First-Flush Device Comparisons
Constant-Volume Containers vs. First-Flush Valves

By Kenneth Vidacovich
February 2004

The Author is the President of FloTrue International Corp in Austin - Texas, USA. His company provides quality, affordable, reliable and low-maintenance products for Rainwater Collection.

This paper compares various advantages and disadvantages of the two most popular, and generally used, first-flush device types: "Constant-Volume Containers" and "First-Flush Valves". The effects of rain intensity as the critical washoff factor, the general variability of rainfall intensity and maintenance issues are detailed in terms of functionality and possible health concerns. Finally, cost estimates are made in order to compare the installation costs of popularly used configurations.


It is generally accepted that when rain first begins to fall it takes a few minutes (at least) to clean the sky, roof and gutters of debris and contaminates. This first few minutes of rain is called the "first-flush" and is sometimes called the "foul-flush".

Many homeowners and institutions are in the process of installing and / or upgrading Rooftop Rainwater Harvesting (RWH) systems that store rainwater for later use. It is important that the rainwater, as collected, be as clean as possible so that it is safe for later use and is odor-free. The generally accepted strategy used to insure rainwater cleanliness, as collected, is to install one or more first-flush (removal) devices as part of the RWH collection subsystem.

Role of First-Flush Devices

It is well understood that main source of contamination in any Rooftop Rainwater Harvesting (RWH) system is the debris and biological/chemical contaminants that are washed from the surface of the roof and gutters, or scoured from the surrounding sky, during the first flush of rain.

A first-flush device´s sole function is to remove the first-flush and thereby keep undesirable elements out of a rainwater storage vessel. Physically, it can be described as an apparatus that fits to the gutter downpipe somewhere between the gutter outlet and the storage vessel inlet.

First-Flush Device Types

There are two general types of first-flush devices that are popularly used on RWH systems:

  1. Constant-volume first-flush containers
    The way these devices work is that they fill to capacity when the rain begins to fall, regardless of the rain intensity, then the excess rainwater is conveyed to the storage vessel. They work on the theory that, if they are sized correctly (for example: 1 gallon capacity per 100 sq.ft. of roof area), then the rainwater that is sent to the storage vessel will be essentially free of contaminates and debris.

  2. First-flush Valves
    These devices, unlike constant-volume containers, are sensitive to the rate of the rainwater flow. The way these devices work is that when the rainwater flow rate reaches a designed minimum a spring-suspended hollow internal container (i.e. valve ball) located inside the device begins to slowly fill with water. Then, when the valve ball has filled the increased weight causes it to contact a valve seat and stop the water flow. The excess rainwater is then conveyed to the storage vessel.

The Rain Intensity Issue

There are two completely independent variables that make each storm event unique: intensity and duration. Studies have shown that intensity is the critical washoff factor for most storm events.

Also according to studies, the general pattern of rainfall is one where intensities start low and gradually rise until a peak is reached. In this scenario, a constant-volume container could fill to capacity with relatively cleaner water (washoff has not occurred) and consequently bypass flows associated with higher intensity rain and mass pollutant loads. (1)

Major Pitfall of Constant-Volume Containers

When rainfall events start at a low intensity (which is the general pattern) it is likely that a constant-volume container will substantially fill with clean water because washoff does not occur effectively at low rain intensities. Then, when more intense rains begin, undesirable contaminates from the roof and gutter can wash into the clean water storage vessel. This is the major pitfall of constant-volume first-flush devices.

Primary Advantage of First-Flush Valves

First-flush valves, unlike constant-volume containers, are actuated by water flow and, if designed correctly, will not actuate during low intensity drizzles. Thus, these devices can substantially avoid conveying water into the storage vessel until the roof is clean. That is, these devices are designed to wait until the rain is intense enough to wash the roof and then, and only then, convey (clean) rainwater into the storage vessel. This is the primary advantage of first-flush valves.

