Rainwater Harvesting System: Pre-Filtering 3

Rainwater Harvesting System Pre-Filtering

Basic Elements of the Rainwater Harvesting System Filtering

What each rainwater harvesting system filtering should include? A functional and effective Rainwater Harvesting System should include at least the following elements:

⦁ Water Catchment Area (see: Container Rainwater Harvesting-Catchment )
⦁ Leaf-Shedding (see: Container Rainwater Pre-Filtering Systems)
⦁Conveyance System
⦁First-Flush Diverter
⦁ Storage Tanks
⦁ Pre-Tank Filtering
⦁ Tank Fittings
⦁ Oxygenation
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

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

Rainwater Storage for Rainwater Harvesting System

How big must be the water tank?

Let’s assume at this point that you know what is your annual demand for water and that your rain-catching surface (most likely the roof) meets (or exceeds) your water needs.

Theoretically, the calculated demand defines the capacity of the water tank. Fortunately, at the majority of geographical locations, rainy days are not “one-time events”. While most likely you will identify in your local weather pattern wetter and drier seasons, precipitations are distributed throughout the whole year. Such weather patterns will reduce the required size of the rainwater tank. The water will be harvested based on seasonal (monthly) weather patterns and distributed following daily-use patterns. The longest annual dry period (based on data from Average Monthly Precipitations) will determine the required size of the tank. But for sure, it will be smaller than your annual water needs.


5,945 gallons (22,500 liters) rainwater tank. Source: Camero Water Tanks (Australia)

Note that both statistics are important. The Average Annual precipitation give you an estimation of how much water you can collect from the catchment area (roof) over the whole year. This number will tell you how big must be the catchment area (roof) to meet your household needs. The Average Monthly Precipitations will help you to identify the longest dry period over which you will have to carry on with what was previously stored in the tank.

Practical Notes for Rainwater Harvesting System:

⦁ What falls on the roof does not necessarily end up in your water tank. Typically, overflows, leaks, first-flush diversion, pre-tank filters, evaporation, etc. will eliminate about 10% to 15% of initial rainwater volume;
⦁ Some suggest that the water tank should be able to collect all water from rainstorms. While mostly unpredictable, they happen, and that’s when after initial flushing of the roof, you get the cleanest water you may have. It may be worth to keep it!
⦁ When it comes to the size of the rainwater tank, there is no clear answer to the question of how big is too big. What is clear however is that it is more practical to have 2 or more smaller tanks (multi-tank system) than one big. It’s because from time to time tanks will have to be disconnected from the system for cleaning (OK, no panic – you can siphon the sludge from the tank filled with water, but the process is far from perfect; more about it in a dedicated chapter). The best scenario (especially at off-grid locations) is emptying the tank, disconnecting it from the system, then cleaning, disinfecting and flushing with water.

Multiple rainwater tanks. Source: Blue Barrel Systems (USA)

⦁ It’s a good engineering approach to add about 10% to 20% capacity to the calculated size of the tank as a safety “buffer”. It won’t hurt but can be priceless in difficult times! Note that average seasonal precipitations are statistical events; real life often twists statistics.

Tank Materials for Rainwater Harvesting System Filtering

Materials for general-purpose (not puttable) rainwater tanks have only one restriction – they should be impenetrable to light in order to prevent the growth of light-dependent organisms (like algae….). The list of common materials includes plastic (polyethylene….), fiberglass, and steel for above-the-ground tanks as well as concrete for underground ones.

The selection criteria usually take into account the initial cost as well as potential health hazards when drinking-quality water is needed. No without merits are also practical aspects like easiness or complexity of installation (DIY appreciated), cost of transport, lifespan, maintenance process, etc. Some will also look at their Eco-friendliness and potential for future recycling.

Plastic Tanks

The most popular storage tanks are made from polyethylene (commonly known as Poly-Tanks). Usually, they are made in the form of barrels although there are also slim versions nicely fitting along the walls of a house. Poly-tanks are easy to install, seamless (one-piece units), UV-resistant, portable (when empty), and represent a low-cost solution. They are offered in a range of sizes and a variety of colors to fit into their surroundings. While available in capacities of a few thousand gallons for larger systems, the majority of poly-tanks have sizes of typical barrels. Required water storage capacity is achieved by interconnecting several modules together. Such multi-module systems have several advantages – starting from inexpensive transport, and easy DIY installation but also easy maintenance without the need to shut down (empty) the whole system.
 Rainwater barrel – simple and inexpensive system to harvest rainwater for gardening. Source: WaterCache

