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Sizing Your First BWRO System: A Practical Guide for Engineers

Ten steps from feed water analysis to a complete brackish water RO design.

Technical Guide By ForeverPure Engineering Team Published November 25, 2026 Read 12 min

BWRO vs SWRO: When Brackish?

Brackish water reverse osmosis (BWRO) and seawater reverse osmosis (SWRO) are mechanistically similar but design-distinct. The dividing line, in practical terms, sits around 5,000 mg/L TDS. Below that, you are firmly in BWRO territory: lower operating pressures (10–25 bar instead of 55–85 bar), higher recoveries (75–85% instead of 35–50%), thinner-wall pressure vessels, and a completely different membrane catalog. Brackish water feed sources include inland wells, agricultural runoff, river water with high mineral content, mine drainage, oil & gas produced water, and the concentrate from previous treatment stages.

Choosing brackish-rated equipment for a feed water that turns out to be marginal seawater — or vice versa — is the single most expensive design mistake we see in BWRO. This guide walks through the ten steps to get the design right the first time.

1Feed Water Analysis

Nothing else in the design matters until you have a complete water analysis. For BWRO sizing, request a minimum lab panel covering:

Sample collection matters. Pull samples from the actual point of intake at the actual season — ideally three samples across the year if the source has any seasonal variation (rivers, shallow wells). A water analysis from a single day in spring will mislead you if the well draws from a brackish aquifer that gets saltier as the water table drops in late summer.

2Establish Permeate Quality Targets

The permeate spec drives the rejection requirements and ultimately the membrane selection and array configuration:

3Choose Recovery

Recovery is the single most consequential design choice in BWRO. Higher recovery means less concentrate to dispose of, less feed flow to pump, and lower operating cost — but it raises the concentration of every scaling species in the brine, demanding more sophisticated antiscalant chemistry and, at the limit, more aggressive cleaning frequency.

The screening calculation that matters: compute the Langelier Saturation Index (LSI) and Stiff & Davis Stability Index (S&DSI) for CaCO₃, and the percentage of solubility limit for CaSO₄, BaSO₄, SrSO₄, and amorphous silica in the projected concentrate. Any species above ~80% of saturation needs an antiscalant rated for it and confirmed by the antiscalant supplier on your specific water.

4Capacity Sizing

Three numbers to nail down:

Design the RO to deliver the average daily demand over an operating window of 16–20 hours per day. Never design at 24/7 unless redundancy is also provided — you need downtime for membrane cleaning (CIP), cartridge filter changes, and unplanned maintenance. The remaining 4–8 hours becomes the maintenance window, and the storage tank covers the gap.

5Membrane Selection

For BWRO, the choice usually comes down to three Filmtec families (or their Hydranautics / LG equivalents):

For Hydranautics, the equivalent families are ESPA (Energy-Saving Polyamide), CPA (Composite Polyamide), and LFC (Low Fouling Composite). For LG, the BW family. See our pages on Filmtec, Hydranautics, and LG membranes for current model availability.

6Multi-Stage Array Design

BWRO recovery dictates the number of stages. Each stage typically achieves 50% recovery on its feed:

Pressure vessels are typically 6 or 7 elements long. Crossflow velocity drops as you move down the array (each element extracts permeate, reducing brine-side flow), so the multi-stage design preserves crossflow by reducing vessel count stage-by-stage. Maintaining adequate crossflow is critical to preventing concentration polarization at the membrane surface.

7High-Pressure Pump

BWRO operating pressures are modest by SWRO standards: 10–25 bar depending on feed TDS, recovery, and membrane selection. The default pump selection is a vertical multistage centrifugal pump such as the Grundfos CR series. Sizing:

Materials: 316L stainless steel for the wet end is standard for BWRO with chloride below 1,000 mg/L. For higher chloride, specify Duplex 2205 or step up to Super Duplex 2507.

8Pretreatment

Pretreatment determines membrane life. The minimum specification for a BWRO on well water:

9Controls

A modern BWRO PLC monitors and trends at minimum:

For solar or off-grid BWRO, soft-start variable frequency drives (VFDs) on the high-pressure pump are mandatory. They protect the membranes from water-hammer at startup and allow flux control as feed water temperature and TDS shift.

10Permitting and Brine Disposal

The often-underestimated step. BWRO concentrate is typically 2–5x the feed TDS and is rarely just "salty water" — it carries the antiscalant chemistry, the pretreatment carryover, and any membrane CIP residual. Disposal options, from cheapest to most expensive:

Worked Example: 25,000 GPD BWRO

Feed: well water, 3,000 mg/L TDS, hardness 350 mg/L CaCO₃, silica 15 mg/L, no iron, no free chlorine.

Permeate target: 25,000 GPD potable water at <500 mg/L TDS, operating 18 hours per day.

Sizing calculation:

This sizing exercise maps cleanly onto our standard BWRO 20,000–50,000 GPD product line, available as either a packaged skid or a containerized unit. For the project specifics — pretreatment customization, controls philosophy, brine disposal — please request a quote with your water analysis attached.

Related reading: Brackish Water Desalination Systems overview, SWRO Design Guide.

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