Situation

A new hospital construction in Ontario requires the installation of two 3500 LB/hr, gas fired steam boilers for equipment sterilization purposes. Make up rate is set at 95% and full load operation would be approximately 16 hours per day. Our in house water analysis revealed conductivity to be at 610 μS/cm and M alkalinity at 246 ppm as CaCO₃. A rule of thumb in high conductivity waters is to immediately steer towards Reverse Osmosis (RO). However, the purpose of this study is to technically and financially compare two boiler treatment approaches in order to understand the key differences that will influence the choice of the system to be installed. The consulting engineer will hence be better informed and at ease in discussing the two processes.

Study Objectives

The following objectives were pursued:

• Expose and underline differences in capital expenditure between the two processes.
• Expose and underline differences in operating costs between the two processes.
• Expose and underline differences in maintenance costs between the two processes.
• Present a financial analysis.

 

The need for these processes

The concentration of minerals in boiler feed water will determine the blow down rate. Total dissolved solids and alkalinity are key parameters in evaluating the number of times water can be concentrated (concentration cycle) in the boiler before it needs to be purged. The main problem in high conductivity waters is the energy consumption and new water demand (energy losses) associated with high frequency blow downs. In this particular case, using only softened water, the blow down rate would be 33.9%, alkalinity being the critical factor. The following table illustrates the effects of an additional dealkalization (DA) or RO treatment to the feed water.

Water treatment	Concentration Cycle (ratio)	Blow down %
None	2.85	33.9
Dealkalization	5.74	16.6
Reverse Osmosis	56.9	1.67

From this table, we can already appreciate the impact of RO pre treatment on this type of water.

The price to pay for performance

The following table presents the capital expenditure for both water treatment options.

Water treatment	Required equipment	Capital cost ($)
Dealkalization	Sodium cycle dealkalizer Water softener	33 645
Reverse Osmosis	Carbon filter Reverse osmosis machine Water softener Distribution system	72 694

Differences in capital costs are caused by the extra carbon filter required for chlorine removal prior to the RO machine and the increase in softener size due to the membrane permeate recovery rate (60% approx.).

Operating and maintenance

To properly evaluate two investment projects, we must absolutely consider the full yearly operating and maintenance costs inherent to both systems.

Expenses	Dealkalization ($/year)	Reverse osmosis ($/year)
Electrical	32.36	3432.53
Water (potable and waste)	5 328.00	8 069.00
Salt et chemicals	25 609.00	14 858.00
Natural gas	28 517.00	2822.00
Maintenance	1 116.00	14 500.00
Total	60 602.00	43 681.00

Average operating and maintenance costs are 39% higher for DA than RO mainly because of the natural gas consumption associated with the lower concentration cycle. Salt and chemical consumption also represents a high percentage of operating costs at 42%. However, maintenance costs for RO are significantly higher than that of DA.

Financial analysis

We have chosen to present a NPV calculation to evaluate the best value between both treatment options.

Basic assumptions:

Study horizon:  15 years
Return Rate: 15%
Amortization rate: 4% (source: revenue Canada)
Income tax rate: 40%
We must also mention that the equipment is not to be sold at the end of the study horizon.

NPV calculations yield the following results:

Sodium cycle dealkalization:     NPV= -381 593.00$
Reverse Osmosis:    NPV= -321 329.00$

The negative value only indicates that this project does not generate any operating gains. In light of these results, reverse osmosis is the better investment based upon the NPV financial measurement test.

Conclusion

In this case study, reverse osmosis won the battle due to savings in operating costs areas.
However, this may not always be the case for the same water supply. In fact, the relation between RO machine pricing and flow rate is not linear. Also, RO recovery rates may not always be as conservative, leading to an increase in water consumption costs that could advantage the DA method. It is good practice to perform a detailed comparative analysis to confirm the best suited process. Furthermore, it is also essential to determine what other water requirements there are on a project in order to optimize water conditioning equipment. This might also influence the choice of equipment to use and may overrule certain financial analysis test methods.