Reducing A Data Center’s Cooling Tower Operating Costs with Ultra-High Recovery Reverse Osmosis

In 2019, Aquatech was approached to consult for a US-based Data Center with high annual water usages of approximately 100 million gallons per year. Cooling tower optimization had previously been limited due to high sulfate (SO4) and chloride (Cl) levels in the city water makeup The makeup water was known to change frequently throughout the year. Due to these factors the plant was limited in cycles of concentration (approx. 3 cycles) in the cooling tower. Our goal was to increase cycles of concentration

The original system characteristics:

• • • The cooling tower used 200 gpm of city water as a make-up.
    • 69 gpm of cooling tower blow-down was sent to the city wastewater treatment plant for disposal.
    • The cooling tower was limited to 3 cycles due to high sulfate & chloride in city water make up.

Figure 1 – Existing System

Table 1. City Water Analysis

Aquatech evaluated water usage, water cost (make-up water + blow down disposal), water chemistry, discharge permit parameters and found that lowering the chloride and sulfate concentrations from city water would enable cooling tower to operate at high cycle of concentration.

High cycle of concentration in cooling tower benefits include:

• • • Less make-up water for cooling tower
    • Less chemical dosing
    • Less cooling tower blow-down and discharge cost

The challenge was now to select a treatment system that met the following criteria:

• • • Minimal wastewater generation while reducing chloride and sulfate from city water to the lower levels.
    • Small footprint since facility didn’t space available for more equipment.
    • Pre-assembled to minimize implementation/installation costs and allow the client to start saving as soon as possible.
    • Easy to upgrade for future expansion.
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• • • ARRO™ also uses osmotic cleaning to reduce RO membrane scaling potential
    • The ARRO™ system generates low volume of wastewater since it operates at 20%+ permeate recovery in comparison to traditional RO.
    • The system is pre-assembled and compact
    • Due to its modular nature, ARRO™ capacity can be upgrades byadding a new skid in parallel.
      
      

Figure 2 – City Water Treatment by Aquatech’s ARRO™ and Blending

As you can see below, the ARRO™ treated city water is much lower in total dissolved solids concentration which would increase the cooling tower COC from COC= 3 to COC = 6.

Table 2. Treated Water Quality from ARRO™

Doubling the cooling tower Cycles of Concentration with ARRO™ treated city water would bring down overall cooling tower operational cost. Subsequently, the make-up water use was reduced from 200 gpm to 169 gpm, and the blow-down waste from 69 gpm to 38 gpm.

The following table compares the annual cost difference with & without city water treatment for the facility.

Table 3. ARRO™ Savings Comparison

The above costs are based on the following values: City water cost $2.92 per 1000 gallons and wastewater disposal cost $5.59 per 1000 gallons.

The cost of city water is expected to continue to increase in the coming yearsmaking the proposed project even more valuable . As you can see from Table 3 above, the water savings will quickly pay back the cost of the equipment. Some additional context on the proposed ARRO™’s operating characteristics are included below. The ARRO Opex to produce 1000 gallons of water is $ 0.57, hence the annual operating cost of ARRO @ 115 gpm flow is $ 34,453.

Table 4. ARRO™ System Characteristics

After adding the ARRO opex to the city water treatment & disposal cost, the actual cost with ARRO treatment option would be $369,400 + $34,453 = $403,853. Currently, the plant spends $509,600 annually on the city water & disposal cost, so total annual savings with ARRO treatment would be $105,747. Considering the CapEx for a 115 gpm ARRO system, the time for full Return On Investment here was projected to be less than 3 years. Opportunities like these have given us valuable experience for future high value water reuse solutions in an increasingly resource-conscious world.