Best Practices for Reverse Osmosis (RO) System Design

What Does a Reverse Osmosis System Do?

Car wash owners use Reverse Osmosis (RO) water for the spot-free rinse portion of the wash. The system uses RO membranes to remove dissolved solids from municipal feed water, producing permeate water with very low Total Dissolved Solids (TDS). RO membranes can remove up to 99.7% of dissolved solids, achieving TDS levels from 0 to 15 ppm. Feedwater TDS varies significantly by region and directly impacts RO output quality—higher-TDS feedwater produces higher-TDS RO water.

When used as a spot-free final rinse, the general guideline is to maintain RO water below 20 ppm TDS to avoid visible spotting when vehicles air-dry.

Understanding RO Membrane Specifications

RO membranes are rated at a standard feedwater temperature of 77°F. Manufacturers use this condition for flow specifications. In many areas, especially northern climates with surface-water supplies, feedwater temperatures can drop to 40°F or lower during winter months.

For each degree below 77°F, RO production flow decreases by approximately 2%. For example, a 10,000 GPD RO system will produce about 6,000 GPD when feedwater temperature drops 20°F—roughly 40% less than its rated capacity. Operators must consider this reduction when specifying systems to avoid running out of RO water during busy winter days.

4″ vs 8″ Diameter Membranes

For decades, standard car wash RO systems have used 4″ × 40″ membranes with ≈ 80 ft² of active surface area. High-volume conveyor washes often require large storage tanks (3,000 gallons or more) and multiple 4″ membranes—sometimes 8 to 12 or more—to meet tunnel demand.

Leading RO manufacturers are now switching to 8″ diameter membranes, long proven in large-scale industrial applications. These 8″ elements provide ≈ 440 ft² of membrane surface area—5.5 times more than a standard 4″ membrane. This allows RO systems to achieve required flow using only 2 or 3 membranes while maintaining smaller footprints and potentially eliminating the need for oversized storage tanks.

Customer Benefits of Using 8″ Membranes

• Fewer membranes (e.g., 16 × 4″ → 3 × 8″) reduce overall membrane and housing cost for equivalent capacity.
• Less maintenance—fewer seals, leak points, and plumbing connections.
• Faster troubleshooting—series vs. parallel flow paths allow sampling from individual membranes to identify cut seals or failures.
• Replacement cost for 8″ membranes is approximately 45% lower than for equivalent 4″ membranes.

Type of Membrane Flush Is Critical for Long Membrane Life

Traditional RO systems use municipal-water flushes to protect membranes from fouling. A valve opens, allowing low-pressure water to rinse the concentrate (high-TDS) side of the membrane. In addition to automated daily flushes, older systems often require periodic chemical cleaning—typically a multi-step alkaline (high pH) wash, flush, then an acidic (low pH) wash. Ideally this cleaning occurs in a separate tank with recirculation and a full rinse before returning the membranes to service. In practice, many operators attempt to clean in place, which yields incomplete results.

Modern high-end RO systems instead use permeate-water flush for membrane protection. After each production cycle, low-TDS permeate water flushes the concentrate from the feed side, leaving clean permeate on the membranes until the next run. This approach has delivered excellent long-term results in many car washes—eliminating the need for frequent chemical cleaning and preventing the gradual flow reduction associated with fouling.

Additional Features of High-End Car Wash RO Systems

• Fully automated “flow-on-demand” control adjusts RO production to match tunnel throughput.
• Automatic compensation for seasonal feedwater temperature changes—consistent RO output year-round with no operator adjustment.

About the Author

Jim is a Mechanical Engineer with over 40 years of product development experience across automotive, aerospace, and high-demand commercial wash applications. His engineering work includes multiple U.S. patents and decades of reliability-focused equipment design. Over the past 10 years, he has worked directly with operators and equipment suppliers to improve uptime and reduce lifecycle cost in industrial water treatment and washdown systems.