Performance comparison of mixed bed and EDI

1. Working principle

Mixed bed
During the effective exchange period of the mixed bed, the effluent water quality is stable, and its resistivity can reach 14MΩ. Once the failure point is reached, the conductivity will rise sharply, and the effluent water quality will also be unstable. Since the exchange cycle is affected by factors such as the operating level of the operator, the quality of the regenerant, the quality of the pretreated water, and the quality of the resin itself, there are factors in which the effective cycle time is uncertain. Therefore, there are at least two mixed beds in the reverse osmosis + mixed bed system, one used and the other prepared to reduce the risk of sudden failure of the mixed bed.

EDI
Also known as continuous electrode ionization (EDI, Electro deionization or CDI, continuous electrode ionization), it combines two mature water purification technologies-electrodialysis and ion exchange. The dissolved salt is removed under low energy consumption conditions. It does not require chemical regeneration during operation, and its effluent resistivity is higher than that of mixed bed effluent, reaching 10-15MΩ.CM, which meets the national electronic grade water level I standard. EDI does not have too high requirements for the conductivity of the effluent of the first-stage reverse osmosis, and the conductivity of the influent is 4-12us/cm, which can be qualified for water production. It may be necessary to add a softening device, or increase the RO membrane scale inhibitor to remove calcium and magnesium ions in the water. If the conductivity is high, only need to adjust the size of the operating current and the dosage (sodium chloride). It is an environmentally friendly technology, the ion exchange resin does not need acid and alkali chemical regeneration, saving a lot of acid, alkali and cleaning water, Greatly reduce labor intensity. More importantly, there is no waste acid and waste lye discharge, which is a non-chemical water treatment system, which does not require acid and alkali storage and treatment, and no waste water discharge.

2. Operation Comparison

Mixed bed
The regeneration time of the mixed bed is relatively long, and a large amount of RO water needs to be used to flush the mixed bed qualified during regeneration. The operation of the mixed bed equipment is relatively complicated in the purified water system. From the beginning of the acid and alkali preparation to the final regeneration, at least two shifts and multiple people are required. The labor intensity is relatively high. At the same time, due to the mixed bed The shortening of the effective exchange period brings about frequent regeneration of the mixed bed, which further increases the labor intensity during regeneration. When the mixed bed is regenerated, the operator needs to come into contact with acid and alkali, which is a dangerous operation. In addition, although the operator wears labor protection equipment during regeneration, the personal safety of the operator is still dangerous. The validity period of the mixed bed after regeneration has a great relationship with the operator’s experience, work responsibility, and the quality of the acid and alkali used for regeneration. Since most of its operations rely on experience, it is inevitable that the mixed bed will be regenerated during the standby period. Things that are invalid and unusable. This may affect normal production.

EDI
EDI is composed of one or several modules with the same water output per hour. The EDI module can be opened or stopped according to the actual pure water usage. Manual operation is relatively frequent, but the operation is relatively simple. You only need to open the EDI inlet valve and the extreme water valve. And the concentrated water valve, and turn on the power supply and adjust the dosage (sodium chloride), electrolysis voltage and current according to the water quality at the same time, which requires a higher sense of responsibility for the operator.

3. Cost comparison

  1. Comparison of investment volume
    Compared with the mixed bed ion exchange facility, the initial investment of the EDI device is about 20% higher. However, from the point of view that the mixed bed requires acid and alkali storage, acid and alkali addition, and wastewater treatment facilities, post-maintenance, and resin replacement, the cost of the two is different. Around 10%. With the improvement of technology and mass production, the investment required for EDI devices will be greatly reduced. In addition, the EDI device is compact, and the required plant space is much smaller than that of a mixed bed
  2. Comparison of operating costs
    The operating costs of EDI devices include electricity consumption, water consumption, pharmaceutical costs, and equipment depreciation, eliminating the need for acid and alkali consumption, reclaimed water, wastewater treatment, and sewage discharge. In terms of power consumption, the EDI device is about 0.5kWh/t water, and the mixed-bed process is about 0.35kWh/t water. The power consumption cost of EDI will be slightly higher.
    In terms of water consumption, the EDI device has a high water production rate and does not require reclaimed water. Therefore, the operating cost in this respect is lower than that of a mixed bed. As for the mixed bed pharmacy cost and the EDI equipment depreciation cost, there is not much difference.
    In general, in terms of operating costs, the operating cost of an EDI device per ton of water is around 2.4 yuan, and the operating cost of a conventional mixed bed per ton of water is around 2.7 yuan, which is higher than that of the EDI device. Therefore, the cost of excessive investment in EDI devices can be fully recovered within a few years.
  3. Comparison of product water quality
    The EDI device is a continuous water purification process, so the water quality of its product is stable. The resistivity is generally 15MΩ•cm and the highest can reach 18MΩ•cm, reaching the index of ultrapure water. The water purification process of the mixed-bed ion exchange facility is intermittent. Just after being regenerated, the product water quality is relatively high, and the product water quality will gradually deteriorate with the extension of use time. Furthermore, it is necessary to regenerate the mixed bed. (Generally, the mixed bed regeneration cycle is about 25 days. With the extension of the resin usage time, the regeneration cycle will become shorter and shorter)

4. Analysis of advantages and disadvantages

advantage
Mixed bed

  1. Low initial investment in equipment
  2. Stable water quality
  3. Simple pretreatment requirements
  4. High water utilization rate

EDI

  1. Thoughtful stacking design
  2. Stable water quality
  3. No need for acid-base regeneration, no hazardous waste liquid discharge
  4. Continuous operation, simple operation
  5. Low operating cost
  6. Small footprint
  7. Easy to install and maintain
  8. High water utilization

Disadvantages
Mixed bed

  1. Low utilization rate of resin exchange capacity and high loss rate
  2. Discharge of hazardous waste liquid from acid-base regeneration
  3. Bacteria are easy to multiply in the bed
  4. There are many valves and complicated operation
  5. High operating weight and large occupied area

EDI

  1. Large initial investment
  2. High requirements for pretreatment

Disadvantages
Mixed bed

  1. Low utilization rate of resin exchange capacity and high loss rate
  2. Discharge of hazardous waste liquid from acid-base regeneration
  3. Bacteria are easy to multiply in the bed
  4. There are many valves and complicated operation
  5. High operating weight and large occupied area

EDI

  1. Large initial investment
  2. High requirements for pretreatment
EDI ultrapure water treatment

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