Operation of One Stage Reverse Osmosis and Cleaning of Membrane
1. Online cleaning of primary reverse osmosis membrane
The normal operation of the primary Reverse Osmosis System can control the water quality of the feed water, which can prolong the operation period of the membrane, otherwise the membrane will be polluted soon. After using reclaimed water, the SDI of the first-stage reverse osmosis equipment is about 3 to 3.5, and the highest is about 4.0. During the heating period, due to the large amount of water supply, the operation period of the primary reverse osmosis equipment is short and cleaning is frequent; while in the non-heating period, the operating period of the primary reverse osmosis equipment is 4 to 5 times that of the heating period.
(1) Cleaning standards for reverse osmosis membranes
The determination of the cleaning time of the Reverse Osmosis Membrane is a general principle, and cleaning is carried out when one of the following situations occurs:
A. Under normal pressure, if the product water flow drops by 10% to 15% of the normal value;
B. In order to maintain normal product water flow, the water supply pressure after temperature correction is increased by 10% to 15%;
C. The water quality of the product is reduced by 10% to 15%, and the salt transmission rate is increased by 10% to 15%;
D. The pressure difference between each stage of reverse osmosis has increased by 15% compared with the initial stage of operation.
Through the analysis of the operation of the reverse osmosis membrane, in order to prolong the life of the membrane and ensure the quality of the product water, the operation should be carried out under the premise of not exceeding the rated water inlet pressure and rated temperature. When the product water flow rate is reduced by 10% to 15% of the normal value when cleaning.
(2) Determination of pollutants
Due to the use of reclaimed water, the content of organic matter, inorganic salts, bacteria, etc. is relatively high. From the perspective of system operation, especially during the heating period, under the same temperature and concentrated water discharge flow, the pressure difference of a section of RO Water System rises significantly. , that is, the inlet pressure rises rapidly, the output of the membrane decreases, the desalination rate of the membrane hardly changes, and the membrane has serious fouling. By checking the front water tank, it is found that there is a rancid smell in the water tank, the inner wall is sticky and slippery, and the color is yellow-green; check the filter element replaced by the 5μ filter, the filter element is mainly polluted by yellow sticky and slippery substances, which are in high concentrations of NaClO and It can be slowly dissolved in HCl, and it is judged that the pollutants are mainly bacterial microorganisms, organic matter and inorganic salt scale.
(3) Determination of cleaning plan
Reverse osmosis pollutants are mainly bacterial microorganisms, organic matter and inorganic salt scale. Considering the actual situation of the site, considering that EDTA sodium salt can chelate iron and decompose organic matter under high pH conditions, it is determined to use EDTA alkali washing, hydrochloric acid pickling, two A cleaning regimen that combines a second EDTA alkaline wash. details as follows:
- Pickling with hydrochloric acid
Chemical cleaning with HCl:
Use 0.2% concentration of hydrochloric acid, PH2～3, to carry out reverse osmosis membrane cycle cleaning;
The cleaning temperature is 28～30℃, the cleaning inlet pressure is less than 0.4 MPa, and the fresh water side pressure is 0.07 MPa;
The cleaning process is dynamic and static alternately, namely: dynamic circulation for 1h and static soaking for 1h. The cleaning time is until the concentration of hydrochloric acid no longer decreases, generally not more than 6h.
- EDTA alkaline washing
Chemical cleaning with NaOH+Na2-EDTA:
Use 0.1% NaOH + 0.5~1% Na2-EDTA mixed liquid, pH 11~12, to clean the reverse osmosis membrane;
The cleaning temperature is 30~32℃, the cleaning inlet pressure is less than 0.4 MPa, and the fresh water side pressure is 0.07 MPa;
The cleaning process is dynamic and static alternately, namely: dynamic circulation for 1h., static soaking for 1h, and cleaning time for 20h.
2. The operation process of primary Reverse Osmosis Water System
The process of the primary reverse osmosis equipment: ultrafiltration water tank → 5μm filter → primary high pressure pump → primary reverse osmosis → primary reverse osmosis water tank. When the water filtered by the ultrafiltration device is added with scale inhibitor and reducing agent, the pressure is raised to 1.22MPa by the first-stage high-pressure pump, and then enters the reverse osmosis device. The product water flows into the first-stage reverse osmosis water tank, and the concentrated water is discharged.
Influent 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 about 65μg/L.