Online Cleaning of Ultrafiltration Membranes and Reverse Osmosis Membranes (Part 1)

Operation and membrane cleaning of the first-stage reverse osmosis

1. Online cleaning of the first-stage reverse osmosis membrane

For normally operating first-stage reverse osmosis equipment, controlling the quality of the feed water can extend the membrane's operating cycle; otherwise, the membrane will quickly become polluted. After using reclaimed water, the SDI of the feed water for the first-stage reverse osmosis equipment is approximately 3～3.5, with a maximum of around 4.0. During the heating season, due to the large water supply volume, the operating cycle of the first-stage reverse osmosis equipment is shorter and cleaning is more frequent; while during the non-heating season, the operating cycle of the first-stage reverse osmosis equipment is 4～5 times that of the heating season.

(1) Reverse osmosis membrane cleaning standards

As a general rule, the reverse osmosis membrane cleaning time is determined when any of the following situations occur:

A. Under normal pressure, the product water flow rate has decreased by 10%～15% of the normal value;

B. To maintain the normal product water flow rate, the feed water pressure after temperature correction has increased by 10%～15%;

C. The product water quality has decreased by 10%～15%, and the salt passage rate has increased by 10%～15%;

D. The pressure difference between the various sections of the reverse osmosis has increased by 15% compared to the initial operation.

Through analysis of the operating conditions of the reverse osmosis membrane, in order to extend the membrane's lifespan and ensure the quality of the produced water, operation is carried out without exceeding the rated inlet pressure and rated temperature. Cleaning is performed when the product water flow rate decreases by 10%～15% of the normal value.

(2) Determination of pollutants

Because reclaimed water is used, the content of organic matter, inorganic salts, and bacteria is relatively high. From the perspective of system operation, especially during the heating season, under the same temperature and concentrate water discharge flow rate, the pressure difference in the first stage of the RO equipment increases significantly, that is, the inlet pressure increases rapidly, the membrane output decreases, and the membrane desalination rate remains almost unchanged, indicating that there is serious membrane fouling. Upon inspection of the pre-filtration water tank, a foul odor was detected, the inner wall had sticky deposits, and the color was yellowish-green; inspection of the filter element replaced from the 5μ filter showed that the filter element was mainly polluted by a yellow, sticky substance, which can slowly dissolve in high-concentration NaClO and HCl. It is judged that the pollutants are mainly bacterial microorganisms, organic matter, and inorganic salt scaling.

(3) Determination of the cleaning scheme

The main pollutants in the reverse osmosis are bacterial microorganisms, organic matter, and inorganic salt scaling. Combining the actual on-site situation and considering that EDTA sodium salt can chelate iron and that high-pH conditions can decompose organic matter, it was decided to adopt a cleaning scheme combining EDTA alkaline washing, hydrochloric acid acid washing, and a second EDTA alkaline washing. The details are as follows:

A. Hydrochloric acid washing

Hydrochloric acid (HCl) chemical cleaning is used:

0.2% concentration hydrochloric acid, pH 2～3, is used for circulating cleaning of the reverse osmosis membrane;

The cleaning temperature is 28～30℃, the cleaning inlet pressure is less than 0.4 MPa, and the freshwater side pressure is 0.07 MPa;

The cleaning process is alternately dynamic and static, i.e., dynamic circulation for 1h, static immersion for 1h. The cleaning time is until the hydrochloric acid concentration no longer decreases, generally not exceeding 6h.

B. EDTA alkaline washing

NaOH+Na2-EDTA chemical cleaning is used:

A mixed solution of 0.1% concentration NaOH + 0.5～1% concentration Na2-EDTA, pH 11～12, is used for circulating cleaning of the reverse osmosis membrane;

The cleaning temperature is 30～32℃, the cleaning inlet pressure is less than 0.4 MPa, and the freshwater side pressure is 0.07 MPa;

The cleaning process is alternately dynamic and static, i.e., dynamic circulation for 1h, static immersion for 1h, with a total cleaning time of 20h.

