How to Determine Whether to Use 4“ or 8” Membrane Elements for Reverse Osmosis?

In water treatment system design, the selection of reverse osmosis membrane elements directly impacts system efficiency and cost. When faced with the two mainstream specifications—4-inch (4040) and 8-inch (8040)—the decision is far from a simple “size” choice. It requires careful consideration based on feedwater characteristics, system scale, and overall economic viability.

I. Source Water Characteristics

Source water quality is the primary determinant for membrane selection, directly influencing membrane flux (water production per unit membrane area, unit: gfd) and the minimum water production capacity per membrane element:

Surface Water: Complex water quality typically warrants a lower average flux (8–14 gfd). The minimum water production per 8040 membrane element is approximately 0.46 tons per hour (t/h). This implies that if the system's designed water production significantly exceeds this value (e.g., several times higher), using 8040 membranes remains theoretically feasible.

Groundwater: Typically exhibits superior quality, allowing for increased flux (14–18 gfd). The minimum water production per 8040 membrane increases to approximately 0.80 t/h.

Secondary RO Feedwater (Primary RO Product Water): Optimal water quality yields the highest flux (20–30 gfd), with a single 8040 membrane consistently producing approximately 1.15 t/h.

II. System Scale

Source water characteristics provide the baseline flux, while the total designed system product water volume directly determines membrane element specification suitability:

Below 3 t/h: At this scale, the total system water demand is relatively low. Selecting 4040 membrane elements allows meeting requirements by increasing membrane quantity, offering greater flexibility to match system recovery rate demands (especially when concentrate recirculation is required), and avoiding resource wastage from “overkill” solutions. Supporting equipment costs for pressure vessels, piping, and racks are also lower, resulting in more economical overall investment.

Above 5 t/h: Scale increases significantly. Continuing with 4040 membranes would require a steep increase in membrane quantity (far exceeding 8040 membranes), leading to an overly complex system structure, crisscrossing piping, and a dramatic increase in footprint. At this scale, the 8040 membrane's high single-unit capacity substantially reduces membrane element count, simplifies system architecture, and conserves valuable space, highlighting its comprehensive advantages (including ease of future maintenance).

3-5 t/h: This range constitutes a “gray area” where both specifications are technically viable. The final decision requires in-depth analysis of the critical factors at the next scale level.

III. Deep-Level Factors

Once the capacity threshold is preliminarily met, the following factors often become decisive in the final decision:

System Recovery Rate and Concentrate Management: High recovery rate requirements typically necessitate concentrate recirculation. Smaller systems (<3 t/h) find this configuration easier to implement and more flexible with 4040 membranes. For systems within 3-5 t/h facing stringent recovery requirements and space constraints, the 4040 solution may be preferable. Conversely, for moderate recovery rates, 8040 can meet requirements through optimized segmented design.

Footprint Constraints: Limited space in plants or equipment rooms is a common challenge. The 8040 membrane module offers high single-unit capacity and requires fewer pressure vessels, significantly reducing the system's footprint. This is critical for retrofit projects with space limitations or compact designs.

Investment and Long-Term Economics: Comprehensive evaluation is required:

Initial Investment: While the 4040 membrane has a lower unit price, the greater number of pressure vessels, valves, piping, and instrumentation required may result in a higher overall initial investment, particularly for medium to large-scale systems.

Maintenance Costs: The simplified structure of 8040 membrane systems, with fewer membrane elements, reduces labor and chemical costs for routine cleaning and replacement. Fewer pipe connections also minimize potential leak points. Their long-term operational and maintenance economics are typically more favorable.

IV. Industry Experience Recommendations

Based on extensive engineering practice, the industry has established the following universal guidelines:

<3 t/h: 4040 membrane elements are the preferred recommendation. The system offers flexible configuration, manageable initial investment, and better meets the need for precise recovery rate control at low flow rates.

3-5 t/h: Flexible selection range, requiring key evaluation:

Is space extremely limited? (Yes → lean toward 8040)

Are recovery rate requirements extremely high and does it require concentrate recirculation? (Yes → lean toward 4040)

Is the project budget more focused on short-term investment or long-term operational costs?

Are there clear future expansion plans? (Yes → Consider 8040 to reserve capacity for future growth)

>5 t/h: 8040 membrane elements are the top recommendation. Their significant advantages—dramatically reducing membrane quantity, simplifying system design, saving space, and lowering long-term operational costs—make them the most cost-effective solution.