How THM Removal Works in Water Storage Tanks

A Practical Guide for Utilities

Trihalomethanes (THMs) are one of those invisible troublemakers in drinking water systems. They form quietly whenever chlorine reacts with natural organic matter, and they tend to accumulate in places where water sits still — especially storage tanks. Left unchecked, THMs can push a system out of regulatory compliance and create long‑term health concerns for the communities relying on that water.

The good news? THM removal inside water tanks is absolutely achievable, and the science behind it is surprisingly elegant.

Let’s break it down.

What Are THMs and Why Do They Build Up in Tanks?

THMs — including chloroform, bromoform, bromodichloromethane, and dibromochloromethane — are volatile disinfection byproducts. Because they’re volatile, they naturally want to escape from water into air. But in a poorly mixed tank, water stratifies, chlorine lingers, and THMs continue forming and accumulating.

Factors that accelerate THM formation include:

• High water age
• Warm temperatures
• Excess chlorine contact time
• Organic matter in the source water

If the tank isn’t actively circulating or ventilated, THMs can climb above the EPA’s 80 ppb TTHM limit.

The Core Idea Behind THM Removal: Let Them Escape

Since THMs are volatile, the most effective strategy is to help them leave the water. Utilities do this through aeration, mixing, and ventilation — a trio of physical processes that work together to strip THMs from the water and release them into the atmosphere.

Here’s how each part contributes:

1. Surface Agitation

Aeration systems often create a disruption at the top of the water column where water meets air. This breaching action releases THMs directly into the air in the headspace of the tank.

2. Active Mixing

Mixing prevents stratification and ensures that water from the bottom — where THMs often concentrate — reaches the aeration zone.

3. Ventilation

Once THMs enter the tank headspace, they must be exhausted. Without proper venting, THMs can re‑dissolve back into the water.

Common THM Removal Systems Used in Tanks can take Different Forms

Utilities can put together a system that fits their needs base on tank conditions and THM saturation:

• Submersible mixers paired with active ventilation Ideal for large tanks with persistent stratification. This also ensures stripped THMs don’t accumulate in the headspace.
• Adding Surface Aeration This can provide additional “horsepower” to speed up THM volatilization when tank turnover is quick and higher levels of THMs need to be evacuated.

Each system can reduce THMs by 30–60%, depending on tank size, water age, and airflow.

Why THM Removal Matters

Beyond regulatory compliance, THM control improves:

• Water quality throughout the distribution system
• Customer confidence
• Operational flexibility (less pressure to flush excessively)
• Long‑term public health outcomes

THM removal is one of the most cost‑effective ways to improve water quality without overhauling upstream treatment processes.