Pre-Filtering Rainwater Harvesting Systems

OFF GRID water harvesting system (2)

Basic Elements of Pre-Filtering Rainwater Harvesting Systems

A functional and effective Rainwater Harvesting System should include at least the following elements:

⦁ Water Catchment Area (see: Container Rainwater Harvesting-Catchment )
⦁ Leaf-Shedding
⦁ Conveyance System
⦁ First-Flush Diverter
⦁ Storage Tanks
⦁ Distribution System
⦁ Overflow System
⦁ Protection from insects & vermin

And, if needed:

⦁ Water Pump
⦁ Bladder or Gravitational System.
Assuming that you are located far from heavily-polluted industrial and/or agricultural zones, such a system if properly designed and maintained, should be able to provide bathing-quality water. (See the chapter: CH -Fundamentals of Rainwater Harvesting).

Rainwater-Harvesting-System-02Basic elements of the Rainwater Harvesting System (in read components being subject of this article). Source: Your Home (Australia’s guide to environmentally sustainable homes)

Pre-Filtering Rainwater Harvesting Systems

In order to minimize the risk of overheating, most off-grid container houses will use trees as natural sun barriers. With trees come much-needed shade and freshness, but unfortunately, there is also another part of the story – countless buds and seeds (spring) as well as leaves (fall). It’s a typical scenario when the house is located next to trees. Once on the roof, carried with rainwater they will block gutters and downspouts preventing them to channel rainwater and discharge it at a specified location. Water will be dropping along the perimeter of the house creating mud. Sounds familiar?

However, when the roof is used as a rainwater catchment, it’s no anymore about mud, leaks, overflows, and in general sort of mess around the house. Leaves, buds, and seeds (plus everything else accumulated on the roof) will significantly limit the efficiency of the water harvesting process. While in both mentioned cases, leaf screens are needed, in rainwater harvesting systems they are a crucial part of the overall filtration scheme.
Gutter Leaf-Screens.

Large leaf screens covering the lower part of the roof and gutters beneath its edge are the most effective solution guaranteeing an undisturbed flow of rainwater through the conveyance system. Thanks to the large surface, they can handle a significant quantity of organic matter with a low probability of being clogged. Installed on angled roofs, they have self-cleaning properties. On the negative side – they add extra cost and complexity. Also, like everything on the roof, they are not easily accessible for cleaning if required.


Pre-Filtering Rainwater Harvesting Systems

Gutter leaf screens keep leaves out of the rainwater collection system. Source: Your Home (Australia) Photos: John Caley (Ecological Design)

Leaf-shedding gutters

Some may prefer installing ready-to-use leaf-shedding gutters. The most common ones have flat screens so regardless of the type of roof (flat or pitched), they are prone to blocking leaves. Especially, it may happen in spring, because falling pollen and buds will have a tendency to firmly stick to the gutter’s guards creating a firm water barrier. It’s not the end of the world when you just want to evacuate rainwater from the roof, however, when you have to collect every drop of rainwater for future use, you cannot afford a such loss.

Gutter with lock-on screen (left) and its inherent tendency to accumulate debris (right). Note that half-round-shaped lead-guards will perform better. Source: Gutter Supply & RainTrade Corporation
There are many advanced leaf-shedding gutters on the market. They may perform better compared to typical flat-screen gutters. However, it is not clear what is their water harvesting efficiency (especially during rainstorms) when they must handle large quantities of water and leaves.

Leaf-Shedding Rain Heads

Another popular solution is a Leaf-Shedding Rain Head. Installed in-line with downspouts, they are visible and easily accessible so they can be frequently cleaned when needed. Thanks to their slanted structure they have self-cleaning property. Unfortunately, they will work well when leaves “happen” only sporadically (they may have problems handling a large number of leaves).

With plenty of leaves, you may expect the following scenarios:
⦁ Gutters will be stuffed with leaves and rainwater will largely overflow over their edges;
⦁ Leaf-Shedding Rain-Head will get quickly clogged and start diverting rainwater out of the downspout.
In both cases, it leads to the significant loss of rainwater, and possibly local ‘flooding” around the house.

Leaf-Shedding Rain-Head: the screen keeps leaves out of the rainwater collection system. Source: Your Home (Australia) Photos: John Caley (Ecological Design)
In contrast, when equipped with fine (mesh-type) screens, leaf-shedding rain heads can improve the pre-filtration system by removing smaller-size elements, not deflected by leaf screens installed at the first line of battle along the gutters (for example dead insects, seeds, etc…).
In general, Leaf-Shedding Rain Heads thanks to their relatively low cost and easy DIY installation are popular solutions.