No wonder poly-tanks are the most popular solution for general-purpose rainwater storage (in other words utility and irrigation water). However, the situation is not that clear when it comes to storing potable water. It’s true that Food-Grade Polyethylene is widely used by relevant industries and so we may assume that water tanks made from such a version of polyethylene are safe. Unfortunately, it may not be so. Note that in contrast to popular one-time-use plastic bottles for mineral water, or plastic boxes for food, poly-tanks are not “disposable”. Over their lifetime, they will be exposed to large daily and seasonal variations of temperatures, UV radiation, and potentially slightly acidic rainwater and potentially, mechanical impacts. It’s not a friendly environment!

To provide some degree of protection, the polyethylene used for outdoor applications needs UV stabilizers (inhibitors) to prevent its degradation (braking-down). BPA is another potentially harmful ingredient; it´s used as a plastic softener adding much-needed flexibility to poly-structures. Unfortunately in the long term, due to potential off-gassing and leaching of these extra components, poly-tanks may become harmful!

Poly-tanks: Summary

⦁ Poly-Tanks are a great low-cost solution for the storage of utility/irrigation quality rainwater. They are lightweight, easy to, install, relatively robust, and available in capacities from typical 60 gallons barrels up to about ten thousand gallons tanks.

⦁ Poly tanks for storage of potable rainwater must be made from BPA-free, food-grade plastic. Keep in mind however that due to harsh weather conditions, meeting these initial requirements may not guarantee their safety over a long period of time. Once again – it’s expected that rainwater tanks despite their continuous exposure to weather elements should last for about 20-to-25 years. Note that mentioned earlier food-grade poly packages used by the food industry are mostly manufactured for one-time use in a controlled environment. That’s why the use of poly tanks for the storage of potable water is rather questionable. Nevertheless – if you decide to go for it, make sure to have all relevant information available from the manufacturer on the paper!

⦁ Poly-tank barrels (including big ones) are “rotationally molded” so by definition, they are seamless. The main advantage: they do not leak (unless mechanically damaged).

⦁ Plastic (especially with the addition of “softeners”) has some flexibility. That’s why small barrels do not need any special foundations while big ones can be placed on a sand bed or typical pavement.

⦁ While available in a range of colors to suit individual preferences, most rainwater tanks designed for potable water are made from dark green or black color effectively inhibiting penetration by sunlight.


            5,000 liters (about 1,320 gallons) Poly-tank. Source: Tank Shop (Australia)
⦁ Polyethylene tanks are recyclable. While recycled plastic cannot be used for another water tank, there are countless possibilities to convert it to other useful products; (note that in most countries regulations require that only “virgin” plastic can be used for food-grade products).

Steel Tanks for Rainwater Harvesting System Filtering

Steel tanks are made from stainless steel, galvanized steel, or popular, lower-cost corrosion-resistant versions of steel like Colorbond, Zincalume (examples of Australian brand names).


Stainless steel (iron with a substantial content of chromium and nickel) seems to be the most robust and long-lasting solution. However, its most important characteristic in this application is safety. It’s because stainless steel does not contain any chemical compounds that may be off-gassing, leaching, or breaking down into harmful substances. Also, its surface is non-reactive, which means highly tolerant to potentially acidic rainwater.

For all these reasons, stainless steel in its food-grade version known as 304 or “18-8” is widely used by the food industry. Compared to plastics, stainless-steel surface is very smooth making it difficult for microorganisms to grow on it. As a result, stainless-steel tanks require a much lower level of maintenance than poly tanks. And – for eco-friendly minds – stainless steel is fully recyclable!

As it could be expected, the safety, long lifespan (100 years does not seem to be an exaggeration) and low maintenance come at a higher initial cost! But in the long run – it may pay off!


5,000 liters (about 1320 gallons) stainless-steel rainwater tank. Source: Tank Shop (Australia)
Robustness, low cost, and recyclability make steel an attractive material for rainwater tanks. No wonder, steel in its corrosion-protected or corrosion-resistant versions are largely used by manufacturers of rainwater tanks.

