CH – Rainwater Harvesting System: Pre-Filtering

Rainwater filters

Basic Elements of the Rainwater Harvesting System

Functional and effective Rainwater Harvesting System should include at least the following elements:
⦁ Water Catchment Area (see: Container Houses: Rainwater Harvesting-Catchment )
⦁ Leaf-Shedding (see: Container Houses: Rainwater Pre-Filtering Systems)
⦁ Conveyance System
⦁ First-Flush Diverter
⦁ Storage Tanks (see: Container Houses: Rainwater Tanks)
⦁ Pre-Tank Filtering
⦁ Tank Fittings

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 the 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 Tanks Fittings

Many of us are more or less familiar with water installation fixtures, so we will keep it short, focusing only on their most important aspects.

Pre-Tank Inlet Filters

Tank is an exterior reservoir used for the collection of rainwater that is subsequently consumed over longer periods of time between subsequent rains. By nature, such water shares some characteristics of “stagnant” water. In other words, it can easily become a breeding ground for insects (mosquitos will love it) as well as a favorite milieu for all sorts of microorganisms. To minimize such scenarios and keep stored water as fresh as possible, the “open” upstream catchment and conveyance system must be separated from the water storage system (tank) by a suitable, fine mesh-type filter. The filter should not only prevent insects but also minimize the amount of organic matter washed out from the catchment area from entering the tank (the latter gives a boost to microorganisms). Ideally, the mesh-filter should have the following characteristics:

⦁ Health-Hazards Free
If the stored rainwater is intended for drinking and cooking, the pre-tank filter must be certified for potable water applications. From this point of view, the safest are stainless-steel mesh-type filters. Plastic used for filter’s frame (if any) must be made from food-grade polyethylene w/UV inhibitors (if exposed to the sun).

⦁ Mesh Aperture
It is commonly accepted that the mesh aperture should be no larger than 40 mils (about 1mm). Filters with larger apertures will not be able to eliminate minuscule insects, their eggs and/or larvae as well as scraps of broken down organic matter from entering the tank.

The market offers a variety of pre-tank intake filters with apertures ranging from 40 mils (1mm) to 4 mils (0.1 mm).

Note that fine mesh can also filter the most common airborne sediments like sand, silt, and dirt. The math is simple: more of them can be prevented from entering the tank, healthier the stored water, less of accumulated sediments in the tank, and lower its maintenance service.

Unfortunately, the fine aperture is a “double-edged” sword – because it also lowers the water filtration rate (in other words – diverts more water from the tank). It may not be a problem in areas with abundant precipitations, but certainly, a big issue when rains are long-awaited sporadic events.

⦁ Robustness
While the pre-tank filters are mainly designed for insects and minuscule sediments, they may also occasionally face some rodents and other “water-thirsty” creatures, so its robust structure is of importance. Another aspect – robustness means also long lifespan! Once again, from this point of view stainless-steel is the best solution (lower cost nylon filters may have short lifespans). Note that the ruptured mesh of the pre-tank filter jeopardizes the quality of all stored water in the tank. It may be a too high price to pay in an off-grid environment!

⦁ Easily accessible for cleaning
The pre-filtering systems: roof-mounted leaf-filters and downspout-mounted First Flush Diverter(s) are not perfect. Keep in mind that they are designed as a compromise between efficient filtering and minimum water overflow due to potential clogging. Pre-tank mesh-filters make the last line of defense facing small organic scraps and airborne sediments (sand, silt, dirt…) that had been able to sneak through the pre-filtration system. They will accumulate on the surface of the mesh-screen and if not removed, they will obstruct the filtration process, lower its rate and lead to water overflows!

But there is more than just overflow – if these mostly organic scraps are left on the screen, they will eventually rot, and then decomposed by microorganism to minuscule bits will inevitably end up in the tank at the first opportunity (rain)!

That’s why it’s important to have easy access to a pre-tank filter for both – monitoring and manual cleaning!

