CALCULATING THE COST OF DI EXCHANGE TANK OPERATION

Gaining Pure Water Production Efficiency Through Reverse Osmosis Application

Recently, industrial facilities have been attempting to streamline production and reduce operating costs by limiting employee responsibilities to specific tasks and removing some non-job related chores from their already full work load. Today, sophisticated production and plant maintenance work is more likely to be contracted out to specialized service organizations who have the tools, manpower and expertise to perform certain service tasks.

In-plant environmental issues are also effecting the industrial workplace. The typical industrial facility is being carefully kept free of hazardous chemicals where ever possible in order to safe guard employee health and avoid frequent O.S.H.A. inspections. Solid and liquid waste streams are being minimized, or totally eliminated, in an effort to reduce sewer and hazardous waste transportation fees and the likelihood of receiving costly fines and public relation problems for discharge infractions.

Years ago, deionized water systems, using acidic and caustic chemicals for regeneration, were considered to be the only way to produce mineral free water for many industrial processes. The fact that they generated a significant waste stream and required considerable maintenance were generally overlooked. Today, many facilities hire service contractors who provide off-site, transportable deionization columns. Although this service adds to the cost of the deionized water, it is usually felt that the service is beneficial in order to reduce maintenance responsibilities from employees and to remove the inherent problems with on-site chemicals.

Portable exchange tank deionizers are less expensive to purchase than on-site regenerable deionized water systems. There is typically however a significant long-term cost burden for the facility in off-site regeneration costs and occasional DI water outages. In order to properly evaluate a deionized water system performance enhancing, or totally alternative, technology, it is necessary to develop a formula which calculates the cost of operation of the portable DI tank operation. Since the deionization tanks contain a resin which purifies a finite volume of water before needing to be regenerated, it is useful to develop the formula to determine the cost of deionized water per gallon.

TYPICAL PORTABLE DI TANK VOLUMES

CALCULATION EXAMPLE:

Feed Water Conductivity: 250 microSiemens/cm (as measured by a field conductivity meter)

Portable DI Tank Dimensions: 9" diameter x 48" height

Regeneration Cost For Each Tank: $ 250.00

First, the available data must be put in a form which is useable by our formula.

To obtain the feed water TDS in parts per million, use the conductivity-TDS relationship from above:

TDS = 250 microSiemens/cm x .667 = 166.75 ppm

Use the tank volume table to determine that the 9" x 48" diameter tank contains 1.7 cubic feet of resin.

Now the regeneration cost per cubic feet of resin (RC) can be calculated using Formula 1.2:

RC = $ 250.00 / 1.7 cubic feet

RC = $ 147.06 per cubic foot

Finally, the cost per gallon of DI water (Q) can be calculated using Formula 1.1:

Q = 166.75 x 147.06 / 171,000

Q = $ .1434 per gallon

If it is assumed that the process requires 1000 gallons of DI water per day, five days per week, the yearly cost of portable exchange tank DI water is calculated as follows:

COST PER YEAR = Q x V x D

COST PER YEAR = $.1434 x 1000 x 260

COST PER YEAR = $ 37,285.30

As can be seen from this example, portable DI exchange tank operating costs can be significant. In order to reduce the on-going maintenance costs of operating portable exchange tank deionizers, either the volume of water used, or the TDS of the feed water needs to be reduced. Water conservation can be evaluated for feasibility and potential cost savings. To allow for economical use of deionized water in all applications critical to production, a method of reducing the actual cost per gallon of deionized water is recommended.

The usable capacity of a portable tank deionizer depends heavily on how much dissolved mineral (TDS) is in the feed water. A feed water with high levels of TDS will exhaust a deionizer more quickly than a feed water with low TDS. The formulas presented above will be used to demonstrate this fact in the following example.

A significant amount of TDS can be removed, prior to water flowing through a deionizer, by employing a membrane separation process known as reverse osmosis. HydroMax water purification equipment uses this technology to generally remove about 98% of the feed water TDS, greatly reducing loading and drastically extending service run time of portable exchange tank or on-site regenerable deionization systems. The HydroMax water purification equipment functions so efficiently that any form of deionization resin treatment can be eliminated for many applications. Reverse osmosis purification will also provide a much better quality process water, free of particulate and low in organic material, than typical filtered, deionized water.

Using the same data as was used in the preceding example, the cost per gallon of deionized water with reverse osmosis pretreatment can be calculated. The TDS of the feed water after being processed by the HydroMax system can be calculated as follows:

TDS (R.O.) = TDS (raw water) x Percentage Of Ions Rejected By R.O.

TDS (R.O.) = 166.75 ppm x .98

TDS (R.O.) = 3.33 ppm

Therefore, using Formula 1.1:

Q (R.O.) = 3.33 ppm x 147.06 / 171,000 = $ .00286 per gallon

And if 1000 gallons of DI water are used per day, five days per week:

COST PER YEAR (R.O.) = $.00286 x 1000 gallons x 260 days

COST PER YEAR (R.O.) = $ 744.59

This is a $36,540.71 savings when using HydroMax reverse osmosis if compared with the initial DI exchange tank only example.

HydroMax has used our reverse osmosis technology to reduce the feed water TDS levels in applications using on-site and portable exchange tank deionization systems. This method of water purification is extremely effective when a facility is striving to reduce operation and maintenance costs. Further savings are experienced when reverse osmosis quality water is acceptable without using any exchange tank deionization process. HydroMax has field proven reverse osmosis technology as a viable, quantifiable means of reducing TDS levels by up to 99%. A HydroMax reverse osmosis system can easily be integrated into an existing deionization system. Pay-back periods for the addition of a HydroMax reverse osmosis system have proven to be typically short and very favorable.