How Engineering Decisions Shape Sustainability

While reading an interview with Mr. Ahmed Bin Shafar, CEO of EMPOWER, one idea stood out clearly:
sustainability in district cooling is achieved through operational efficiency, not slogans.

One of the most influential — and often misunderstood — operational decisions in district cooling plants is how cooling tower water cycles are managed.
This article explains the concept technically, clarifies common misconceptions, and shows how technologies such as reverse osmosis (RO) dramatically improve water efficiency at district scale.

1. Why Cooling Towers Dominate Water Consumption

In a typical district cooling plant:

Chillers consume electricity
Cooling towers consume water

Field data consistently shows:

70–80% of total plant water consumption occurs at cooling towers
Evaporation is unavoidable (it is how heat is rejected)
Blowdown is the controllable loss

Therefore, any meaningful water-saving strategy must focus on cooling tower operation.

2. What Are “Water Cycles” (Cycles of Concentration)?

Definition

Cycles of Concentration (COC) describe how much dissolved material accumulates in cooling tower water compared to the incoming makeup water.

$\boxed{ \text{COC} = \frac{\text{TDS (or conductivity) of circulating water}}{\text{TDS of makeup water}} }$

COC=TDS of makeup waterTDS (or conductivity) of circulating water

Physical meaning

Evaporation removes pure water
Dissolved salts remain
Their concentration increases
Blowdown limits this increase

3. The Most Common Misconception

“3 cycles means the system water is replaced every 3 cycles”

This is incorrect.

Reality:

The circulating water remains in the system
A small portion is continuously or intermittently discharged
Makeup water replaces only what is lost

A cooling tower operates on controlled bleeding, not batch replacement.

4. Cooling Tower Water Balance (Fundamental)

Every cooling tower follows this balance:

M=E + D + B

Where:

M = Makeup water
E = Evaporation loss
D = Drift loss
B = Blowdown

Drift is typically very small in modern towers (≈0.001–0.005%), so water optimization focuses on E and B.

5. Relationship Between Cycles and Blowdown

Blowdown is inversely related to cycles:

$\boxed{ B = \frac{E}{\text{COC} – 1} }$

This single equation explains why higher cycles mean lower water consumption.

6. Worked Example — District Cooling Plant

Assumptions

Cooling tower evaporation: 100 m³/day
Drift ignored for simplicity

Case 1: 3 Cycles (Common Practice)

$B = \frac{100}{3 – 1} = 50 \text{ m³/day}$ $M = E + B = 100 + 50 = 150 \text{ m³/day}$

M=E+B=100+50=150 m³/day

Case 2: 10 Cycles (Advanced Practice)

$B = \frac{100}{10 – 1} = 11.1 \text{ m³/day}$

B=10−1100=11.1 m³/day $M = 100 + 11.1 = 111.1 \text{ m³/day}$

M=100+11.1=111.1 m³/day

Result

Daily saving: ~39 m³
Annual saving: ~14,000 m³ (per tower)

At district scale, this becomes a strategic sustainability lever, not a marginal gain.

7. Why Many Plants Stop at 3 Cycles

The limitation is not philosophy, but water chemistry.

Typical GCC makeup water:

TDS: 400–800 ppm
Calcium hardness: high
Silica: 20–40 ppm
Chlorides: elevated

At 3 cycles:

TDS ≈ 1,200–2,400 ppm → manageable

At higher cycles:

Scaling risk increases rapidly
Silica becomes the dominant constraint
Chemical treatment alone becomes inefficient

Thus, many plants remain conservative.

8. Where Sustainability Actually Improves

True sustainability improvement does not come from:

Adding more chemicals
Increasing blowdown
Accepting water waste

It comes from:

Reducing the amount of dissolved solids entering the system

This is where Reverse Osmosis (RO) plays a critical role.

9. How Reverse Osmosis (RO) Works (Practically)

Reverse osmosis uses semi-permeable membranes to:

Allow water molecules to pass
Reject 95–99% of dissolved salts

RO produces:

Permeate: low-TDS water (used as makeup)
Reject: concentrated brine (disposed or reused)

Important clarification:

RO treats makeup water, not circulating tower water.

10. Why RO Enables Higher Cycles

Without RO

Makeup TDS = 600 ppm
Max allowable circulating TDS = 1,800 ppm

$\text{COC}_{max} = \frac{1800}{600} = 3$ COCmax=3

With RO

Makeup TDS = 50 ppm
Same allowable limit

$\text{COC}_{max} = \frac{1800}{50} = 36$ COCmax=36

In practice, limits such as silica and corrosion reduce this to:

8–12 cycles reliably
15+ cycles in optimized systems

11. Water Saving vs RO Cost (Conceptual)

RO adds:

Capital cost
Energy (≈1–2 kWh/m³)
Membrane maintenance

But it reduces:

Makeup water cost
Blowdown disposal
Chemical consumption
Corrosion and scaling risks

At district cooling scale, RO systems typically achieve short payback periods, especially where:

Water tariffs are high
Sustainability targets are mandated
Plant life exceeds 25–30 years

12. Typical District Cooling Strategies

Level	Strategy	Typical Cycles
Basic	Chemical treatment only	3–4
Improved	Side-stream filtration	5–6
Advanced	RO makeup	8–12
Best-in-class	RO + ZLD	12–20

13. Final Takeaway

Water cycles are not a chemical setting.
They are an engineering decision with long-term consequences.

At district scale:

Small cycle increases → massive water savings
Operational efficiency → measurable sustainability

This is how sustainability becomes real — exactly the philosophy highlighted by leaders such as Mr. Ahmed Bin Shafar in district cooling operations.

climalogic.net