Thanks to Fine Screen (Clean Shield™), the traditional Rainwater Leaf-Beater can greatly improve the efficiency of the filtration system. Source: Bunnings Warehouse (Australia).
When equipped with fine screens they can separate smaller objects: dead insects, tree seeds, needle-like coniferous leaves, and so on … For example, the Clean Shield™ aperture of about 1mm (40 mils) qualifies it as a mosquito screen (necessary in rainwater harvesting systems to prevent the breeding of insects in the tank). Unfortunately, fine mesh-type screens will be quickly overwhelmed by a large quantity of debris so they definitely need an upstream pre-filtration stage like mentioned earlier leaf screens.

Due to their slanted/open structures, Leaf-Shedding Rain Heads may deflect not only debris but also water, especially during rainstorms. To choose the right screen, you may have to experiment with one of the larger apertures to make sure that the filtration process works well without the noticeable loss of rainwater!


⦁ The coarse (pre-filtration) stage is necessary regardless of how you plan to use collected rainwater. If not removed, debris may clog the rainwater conveyance system, will accumulate in the tank forcing its frequent cleaning, may block distribution pipes and most likely, the whole system will become a breeding ground for all sorts of insects and micro-organisms.
⦁ In some countries (states), the use of Leaf-Screens in rainwater harvesting systems is mandatory!

First-Flush Diverters (FFD)

Unfortunately, roofs accumulate not only leaves, buds, flowers, dead insects, etc. From the point of view of water’s end-use, much more dangerous is aerial pollution. It includes dust, organic matter (pollen, birds’ droppings….), industrial and agricultural compounds (particles and chemicals) as well as zillions of potentially harmful (if not deadly) microorganisms thriving on accumulated organic matter. Their presence in harvested rainwater may disqualify it even from the use of gardening, showering, and washing clothes – all major household recipients of the freshwater. Strict decontamination and disinfection is a complex and costly process and frankly, in the case of non-potable water not fully justified. That’s where First-Flush Diverters come.

First-Flush diverters act like a pre-washing program. Their job is to divert the first wave of rainwater from the roof away from the main storage tank. It is assumed that “first-flush” rainwater carries plenty of sediments accumulated on the roof since the previous rain and so it is contaminated beyond use.

It sounds like a great idea, unfortunately, its practical implementation is quite challenging. The truth is that existing First-Flush Diverters are not the “intelligent devices” we got used to so much these days. They act based on “statistical data” and once designed and installed, cannot be “re-programmed”.
Pre-Filtering Rainwater Harvesting Systems

First-Flush Diverter: Concept. Source: Harvest H2O

First-Flush Diverter is a passive element. It collects the first wave of rainwater (presumably the most contaminated). Once its capacity is filled, water starts to flow to the main system. The diverted water is drained at a slow rate to make it ready for the next rain. It’s as simple as that, although its practical implementations are a bit more complex.

First-Flush Diverter: Detailed concept with a sealing ball. Source: Blue Mountain Co. (Australia)

First-flush diverters are designed individually for a given catchment area (roof) taking into consideration the local environment (pollution), local weather patterns, and planned end-use of water. Let’s analyze each of these factors:

Catchment area
For a given catchment area, its profile (flat or sloped), and finish (smooth or textured) there is a minimum level of rainwater (in inches/square foot) necessary to wash the majority of sediments and contaminants. Usually, the First-Flush diverter is a “One-Point” system (all harvested rainwater must pass through it), and it is designed to divert a specific amount of water in gallons.

As an example: If we assume the catchment area of 600 sq. ft. and the minimum level of rainfall necessary for first flush of 0.05”/sq. ft., the First-Flush Diverter should have the capacity of:
600 [sq. ft] x 0.05 [inch/sq.ft] x 0.623 [gallons/inch] = 18.7 gallons.
This is the amount of water that will be “automatically” diverted from the rainwater harvesting system, regardless if at a given moment it is justified by the level of contamination or not!

Industrial and/or agricultural areas with a higher level of pollution will need more water to wash the catchment area. In contrast, countryside locations in a clean environment may need very little water, mainly to get rid of omnipresent dust. Note that such an “idyllic” picture can be disturbed by wildlife (birds’ droppings, insects, etc (all potential sources of pathogens). What makes the design of FFDs even more difficult is the seasonality of pollution. While industrial pollution is pretty much consistent throughout the year; the agricultural as well as natural ones (pollen, birds droppings, insects, etc…) are very seasonal (spring-early summer).

Weather pattern
During dry seasons, industrial and organic pollution accumulated on the catchment area (roof) will harden and eventually firmly adhere to the surface. Understandably, it will take more first-flush water to wash it out. In contrast, in areas with frequent, more or less evenly distributed precipitations over a year, pollution will not have a chance to built-up and solidify, so significantly less water will be necessary to carry it away.