Corrosion-Protected Steel

Galvanized Steel

For a long, galvanized steel pipes were widely used for water installations at residential homes (they were eventually replaced by copper pipes and then recently by flexible PEX-pipes). No wonder that this proven combination of metals found the application for the construction of water tanks. Steel guarantees strength, while the zinc-coating (usually hot-dip galvanization process) offers protection against corrosion. Unfortunately, their performance (quality of water and lifespan) largely depends on the thickness of zinc’s coating as well as on the” neutrality of water”. The latter factor is determined by atmospheric pollution, and so it’s hard to control. Another potential problem – any mechanical damage (like dents etc…) may disrupt the continuity of the zinc coating, enabling the corrosion process.

Also, users often point to the specific “metallic” taste of water stored in galvanized steel tanks due to the leaching of zinc, although this effect largely diminishes with time. In order to overcome these problems, most galvanized steel tanks are internally poly-coated. Unfortunately, this brings us back to problems related to poly-tanks. There is however one important exception – now the poly-coating is fully protected from exposure to sun and UV so the polyethylene does not need extra chemical compounds preventing its degradation. At least we can say – it’s a bit safer.


Galvanized rainwater tank: The Aesthetic Choice for Rainwater Storage. Source: Innovative Water Solutions LLC (Austin, Texas, USA)


Aquaplate® steel is a sandwich consisting of galvanized steel on the inner side coated with a specially formulated food-grade polymer film, while on the opposite (outer) one lined with weather-protection coating. It was specially developed by BlueScope (Australia) for the construction of water tanks (note that Australia is a leading country when it comes to rainwater harvesting).

                                            Aquaplate® steel structure. Source: Blue Scope (Australia)
Aquaplate® steel tanks are more affordable than stainless-steel ones. For some, however, the real questions will always be the potential impact of “chemical components” (interior poly-lining). It’s true that the polymer film meets quality requirements for the storage of potable water established by relevant agencies of the Australian Government.

It’s the hard fact that we are living in a more and more polluted world, and new composite materials are eliminating old and proven natural ones. But it is also true is that consumers are the last to learn the details!
A similar approach (lining) is used for tanks made from Colorbond or Zincalume steels.

Metal Tanks: Summary

⦁ Stainless steel tanks are the safest and practically lasting forever solutions for storing potable water. While their initial cost is high, low maintenance requirements and longevity may balance it out over long years of service.
⦁ The lifespan of water tanks from corrosion-protected steel is limited. It can be affected by the acidity of harvested rainwater (atmospheric pollution), and the leaching of chemical components from the catchment area (especially if metallic) – both causing spontaneous chemical reactions leading to the corrosion of tanks. Usually, a thin layer of coating/lining is prone to abrasion, which may expose raw steel to water. Special attention must be also paid to fittings. Other metals (copper, brass, bronze, regular galvanized steel….) may lead to intermetallic reactions and corrosion. It is suggested to use plastic (PVC) ”buffer pipes” (length of about 6ft/2m) to substantially lower the intensity of electrolytic reactions inside of the tank.
⦁ Metal tanks are rigid, so they require adequate foundations (flat and able to support the load). They are NOT seamless! Their joints (typically welded) are potentially weak spots in the overall structure.

Fiberglass Tanks for Rainwater Harvesting System Filtering

In short words- Fiberglass is a plastic reinforced with glass fibers and mixed with resins to hold it together. One of the advantages of fiberglass is its strength-to-weight ratio (much higher than steel) as well as its non-corrosive nature. What also makes fiberglass attractive for rainwater storage applications is its rigidness and weather resistance. These properties made fiberglass suitable for the construction of rainwater tanks for above-the-ground as well as underground applications.

No wonder, fiberglass tanks are more expensive than popular poly tanks. That’s why they are mostly offered as large-capacity models (thousands of gallons and up). For increased reliability (note that huge stress is imposed on sidewalls either by water or by soil (if buried)), large fiberglass tanks are made using a woven mat or filament twisted fiber.

Due to safety requirements, fiberglass tanks for potable water must be internally coated with food-grade poly-coating. On top of that – in above-the-ground versions, fiberglass must be gel-coated, generously pigmented or dark-painted on its external side to prevent infiltration of sunlight.