“Basket”-type Pre-Tank Filters

Inexpensive plastic basket-type filters are popular for traditional garden-use rainwater harvesting systems. Due to its shape, a large dirt-collecting basket is able to filter rainwater while building-up sediments. At some moment, however – (quite often as a matter of fact), the basket-type filter must be cleaned.
These are usually low-cost filters, entirely made from the plastic (including the crucial mesh).

basket-filter-01

Plastic, basket-type pre-tank Garden Filter with 40 mils (1mm) mesh aperture. Note the outlet pipe – it’s necessary to evacuate rainwater in case the filter is fully clogged by solids. Source: 3P Technik (UK)

Tanks storing potable rainwater (or even utility-quality water) need better and more reliable pre-tank filters with stainless-steel mesh-screens. The basic however are the same.

Depending on the efficiency of the upstream pre-filtering system, sooner or later all kinds of debris will build-up on the surface of any standard basket-type pre-tank filter. Similarly, as in the more familiar case of swimming pools’ filters –periodic maintenance service (removing, cleaning, and re-installing) will be required. You like it or not – if you do not want to lose the precious water due to overflows, or farther contaminate the stored water with decomposed organic matter and a plethora of not always friendly microorganisms – the frequent cleaning service is mandatory!

To make it easier, some manufacturers offer a supplementary Maintenance Tray. It’s a sort of easy to remove secondary strainer placed on top of the primary pre-tank filter. When full of leaves and debris, it can be pulled out using the handle, turned upside-down, rinsed, and re-installed – all in a matter of seconds!

Example of a Pre-tank rainwater stainless-steel strainer filter (Model TATS11) and a Maintenance Tray (Model TMTG01). Source: Tank Shop (Australia)

Practical notes:

⦁ If properly maintained, removable pre-tank filters offer the best filtering rate. In other words: All water that comes in from the conveyance system gets through the filter and ends up in the tank! It’s an ideal solution for areas with low annual precipitations where every raindrop counts!

⦁ Removable filters are Maintenance Intensive! During wet periods you may be forced to clean the filter more than once (especially if the catchment area is surrounded by trees)! Finer the mesh aperture, more frequent service is required. Good news – typical airborne sediments like sand, silt, and in general dirt are all “grounded” by rains, so in the absence of leaves, the thorough cleaning at the beginning of the wet season may do the job.

⦁ Importance of the upstream pre-filtering system.

The Maintenance Tray in action (Model TMTG01). Source: Tank Shop (Australia)

Most of container houses are located out of urban areas and most likely are surrounded by Mother Nature. In fact, trees are part of the eco-friendly strategy (protection from direct exposure to the sun, part of the natural ventilation “environment”’ etc….. The bottom line – unless you are located in the middle of the desert where a “wandering” leaf is as rare as a snowflake on the Waikiki beach (Hawaii), you must have a well-functioning, efficient leaf-screen system.

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

With the large active area, roof-mounted self-cleaning leaf-screens have a very low probability of being clogged. They can maintain a high filtering rate over consecutive days of rain-and-leaf-fall on the roof. In contrast, just a few leaves going down the rainwater conveyance system can substantially obstruct the pre-tank filter or force you to (hopefully with an umbrella in one hand) keep cleaning it over and over again! Well, it’s not a dream job . While some may promote the idea that just a one, single-point pre-tank filter can do all this job, such a concept is lightyears away from the truth!

The next obvious question is:
Are there any self-cleaning pre-tank filters and what are their pros and cons|?

Self-cleaning Pre-Tank Filters

Vortex Pre-Tank Filters

Vertical “inlet” filters mounted in line with downspouts have self-cleaning properties. Unfortunately, self-cleaning comes at the price of lower water efficiency, because a portion of harvested rainwater will be used for washing-out scraps and dirt from the surface of the screen. In practice, hydraulic efficiency (filtration rate) of 90% will be considered as quite a good achievement (it means 10% of water diverted from the storage tank). Self-cleaning filters use the concept of popular Leaf-Shedding Rain-heads, although due to much smaller mesh aperture (from 30 mils (0.75 mm) to 4 mils (0.1mm)), they have a more complex structure.

