The Ultimate FAQ Guide:Double Reverse Osmosis

Reverse osmosis technology is commonly used to treat all forms of raw water because of its efficiency, reliability and adaptability. Bipolar reverse osmosis is also widely used to improve the quality of drinking water because of its ability to produce impeccable quality product water.

In this article, we will focus primarily on the process, application areas, and product characteristics of bi-stage reverse osmosis.

Figure 1 Double reverse osmosis system.

What is Double Reverse Osmosis?

Double reverse osmosis is an advanced water treatment technology that utilizes two reverse osmosis units to generate higher quality product water than one pass reverse osmosis. Essentially, the product water (permeate) from the first reverse osmosis unit is pumped into the adjacent RO unit as the feed water. The permeate quality in double-pass reverse osmosis is tremendously high given the water has been subjected to two reverse osmosis systems.

It is vital to distinguish double-pass reverse osmosis from two-stage reverse osmosis. Two-stage reverse osmosis augments the system’s recovery rate by subjecting the concentrate from the first stage to further reverse osmosis treatment. The reverse osmosis permeates from the first and second stages is then combined ready for use or further treatment.

How Does Double Pass Reverse Osmosis Function?

Characteristically, double-pass reverse osmosis systems consist of two distinct RO systems known as a first pass and a second pass. The feed water is initially pretreated using sediment filters, carbon filters, chemical dosing, or other appropriate water pretreatment technologies. Consequently, it is driven into the 1st pass RO system and forced through the semi-porous membranes.

The dissolved solids, minerals, viruses, bacteria, microorganisms, and proteins are alienated from the pure water based on size exclusion. The water as well as other low-molecular-weight substances pass to the permeate side while the alienated constituents are left on the concentrate stream. The permeate from the 1st pass is subsequently reserved in a break tank before being injected into the 2nd pass RO system for further treatment.

A similar process is repeated in the second pass reverse osmosis system ultimately refining the RO permeate.

Double pass reverse osmosis is well illustrated in the following video.

Figure 2 Double pass reverse osmosis process.

Why Should I Use Double Reverse Osmosis?

Ideally, you will need a double pass reverse osmosis system if you desire product water with comparatively higher quality. Double reverse osmosis is essentially an advanced reverse osmosis technology that refines the quality of RO water generated by one pass reverse osmosis systems. Additionally, they elevate the recovery of your reverse osmosis system up to 90% from approximately 80%.

A double pass ro system will help you minimize the quantity of reject water generated and provide you with premium-grade RO water. When treating seawater, you can have your 1st pass RO system as a seawater reverse osmosis system and the 2nd pass unit as a brackish water RO system. When treating brackish water, both the first and second pass RO machines should be customized brackish water reverse osmosis systems.

What are The Main Components of Double Pass Reverse Osmosis?

Double pass reverse osmosis systems are primarily made of two separate reverse osmosis systems. The composition of each system is quite similar given that even the second pass RO system comprises a pretreatment unit. Beneath, we break down the primary components that work cohesively to make your double pass reverse osmosis system fully functional.

● Pretreatment system.

Primarily, sediment filters and carbon filters are deployed to eradicate suspended solids and chlorine respectively. The pretreatment system employed in a double pass RO system can contain varying equipment such as ultrafiltration systems, water softeners, or media filters depending on the composition of the inlet water.

● Pressure booster pumps.

Essentially, high-pressure pumps are deployed in double pass RO systems to reverse the osmotic pressure and flow. The pressure booster pumps are equipped with varied horsepower depending on the inlet water quality and the system’s capacity range.

● Reverse osmosis membranes.

The reverse osmosis membrane filter is the chief component in reverse osmosis systems. The membranes used essentially have high rejection capabilities and are composed of microscopic pores (approximately 0.0001 microns). They alienate the concentrate stream from the permeate stream. They eliminate up to 99.7% of the dissolved solids as well viruses, proteins, organic chemicals, and minerals.

● Storage tanks.

A pressurized storage vessel often situated between the 1st pass and 2nd pass reverse osmosis systems is mandatory. It holds the permeate generated by the 1st pass RO system before it is driven into the 2nd pass RO unit for further filtration. Other vital components include;

  • Membrane cleaning systems.
  • Post-treatment units.
  • User controls.
  • Control valves.

Figure 3 Two-stage reverse osmosis system.

Does Double Reverse Osmosis Necessitate Pretreatment?

