Why is forward osmosis a good method for water recycling?
When recycling industrial wastewater, your ability to remove contaminants from the permeate is essential for the quality of the recycled water. And Forward Osmosis (FO) is an excellent method for separating water from dissolved solutes. By using natural osmotic pressure as the driving force, a forward osmosis system transports water through a filtration membrane while retaining all other compounds in the feed stream. An osmotic pressure gradient across the semipermeable membrane delivers the driving force to facilitate water transport from the feed solution (low osmotic pressure) to the draw solution (high osmotic pressure).
How does a forward osmosis system purify water for recycling?
This is a brief overview of the main components of a forward osmosis system and their function. For a more detailed explanation, visit our FAQ about forward osmosis.
A forward osmosis solution with a draw recovery system to separate the clean water from the draw solution.
The forward osmosis membrane
The FO membrane allows water to permeate the feed solution to the draw side but prevents other substances from permeating. The most important qualities for a forward osmosis membrane are a high rejection rate and the ability to keep reverse flux to a minimum.
The feed solution
In an industrial forward osmosis system, the feed solution is the effluent stream you wish to dewater in order to increase water available for recycling. When the feed solution flows through the forward osmosis system, it gradually concentrates as water permeates into the draw solution due to the osmotic pressure difference.
The draw solution
The high TDS level of the draw solution creates the osmotic pressure difference which induces water to flow from the feed solution through the membrane and into the draw solution. To achieve optimal results, the draw solution must be precisely formulated to match the composition of the effluent. As the water permeates through the membrane and the feed solution is concentrated, the draw solution becomes diluted and ultimately ineffective unless it is regenerated in a draw recovery system.
The draw recovery system
The role of the draw recovery system is to restore the TDS concentration of the draw solution and extract clean water for reuse. This is usually accomplished through a reverse osmosis system designed to match the appropriate TDS level. With a conventional sea water RO membrane, the draw recovery system can handle TDS up to 60,000 to 70,000 parts per million (ppm), while a high-power reverse osmosis (HPRO) membrane allows for a TDS of 90-120,000 ppm and an osmotically assisted reverse osmosis (OARO) membrane enables a TDS of 120-200,000 ppm. The permeate from the draw recovery system is clean water that is available for recycling.
Important considerations when using forward osmosis for industrial water recycling
The water recycling potential of using forward osmosis in industrial wastewater treatment depends directly on several factors, including the feed and draw solutions’ quality and composition.
1) What is the composition of the industrial wastewater stream
The characteristics of the industrial wastewater stream must be examined to evaluate the feasibility of using forward osmosis technology to extract water for recycling.
This includes:
- Osmotic pressure (π)
- PH value
- Total dissolved solids (TDS)
- Chemical oxygen demand (COD)
- Total suspended solids (TSS)
- Oil and grease (O&G)
- Detailed chemical composition
2) Do you want a batch or continuous forward osmosis process
When using forward osmosis for industrial effluent treatment, both the feed and draw solution can be operated in either batch mode or in continuous (single-pass) mode.
Running the feed solution in batch mode enables higher concentration factors with less membrane area. A batch process is terminated when the required concentration factor or water recovery is reached or when the osmotic pressure of the feed solution equals the osmotic pressure of the draw solution.
Running the draw solution in single-pass mode ensures a constant driving force from the draw side. This operation mode typically requires a draw regeneration step (for example, reverse osmosis in the draw loop) to be included to remove water from the draw solution and keep the TDS level constant to ensure the same osmotic pressure across the membrane throughout the FO process. This permeate is then available for reuse and recycling, both in other processes or for alternative purposes such as cooling or irrigation.
3) How do you want to use the recycled water?
With forward osmosis, you will be able to extract a significant quantity of high-quality permeate from your industrial wastewater stream. The forward osmosis treatment can be adjusted and designed to produce the level of purity and the specific characteristics of the recycled water necessary to meet the standards you need for the intended use of the recycled water.
Reclaimed water can be routed back to your existing processes, either with or without additional treatment. While it may be too costly or impractical to achieve a water quality sufficient to recycle the water for its original purpose, it can still reduce your total water consumption and freshwater intake. Examples of reuse include cooling, rinsing and irrigation.
4) Will you need to scale your water recycling capacity?
The quantity of wastewater you need to treat and recycle may vary, for example if you run different types of processes at your site or expect to either increase or decrease your activity level in the future.
Since forward osmosis uses a modular membrane technology, scaling the capacity of your forward osmosis water treatment system is relatively easy, compared to other water recycling technologies.
How to determine if your water treatment process is suited for forward osmosis
Since industrial wastewater streams can have great variance in terms of composition and difficulty to treat, it is always recommended to conduct careful analysis before selecting a forward osmosis solution. Laboratory pre-testing and pilot trials are needed to establish the exact content and composition of the effluent stream and determine the right technology and configuration of a full-scale forward osmosis solution.
Aquaporin recommends following a 3-phase approach to investigating the feasibility of forward osmosis in your industrial wastewater treatment setup.
Phase 1 – desk evaluation
Based on data provided through a detailed questionnaire, Aquaporin will evaluate the feasibility of incorporating forward osmosis as a method for water recycling in your industrial wastewater treatment process. This explores if there is a theoretical match between your desired application, the specific targets and the forward osmosis technology. This phase can typically be completed in 2 weeks.
Phase 2 – feasibility screening
If the desk evaluation indicates that forward osmosis is a potential match for your application, samples of your wastewater will be collected and tested in a laboratory setting. This includes processing the wastewater with Aquaporin Inside® modules in a small-scale test setup, with the goal of quickly confirming how the membrane reacts to wastewater and if the desired quality and performance (eg. sufficient levels of flux, rejection and recovery) can be achieved. This feasibility screening typically lasts 1-2 weeks and the results will be used as the premise for a scale-up pilot trial.
Phase 3 – Forward osmosis solution pilot
In this phase, a pilot-scale forward osmosis unit is installed on site and connected to your treatment process, running for a minimum of one month of continuous operation. This demonstration, which includes regeneration of the draw solution, will demonstrate the actual feasibility of using forward osmosis for water recycling in your process. During the pilot phase, the system is validated to ensure that it will meet the required water quality, energy consumption and plant capacity. In addition, the frequency of cleaning needed to maintain forward osmosis system performance is evaluated. The learnings from the pilot will be used to tweak the system design, finalize the full-scale project proposal and devise the plan for implementing this forward osmosis solution in your operations.
Implementing forward osmosis in your wastewater treatment processes
The optimal solution for implementing forward osmosis in your new or existing industrial effluent treatment process depends on your plant and specific goals, including how much water you want to recycle or reuse.
In effluent treatment plants with discharge to river or water bodies the primary, secondary or tertiary treatments can be partly or fully replaced by a forward osmosis system. A forward osmosis system can also be added as a final treatment step to further reduce the effluent volume to be discharged. The clean water extracted in the forward osmosis system can be recycled or reused in the factory.
For effluent treatment plants with ZLD, the ultrafiltration (UF), nanofiltration (NF) or reverse osmosis (RO) stages can be partly or fully replaced by a forward osmosis system. Forward osmosis can also be added after the reverse osmosis stage to further reduce the volume sent to the evaporator.
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