difference between brackish water system and seawater desalination system

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25 Nov 2022

The difference between reverse osmosis brackish water system and seawater desalination system

Any membrane system is designed based on a predetermined set of parameters, such as influent composition, water temperature, permeate volume, and permeate quality. In actual operation, the system must have operational flexibility to meet changing conditions.

1. Brackish water system
Currently there is no strict definition for brackish water. In my country, some people call water with a salt content greater than 1000mg/L brackish water, and some people call water with a chloride content greater than 800mg/L or sulfate content greater than 400mg/L brackish water. Americans refer to surface water and groundwater with a salt content of 1500-5000mg/L as brackish water. Brackish water refers to water whose alkalinity is greater than hardness, contains a large amount of neutral salt, and has a pH value greater than 7. According to the provisions of my country's drinking water quality standards that the salt content should be lower than 1000mg/L, surface water and groundwater with a salt content greater than 1000mg/L should be called brackish water.

The correct way to operate the brackish water reverse osmosis and nanofiltration system is to keep the product water flow rate, recovery rate, total concentrated water discharge and operating pressure within the design range. Any change in membrane flux due to temperature or pollution needs to be adjusted. Inlet pressure to compensate, however, do not exceed the specified maximum inlet pressure, nor allow the membrane to hold too much dirt.

If the influent water quality analysis report changes, resulting in an increase in scaling tendency, the discharge of concentrated water from the system should be increased, the recovery rate of the system should be reduced, or other measures should be taken to meet the new system conditions.

The most common situation is that the water production rate of the water treatment system must be adjusted according to the needs. During design, the system scale is usually determined according to the peak water consumption. Therefore, the operation exceeding the designed water production cannot be adopted, and the adjustment of the system water production can only refer to reducing the system output.

When no water production is needed, the easiest way is to stop the operation of the system. However, the frequent start and stop of the system will affect the performance and life of the membrane. In order to obtain a relatively stable operation, a water production buffer storage tank can be designed; Pressure is another way to reduce the water production of the system. At this time, choosing a high-pressure water pump with adjustable speed can also save energy.

When reducing the permeate, if you want to keep the original recovery rate of the system unchanged, it must be calculated by the computer membrane system analysis software to ensure that the recovery rate of a single element will not exceed their limit. Sometimes during the operation of low permeate water flow, the system The salt rejection rate will be lower than the design water flow operating conditions, and you must also pay attention to ensure that the concentrate flow of the system exceeds the minimum concentrate flow when operating at low flow.

Another way to reduce the water production is to return the excess water to the raw water before reverse osmosis or nanofiltration, so as to ensure that the hydraulics and pressure of the membrane are basically constant, and the quality of the final product water will be improved. The returned product water has a certain cleaning effect on the membrane.

2. Seawater desalination system

In principle, desalination plants are tuned in a similar manner to brackish water, but the maximum operating pressure of 6.9MPa (1,000 psi, some systems may allow higher operating pressures) and the TDS content of the product water are often constraints .

When the inlet water temperature decreases, it can be compensated by increasing the operating pressure. Once it is close to the specified maximum operating pressure, the water production can only be reduced with the further reduction of the inlet water temperature; and the increase of the inlet water temperature can be achieved by reducing the operating pressure. To maintain the same system water yield, the system water TDS will rise at this time; another way is to reduce the number of pressure vessels put into operation, by reducing the effective membrane area, the inlet water pressure and the salt content of the product water can be kept constant, It must be calculated by computer membrane system analysis software to ensure that the maximum unit water production will not exceed the specified value, and the removed pressure vessel must be isolated from the system and properly preserved and maintained.

When the intake salt content increases, the operating pressure can be increased to make up for the decrease in water production, but it is not allowed to exceed the maximum membrane element allowable operating pressure, if the operating pressure is close to the upper limit, but the water production still does not meet the requirements, it can only be adopted to reduce the water production and system recovery mode, when the intake salt content is reduced, the operating pressure can be reduced accordingly, or the recovery rate can be increased. Or increase water production.

When the required water production drops, it can be solved by setting a large enough buffer water production tank. Large water treatment plants are usually designed into multiple sets of identical series, by adjusting the number of the number of series in operation to meet the user's demand for changes in water production.


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