Ultrapure water

Moreover, for each of the above parameters, the permissible values ​​lie in the range of one / billion or one / trillion. It becomes obvious that the use of simple pre-filters, in this case, is simply impossible.

These requirements are presented for a reason. As you know, micro and nanoscale functional devices are made of silicon or other (for example, glass or sapphire) plates by carrying out a sequence of technological operations. More than half of these operations include chemical modification or surface treatment of the material, as well as washing processes. In all these cases, ultrapure water is required that does not contain insoluble particles, ions, and other foreign inclusions. Otherwise, falling onto the device of such particles, the sizes of which can vary from hundreds of nanometers to tens of microns, can lead to its complete inoperability.

That is why when designing FMN Laboratory premises, we paid special attention to the process of preliminary water purification. Together with the leading German manufacturer of ultrapure water supply systems, we made an analysis of the tap water entering the building, calculated the load and water consumption at all technological sites, and designed a mini plant providing multi-stage water filtration and recirculation.

This system is a five-stage closed loop of sequential circulation and water purification, which occupies a separate room and works 24/7. Any short-term shutdown of water circulation in an ultrapure circuit can adversely affect such water parameters as pH and conductivity, however, due to the inherent flexibility of the system, after short shutdowns of the system, it will return to the set mode within a few minutes. In the case of a longer stop, from several hours or more, irreversible consequences are possible, such as the formation of bacteria and particles, which will lead to the need for a complete stop of the system and its disinfection and cleaning. That is why it is extremely important to ensure uninterrupted operation of the water treatment system.

Among the main requirements for the system are the following:

• Productivity: 1,2 m³ / h (20 l / min),
• Pressure: 5 +/- 1 bar,
• Temperature: 22 +/- 1 ° C,
• Resistivity: 18.2 MΩcm,
• Total organic content: 5 ppb (parts per billion),
• Dissolved silicic acid: 3 ppb,
• Dissolved oxygen: 25 ppb,
• Solid particles:

- Size of 0.1-0.2 microns - not more than 1000 particles / l,

- With a size of 0.2-0.5 microns - not more than 500 particles / l,

- Size of 0.5-1 microns - not more than 50 particles / l,

• Various requirements for the content of a wide range of materials (ammonium, aluminum, boron, barium, calcium, chromium, copper, iron, potassium, lithium, magnesium, manganese, sodium, nickel, mercury, strontium, zinc, bromides, chlorides, fluorides, nitrites nitrates, phosphates).

Pre-treatment serves to treat water in such a way as to improve its quality to the level necessary for its purification in subsequent stages. When filtering water through a multilayer filter, pollutants and organic compounds are retained in the filter layer. Filtered water moves into the filtrate tank for storage before it is fed to the subsequent technological stages of purification.

The ultrapure water preparation system includes the following technological stages of purification:

• Preliminary processing
• Preparation of the first stage and membrane degassing
• Preparation of the second stage
• Final processing
• Distribution loop (ultrapure water distribution).

At the stage of preparation of the first stage, using the filtrate pump, filtered water is supplied from the filtrate tank to the reverse osmosis unit of the first stage. Reverse osmosis reduces salinity from about three to 5% of the original value. In addition, when treating water in a reverse osmosis unit, a decrease in the content of TOC (total organic content) also occurs, since some of the substances forming TOC cannot pass through the reverse osmosis membrane.

On the second stage, in the installation of membrane degassing, dissolved oxygen and carbon dioxide are removed from the water. After the membrane degasser and dosing of alkali, water enters the second stage reverse osmosis unit through a high-pressure pump. The water purification process at the reverse osmosis unit of the second stage proceeds in the same way as when using reverse osmosis of the first stage, except that the water entering the second stage is purified more than 50 times earlier using the main set of impurities and salts.

For the required constant water quality, both stages of treatment (first and second) must work continuously with a constant flow rate. Pure water that is not used in the subsequent purification step (finishing) is returned to the filtrate tank. The combination of reverse osmosis and electrodeionization processes ensures the production of water of the required quality.

At the final stage of processing, water quality is brought to the required levels.

The distribution loop supplies consumers with ultrapure water and consists of after-treatment systems for particles larger than 20 nm, PVDF pipelines (Teflon), temperature control and membrane valves. The ultrapure water pump delivers ultrapure water through a finishing unit to a constant quantity dispenser.