The design of water treatment systems often is based on the application of current solutions. Determining the best system at the time of installation can be done with information gained on site, or from experience harvested from other similar, successful applications.
In early 2013, an opportunity to take an existing technology and improve its application with new equipment arose at a manufacturing facility for thermal management products in Redwood City, Calif. One of the final manufacturing processes involves rinsing thermal tape products with ultrapure water. This water must have resistance measurements of at least 1 MOhm. This is not particularly difficult to achieve, and can be accomplished with a pass through a mixed-bed deionizer tank. Making the job somewhat easier are the relatively low levels of total dissolved solids (TDS)—120 to 160 ppm on average—in the Redwood City municipal water supply.
When the process line was built, a local water treatment dealership was consulted to install the necessary equipment. Its recommendation involved portable exchange tanks that it would replace periodically based on the usage of the manufacturing line. This system worked well in some key areas:
- Product water easily met quality standards, usually exceeding 2 MOhm.
- The cost per gallon of treated water was economical, averaging $0.23 per gallon (with 9 x 42 exchange tanks, 162,000 mg of exchange capacity, and $275 per replacement).
- Tank footprints were minimal.
- No onsite chemicals were required for regeneration of the beds.
The portable exchange system provided treatment for the process water for several years. Additional treatment included a carbon block prefilter to remove chlorine and trap any sediment. The demand of the line is light—approximately 15 gal per day—and the capacity of the exchange tanks led to long service runs. A monitor on the product line would indicate when the service company needed to be called for a replacement. The following disadvantages became apparent after the system was in use:
- Access to replace the tanks needed to be planned. This can be challenging in a secured facility, especially when the replacements occur with irregular frequency.
- Long run times and low demand rates can result in organic or microbial growth, especially in an environment free of chlorination.
Updating the System
When the contracted service provider changed hands, the facility manager sought possible alternatives to this system. A new water treatment device using capacitive deionization (DI) was recommended. The DI process involves the removal of ionized TDS by passing the water between positive (cathode) and negative (anode) charged electrodes. “Regeneration” is accomplished by reversing the charge on the electrode to flush, then returning to the service state when complete.
The lower power consumption of the new unit means that the removal of TDS is not complete, but can easily run at 75% to 90% rejection. The adjustable control on the capacitive DI unit provided an 80% removal. This setting produced water quality from the unit of 24 to 32 mg/L TDS, depending on the inlet water at the time. The treated water is stored in a hydropneumatic tank, which mixes to a consistent 30 mg/L. This resulting water quality is good, but does not meet the process requirement of 1 MOhm, so the water gets a final polish by a 20-in. Pentek PCF deionization cartridge.
The new cart-mounted system provides several benefits:
- The necessary replacement of both the prefiltration cartridges and the DI cartridge is controlled by onsite staff. No coordination with a service agent is necessary.
- The mobile treatment cart allows staff to move the system if additional floor space is needed or when the layout of the facility changes. Simple, quick connection to the system on the inlet and outlet minimizes the amount of time required to change plumbing when the system is moved.
- The continual regeneration of the capacitive DI unit does not involve the use of chemicals, and any flushed/excess water is collected. The excess water can be reused for purposes like watering outdoor landscapes.
- Pretreatment before the DI cartridge led to cartridge replacements every 90 days, based on the manufacturing schedule. This was easily indicated with the unit controller’s onboard service reminder.
Sometimes we find breakthrough technologies that can change the way that we treat water. Oftentimes we can find a better way to apply the technologies we have used for many years to improve the effectiveness of the treatment. When technologies are combined, we can develop new solutions to better fit the needs of customers.
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