The concept of vertical filters is based on the effect of vortex – characterizing the flow of fluid revolving around an axis line of the drain. In the case of rainwater – it is conveyed by gutters to downspouts (drains).
Without going deeper into physics, for the whole filtering system to work efficiently, it is important that rainwater on the way dawn develops the laminar flow clinging to the inner walls of the downspout. Such flow (swirling around downspout sidewalls) allows rainwater to “evenly” pass over the surface of “strategically” located mesh-screen. Most of the water will get through the mesh, while debris will be washed away!

For that to happen few conditions are necessary:

⦁ Water flow has to create the vortex pattern and for that to happen, it needs about 12ft (4 m) of downward flow in the downspout. Sometimes, it may be difficult to create such conditions.

⦁ The amount of water flow must be limited (easy to say, because it depends on the intensity of rain). A large amount of rainwater will result in a turbulent downward flow and will substantially decrease the hydraulic efficiency of the filtration system.

The bottom line: Vortex filters are very efficient at low rainwater flow rates. One could say – that’s when it really matters. In fact, during rainstorms, you may harvest so much of rainwater that the storage tank will anyhow overflow (so why bother?)

The following video (courtesy of Rain Harvesting Systems Ltd (UK) beautifully shows the idea behind the vortex filters.

As mentioned earlier, the key to the efficient operation of Vortex filters is the vortex effect. With the little help of ingenuity, to some extent, the effect can be “artificially” created by distributing rainwater in a “rotational” pattern over the cylinder-shaped mesh. The water passes through the vertical mesh walls and then is directed to the storage tank, while washed-out debris accompanied by rinsing water are diverted out of the system …

The concept of a Vortex Pre-Tank filter- model URVF01 in its underground version with the maintenance access from the top. Source: Wisy AG (Germany)/ Polymaster (Australia)

Vertical (Downspout) Pre-tank Filters

To overcome the problem of the Vortex effect, several manufacturers offer modified versions of vertical filters. While their filtering performance is better when rainwater flows downward in an orderly way (so the Vortex is helpful), thanks to the “angled” structure (water hits the mesh at about 45 degrees) they can better handle turbulent water flows.

Left: Concept of vertical self-cleaning pre-tank filter
Right: Practical implementation – according to the manufacturer, the Filter Regenwasser T50 has efficiency over 90% and with a mesh aperture of 4 mils (0,1mm) it can filter dirt, sand grains….
Source: Amazon.com

Horizontal Mesh-type Filters

In some situations, placing a pre-tank filter inside of the tank may have some merits. The most important benefits are:

⦁ Protection of filters from weather elements (direct sun exposure and UV radiation) ;

⦁ Protection from mechanical damage.

⦁ Single-point pre-tank filter design (note that for larger catchment areas with two of more downspouts, you will need more Vortex-type (or in general Vertical) pre-tank Filters).

While it will be hard to have easy maintenance access in big above-the-ground tanks, the underground ones equipped with manhole are quite good candidates for such a scenario. For practical reasons, most “In-Tank” filters will have a horizontal structure. And such configuration may have an extra advantage. As we discussed earlier, one of the most important conditions for the efficient operation of vertical filters is the Vortex Effect (in other words smooth, evenly distributed along downspout’s sidewalls flow of rainwater. For that to happen, rainwater needs at least 12ft of downward flow before hitting the filter. Usually, “In-tank” filters will be fed by long horizontal pipes, so the turbulent water flow can get an “orderly” (smooth) character before hitting the fine mesh-type filter. This in turn may create favorable conditions for the efficient flittering process.

Structure of the LineAr filter (Model 100 K). Source: Wisy AG (Germany)/ Rain Harvesting System Ltd (UK)

LineAr filter (Model 100 K) – Installation scheme inside of the water tank. Source: Wisy AG (Germany)/ Rain Harvesting System Ltd (UK)

Well, if one picture is better than thousand words, what you will say about watching the operation of the LineAr filter on video?

