DTRO (Disc Tube Reverse Osmosis) and STRO (Spiral Tube Reverse Osmosis) are essentially reverse osmosis technologies, but their differences in membrane structure and working principle result in significant differences in their structural characteristics and application fields. Understanding these differences and mastering their applications helps us choose appropriate processes to efficiently and effectively carry out membrane separation processes in order to achieve our goals.
Below, we will briefly compare or introduce several aspects such as membrane structure, working principle, membrane characteristics, application fields, inlet and outlet water quality standards, and common process packages.
1、 Comparison of DTRO and STRO membrane structures
DTRO membrane structure
The DTRO membrane is a composite membrane composed of three layers: a support layer, a dense layer, and a contact layer (see the zoomed in image below). Among them, the support layer provides the mechanical stability of the membrane, the dense layer controls the permeability of the membrane, and the contact layer provides the anti fouling performance of the membrane.
The DTRO membrane column is formed by stacking a reverse osmosis membrane and a guide plate together, fixing them with a central rod and end plate, and then placing them into a pressure resistant membrane shell. Its main components include: pressure bearing nut, water production joint, inlet and outlet joint, upper and lower pressure bearing flanges, upper and lower water passing flanges, membrane shell, center rod, guide plate, membrane package, etc.
Integration: Merge the pressure bearing flange and the overflow flange into one, balancing the functions of pressure bearing and overcurrent.
Corrosion resistance: Made of duplex steel material, it not only enhances the overall corrosion resistance but also facilitates disassembly and assembly.
Pressure bearing strength: As a component of the water production channel and pressure bearing parts, the integrated water production jacket is made of duplex steel material and matched with the integrated water passage flange.
STRO membrane structure
STRO membrane also has a composite structure and is a form of roll type RO membrane, but the design is more complex.
Firstly, there are significant differences between STRO membrane and traditional roll RO membrane in terms of water supply channel and rolling method, as follows:
① Comparison of water supply channels
The traditional roll type membrane mesh is prone to clogging and scaling, which limits the inlet flow channel and increases the pressure difference. The STRO pipeline type reverse osmosis membrane has a 45 ° diamond shaped double-layer open channel structure, which greatly optimizes the effective area of the inlet channel and membrane, and makes the pressure more stable.
② Comparison of Rolling Methods
STRO reverse osmosis membrane shortens the blades while increasing the number of blades, shortens the length of the freshwater channel, reduces the pressure loss of the freshwater channel, and the net pushing pressure along the membrane tends to be the same, which can maintain the same water flux at different parts of the membrane surface as much as possible, thereby reducing the degree of concentration polarization.
Secondly, the STRO membrane is fixed in a fiberglass container with a high-pressure end plate and a high-pressure resistant center rod, allowing the STRO membrane to withstand a maximum pressure of 120 bar (different grades of STRO membranes have varying degrees of pressure resistance).
2、 Comparison of the working principles of DTRO and STRO membranes
Working principle of DTRO membrane
The DTRO disc tube reverse osmosis membrane component adopts the principle of reverse osmosis to separate, purify, and desalinate molecules in liquids. It can remove various organic or inorganic impurities, including heavy metals, ammonia, various organic substances, inorganic materials and other harmful substances.
The DTRO disc tube reverse osmosis membrane component is mainly composed of disc type membrane (filter membrane), guide plate, O-ring rubber gasket, center pull rod, shell, two end flanges, various sealing components and connecting bolts. Stack the filter membrane and guide plate together, and fix them with a central rod and end flange. Then it is placed into a pressure resistant shell, forming a disc-shaped membrane module. Each component in the membrane column has different functions.
The membrane consists of two concentric circular reverse osmosis membranes, with a layer of filamentous support sandwiched in between. The outer rings of these three layers of replacement materials are connected, and the inner ring is open, serving as the outlet for clean water.
The guide plate (replacing the mesh support layer in the rolled membrane) clamps the membrane in the middle, but does not directly contact the membrane, widening the fluid channel. The surface of the guide plate has protrusions arranged in a certain way, which create turbulence in the leachate under high pressure, increasing the permeation rate and self-cleaning function.
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The O-shaped rubber gasket is fitted onto the central pull rod and placed in the groove on the side of the guide plate beam, serving to support the membrane, isolate sewage and clean water. The purified water is discharged through the purified water outlet in the middle of the membrane along the outer periphery of the center rod of the filamentous support channel.
Working principle of STRO membrane
The membrane of STRO component adopts industrial anti pollution reverse osmosis membrane, and the grid channel adopts a parallel grid structure different from the general roll membrane. The grid of STRO components adopts a trapezoidal structure, and wastewater/liquid flows in the channels formed by the grid, just like flowing in a tubular membrane, with much lower resistance than a diamond grid; At the same time, the internal transverse reinforcement ribs can increase the turbulence during the flow of the material liquid, reduce the concentration polarization effect of the membrane, and improve the pollution resistance of the STRO component. The basic product structure and working principle of the structure are as follows:
3、 Characteristics and application fields of DTRO and STRO membranes
DTRO Disk Tube Reverse Osmosis Membrane - Characteristics
① Resistant to the "four highs":
High pressure resistance: 75~160bar
High SDI resistance: (SDI pollution index)<15
High COD resistance: 1000~20000mg/L
High TDS resistance: 2000~80000 mg/L
② Strong anti pollution:
Convex point wide flow channel: The guide plate adopts a unique convex point and turbulence optimization design. The open flow channel increases the surface velocity (Reynolds coefficient ≥ 5000, 3-5 times that of the roll film) to avoid solid matter from being trapped on the surface and has self-cleaning function. Effectively reduce concentration polarization.