2. Operating process of the first-stage reverse osmosis

The process flow of the first-stage reverse osmosis equipment is: ultrafiltration water tank → 5μm filter → first-stage high-pressure pump → first-stage reverse osmosis → first-stage reverse osmosis water tank. After the water filtered by the ultrafiltration device is treated with antiscalants and reducing agents, the first-stage high-pressure pump increases the pressure to 1.22 MPa, and the water enters the reverse osmosis device. The product water flows into the first-stage reverse osmosis water tank, and the concentrate water is discharged.

Inlet water quality: conductivity between 800～1200μs/cm, CODCr 11～20mg/L, SDI value 3.0～4.0, strontium 0.6～0.9 mg/L, manganese 56～85μg/L, barium approximately 65μg/L.

Operation of the ultrafiltration equipment and cleaning of the ultrafiltration membrane

As a pretreatment device for reverse osmosis, the operation of the ultrafiltration equipment will affect the operation of the reverse osmosis. Due to the characteristics of reclaimed water, the ultrafiltration membrane is easily polluted by organic matter, microorganisms, metal ions, and suspended solids.

1. Online cleaning of the ultrafiltration membrane

During the operation of ultrafiltration membrane components, the membrane surface will be covered by various harmful impurities it retains, forming a filter cake layer. Even the membrane pores can be blocked by finer impurities, increasing the transmembrane pressure of the water. The quality of the raw water only affects the speed of membrane fouling, not the fundamental solution to the pollution problem. Even with thorough pretreatment, a very small number of impurities in the water will gradually affect the separation performance of the membrane over time. Therefore, when the ultrafiltration membrane is fouled to a certain extent, it should be cleaned.

2. Selection of Chemical Cleaning Agents: Generally, regardless of the type of pollution, acid washing should be followed by alkali washing, or sterilization followed by acid and alkali washing for the best results. Alkali washing alone is not effective; acid washing followed by alkali washing can also prevent scaling when the solution pH exceeds 9.

The chemical cleaning agents are sodium hypochlorite (NaClO), sodium hydroxide (NaOH), and hydrochloric acid (HCl).

A. Purpose of backwashing with hydrochloric acid: To remove metal ion deposits on the membrane, such as calcium salts, magnesium salts, and iron salts.

B. Purpose of backwashing with sodium hypochlorite: Sterilization and virus inactivation.

C. Purpose of backwashing with sodium hydroxide: Cleaning organic matter and other impurities.

3. Determination of Pollutants

In the early stage of ultrafiltration operation (approximately six months, with city tap water as the source), the operating pressure difference increased rapidly. A sample UF end cap was opened, and the UF membrane was removed. A thin layer of yellow, sticky substance was found on the UF membrane end face and outer shell. This substance did not completely dissolve in HCl or NaOH, but faded in acid. It was determined that the pollutants were mainly inorganic salt scaling and colloidal clogging.

With city reclaimed water as the source for desalination water production, the clogging in ultrafiltration operation intensified, especially during the continuous operation of ultrafiltration during winter heating. The operating pressure difference increased rapidly. By checking the pre-treatment water tank, a foul odor was found, along with sticky deposits on the inner wall, black-yellow-green in color. This substance slowly dissolved in high concentrations of NaClO and HCl. It was determined that the pollutants were mainly bacteria, microorganisms, organic matter, and inorganic salt scaling.

Cleaning with HCl and NaOH + NaClO, with a backwashing flow rate of 360 t/h.

4. Determination of Cleaning Scheme

Due to the characteristics of ultrafiltration equipment pollution, the pollutants of the ultrafiltration membrane are mainly organic matter, microbial pollution, and inorganic salt scaling. Through long-term research, an alternating cleaning scheme of HCl cleaning and NaOH + NaClO cleaning has been developed, i.e., acid washing followed by alkali washing. The ultrafiltration equipment uses an integrated backwashing chemical cleaning method. Appropriate chemicals are added to the backwashing inlet pipeline of the ultrafiltration equipment to ensure normal cleaning and operation of the ultrafiltration equipment.

When performing chemical cleaning of the ultrafiltration equipment, the cleaning method must ensure that there is no air in the ultrafiltration equipment. The concentration of the chemicals, the soaking temperature and time must be considered. According to the operating equipment pressure difference, enhanced cleaning should be performed every 1-2 months. The dosage is twice that of normal chemical cleaning, and the soaking time depends on the degree of clogging. Generally, the soaking time should not exceed 6 hours.