How to Select an RO Membrane Housing: Pressure, Material, and Configuration

The RO membrane housing — also called a pressure vessel — is the structural component that holds your RO elements under operating pressure. A poor housing choice leads to membrane bypass, O-ring failures, or structural failure. Here’s how to get it right.

Understanding the Basics

A standard RO membrane element is 8 inches in diameter × 40 inches long (8040 format) or 4 inches × 40 inches (4040 format). The housing is a tube that holds multiple elements end-to-end in a pressure-sealed enclosure.

Water enters the feed end, passes through the membrane spiral-wound elements, and exits at two points:

• Permeate (product): collected via a permeate tube through the center of each element
• Concentrate (reject): exits the tail end of the housing

Step 1: Choose Your Element Format

For most industrial applications over 5 m³/h, the 8040 format is the standard choice. It offers the widest membrane selection from all major manufacturers (Dow Filmtec, Toray, Hydranautics, etc.) and the lowest cost per m³ of permeate.

Step 2: Determine Elements Per Housing

Standard housings hold 1 to 7 elements (most common: 4-element and 6-element for 8040).

Why does element count affect recovery? More elements in series means the water spends more "contact length" with membranes, increasing recovery. However, too many elements increases concentrate-side osmotic pressure and reduces flow in the tail elements.

Standard recommendation: 6 elements per housing for brackish water RO (< 5,000 ppm TDS), 4 elements for seawater RO (> 15,000 ppm TDS).

Step 3: Select Operating Pressure Rating

Housings are rated for maximum operating pressure. Selecting under-rated housings is a safety hazard.

Always specify housings with a 20–30% pressure margin above your operating point. A 10-bar system should use a 21-bar housing, not a 10-bar one.

Step 4: Choose Housing Material

FRP (Fiberglass Reinforced Plastic)

• Most common for standard industrial and commercial RO
• Excellent corrosion resistance to brackish and seawater
• Lighter than stainless steel, lower cost
• Available up to 83 bar (1,200 psi)
• Limitation: Cannot be opened under pressure; requires end-cap tool for element change

316L Stainless Steel

• Required for pharmaceutical (USP purified water) and food-grade applications
• Can be sanitized with hot water (80°C) or steam
• Electropolished interior prevents biofilm adhesion
• More expensive, heavier
• Best for: GMP environments, sanitary applications, where FDA compliance is required

Carbon Steel (Epoxy-lined)

• Used for very large vessels (> 10" diameter) in industrial settings
• Lower cost at large sizes
• Requires careful lining integrity inspection

Step 5: Select End Plug Configuration

The end plugs seal the housing and allow element loading. There are three main configurations:

For systems with more than 4 housings, side-port housings connected via a permeate header manifold are the industry standard — easier to service and allows individual housing isolation.

Step 6: Array Configuration

For larger systems, housings are arranged in arrays (banks) with a specific ratio between stages:

2-stage system (most common for 75–80% recovery):

Stage 1: 2 vessels × 6 elements = 12 elements
Stage 2: 1 vessel × 6 elements = 6 elements
Ratio: 2:1

The 2:1 ratio maintains balanced flux across both stages as recovery increases.

For high-recovery systems (> 80%), a 3-stage array (4:2:1 ratio) is used, though this increases system complexity and fouling risk.

Common Installation Mistakes

1. Horizontal mounting without support — Housings > 1,200 mm long need mid-span support to prevent sag and O-ring distortion 2. Over-tightening end plugs — Follow torque specs (typically 14–20 Nm for 8040); over-tightening cracks FRP end caps 3. No flush valve on startup — Always flush with low-pressure permeate or tap water before applying full operating pressure to a new membrane 4. Wrong O-ring material — Standard is EPDM for most RO applications; use Viton for high-temperature or chemical exposure applications

Quick Reference: ANLISI Housing Models

Custom pressure ratings, lengths, and sanitary finishes available on request.

Selection Checklist

• [ ] Element format determined (4040 or 8040)
• [ ] Elements per housing specified based on recovery target
• [ ] Operating pressure confirmed — housing rated ≥ 20% above operating pressure
• [ ] Material selected: FRP (standard) or 316L SS (pharma/food)
• [ ] End plug configuration chosen (side port for arrays)
• [ ] Array ratio calculated (2:1 for standard 75% recovery)
• [ ] Support brackets specified for horizontal installation
• [ ] O-ring material confirmed (EPDM standard, Viton for chemical exposure)

Have a system design you’d like reviewed? Send your water analysis and flow requirements — we’ll review your housing specification and confirm it before you commit to purchase.