What Is the Main Purpose of Low-Pressure Flushing Before and After RO System Startup and Shutdown?| Insights by AQUALITEK

Why Low-Pressure Flushing Is Essential Before and After RO System Startup and Shutdown

Low-pressure flushing, also known as “low-pressure rinse,” is performed by feeding clean water through the RO system at minimal pressure. While simple, it plays a critical role in protecting membrane elements and maintaining long-term system performance.

Below is a complete breakdown of its purposes, benefits, and best practices.

1. Main Purpose of Low-Pressure Flushing

1.1 Before Startup: Remove Stagnant or Contaminated Water

When an RO system is idle, particularly for hours or overnight, water inside the pressure vessels becomes:

•Stagnant

•High in microbial activity

•Potentially contaminated with debris

•Chemically unstable, especially after dosing antiscalants or coagulants

Low-pressure flushing replaces this degraded water with fresh feed water, preventing the membranes from being exposed to harmful or concentrated contaminants when the system ramps up to high pressure.

1.2 After Shutdown: Remove Concentrated Brine and Prevent Scaling

At the moment of shutdown, the concentrate side contains:

•High-TDS brine

•Concentrated hardness ions (Ca²⁺, Mg²⁺)

•Suspended solids

•Residual chemicals

If left inside the membrane housing, these concentrated substances will:

•Rapidly form scale

•Settle and cause fouling

•Increase biofilm growth

•Damage the membrane surface

Low-pressure flushing removes this concentrated brine and replaces it with lower-TDS water, protecting the membrane during downtime.

2. Secondary Purposes and Additional Benefits

2.1 Reduce Mechanical Stress on Membranes

Sudden high-pressure startup can damage membrane elements.
Flushing allows:

•Smooth, gradual pressure increase

•Lower risk of membrane telescoping

•Protection for O-rings and adaptors

2.2 Reduce Fouling Potential During Start/Stop Cycles

Frequent start-stop cycles promote fouling. Flushing helps to:

•Wash away particles before pressure is applied

•Prevent mudballs and deposit formation

•Limit biological attachment

2.3 Remove Residual Chemicals After Cleaning

If the system has just been chemically cleaned:

•Low-pressure flushing helps remove cleaning chemicals

•Prevents chemical incompatibilities during startup

•Protects the membrane from oxidant residues or extreme pH

2.4 Improve Overall System Stability

RO systems run more consistently when:

•Pressure ramps up gradually

•No stagnant or concentrated water remains inside

•Fouling is minimized from the beginning

3. Best Practices for Low-Pressure Flushing

•Use clean, filtered feed water or permeate for flushing
(Avoid using raw water with high solids content.)

•Flush until permeate TDS stabilizes at normal feed levels

•Flush for 1–2 minutes before startup (depending on system size)

•Flush for 1–3 minutes after shutdown

•Ensure pressure < 30 psi (2 bar) during flushing

•Do not allow the system to stop with brine in the housings

Proper low-pressure flushing can extend membrane lifetime by 20–40% and reduce annual cleaning frequency.

4. Summary: Why Low-Pressure Flushing Is Necessary

Before Startup

•Removes stagnant water

•Reduces microbial load

•Prevents fouling at high pressure

After Shutdown

•Flushes out high-TDS brine

•Prevents scaling and deposition

•Protects membranes during idle periods

Overall

Low-pressure flushing is one of the simplest yet most effective steps to protect RO membranes, stabilize system performance, and reduce operating costs.