Rainwater Harvesting System

    Underground fiberglass water tank. Source: Climate Inc. – Water Collection Systems


           7,925 gallons (30 cubic meters) of fiberglass water tank. Source: Izoplace (Turkey)

Fiberglass Tanks: Summary

⦁ In contrast to Poly-Tanks, fiberglass tanks are not one-piece modules. In other words, they have seams and these are usually the weakest points of the overall structures when it comes to leaks.
⦁ They are more expensive (compared to Poly-tanks) and available mainly in capacities higher than 1,000 gallons (note that due to lower strength, the capacity of poly-tanks is limited to a few thousand gallons).
⦁ Fiberglass (especially one exposed to severe weather elements, UV, etc., will degrade with time. Bristles and cracks are the most common effects.
⦁ Due to good strength and inherent resistance to corrosion, fiberglass tanks are ideal for underground applications.
⦁ So far fiberglass is not recyclable (in the sense that it cannot be easily broken down to the level of individual chemical components). On the positive side, after incineration or grinding it can be reused as a filling component for concrete, engineered wood, roofing materials, tires, etc…It’s also worth noting that the production of fiberglass requires less energy than other composite materials.


All rainwater tanks require a flat base (foundation) with strength adequate for the load they will have to support. It’s especially important for steel and fiberglass tanks (by nature more rigid than plastic ones that thanks to some level of flexibility can accommodate uneven surfaces). In fact, the lack of a required foundation may void the manufacturer’s warranty on the water tank!

Concrete Tanks

Concrete for a long was (and still is) used in municipal water systems. It was also a preferred construction material for small wells in rural areas. Concrete, in its natural form consisting of sand, gravel, lime, and water is considered not harmful to health. Actually, due to the presence of lime (a mix of calcium-oxide and calcium-hydroxide), concrete tanks create an alkaline environment, balancing-out the potential acidity of the rainwater. To make it clear – originally, raindrops represent the purest form of water (condensed vapor has the purity of distilled water). However omnipresent in the atmosphere industrial gases (sulfur dioxide, nitrogen dioxide, carbon monoxide…) and all sorts of particles are easily absorbed by raindrops, on their “journey” through the air.

Most concrete tanks are built as underground structures. This single fact has several positive consequences:

⦁ They better survive lower winter temperatures (surrounding soil minimizes the probability of water freezing or at least delays the process). By the same token, they are not directly exposed to scorching sun and UV radiations as all above-the-ground tanks. In other words – they stabilize the rainwater temperature!

⦁ Architectonically, tanks do not “spoil” surroundings (which is not the case with big above-the-ground tanks.

⦁ Buried and hidden underground they can be located very close to the principal catchment area (roof) without the need for an extensive, underground water conveyance/distribution system.


Most concrete rainwater tanks are buried underground. Source: Panthers Concrete Tanks (Australia)

But not everything is “shining”. First of all, in the overwhelming majority of cases, concrete tanks have to be made “on-site”. The construction (including digging) takes much more time than just transporting, almost ”ready-to-use” plastic, fiberglass, or metal tanks. Concrete is porous, and its surface is not only a great natural habitat for algae and all sorts of microorganisms but also difficult to clean (it means more demanding maintenance). If poorly designed and constructed, concrete tanks may crack and that may mark the end of their life). If the steel-reinforced concrete is used for the construction of the tank, due to the concrete’s porousness, water will reach the metal. Corroded steel will not only contaminate the water, but it may also cause cracks in the concrete’s structure.

Rainwater Harvesting System

                        Concrete rainwater tank. Source: Apollo Concrete Tanks (Australia)

And the last (but not least) on the list of problems is the potential chemical contamination of concrete. Adding ash from the coal-burning industry to increase concrete’s strength and lower its cost (what is quite common practice these days) has a potential impact on the “idyllic” picture of harmlessness. Ashes are known for having a high concentration of chemical components that will eventually leach and contaminate the water. So, the least you can do when making the decision to build the concrete rainwater tank is to make sure that the concrete is all-natural (or to be sure – use suitable cement and mix it on-site with sand and gravel).

Fabric-based poly-liners can be used to eliminate these dangers (unfortunately, together with the benefit of alkalinity). You have to also take into account an extra cost!
Note that concrete is not recyclable, so at the end of its life, it will be buried forever or will end up (in pieces) in a landfill.

Rainwater Tanks: General Notes

⦁ Above-the-ground tanks should be located (if possible) in the shade. Exposure to direct sun will cause excessive evaporation of water. High water temperature will also facilitate bacterial growth and eventual y – unwanted chemical reactions.

⦁ For increased security, new tanks should be washed (flushed) before use for the storage of potable water. In practice (especially at off-grid locations with no access to municipal water), it may be suggested to not use harvested rainwater for drinking until after a few rainstorms to make sure the whole Rainwater Harvesting System was washed-out. Another approach is to negotiate that the tank is thoroughly flushed by the supplier before delivery.

