Is There A Difference Between Passive And Active First Flush Systems?

Understanding First Flush Systems

First flush systems are designed to divert the initial flow of rainwater, known as the “first flush,” which can contain high concentrations of contaminants, such as leaves, twigs, and debris, from the roof and downspout. This is crucial in maintaining the water quality in the collection tank. A 10% to 20% rule of thumb is used to determine the size of the first flush device, where the device should divert at least 10% to 20% of the total downspout flow.

Passive vs Active First Flush Systems

Passive first flush systems rely on gravity and the flow of water to divert the initial contaminants, often using a baffle or a small tank. However, they can be less effective, especially during heavy rainfall events. Active first flush systems, on the other hand, use a mechanical device, such as a valve or a float-activated mechanism, to divert the initial flow of water. These systems are more effective, but they require more maintenance and can be more expensive. A common configuration for an active first flush system is a 50-gallon tank with a 10% to 20% diversion ratio, where the first 50 to 100 gallons of rainwater are diverted before entering the main collection tank.

Designing and Installing First Flush Systems

When designing and installing a first flush system, it’s essential to consider the roof size, pitch, and material, as well as the downspout size and location. A well-designed system can be simple and cost-effective, while a poorly designed system can lead to contamination and maintenance issues. In general, it’s recommended to install a first flush device at the roof level, near the downspout, to minimize the amount of contaminants entering the collection tank.