Virtually all reverse osmosis systems are connected to pretreatment units to facilitate optimal functioning and mitigate untimely fouling. Similarly, double-pass and two-stage reverse osmosis systems incorporate varied pretreatment technologies based on the inlet water concentration. Here is a brief elaboration of the primary pretreatment mechanisms used in double reverse osmosis.

● Ultrafiltration.

Ultrafiltration (UF) filter systems are considered one of the best pretreatment solutions for seawater reverse osmosis systems. They utilize membrane technology to alienate sediments and other high-molecular-weight solids from the feed water thereby elongating the lifespan of the double reverse osmosis system. The perforations in UF membranes measure approximately 0.005–0.1 μm.

● Multimedia and pressure sand filtration.

To reduce the TSS levels in the feed water, multimedia filters or pressure sand filters can be employed. Pressure sand filters are characteristically made of multiple layers of sand while multimedia filters comprise several layers of anthracite, sand, and gravel. They shrink the presence of suspended solids by 25-50 microns and 10-25 microns respectively.

● Carbon filters.

Chlorine, VOCs, pesticides, and organic chemicals could potentially corrode the double RO equipment. To mitigate this, activated carbon filters are employed appropriately. They also help get rid of unwanted smells and tastes.

What Quality of Water Is Generated Using Double Reverse Osmosis?

Reverse osmosis generally produces high-quality product water with acceptable TDS levels and low microbial count. However, the use of double-reverse osmosis refines the quality of RO water from the 1st pass RO system. Using conventional water purity grading parameters, typical RO permeate falls under Type III purified water.

A wide range of contaminants is eliminated via double reverse osmosis thereby guaranteeing first-rate product water. The quality of the RO permeate can be established by measuring its TDS concentration, electrical conductivity, acidity level, and turbidity.

The conductivity of double-reverse osmosis water has a resistivity of >4 MΩ-cm, TOC level of <200 ppb, and conductivity of <0.25 µS/cm. A higher grade is attainable once you deploy an appropriate post-treatment process.

What Is The Difference Between Double Pass Reverse Osmosis and Two-Stage Reverse Osmosis?

Many people find it hard to distinguish double pass reverse osmosis from two-stage reverse osmosis. They can be puzzling concepts but they are characteristically very distinct. Double pass reverse osmosis utilizes two reverse osmosis plants to initially treat feed water and subsequently refine the permeate from the 1st system.

Two-stage reverse osmosis on the other hand subjects the concentrate from the 1st stage to treatment in the second stage. This shrinks the quantity of the rejected water produced and ultimately augments the system’s recovery rate. The permeate from the 2nd stage and 1st stage are collectively harvested and ready for use or further treatment in subsequent stages.

Figure 4 Industrial double pass RO plant.

What are The Advantages of Using Double Pass Reverse Osmosis Systems?

Double pass reverse osmosis systems are undoubtedly more rewarding than one pass reverse osmosis systems. This is chiefly because they generate premium-grade product water and have an elevated water recovery rate. Double pass RO systems generally reduce TDS levels in feed water by up to 99.9% and have recovery rates of 90%.

The high recovery rate of the double-reverse osmosis system provides economical solutions to users who purchase the feed water. Contemporary systems exploit energy recovery devices, which fundamentally decrease their energy consumption. Such cost-efficiencies make double pass RO systems low maintenance water treatment solutions. Additional rewards gained from utilizing double pass RO systems include:

  • High and varied service flow rates.
  • Diverse daily capacity ranges.
  • Low wastage of feed water (as low as 5%).
  • Customizable designs and stress-free operation.

How Much Does Double Reverse Osmosis Cost?

Innovations in water treatment mechanisms and systems have played a significant role in making water treatment efficient, economical, and highly dependable. For instance, the use of energy recovery devices and efficient membrane technologies has lowered the treatment costs from $1.60/m3 in the 90s to approximately $0.40-0.50/m3.

These projections are arrived at by computing the equipment cost and the cumulative operational expenses. Distinct double pass RO systems command varied prices due to the distinction in functional capabilities and other features. The operational costs cover essential expenses such as power usage, which customarily ranges from 5.3kWh/m3 to 9.7kWh/m3, labor, installation, and repairs.

In addition, the cost of double-reverse osmosis will certainly fluctuate based on factors such as the type of feedwater treated, the region, and pretreatment processes deployed. To get a quotation of your preferred double pass RO system, kindly send us your inquiry via our website.

Does Double Reverse Osmosis Necessitate Polishing?

Post-treatment in reverse osmosis water treatment is mainly optional. Its necessity is determined by the quality of potable water needed. Although the 2nd pass reverse osmosis process is considered a polishing process, other water treatment technologies can be deployed to purify the RO permeate.