⦁ In contrast to municipal water, harvested rainwater does not contain fluoride. While it does not matter when it comes to utility or irrigation water, it is importance when rainwater is used for cooking and drinking. Note that fluorides are a vital part of the dental health strategy. If rainwater is the only source of your drinking water, you should seek advice from dental professionals (or National Health Agencies) on how to proceed with fluorides!

Poly-Linings for Rainwater Harvesting System Filtering

It seems that these days we cannot live without chemical components. Poly-coating in its food-grade version is a reality in the majority of potable water tanks. Practically, only stainless-steel tanks can meet relevant requirements without any interior coating/lining. Poly-coatings are necessary to ensure that water remains odorless, taste-free, and safe for drinking (obviously, after an appropriate filtering process is required for potable-quality rainwater).

Unfortunately, customers very rarely (if at all) can get detailed information about the chemical composition of food-grade linings. Our choice is limited to the manufacturer’s certification that in fact, the tank meets requirements imposed by national agencies for the storage of potable water.

On the positive side, however, we can assume that interior poly-linings are not exposed to UV, so at least they do not include UV inhibitors (as poly-tanks do). Fewer chemicals mean – a lower probability of health hazards. Another positive factor applies to underground water tanks as they will be exposed to moderate variations of temperature than their above-the-ground versions. Intuitively, we all understand that direct exposure to the scorching sun accelerates the breakdown process of complex chemical compounds.

Many water tank coatings are sprayed and cured by manufacturers. In a way, it is a more convenient version of the lining. It makes an integral part of the tank that is delivered on the site in a “ready-to-use” form. On the negative side – any mechanical damage to the coating puts in jeopardy the whole concept of drinking water protection. Unfortunately, any mechanical impact (bent, dent, etc…) on sidewalls may destroy the continuity and integrity of the interior coating, and with it, make the safety questionable!

Field-installed liners are made in the form of portable “fabrics”. Usually, it’s a multi-layered material with added strength and durability. It’s a good combination – the steel tank guarantee robustness, while the lining, eliminates potential health hazards. Note that such lining can be on-site repaired (a qualified person required) or replaced by a new one, prolonging the lifespan of the water tank system.

Below is an example of a food-grade water tank lining developed by Rhino Tanks. The good news is that it can be custom-made for a given shape of the tank and according to the manufacturer, it can last for up to 60 years. On the slightly negative side – it must be installed by authorized technicians.

Rainwater Harvesting System

“Infinity” – Food-grade lining for the water tank. – The critical layer called Metallocene (the one in contact with water) is laminated to reinforced layers of polyethylene. Source: Rhino Tanks (Australia)


Installation of the Poly-lining in a metal rainwater tank. Source: Peter Jefferies (Rhino Tanks, Australia)

The following video shows the step-by-step assembly process of the Rhino water tank and the installation of its interior lining.

Another example of a functionally similar lining is the MDPE Food Grade Certified Liner made by Titan Environmental Containment (certified by Health Canada).
Another example of a functionally similar lining is the MDPE Food Grade Certified Liner made by Titan Environmental Containment (certified by Health Canada).
Summary: Water Tank Linings
⦁ Most interior linings are sensitive to UV, so tanks must be opaque and their top covers closed to prevent penetration of UV radiation.
⦁ Linings are designed for the storage of potable water. While they should tolerate a small amount of acidity typical for rainwater, any exposure to extreme chemical contamination from heavy industrial or agricultural pollution may damage the lining! Check with the manufacturer about the warranty and its limitations.
⦁ Spray-type coatings are permanent, they come with the tank and most likely, as a weaker part of the structure, they determine the lifespan of the tank. Fabrics-type linings are fully detachable and can be replaced if needed. Keep in mind that health is above everything!
⦁ Suitable Inner linings guarantee that the tank can be used for storing potable water. It does not guarantee however that the stored rainwater will be safe for drinking. Rainwater quality depends on many factors including local pollution, the effectiveness of pre-tank filters and diverters, maintenance of the whole Rainwater Harvesting System, as well as water post-treatment methods.
⦁ Note that linings of rainwater tanks are severely stressed by organic and non-organic contaminants and sediments washed from the catchment area (roof), but not redirected by First-Flush diverters or pre-tank mesh filters. As a result, tanks (and with them linings) must be periodically cleaned (not only chemically to eliminate microorganisms, but also mechanically to remove sediments.




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.

Leave a Comment

Your email address will not be published. Required fields are marked *