Cost Considerations And Comparisons To Ro

The cost of resins is a major consideration. Heavy-metal-selective chelating resins are the most expensive. Table 4 provides some cost ranges for commercially

Table 4. Cost Ranges of Commercial Resins

Resin Type

Cost ($/f3)

Strong acid cation

70-120

Weak acid cation

150-200

Strong base anion

180-250

Resin Type

Cost (S/f3)

Weak base anion

180-200

Chelating cation

330-600

The technology that competes with ion exchange in wastewater application is reverse osmosis (RO), therefor it is appropriate to make some comparisons. Direct cost comparisons are not straightforward, and requires comparison of some of the hidden cost parameters. Since there appear to be few detailed comparisons in the open literature, there exists the general impression that RO is more economical than ion exchange. Whereas this may be true in a number of applications, as a general rule this is not the case.

The following are factors that should be taken into consideration when making case-specific cost comparisons between the two technologies. First, ion exchange is generally run batch, whereas RO is essentially continuous. This implies a higher degree of operator attention for ion exchange. However, one must remember that RO membranes must be cleaned, and this can be frequently depending upon the treatment application. Furthermore, although ion exchange systems are discontinuous, commercial systems are fully automated, and hence operator attention can be brought to a minimum.

A second consideration is that RO tends to be sensitive to incoming suspended matter. Comprehensive and sometimes expensive pre-treatment technologies are generally needed with RO, whereas ion exchange is less sensitive to the suspended matter. Further, RO systems are sensitive to hardness, so that softening is usually required as a pre-treatment. As a rule, RO membranes cannot handle high silica waters.

RO systems are quite sensitive to certain temperature ranges. Most critical is the range from 25 to 15° C, where RO systems have been reported to loose up to 30 % performance. RO has increased salt passage if the temperature increases, whereas ion exchange is insensitive to temperature variations. With the new generation of high performance resins, ion exchange can be kept fairly small using short operating cycles, and regeneration utilization approaches stoichiometric theoretical values. This translates into lower running costs.

Between the two technologies, ion exchange can be thought of as more within the arena of a pollution prevention technology. We can state this because of the relative recoveries or yields. If we take a boiler application as an example, with ion exchange, the difference between "net throughput" (the water you produce for the boiler) and "gross throughput" (the water you consume is minimal. You simply need a few cubic meters for the dilution of regenerants and for rinse. Typically, for a medium TDS water, the wasted water is about 5 % or less. With TDS and older co-flow regeneration systems, it can reach or exceed 10 %. In comparison, with

RO, only about 70 to 75 % of the water that is pumped into the system can be recovered. RO rejects large volumes of concentrate. Ion exchange removes all ions down to extremely low residuals. It does not remove non-ionic species, however (only partially). In contrast, RO removes all compounds based on their size. Very small ions or molecules, such as Na, CI, C02 are only partially removed. Other ions (Ca, S04) are harmful to the membrane. Globally, RO is a partial demineralization process, wehreas complete demineralization can be achieved with a simple ion exchange plant. To achieve the same salt residual as obtained with a simple ion exchange plant, a double-pass RO system is needed, and is considerably more expensive. The company Rhom and Hass (go to www.rohmhaas.com for details) has done a cost comparison between several design cases for the two technologies. Although Rhom and Haas favors ion exchange because they are marketing their technology, the comparisons are well done, and clearly support a cost advantage ion exchange. The following is one cost comparison summary. In this case the plant specs are as follows:

• Daily water production = 91 mVhr

• Operating basis = 300 days per year

• Treated water specifications: Conductivity = 1 ftS/cm; Residula silica = 30 /ig/Liter as Si02;

• Feed water salinity = 6.54 meq/Liter (327 mg/Liter as CaC03)

The comparison is made against a simple ion exchange plant versus a single pass RO system. The following is the overall cost estimate comparison, followed by a breakdown of the individual cost item details, including operating and energy costs. In reviewing these comparative costs, you see that there most definitely a significant cost savings in favor of ion exchange over RO, and given the water conservation advantage of ion exchange over RO, clearly it is more within the arena of pollution prevention technologies. This is not necessarily the case for every situation, and a detailed cost comparison for any investment needs to be made on a case-specific basis. When you visit Chapter 12, some of the principles for cost estimating and making technology comparisons are discussed.

Table 5. Overall Investment and Operating Cost for Design Case Study

Investment Cost

Ion Exchange

Reverse Osmosis

U.S. $

840,000

1,020,000

Operating Costs (Refer to Table 6 for detailed breakdown)

U.S. $/m3 treated water

0.442

0.690

U.S. $ per annum (330 day basis)

241,330

376,500

Table 6. Unit Cost Details for Design Case Study. (Note: Usage is expressed per m3 produced water, and Cost is in U.S. $ per m3 produced).

Detailed

Cost Estimate

Unit Cost

Ion Exchange

Comments

RO

Comments

Component

U.S. $

Usage

Cost

Usage

Cost

Energy, kWh

0.06

0.25

0.015

0.99

0.059

H2S04, kg

0.11

0.40

0.044

0.02

0.002

for MB

NaOH, kg

0.56

0.15

0.084

0.06

0.032

for MB

Neutralization, kg

0.11

0.00

0.000

Self neutra 1

0.15

0.017

forRO

Anti-scale, kg

3.89

0.00

0.000

0.01

0.019

5 ppm

Extra Water, m3

0.26

0.03

0.008

Dilution and rinse

0.45

0.117

Rejected

Wastewater, m3

0.33

0.03

0.011

Spent reg.

0.45

0.149

Concentrate

Resin Replacement

0.020

0.003

for MB

Membrane Replacement

0.000

0.031

Subtotal

0.182

0.430

Net feed water, m3

0.26

1.00

0.260

1.00

0.260

Total

0.442

$/m3

0.690

$/m3

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