● Ultraviolet water sterilization.

UV water disinfection is primarily employed to shrink the microbial count in RO water. Double reverse osmosis eradicates viruses and other waterborne microorganisms but not as efficiently as some applications demand. UV water sterilizers, which utilize germicidal radiations rather than chemicals are employed to eradicate up to 99.9% of the harmful microbes in the RO water.

● Electrodeionization.

Dissolved gasses and ionized constituents that penetrate the RO membranes are often removed using traditional water deionizers or electrodeionization systems. The deployment of EDI in refining RO permeate results in the production of high-purity water with a low resistivity of 18.2MW/cm.

Figure 5 Double reverse osmosis EDI water system.

● Activated carbon filters.

Carbon filtration is prevalently utilized to eliminate Volatile Organic Compounds, pesticides, herbicides, chlorine, and chloramine. When treating drinking water, they can be used to remove organic chemicals responsible for unpleasant tastes and odors.

How Much Power is Used in Double Reverse Osmosis?

Double pass and single-pass reverse osmosis systems are pressure-driven processes, which necessitate a substantial amount of energy to power the pressure booster pumps. In the past years, the power consumption of double-reverse osmosis systems, especially in seawater desalination was a major concern for most prospective users. However, recent strides made in energy recovery have significantly decreased the power burden associated with reverse osmosis.

A contemporary double pass RO system is likely to consume approximately 5.3 kWh/m3 to 9.7 kWh/m3. A single-pass RO unit on the other hand consumes approximately 4.7 kWh/m3 to 8.94 kWh/m3. However, the amount of energy consumed in double reverse osmosis is likely to be influenced by the inlet water concentration, the production capacity, and the power supply options.

Which Type of Double Reverse Osmosis Systems Do You Provide?

Double reverse osmosis systems are one of the most popular categories of reverse osmosis equipment. Nonetheless, they can further be classified according to:

  • Area of specialization such as industrial or commercial double pass ro system.
  • Design such as skid-mounted or containerized double reverse osmosis systems.
  • Feedwater treated e.g. seawater or brackish water double RO system.

● Seawater double reverse osmosis system.

Double pass seawater RO systems are generally used in offshore and onshore seawater desalination exercises. The first pass RO system typically consists of a seawater RO unit with a TDS range of 35,000 to 45,000ppm. The second pass RO system can be a brackish water RO system given that the feed water (1st pass RO permeate) has a significantly lesser TDS level.

● Brackish water double pass reverse osmosis system.

This is an advanced brackish water treatment system used to generate first-rate potable water from slightly saline water (1000ppm to 15,000ppm). Double pass reverse osmosis is quite inexpensive when treating brackish water and can be used in residential, commercial, or industrial applications.

● Containerized double reverse osmosis systems.

Containerized equipment essentially houses all double osmosis water filter in a self-contained container. Typical containers measuring 20ft, 10ft, or 40ft are dominantly used thereby augmenting movability and making operations easy.

Figure 6 containerized double pass reverse osmosis system.

● Skid-mounted double reverse osmosis systems.

Reverse osmosis skids are technically RO units housed in self-sufficient metallic structures. They are portable units whose reduced footprints facilitate the economic utilization of space. NEWater manufactures double pass RO skids for households, commercial entities, and industries. They can also be deployed to treat seawater, brackish water, well water, and surface water.

Double Reverse Osmosis from NEWater

NEWater’s Dual Reverse Osmosis is a modified two-stage reverse osmosis ultrapure water system with two channels with different water flow capacities.

Accessories include a filter cartridge, a primary diaphragm pump, a primary reverse osmosis module, a secondary diaphragm pump, a secondary reverse osmosis module, a primary pure water discharge line, and a secondary wastewater discharge line. A return channel is provided between the secondary wastewater discharge pipe and the primary pure water discharge pipe in parallel with the secondary diaphragm pump and the secondary reverse osmosis assembly.

The tank then pushes the stored water into the second pass for further purification. As a result, these dual reverse osmosis systems can perform more powerful purification than single-stage RO systems. These dual reverse osmosis systems can perform a more powerful purification than a single-stage RO system. Do you want an environmentally friendly and efficient water purification system for your business or home?

It is small, easy to operate, and can perform the dual functions of secondary wastewater return conditioning and pH adjustment solution micro-dosing, using it to produce secondary RO purified water that meets national secondary standards for laboratory water, while maintaining its original benefits.

Are you inspired to get an eco-friendly and potent water purifying system for your business or home? Then, get in touch with NEWater today.

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