First, I believe I've already provided a detailed explanation regarding the necessity of reverse osmosis pretreatment and the selection and sequencing of related processes.
As a reminder, for comprehensive understanding of reverse osmosis (RO), pure water, and ultrapure water, I recommend systematically reviewing the following two collections. If you have questions about the content or need further discussion, feel free to contact me anytime.
Recently, a consortium comprising France's Meridiam Group and Suez secured the bid for the world's second-largest seawater desalination project: Jordan's Aqaba-Amman Water Desalination and Conveyance Project, with a total investment of approximately 4 billion euros. The project involves constructing a new desalination plant in the Jordanian port city of Aqaba near the Red Sea, with a daily production capacity of 851,000 tons. The treated water will then be transported via a 450km pipeline network to the Amman metropolitan area. To be honest, my current company also engages in seawater desalination, though neither our scale nor technical capabilities can compare to those of the aforementioned water treatment giants. The buzz around this topic likely began early this month when my system kept pushing articles like “Why Did the Mega Desalination Project Ditch Ultrafiltration?” and “Why Isn't Ultrafiltration Used in Desalination—XXX.”
Most related content analyzed the Jordan project from cost and process perspectives, presenting well-reasoned arguments packed with substantive insights. However, while the articles and videos themselves were fairly balanced, the comment sections proved far more entertaining—after all, controversy fuels engagement. The core debate expanded from questioning whether ultrafiltration is necessary for desalination projects at all to whether using ultrafiltration as reverse osmosis pretreatment is fundamentally a waste of money!
Let's start with a simple conclusion: In the vast majority of cases, ultrafiltration as reverse osmosis pretreatment holds distinct advantages.
I. What Constitutes Relative Advantages
To discuss advantages, we naturally need a point of comparison. Most people likely immediately think of disc filtration + ultrafiltration versus sand filtration + carbon filtration. Although this has been elaborated upon earlier, for the sake of “padding the word count—and clarity,” let's briefly recap the feedwater quality requirements for reverse osmosis and the fundamental roles of these four processes.
Disc Filtration: As pre-treatment for ultrafiltration, typically employs 50/100μm filtration precision, primarily intercepting larger particulates.
Ultrafiltration: Utilizes hollow fiber membrane filtration technology, commonly with 0.01μm (0.002–0.1μm) precision, primarily intercepting colloids, bacteria, and fine particulates. Note: Ultrafiltration membranes vary widely in material and precision range. General reference diameters for primary targets include: colloids in water ≥0.1 μm, latex ≥0.5 μm, bacteria ≥0.2 μm, particulate matter ≥5 μm.
Quartz Sand (Multi-Media) Filtration: Retains and adsorbs silt, colloids, suspended solids, and particulate matter, effectively reducing water turbidity.
Activated Carbon Filtration: Utilizes physical adsorption and chemical reactions to effectively remove residual chlorine, organic compounds, suspended solids, and associated odors/colors. As a broad-spectrum adsorbent, activated carbon has extensive applications.
By understanding the primary principles and functions of these four pretreatment processes, we can briefly summarize the advantages and disadvantages of their corresponding combinations:
① Particle Removal Advantages: Offers negligible benefits for standard industrial pure water but provides some assistance for ultrapure water systems.
② Organic Matter Advantages: Given the heightened focus on inorganic salt scaling risks (which take priority for exclusion), organic contamination has become the primary factor affecting the stable operation of modern reverse osmosis systems—the main driving force behind the adoption of ultrafiltration. Consequently, ultrafiltration pretreatment systems are designed with higher flux capacities than traditional pretreatment systems.
③ Residual Chlorine Disadvantage: Disc filtration and ultrafiltration cannot remove residual chlorine, meaning they cannot effectively mitigate the impact of oxidizing substances on membrane materials, including ultrafiltration membranes.
④ Footprint: Disc filtration + ultrafiltration offers a significant advantage, with larger systems demonstrating greater efficiency.
⑤ Equipment Cost: Initial capital investment favors sand filtration + carbon filtration, though this relative advantage diminishes with larger system sizes.
⑥ Operational Costs: Considering membrane replacement, filter media, and electricity consumption, existing project data indicates minimal difference between systems.
Note: Regardless of whether evaluating disc filtration's high throughput or the cost curve resulting from ultrafiltration's structural design, disc filtration + ultrafiltration demonstrates greater advantages in large-scale projects from both footprint and equipment cost perspectives.
II. Process Optimization and Applicability
In a sense, ultrafiltration cannot replace activated carbon filtration for residual chlorine removal nor substitute sand filtration alone. Accordingly, the process logic can be optimized as follows:
① Disc Filtration + Ultrafiltration + Reducing Agent: The addition of a reducing agent replaces the residual chlorine removal function of activated carbon.
② Disc Filter + Ultrafiltration + Carbon Filter: Retain activated carbon adsorption. When raw water quality is poor or ultrafiltration material is PVC/PAN (prone to oxidation), place activated carbon upstream. If raw water quality is acceptable or ultrafiltration material is PES/PVDF (resistant to oxidation), place activated carbon downstream. Note: Generally suitable for municipal tap water quality. Other surface water sources present complex conditions; some require preliminary oxidation disinfection (non-oxidizing disinfectants are costly). In such cases, refer to the above guidelines based on ultrafiltration membrane material and overall raw water characteristics.
③ Sand Filtration + Carbon Filtration + Ultrafiltration: Though seemingly cumbersome, this combination offers the broadest applicability. However, it inevitably involves functional overlap, resulting in higher initial and operational costs. Flexible placement of activated carbon (pre- or post-treatment) remains an option.
III. Non-Typical Influencing Factors for Sand Filtration, Carbon Filtration, and Ultrafiltration
Sand Filtration: Reasonable flow rate, multi-stage distribution, appropriate precision—switching from multi-media to single-media causes flow rate to exceed standards;
Carbon Filtration: Reasonable flow rate, qualified iodine value, etc.—insufficient iodine value leads to oxidative damage to RO;
Ultrafiltration: Material selection, backwash system compatibility, etc. — Designers of ultrafiltration pretreatment for small industrial pure water systems are often detached from practical realities (except where space constraints apply).
Why are these considered atypical factors?
Because they represent elements that can and should be executed properly, yet are often neglected due to oversight, cutting corners, or complacency. Such issues stem not from objective constraints like feedwater quality, space, or budget, but rather from “human negligence.” It is unfair that these factors ultimately become sources of system instability.
Reiterating the core principle for pretreatment selection: Ensure necessity and appropriateness, while fundamentally adhering to the sequence of lower precision before higher precision, and lower cost before higher cost.
Add one more word—flexibility.
