Design Aspects for RO

Summary

This document outlines various design considerations and limits for Reverse Osmosis (RO) systems utilizing Hydranautics membranes. It covers system components, flow configurations, and sizing guidance to optimize performance.

System Components

RO systems typically consist of the following basic components:

• Feed water supply unit
• Pretreatment system
• High pressure pumping unit
• Membrane element assembly unit (RO block)
• Instrumentation and control system
• Permeate treatment and storage unit
• Cleaning unit

The membrane assembly unit comprises a stand supporting pressure vessels, interconnecting piping, and feed, permeate, and concentrate manifolds. Each pressure vessel can house one to seven membrane elements connected in series.

Staging and Recirculation Concepts

Concentrate Staging and Pyramid Design

A system can be divided into groups of pressure vessels called concentrate stages. Pressure vessels within a stage are connected in parallel. The number of pressure vessels in subsequent stages usually decreases in a ratio of 2:1, creating a "pyramid" structure. This design maintains appropriate feed/concentrate flow rates per vessel, preventing high pressure drop and structural damage from high flow, or insufficient turbulence and excessive salt concentration from low flow.

• Recovery rate per individual membrane element should not exceed 18%.
• Common engineering practice for brackish RO systems is an average recovery rate of about 9% per 40-inch long membrane element.
• For RO units with 6 elements per pressure vessel:
  • Two concentrate stages are used for recovery rates over 60%.
  • Three concentrate stages are used for recovery rates over 75%.
• For RO units with 7 elements per pressure vessel:
  • A two-stage configuration is sufficient for recovery rates up to 85%.

Concentrate Recirculation

Used mainly in very small RO units to increase overall system recovery ratio while maintaining acceptable concentrate flow. A portion of the concentrate stream is returned to the suction of the high pressure pump. This design offers a compact unit but requires a larger feed pump, leading to higher power consumption and increased feed pressure/permeate salinity due to blending.

Permeate Staging

For applications requiring very low salinity permeate (e.g., seawater RO systems, brackish RO for ultrapure water), a single pass RO system may be insufficient. Permeate staging, or a "two-pass design," involves desalinating the first pass permeate in a second RO system.

• First pass permeate is very clean, requiring no significant pretreatment.
• Second pass system can operate at a relatively high average permeate flux and high recovery rate.
• Common design parameters for a second pass RO unit:
  • Average flux rate: 20 gfd
  • Recovery rate: 85% - 90%
• Concentrate from the second pass unit can be returned to the suction of the first pass high pressure pump to slightly reduce feed salinity and increase overall feed water utilization.

Flow Distribution Control

To equilibrate permeate flow between stages (decrease first-stage flow, increase last-stage flow), two configurations are possible:

1. Permeate Throttling: Installing a valve on the permeate line from the first stage. Throttling increases permeate back pressure, reducing net driving pressure and permeate flux from the first stage. The second stage operates at a higher feed pressure to compensate. This design offers simplicity and low capital cost but results in additional power losses and higher power consumption.
2. Interstage Pump: Installing a booster pump on the concentrate line between the first and second stages. This increases feed pressure to the second stage, resulting in higher permeate flow. This design has a higher investment cost but lower power consumption compared to permeate throttling.

RO Sizing Guidance

To determine approximate system size (number of membrane elements and pressure tubes) for a desired permeate quantity:

1. Select membrane type and model.
2. Select flux rate (GFD) based on expected feed water quality.
3. Divide desired plant capacity by design flux rate and membrane element surface area.
4. Divide total elements by elements per pressure vessel; round up.
5. Select appropriate array for desired recovery, increasing pressure vessels if needed.

A safety margin of 10% for recommended pump pressure over calculated feed pressure (+ 3 psi / 0.2 bar for entry losses) is suggested, or a 10% higher number of elements as a contingency if fouling rate cannot be predicted.

Hydranautics Design Limits

Average Flux Rates and Expected % Decrease in Flux per year

• Surface water (SDI 2 - 4): 8 - 14 GFD; 7.3 - 9.9 % Flux Decline/year
• Well water (SDI < 2): 14 - 18 GFD; 4.4 - 7.3 % Flux Decline/year
• RO Permeate (SDI < 1): 20 - 30 GFD; 2.3 - 4.4 % Flux Decline/year

Expected % Salt Passage Increase per year

• Cellulosic membrane (CAB1, CAB2, CAB3): 17 - 33 % SP Increase/year
• Composite Membrane (Brackish, Low Pressure ESPA1, ESPA2, ESPA3; Brackish, High Rejection CPA2, CPA3, CPA4; Low Fouling LFC1, LFC2; Seawater SWC1, SWC2, SWC3; Softening, PolyVinyl Deriv. PVD1, ESNA1, ESNA2): 3 - 17 % SP Increase/year

Maximum Feed Flow and Minimum Concentrate Flow Rates per Vessel

• 4 inch Membrane Diameter: Max 16 GPM (3.6 m³/hr), Min 3 GPM (0.7 m³/hr)
• 6 inch Membrane Diameter: Max 30 GPM (8.8 m³/hr), Min 7 GPM (1.6 m³/hr)
• 8 inch Membrane Diameter: Max 75 GPM (17.0 m³/hr), Min 12 GPM (2.7 m³/hr)
• 8.5 inch Membrane Diameter: Max 85 GPM (19.3 m³/hr), Min 14 GPM (3.2 m³/hr)

Saturation Limits for Sparingly Soluble Salts in the Concentrate

• CaSO₄: 230%
• SrSO₄: 800%
• BaSO₄: 6000%
• SiO₂: 100%

Limits of Saturation Indices (Langelier and Stiff & Davis Saturation Indices)

• LSI and SDSI without scale inhibitor: < -0.2
• LSI & SDSI with SHMP: < 0.5
• LSI & SDSI with organic scale inhibitor: < 1.8

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Disclaimer: This summary is based on the provided text extraction and may not include all figures, footnotes, or the latest revisions from the original OEM PDF. Contractual data for any project must match the specific OEM PDF revision used for that project.

Official datasheet (PDF)

PDF datasheet