How to size an industrial RO system

Here is a step-by-step guide to the key parameters and calculations involved in sizing an industrial RO system.

Step 1: Define the Core Requirements (The "What")

1. Product Water Flow Rate (Qp): This is your primary requirement. How many gallons per minute (GPM) or cubic meters per hour (m³/h) of purified water do you need continuously? (e.g., 100 GPM for a manufacturing plant).
2. Product Water Quality: Define the maximum allowable Total Dissolved Solids (TDS) or specific ion concentrations (e.g., silica, sodium, chlorides) in the permeate water. This determines the required salt rejection percentage.
3. Feedwater Source & Analysis: This is the most critical data set. A comprehensive water analysis is non-negotiable. Key parameters:

    ◦   TDS, Conductivity

    ◦   Silt Density Index (SDI) or Turbidity (measures fouling potential)

    ◦   Concentrations of: Calcium, Magnesium (hardness), Barium, Strontium (scale formers), Silica, Iron, Manganese

    ◦   pH, Temperature

    ◦   Free Chlorine (must be zero to protect membranes)

4. Operating Conditions: Hours per day, days per week, required system availability.

Step 2: Key Design Calculations (The "How Much")

With the requirements above, you perform these calculations:

1. System Recovery Rate (%R): The percentage of feedwater converted to product water.

    ◦   Formula: R (%) = (Product Flow / Feed Flow) x 100

    ◦   The Limiting Factor: Recovery is limited by the scaling potential of the feedwater. Higher recovery means the concentrate (reject) stream becomes more concentrated, risking scale precipitation on the membrane. Software (like ROSA from Dow/Chemours) is used to model the maximum allowable recovery based on your water chemistry, antiscalant dosing, and pH adjustment.

2. Concentration Factor (CF): Related to recovery.

    ◦   Formula: CF = 1 / (1 - R) or CF = Feed TDS / Concentrate TDS

    ◦   Example: At 75% recovery, the concentrate is 4 times more concentrated than the feed (CF = 1/(1-0.75) = 4).

3. Feed Flow Requirement (Qf):

    ◦   Formula: Qf = Qp / R

    ◦   Example: If you need 100 GPM of product (Qp) at 75% recovery (R), you need a feed flow of: 100 / 0.75 = 133.3 GPM.

4. Concentrate (Reject) Flow (Qc):

    ◦   Formula: Qc = Qf - Qp

    ◦   Example: 133.3 - 100 = 33.3 GPM.

5. Membrane Array Design:

    ◦   Industrial systems are built in "arrays" (e.g., 2:1, 3:2:1 array) to maintain adequate cross-flow velocity across the membrane surface, which minimizes fouling.

    ◦   The number of pressure vessels (housings) and membranes is determined by the required feed flow and the manufacturer's specifications for flux.

6. Flux (GPD/ft² or LMH): The flow rate of permeate per unit area of membrane.

    ◦   Formula for total membrane area needed: Area = Qp / Flux

    ◦   Flux Guidelines: Chosen based on feedwater quality (lower flux for poor quality water to reduce fouling).

        ▪   RO Permeate (2nd Pass): 20-30 GFD

        ▪   Well Water: 12-18 GFD

        ▪   Surface Water: 8-14 GFD

        ▪   Wastewater: 7-12 GFD

Step 3: Pretreatment & Post-Treatment Sizing (The "Support System")

The RO cannot operate alone. Its size dictates the size of all supporting equipment.

1. Pretreatment (MUST be designed based on feedwater analysis):

    ◦   Multimedia Filter: For removing suspended solids. Sized for the Feed Flow (Qf) at a typical service flow rate of 5-10 GPM/ft².

    ◦   Softener or Antiscalant Injection: To control scaling. Softener is sized for the Feed Flow (Qf) and hardness loading.

    ◦   Carbon Filter or Sodium Bisulfite Injection: For chlorine removal. Sized for Qf.

    ◦   Cartridge Filter (5µ safety filters): Sized for Qf.

2. High-Pressure Pump Selection:

    ◦   Must deliver Qf at the required pressure to overcome the osmotic pressure of the concentrated feed and system pressure losses.

    ◦   Pressure typically ranges from 150-300 psi for brackish water, 800-1200 psi for seawater.

3. Post-Treatment:

    ◦   Degasifier or Membrane Contactor: To remove CO₂ if pH was adjusted.

◦   Mixed Bed Polisher (CEDI/EDI): For ultra-high purity water. Sized for Qp.

Step 4: System Layout & Staging

• Single Pass vs. Double (Two) Pass: A single pass may suffice. If very low TDS is needed (e.g., for high-pressure boilers, semiconductors), the permeate from the first pass becomes the feed to a second RO system.
• Concentrate Staging: To increase overall recovery, the concentrate from the first bank of membranes becomes the feed to a second bank. This is the "2:1 array" concept.

Summary: The Sizing Checklist

Parameter Symbol How it's Determined

Product Flow Qp End-user process requirement

Feedwater Analysis - Lab test. The foundation of the design.

System Recovery R Limited by scaling potential (via software modeling)

Feed Flow Qf Qp / R

Concentrate Flow Qc Qf - Qp

Flux Rate J Based on feedwater source quality (guidelines)

Membrane Area Am Qp / J

# of Membranes N Am / Area per membrane (e.g., 400 ft²)

# of Pressure Vessels - N / (membranes per vessel, typically 6-7)

Array e.g., 4:2:1 Designed to balance flow and velocity

High-Pressure Pump - Must deliver Qf at required pressure

Critical Warning & Recommendation

• Do Not Wing It. An undersized pretreatment or an over-ambitious recovery rate will lead to rapid membrane fouling or scaling, resulting in chronic downtime and high operational costs.
• Use Professional Software. Membrane manufacturers (DuPont, Hydranautics, LG) provide free design software (e.g., ROSA, IMSDesign) that models all the interactions between water chemistry, recovery, flux, and scaling.
• Consult an Expert. For any true industrial application (> 50 GPM), it is standard practice to involve a water treatment systems engineer or an integrator. They will perform a pilot test on the actual feedwater, which is the gold standard for de-risking the design and ensuring optimal sizing. HID is professional RO membrane and RO system manufacturer. Welcome consult HID for a professional solution or suggestions.