Also, note that heavy rain (due to its intensity) will be more efficient in cleaning the catchment area than light shower rain. The first one will act like a metal brush, the latter as a “gentle massage”…..

To Use or Not to Use First-Flush Diverters

All mentioned above factors make the design of FFD’s quite difficult and hardly optimal. It’s a rigid design, once built and installed, it will always divert the same amount of water regardless of the real needs at a given moment. In areas with a high level of annual precipitations, most likely it would not be a problem at all. However, in predominantly dry areas, characterized by light rain showers rather than rainstorms, the decision to use First-Flush Diverters may be a tough one!

If you divert a lot of first-flush water, you get cleaner water in the storage tank, but it may not be enough to meet your needs. However, if you limit the first-flush process, you risk getting low-quality water that may not meet quality requirements for the planned end end-use.

The bottom line – is that each First-Flush diverter (it is capacity- to be clear) must be individually designed for a given Catchment and its Location. Unfortunately, it doesn’t matter how hard you will work on the design, it will not be the Optimal One. Optimization, given so many random factors is a Mission Impossible! In fact, the optimum FFD’s design may be truly unique for each particular rain event (be it an intense storm, light shower, or just drizzling). This is the most important fact you have to realize before committing to such a system.

Possible Scenarios for  Pre-Filtering Rainwater Harvesting Systems :

⦁ From the practical point of view, there are not many “knobs” to “tune” a given First-Flush Diverter to the real (not statistical) events. In fact, the only one is a “Weep-Hole”, draining the tank holding diverted water. In areas with frequent, but light rain showers you can slow the draining process and that minimizes the amount of diverted water from the next rain (note that most FFDs come with a set of drains with various diameters). It is a justified approach because, in shorter periods of time between consecutive rains, less pollution and sediments will accumulate on the roof, so shorter pre-wash (less water) may do the job.

⦁ Diverted water does not have to be The Lost Water. You can install a generous-capacity FFD (to make sure the harvested water is cleaner) but at the same time, store the diverted rainwater in a sort of “Gray Tank”. Such water can be always used for flushing toilets and watering lawns as well as (in areas with a low level of chemical pollution), for watering gardens. This should decrease the stress on mainstream water tanks.

⦁ Another solution to get rid of accumulated sediments and pollution is to wash the roof before the season starts. It can be done manually (if you have a pressure washer), or just by letting the first serious rain do the job. In both cases, you have to divert all water out of the main path (usually by disconnecting the pipe carrying the water to the tank.

⦁ If the rainwater is collected only for irrigation, the installation of the First-Flush Diverter will be actually counterproductive. Note that organic pollution is actually beneficial for gardening, lawns, etc as it carries elements of natural “fertilizers”. The exception may be industrial areas, where due to heavy pollution it may be unwise to use such water for veggies and fruits due to potentially increased concentrations of chemical contamination. One may say – it happens anyhow (the garden does not have an umbrella), but it does not mean that “more is better”! And it may be even forbidden to use such water for veggies and fruits growing in greenhouses.

Maintenance of FFDs.

First-Flush diverter operates automatically. It temporarily traps a pre-determined amount of first-flush water from the roof and then once full, it “automatically” lets the remaining rainwater to pass directly to the tank. Thanks to a drain (let’s call it a “controlled leak”), within a predetermined time frame, its tank is also automatically emptied to be ready for the next rain. In the perfect world, it will all work well, but the world is far from perfect.

The first-flush water carries all sorts of dirt, residues, and contaminants accumulated on a roof since the last rain. Water-soluble compounds will escape from First-Flush Diverter with water passing through a drain. However, most solid residues will accumulate in the diverter’s tank. If left, they may considerably delay the draining process or even completely stop it by clogging the drain. As a result, the First-Flush Diverter will lose its ability to divert the water from the next rain, or even worst, it will add extra contamination to already dirty rainwater flowing from the roof. Unfortunately, increasing the size of the drain to minimize the probability of clogging is not a solution. If the FFD’s tank is emptying too fast, the majority of water from the light rain shower will be diverted from the harvesting system.
In reality, despite some aspect of “automatization” (as described above), First-Flush Diverters need frequent manual maintenance!

Bottom line: In order to work as intended, First-Flush Diverters need periodic maintenance service. Otherwise, they may actually do more harm by becoming part of the problem instead of being part of the solution! The good news in the whole story – the First-Flush Diverter is the only component of the Water Harvesting System requiring frequent maintenance service. For some, it may be too much, but frankly, cleaning this “One-Point” element of the whole system offers you better quality water and saves your downstream system (tanks) from the more difficult cleaning process and water pumps (i used) from damage.
To facilitate visual monitoring and maintenance, First-Flush Diverters should be installed in places with good access!

