Chemical Cleaning and Water Flushing of Reverse Osmosis Membranes

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During cleaning, the cleaning solution is circulated at low pressure and high flow rate on the high-pressure side of the membrane. The membrane element remains inside the pressure vessel, and this process requires a specialized cleaning device.

General steps for cleaning reverse osmosis membrane elements:

1. Use a pump to pump clean, chlorine-free reverse osmosis product water from the cleaning tank (or a suitable water source) into the pressure vessel and discharge for a few minutes.

2. Prepare the cleaning solution in the cleaning tank using clean product water.

3. Circulate the cleaning solution in the pressure vessel for 1 hour or a pre-set time. For 8-inch or 8.5-inch pressure vessels, the flow rate should be 35–40 gallons/minute (133–151 liters/minute); for 6-inch pressure vessels, the flow rate should be 15–20 gallons/minute (57–76 liters/minute); and for 4-inch pressure vessels, the flow rate should be 9–10 gallons/minute (34–38 liters/minute).

4. After cleaning, drain the cleaning tank and rinse it. Then fill the cleaning tank with clean product water for the next rinsing step.

5. Use a pump to pump clean, chlorine-free product water from the cleaning tank (or an appropriate water source) into the pressure vessel and discharge it for a few minutes.

6. After flushing the reverse osmosis system, run the system with the product water drain valve open until the product water is clean, free of foam or cleaning agent (usually 15-30 minutes).

Table 1. Characteristics and Treatment Methods of Reverse Osmosis Membrane Fouling

pollutants

General characteristics

processing method

Calcium deposits

(Calcium carbonate and calcium phosphate, generally occurring in the second stage of the system)

The desalination rate decreased significantly.

Increased system voltage drop

The system's water production decreased slightly.

Clean the system with solution 1.

oxides

(Iron, nickel, copper, etc.)

The desalination rate decreased significantly.

The system voltage drop increased significantly.

The system's water production has decreased significantly.

Clean the system with solution 1.

Various colloids

(Iron, organic matter, and silica gel)

The desalination rate decreased slightly.

The system voltage drop gradually increases

The system's water production gradually decreased

Clean the system with solution 2.

Calcium sulfate

(This usually occurs in the second segment of the system)

The desalination rate decreased significantly.

The system voltage drop increased slightly or moderately.

The system's water production has decreased slightly.

Clean the system with solution 2.

Use solution 3 to clean when the contamination is severe.

Organic matter deposition

The desalination rate may decrease.

The system voltage drop gradually increases

The system's water production gradually decreased

Clean the system with solution 2.

Use solution 3 to clean when the contamination is severe.

Bacterial contamination

The desalination rate may decrease.

The system voltage drop increased significantly.

The system's water production has decreased significantly.

Based on the possible types of pollution

Choose three solutions

A cleaning system

Note: The cause of the pollution must be identified and the source eliminated. Please contact Hydranautics for assistance.

Table 2. Recommended common cleaning solutions

Cleaning solution	Ingredients	Dosage for preparing 100 gallons (379 liters) of solution	pH adjustment
1	Citric acid Reverse osmosis product water (free chlorine)	17.0 lbs (7.7 kg) 100 gallons (379 L)	Adjust the pH with ammonia. 17.0 lbs (7.7 kg) 100 gallons (379 L)	Adjust the pH to 3.0 with ammonia.
2	Sodium tripolyphosphate EDTA tetrasodium salt Reverse osmosis product water (free chlorine)	17.0 lbs (7.7 kg) 7 pounds (3.18 kg) 100 gallons (379 L)	Adjust the pH to 10.0 with sulfuric acid.
3	Sodium tripolyphosphate Sodium dodecylbenzenesulfonate Reverse osmosis product water (free chlorine)	17.0 lbs (7.7 kg) 2.13 lbs (0.97 kg) 100 gallons (379 L)	Adjust the pH to 10.0 with sulfuric acid.