsorption and migration processes of water molecules within the membrane.

Introduction:

The fundamental principle of reverse osmosis is to use a specially designed high-pressure pump to pressurize the raw water to a pressure of 6 to 20 kilograms. This pressure forces the raw water through a reverse osmosis membrane with pores as small as 0.0001 micrometers. Chemical ions, bacteria, fungi, and viruses cannot pass through these pores and are discharged with the wastewater. Only water molecules and solvents smaller than 0.0001 micrometers can pass through.

The security filter contains filter cores with a pore size of 5 μm. These cores effectively filter out particles larger than 5 μm, protecting the downstream RO membrane from scaling. The most commonly used type of permeation membrane is the polyamide composite membrane, which has a salt rejection rate of up to 99.5%.

Parameter definition：

Recycling rate: This refers to the percentage of feed water that is converted into product water or permeate in a membrane system. The design of the membrane system is based on the preset water quality of the feed water. A concentrate valve installed on the concentrate pipeline can be used to adjust and set the recycling rate. The recycling rate is often aimed at being maximized to achieve the highest possible product water yield; however, it should not exceed a limit value to prevent precipitation of salts and other impurities due to oversaturation within the membrane.

Desalination rate: the percentage of total soluble impurities removed from the system feed water through a reverse osmosis membrane, or the percentage of specific components such as divalent ions or organic matter removed through a nanofiltration membrane.

Permeability: the opposite of desalination rate, it is the percentage of dissolved impurities in the feed water that pass through the membrane. Permeate: purified water produced by the membrane system.

Flow: Flow refers to the water rate entering the membrane element, typically measured in cubic meters per hour (m³/h) or gallons per minute (gpm). Concentrated water flow is the ‘inflow’ of the portion that does not pass through the membrane when it leaves the system. This concentrated water contains soluble components from the raw water source and is often measured in cubic meters per hour (m³/h) or gallons per minute (gpm).

Flux: The flow rate of liquid through a unit membrane area, usually expressed in liters per square meter per hour (l/㎡h) or gallons per square foot per day (gpm).

Solutions in dilute form: purified aqueous solutions, which are the water produced by reverse osmosis or nanofiltration systems.

Concentrated solution: the portion of the solution that does not pass through the membrane, such as concentrated water in a reverse osmosis or nanofiltration system.

Preprocessing system

The setup of the pretreatment system is designed to improve water supply conditions, meeting the feed water requirements of the reverse osmosis system. This not only protects the reverse osmosis unit but also extends the lifespan of the membrane. In water treatment systems, pretreatment designs are often tailored to different water qualities. The most common design includes a multi-media filter (to remove suspended particles larger than 10um), activated carbon (to eliminate odors, residual chlorine, heavy metal ions, and organic carcinogens), an anti-scaling agent dosing device (or a softening device) (to significantly reduce the formation of scale from calcium and magnesium), and a precision filter (with a filtration accuracy of 5um) (to further remove residual particulate matter from the water).

Reverse osmosis unit (pure water unit)

(1) Function: The reverse osmosis unit is a key component of the water purification production line. This unit uses energy-efficient composite membrane ESPA-type reverse osmosis membrane elements produced by the American Hydranautics Company, a subsidiary of Japan’s Nippon Electric Works Group. The ESPA series is designed for high desalination rates in brackish water desalination. It can achieve high water flux at lower operating pressures, with an average desalination rate of 99.5%. Due to these advantages, the ESPA membrane offers a wider range of options for selecting equipment such as pumps, pressure vessels, pipelines, and valves. Additionally, a smaller motor can meet the operational requirements. The high water flux and high desalination rate of the ESPA membrane significantly reduce the manufacturing costs and system investment expenses, while also saving a significant amount of energy, reducing operating costs, and improving water quality.

(2) Selection: American Hyde Energy ESPAI-4040/8040. The designed water production temperature of the reverse osmosis device is 25℃, and the water utilization rate is 70%.

Supercharged pump

(1) Function: Provide the necessary working pressure for the downstream mixed bed.

(2) Control: The pressure and water intake are adjusted by the regulating valve after the pump (manual operation).

Classification

mainly includes the system pump, reverse osmosis device (reverse osmosis membrane and shell, frame, electrical control box), flushing/cleaning device and intermediate water tank.

Reverse osmosis mechanism model

There are several differences between the classic models:

1. Priority Adsorption Capillary Model: Under dry electron microscopy, no pores were observed. Wet membrane samples are not suitable for electron microscopy.

2. Dissolution Diffusion Model: No pores are considered to exist.

3. Dry Closure Wet Opening Model: Proposed by Deng Yu and others in the 1980s and 1990s, this model explains the unification of the first two models and is the most modern and accurate reverse osmosis mechanism model. It is known as the ‘dry closure wet opening’ reverse osmosis model, which unifies the two most classic reverse osmosis mechanism models: the micropore model and the dissolution diffusion model.