Understanding Makeup vs. Distribution Flow in High Purity Water Systems

In high purity water systems, the storage tank is more than a container. It is the dividing point between two distinct functions: makeup and distribution. These two flows are rarely equal and often operate on very different timelines.

Makeup refers to water produced and sent into the tank, typically by a reverse osmosis (RO) or RO/DI system. This process is steady, predictable, and often continuous or governed by tank-level controls.

Distribution, by contrast, is driven by demand. It may spike suddenly as equipment across several floors calls for water simultaneously.

This disconnect between production and usage is precisely why storage tanks are essential. They allow water to be made slowly and used quickly, creating a buffer that prevents overbuilding treatment capacity.

Satisfying Demand Over Time

To design a system that works, we need to understand the facility’s actual water usage. Does the operation need to produce 4,000 gallons in four hours, or is a 24-hour continuous fill sufficient for steady-state processes like humidification?

Sizing the RO system appropriately based on that usage window allows us to avoid oversizing. The system fills the tank over time, while the tank satisfies high-flow, short-burst distribution demands.

The Role of the Storage Tank

Tank sizing directly affects both performance and water quality. A widely accepted rule of thumb is:

Tank Volume ≥ 10 × Distribution Flow Rate (in gallons)

For example, a 120 GPM distribution system would require a minimum 1,200-gallon tank. Larger tanks are often used to provide reserve capacity or redundancy.

This sizing ensures:

  • Full tank turnover every 10 minutes
  • Thermal stability
  • Bubble control at the pump suction

Too small a tank can introduce heat and air into the loop, causing inconsistent pressure or pump cavitation.

Pipe Sizing and Flow Velocity

When water leaves the tank and enters the distribution loop, piping design becomes the next key factor. Flow velocity should be maintained between 3 and 5 feet per second (fps).

  • Below 3 fps: May support bacteria growth
  • Above 5 fps: Can cause high pressure drop and long-term wear on fittings and valves

Properly sized piping ensures the loop turns over regularly, minimizes stagnation, and supports long-term mechanical reliability.

Application-Specific Purity Needs

Each point of use may require a different water quality level. Examples include:

  • RO water: Often sufficient for humidifiers, autoclaves, and utility rinses
  • RODI water: Required for sterilizers, lab faucets, and critical POU locations where low conductivity and bacteria control are essential

Designing the system around these needs allows us to allocate purification resources efficiently without overdesigning.

Multi-Zone Distribution Scenarios

In multi-floor buildings, the distribution flow rate can be high even if makeup is modest. For example, a hospital or university may only need a few hundred gallons per day but require fast delivery to sterilizers, humidifiers, and faucets across multiple departments and elevations.

In these scenarios, the tank, pump, and piping must be sized for peak simultaneous demand, not just average daily consumption.

Conclusion

In high purity water systems, the storage tank represents the true handoff point between steady production and variable distribution. By:

  • Sizing tanks based on peak distribution flow
  • Maintaining proper flow velocity
  • Tailoring purity levels to application

…we ensure long-term system stability, efficiency, and performance – regardless of the facility’s size or complexity.