Octagonal structure membrane package:
During high-pressure operation, the patented octagonal structure design causes uneven force points, which will generate high-frequency vibrations under the impact of water flow, driving the water flow to form a 'S' flow towards full turbulence and continuously scouring, avoiding pollution and scaling.
③ Low cost:
Adopting standardized and modular configuration, easy assembly and disassembly.
Easy cleaning and good performance parameter recovery.
Easy to maintain, can be replaced and reused separately.
Membrane components have a long service life.
④ Strong adaptability:
High voltage equipment with a recovery rate of over 90%.
Stable water production, with a single processing capacity of 5-100m3/h.
Application Fields of DTRO Membrane
DTRO membrane is a membrane module specifically designed for treating high concentration wastewater, which was first applied to the treatment of leachate from garbage. With its good treatment effect on high turbidity, high SDI value, high salinity, and high COD, as well as its resistance to high pressure and pollution, and ability to operate effectively and stably even in situations with large water quality fluctuations and complex components, the application areas of DTRO are constantly expanding. Below are some common DTRO application areas besides landfill leachate:
① Sewage treatment field
The typical characteristics of desulfurization wastewater, represented by coal chemical wastewater and mine wastewater, are high turbidity, high salt content, and high hardness. The DTRO disc tube reverse osmosis membrane can pre concentrate the TDS mass concentration of desulfurization wastewater from 25-40g/L to 80-100g/L, and the treated wastewater can also achieve near zero discharge.
② Water desalination field
The main purpose of wastewater treatment is to concentrate and reuse, while desalination in the field of water supply mainly deals with preliminary desalination under complex raw water conditions. The common desalination fields include desalination of saline alkali land, brackish water, commercial or military seawater, and freshwater.
③ Zero discharge of industrial wastewater
The industrial high salt wastewater represented by chemical park wastewater and electroplating wastewater is essentially similar to ①, with the biggest difference being that the DTRO process in ① field is often in the early stage of membrane process, while in the zero discharge membrane process, DTRO mainly treats the concentrated water of the early stage membrane process, which is often in the late stage of membrane process.
④ Other fields
High concentration acid-base or waste liquid recovery; Biological protein and collagen recycling; Functional sugar concentration, etc.
STRO Pipeline based Reverse Osmosis Membrane - Characteristics
① High desalination rate:
Under standard testing conditions, the desalination rate reaches 99%, and the performance is stable. The product water quality is better, achieving higher water use indicators.
② Anti pollution blockage:
Special fluid hydraulic design reduces concentration polarization, improves fouling and scaling trends.
③ Special flow channel:
Adopting a 45 ° diamond shaped double-layer flow channel design, the cleaning effect is better and the performance recovery is easier.
④ Easy to maintain:
Adopting a modular design with 1-6 units that can be assembled in series, it is easy to clean and maintain.
⑤ Small footprint:
High membrane packing density, smaller equipment footprint, and reduced project cost.
⑥ Low cost:
The number of seals is small, maintenance is easier, service life is extended, and replacement costs are greatly reduced.
STRO membrane application field
① Sewage treatment field
In terms of sewage treatment, STRO membrane can effectively remove harmful substances and suspended solids from sewage, improving water quality.
② Food solid-liquid separation field
In the food industry, STRO membranes are particularly suitable for handling liquids with high viscosity, easy scaling, or high suspended solids content, such as milk, juice, and other food industries, due to their anti pollution performance and washability.
③ Biomedical field
In the field of biomedicine, STRO membranes are used for separating and purifying biomolecules, preparing drugs, etc.
④ Other fields
In addition to the above-mentioned fields, STRO membranes also have good applications in circulating cooling water, garbage leachate, mine wastewater, chemical wastewater, coal chemical wastewater, coking wastewater, electroplating wastewater, steel wastewater, and other areas.
Although DTRO and STRO have vastly different structures and functional focuses, they both have good salt and COD resistance characteristics. Therefore, in practical applications, we often see that they complement each other and leverage their respective advantages. For example, in some cases, DTRO membranes can be used for initial liquid purification, followed by STRO membranes for fine liquid separation. This can fully utilize the high-throughput performance of DTRO membrane and the excellent anti fouling performance of STRO membrane, achieving better separation effect.
4、 Comparison of inlet water quality requirements for DTRO and STRO membranes
Requirements for inlet water quality of DTRO membrane
As a polymer organic material membrane, DTRO membrane has certain requirements for the inlet water quality to ensure stable and efficient membrane operation, as shown in the table below:
Requirements for inlet water quality of STRO membrane
STRO membrane, as a polymer organic material membrane, has certain requirements for the inlet water quality to ensure stable and efficient membrane operation, as shown in the table below:
Parameter Table of DTRO and STRO Membrane Components from a Certain Brand Manufacturer
DTRO membrane element parameter table
STRO membrane element parameter table
5、 Reference for DTRO and STRO membrane process flow
① Secondary DTRO process flow of leachate from garbage
The secondary DTRO process for leachate is currently the preferred treatment process for most landfills and incineration plants in the industry. It has stable water production quality and can achieve a recovery rate of over 70% for leachate, as follows:
② Garbage leachate 3.0 process package
The 3.0 process package for garbage leachate incineration plant can achieve a recovery rate of over 85%, as follows:
③ Industrial wastewater zero discharge 4.0 process package
The 4.0 process package for zero discharge of industrial wastewater can achieve a recovery rate of over 90%, as follows:
④ TSD Industrial Wastewater Zero Discharge Integrated Process
TSD (TUF+STRO+DTRO) Industrial Wastewater Zero Discharge Integrated Process Package (as above)
At the end of this article, a large amount of content has been excerpted from the network of Disc Filter Technology Co., Ltd. If there is any infringement, it will be deleted. The relevant parameters in the article are for reference only, and we would like to express our gratitude to Butterfly Membrane Technology Co., Ltd.