Combined First-Flush diverters for higher diversion capacity. Source: Blue Mountain Co.(Australia)

First-Flush Diverters in cold climate zones

Potentially, freezing temperatures can create an extra problem for FFDs. Water from thawing snow will be captured by the conveyance system and firstly, directed to the First-Flush Diverter. During winters, it will be a slow process, initiated by usually higher temperatures of the roof due to interior heating. In springtime, the thawing will take a much larger scale. In both cases, there is a high probability that the drain will freeze trapping all water in the FFD. Another potential danger – is at night, when temperatures will drop down, the water may freeze and damage the First-Flush diverter. To prevent this from happening, in most commercial FFDs the bottom ends can be completely detached from the tank, preventing the accumulation of water.

The negative side (if you go for it) – all water from the melting snow will be diverted from the main water tank. In certain circumstances (off-grid location) it may not be an acceptable solution.

Note that in wintertime, the level of pollution is much lower as there are no agricultural or natural activities (birds, insects, pollen, etc). As a result, first-flush diverters may not have a big impact on the quality of collected water from the melting snow. However, industrial pollution and individual heating systems (especially based on wood or coal) may drastically change the situation.

Whatever is your decision, once the winter is gone, your First-Flush Diverted should be inspected for potential damages.

Practical Notes: Pre-Filtering Rainwater Harvesting Systems

⦁ For increased performance, most First-Flush Diverters when fully filled with contaminated water will seal their tanks isolating them from passing, hopefully, cleaner rainwater.

⦁ First-Flush diverters cannot work reliably without the first line of defense established by Leaf-Screens. FFD’s are designed to divert dust, sediments, and contaminations. Big items like leaves will quickly clog diverters effectively disabling their operation.

⦁ Most First-Flush Diverters come with a set of drain control valves determining the time necessary to empty the tank. The rate of flow (controlled leak) is determined by the size of the hole in the drain. That gives you the possibility to adjust its operation according to your local weather patterns and level of pollution. A larger hole will speed up the draining process. It may also help to evacuate solid sediments, minimizing this way the need for maintenance. But there is a trade-off: a wider drain hole means also more water will be diverted and eventually lost.

Pre-Filtering Rainwater Harvesting Systems

90mm First Flush Rain Water Diverter: Note the sealing ball, transparent tank to facilitate inspection and selectable drain. Source: Rain Harvesting (Bunnings Warehouse, Australia)

⦁ There is no commonly accepted consensus about how much rainwater is necessary to efficiently wash out sediments and contaminants from the catchment area. Some sources provide guidelines that at least 0.039” (1mm) of rain is necessary to wash the roof (it results in 23.3 gallons of water from the 1,000 sq. ft. (92.9 m2) roof). Others claim that it is far cry from what is really needed. As usual, use common sense taking into account local weather patterns and the level and nature of pollution. For example, typical dust will be washed quickly, but birds’ droppings and pollen (everything that will firmly adhere to the surface) will need more time and water to be washed away.

⦁ Many First-flush diverters available on the market look similar to downspout pipes. Their capacity is determined either by pipe length, diameter, or both. Usually, they are installed in line with downspouts. Unfortunately, as nice as they look, they may not be adequate for larger rainwater harvesting systems where the amount of first-flush water is measured in tens of gallons.

Wall-mounted First-Flush Diverter. Source: Marley Co. (New Zealand)

⦁ Installation of more FFDs in the system may solve the problem of capacity, but on top of an extra cost, it will be rather a nightmare from the maintenance point of view. Keep in mind – First-flush diverters are NOT maintenance Free, just the opposite, they need frequent maintenance to work as designed.

⦁ Commercial First-Flush Diverters with capacities measured in tens of gallons of water have clearly distinguishable tanks. Due to their high weight, they are typically installed on the ground. The positive aspect – they offer easy access for visual inspection and maintenance!

Conveyance System

Fortunately, traditional gutters and downspouts are suitable for rainwater Conveyance Systems. The most important factor is that they are functional (no leaks and overflows), protected by leaf-screens, and (when the end-use is potable water) – made from suitable for that purpose materials.


Disclaimer: Our intention is to give you some ideas about the possibility, legality, and benefits of rainwater harvesting for use in individual households. Note, however, that the final words belong to local jurisdictions. It is then strongly recommended to check with local authorities what is legal, permitted or eventually restricted when it comes to harvesting and using rainwater in an individual household.

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