A complete water treatment system generally consists of a pretreatment section, a membrane separation section, and a post-treatment section. The purpose of pretreatment is to improve the quality of the influent to meet the system’s influent water quality requirements. The purpose of the post-treatment process is to adjust the quality of the product water to meet customer requirements, and simultaneously adjust the quality of the concentrate to meet discharge standards.
There are two important parameters for evaluating the performance of a water treatment system: permeate flow rate and permeate quality. The designer’s main responsibility is to design a system that minimizes energy consumption and membrane element costs while maximizing permeate flow rate and recovery rate, based on the raw water quality and required permeate requirements, and reducing long-term operational risks and cleaning and maintenance costs of the membrane system.
Generally, the recovery rate of brackish water membrane systems depends on the solubility of sparingly soluble salts, with a maximum value of about 90%; seawater desalination systems, due to the high osmotic pressure of the concentrate and the pressure limitations of the components, are generally designed to have a recovery rate of around 45%.
The design of permeate flow rate and recovery rate must meet safety standards while retaining a certain degree of design flexibility. Operation modes are generally divided into batch operation and continuous operation. Batch operation refers to storing a fixed quantity of influent, pre-treating it, and then processing it through the system; this is generally used for small-scale water treatment projects. Continuous operation involves continuously treating raw water at a certain operating pressure, based on a predetermined recovery rate and permeate flow rate.
(1) Batch operation:
In applications with limited water volume and where continuous water supply is not possible, batch processing is typically employed. The incoming water is pre-collected in a feed container, and once the water volume meets the system’s operational requirements for a certain period, it is circulated for further processing. Permeate is continuously produced, and the concentrate is returned to the feed container. As the raw solution is continuously concentrated in the feed container, the membrane system’s processing speed decreases accordingly. Therefore, the batch processing capacity of the membrane system should be designed according to its capacity. After each batch of material is processed, the membrane needs to undergo flux recovery cleaning.
(2) Continuous operation:
RO/NF systems typically operate continuously, and the parameters of each membrane element remain stable during operation, as shown in the figure below:
