The decision to install an ion exchange (IX) system or reverse osmosis (RO) system is contingent on several economic factors including capital and operating costs as well as chemical and wastewater disposal requirements.
As a manufacturer of both technologies, Dow can provide an objective view without bias for one technology over another.
To aid with the decision-making, there are several factors to consider when determining an economic comparison between IX and RO. The economic factors affecting a break-even analysis include chemicals, resins, membranes, energy, operating labor, maintenance, and capital related items.
Occasionally, technical considerations will outweigh the economics in determining the water treatment system. Such technical factors may include the size of the system, the water quality and flow rate.
Whether the deciding factor boils down to economic or technical considerations, you must determine the acceptable number of years for payback.
Ion Exchange or Reverse Osmosis?
The first question that must be addressed in the design of a new water treatment plant is whether to install an ion exchange system or reverse osmosis. The principal drivers for such a decision will be economic in terms of capital and operating costs, as well as regional requirements for chemical and waste water disposal. In many cases, familiarity with one or other technology is also a factor in the decision process.
Comparison between these two technologies as options for water treatment applications has been the subject of a number of studies and continues to generate a high level of interest in the literature1. As technical developments in both product areas continue to be made, such as counter-flow regeneration packed bed systems, narrow particle sized ion exchange resins and high rejection, lower energy membranes, there is a need for monitoring economic performance. In addition, external factors such as water costs and disposal, power and chemical costs continue to change and are different around the world, further affecting the economics.
Economics of Ion Exchange and Reverse Osmosis
Studies carried out using a cost model which includes capital investment and depreciation, chemical, utility and water costs have been made over a range of water salinities and service run-lengths with water analyses based on averaged compositions from a number of locations.
Two different types of water treatment systems were used: ion exchange only (IX) and reverse osmosis followed by ion exchange polishing mixed beds (RO-IX). The systems were sized to continuously produce mixed bed quality water (<1 µS/cm and 10 ppb SiO2 ) at flow rates of 50 and 200 m3 /hour net (220 and 880 gpm). Operating costs include chemicals, power, labor and maintenance, together with water and waste water, which are an increasing consideration in water treatment economics 3,4. Surface water was used for both size plants with pretreatment consisting of flocculation,clarification and sand filtration. Acid and antiscalant were dosed prior to RO and 5 micron filters were used. Water storage facilities and the cost of neutralizing the waste water were also included together with the disposal costs of the waste effluent. Labor costs were not assumed to vary across the options studied and are based on 1.5 man years. Annual maintenance costs were included but not the cost of land, buildings or taxes.
The IX plant considered was a packed bed counter-flow regenerated design consisting of 2 x 100% streams with cation-degasser-layered bed anion-mixed bed polishers containing uniform particle sized resins. Regeneration with both H2SO4 and HCl were evaluated, although it was found that the cost per unit of treated water were similar for both acid regenerants, as the increased chemical efficiency of HCl and lower resin inventories are off-set by the higher cost of the chemical. Only the economics using H2SO4 are therefore reported.
The RO system consists of 1 x 100% line with RO-degasser-mixed bed polisher for 50 m3 /hour (220 gpm) and 2 x 50% lines for 200 m3 /hour (880 gpm). A system recovery of 80% was used. The mixed bed design was the same as for the IX system.
The results of the calculation indicate that the cost to produce water using only ion exchange increases with feed TDS as expected, principally due to regenerant chemical costs. Although the regenerant costs increase proportionately, the effect of increasing plant size lowers the cost to produce water, since the capital, raw water, labor and maintenance costs are relatively lower for the larger plant.
The costs for IX vary between $0.5-0.7/m3 U.S. ($1.9-2.6/1000 gal) at 50 m3 /hour (220 gpm) and $0.25-0.45/m3 ($1.0-1.7/1000 gals) at 200 m3 /hour (880 gpm). At the lower flow rate, operating costs account for ~70% of the total cost with regenerants, raw water, labor and maintenance making the most significant contributions. At 200 m3 /hour (880 gpm), operating costs increase to ~80%.
The cost of producing water using RO/IX is also dependent on feed TDS, but much less so than for the IX system, due to the fact that the main cost contributors (power, water, labor, maintenance and capital) are relatively constant over a range of water salinities.
RO/IX costs are $0.6 and 0.4/m3 ($2.3 and $1.5/1000 gals) for the 50 m3 /hour (220 gpm) and 200 m3 /hour (880 gpm) plants respectively. Operating costs are 72-80% of the total cost for the two plant sizes.
The salinity break-even point for the two technologies was found to be between 7 and 8 eq/m3 (350-400 ppm as CaCO3 ) TDS for the two flow rates. It should be emphasized that these break-even points are derived from one set of assumptions, so sensitivity studies were also made to assess the effect of the changes in the cost of power, chemicals and water on the economics.
Varying power costs over the range of $0.05-0.16/kWh resulted in a change in the cost to produce water of ±$0.04/m3 ($0.15/1000 gals) for the RO/IX system compared to the base case above, thereby affecting the break-even point with IX by ±1.5 eq/m3 (± 75 ppm as CaCO3 ).
The sensitivity of caustic regenerant price over the range $200-400/ton on IX economics yields a break-even point change of ± 1.2 eq/m3 (± 60 ppm as CaCO3 ).
Finally the effect of varying low cost raw water/effluent (e.g. surface water) was considered and this had a marginal influence on the breakeven point of ± 0.6 eq/m3 (± 30 ppm as CaCO3 ). If, however, mains water is taken or the cost of effluent treatment is expensive, the cost of RO/IX vs IX increases and the break-even point is above 10 eq/m3 (500 ppm as CaCO3 ).
This economic evaluation considers the major factors contributing to the total cost of treated water by RO/IX and IX. The effect of system size and the latest technology in both resins and membranes has been included. The main conclusions from this study are summarized below:
- The break-even point above which it is more economical to use RO/IX versus IX alone is 7-8 eq/m3 (350-400 ppm as CaCO3 ). This is higher than earlier studies1,5,6 and reflects developments in packed bed counter-flow regenerated IX systems compared to co-flow systems and also regional differences in power costs.
- Chemical costs for IX and electrical power costs for RO are the most important operating expenses and those that need to be carefully considered in the decision for a new plant.
- Although capital has a significant effect on the total cost of water for all options considered, operating costs represent the major portion at 70-80% of the total.
- This study considers mainly surface water and low cost discharge of effluent from the water treatment plant into a river. More expensive water sources (e.g. mains) will have a higher impact on RO costs, unless the concentrate from the RO plant can be used elsewhere on site.
1. See for example, A.F. Ashoff, UltraPure Water, July/August 1995 p. 39
2. P.A. Newell, S.P.Wrigley, P.Sehn & S.S.Whipple, Proceedings of SCI Conference IEX '96, 15. July 1996
3. VGB-Kraftwerkstechnik GmbH literature, May 1995
4. K. Grethe and C. Beltle, "Power station make-up water using RO and ion exchange for demineralisation" Steinmuellertagung 1993
5. S. Beardsley, S. Coker and S. Whipple, Watertech Expo '94, 9. Nov 1994
6. S.S. Whipple, E. Ebach and S. Beardsley, UltraPure Water, October 1987
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