Filling versus Dumping – a Potential Health Issue

Constant-volume containers, by design, fill with rainwater that is contaminated by debris washed from the roof and gutters. There are two primary concerns as a result. First, the water that eventually flows into the storage vessel can undesirably mix with the contaminated water stored inside the container. Second, the container will typically require manual cleaning in order to prepare it for the next rain. Consequently, if the owner of a constant-volume container does not clean the device without fail, then a bloom of bacteria could grow in the container only to be washed into the clean water storage vessel during the next rain. Thus, unless constantly maintained, the use of constant-volume containers poses a potential health concern.

First-flush valves, on the other hand, dump contaminated water and debris to the ground or into a secondary tank. There are two primary advantages as a result. First, contaminated water is not likely to mix with (clean) water going into the storage vessel. Second, these devices do not typically need to be manually cleaned or emptied in order to prepare for the next rain. Hence, since valves dump rather than fill they can keep the water in the storage vessel cleaner (no mixing) and do not require constant cleaning in order to keep potent blooms of bacteria from becoming a potential health hazard.

Popular Device Configurations

Larger RWH systems (defined here as RWHs with tank holding capacities between 1,500 and 40,000 gallons) are the typical users of these first-flush devices. Depending on the region of the world, and the application, the typical piping use to convey the rainwater from the gutter to these larger RWH tanks is PVC and comes in the following nominal diameters: 3 inch, 90mm, 4 inch, 100mm, 6 inch and 150mm.

Constant-volume first-flush containers used with these "larger" systems have various capacities and configurations. Generally speaking, there are three main configurations used:

First-flush valves are available for use on larger RWH systems as well. There are two main types that are available commercially today:

Cost Comparisons

For purposes of discussion, lets consider a house with a 2000 square foot roof catchment area and a ten-foot drop from the gutters to the ground. Lets say that this house is going to require 30 gallons of water to be diverted during the first-flush of rain and that the conveyance piping used is 3 inch diameter PVC.
[Note: 90mm piping is the metric "equivalent" to 3 inch diameter piping]

For comparison purposes, all of the First-Flush devices described heretofore (shown in green, below) are configured to fit identical piping layouts.

First-Flush Devices

It is interesting to note that all of the above devices easily have the capacity to divert the required 30 gallons of water except for the 3" diameter Homemade Pipe Diverter.
Each Homemade diverter (a 10ft long pipe) can divert only 8 gallons, so it will require a total of four Homemade devices in order to divert the required 30 gallons of water.

Material Cost Estimates

Estimated total (retail) material costs for each first-flush device and associated equipment is shown below. Every effort was made to identify the low cost options for each diverter type. Most of the pipe components reflect retail pricing from Home Depot, a large building supply chain in the United States.
We did not include the cost of other hardware needed such as pipe clamps or gluing materials. Tax, installation labor, and shipping costs were also not included.

First-Flush Cost Estimates

It is interesting to note that we found some well-known RWH designer / installers in the Central Texas area recommending and putting in (up to) 500 gallon tanks for rainwater diversion purposes, with costs up to $1,100 dollars per diverter tank! To be fair, it must be pointed out that these RWH systems are usually much larger in terms of catchment area and storage vessel capacity.

Cost Analysis

According to the above cost totals, the least expensive device is the Valve Kit, with a total material cost at $67.89. The Homemade Pipe Diverter and the Diverter Tee material costs are moderate at $77.44 and $84.14 respectively. The Diverter Tank is somewhat higher at $96.18, but is still under $100. Finally, the Roofwasher Box cost is highest at $600.

We found it surprising that the homemade device was not the least cost option. It was decided to redesign the Homemade Pipe Diverter using 6 inch pipe so that only two units (instead of four) were required. In doing so we found that the total cost was not reduced; instead the more expensive 6 inch piping components made the cost higher instead of lower. The design illustration and cost analysis for the 6 inch Homemade Pipe Diverter system is shown below.

First-Flush Cost Analysis



Authored by: Thomas R. Adams, P.E. and Robert A. Strong, Jr.
Vortechnics, Inc.,, 1997 Vortechnics, Portland, Maine
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