Water Handling Capacity

Most pre-tank mesh filters are designed for the specified size of the catchment area. The number (for example filter suitable for roofs of up to 500 sq.ft, or 1,000 sq.ft and so on) reflects the fact that the filtering efficiency of a given filter (especially self-cleaning ones) will steeply decrease at higher inlet flow rates (in gallons/second). Unfortunately, such specification is quite rough and may be misleading, because the volume of the water flow is not only proportional to the surface of the catchment area, but also to the intensity of the rain. And the latter depends on the geographical location. Anyhow, knowing that the pre-tank filter is typically the single point in the rainwater harvesting system (in other words, all captured rainwater have to flow through it), the filter’s specified Water Handling Capacity is a number to look at.

Pre-Tank Filters: Practical Notes

⦁ Pre-Tank mesh filter must be placed ahead (upstream) of any area that can accumulate rainwater. If the rainwater conveyance systems (downspouts and pre-tank pipes) can accumulate the water, they will also become the breeding ground for insects.

⦁ With typical mesh apertures of about 10 to 30 mils (0.25 to 0.75mm), pre-tank filters can eliminate from the water stream even minuscule sediments (sand, silt, dirt….). They are critical elements of the Rainwater Harvesting System designed for potable water. If the captured and stored rainwater is intended only for the use as utility-grade water (flushing toilets, washing car & driveway, irrigation…), fine mesh-screens can be replaced by larger (40 mils/1mm) serving as protection from insects. It will lower filters’ maintenance effort (basket-filters) and increase filtering efficiency (self-cleaning filters), but at the price of faster build-up of sediments in the tank.

⦁ When clean, basket-type mesh filters (w/o self-cleaning properties) have the highest filtering efficiency. Unlike the self-cleaning counterparts, their hydraulic efficiency (filtering rate), does not depend on the amount of inlet water. They will perform as well during rainstorms as during light shower rains. However, such filters capture and trap debris, which in turn will obstruct the water flow and may lead to overflows. To guarantee their efficient operation they must be monitored, frequently cleaned, and supported by an efficient, upstream pre-filtering system.

⦁ Self-cleaning mesh-filters, similarly as basket filters capture debris and sediments carried by the water. However, in contrast to the latter, they will use some amount of water to wash-out debris and sediments from the filter screen and carry them out of the system (usually to a drain or stormwater tank). The self-cleaning comes at the price of lower filtering efficiency. It may be an easy compromise at location with generous precipitations, but a tough-one in dry areas. Note that even self-cleaning rainwater filters need periodic maintenance and manual cleaning!

⦁ For efficient operation, fine-mesh pre-tank filters (even those self-cleaning) must be protected from larger debris by leaf-screens over gutters and First-Flush Diverters (FFD). BTW – note that fine-mesh pre-filters cannot replace First-Flush diverters. It’s true, that FFDs will also divert sediments and debris washed-away with the first wave of rain; however, they also eliminate potentially toxic rainwater with diluted chemicals (agricultural and industrial pollution) as well as microorganisms thriving on dead insects and wildlife deposits washed with first drops of rain. Liquid toxins and microorganisms will easily pass even through the finest mesh-type screen. The bottom line is that all these filtering subsystems are necessary parts of the well-designed, efficient rainwater harvesting system.

⦁ Most manufacturers claim that the “hydraulic efficiency” of fine-mesh, self-cleaning filters is well over 90% (in other words only 10% of water is “lost” in the cleaning process). It seems however that it may be an overstatement representing the case when “all stars are aligned”. In practice, the filtering efficiency depends on the pattern of the water flow. In general, the evenly distributed, smooth water flow results in higher filtering efficiency than the turbulent one (turbulent, gravity-driven high-velocity water flow, which often leads to a “bouncing” effect from the mesh screen rather than filtration). Efficiency also depends on the water flow rate. At high water flows, the amount of overflowing water rapidly increases, lowering the filtration rate. On the other side, at low water flows (characteristic for light shower rains), the filtering rate may be almost 100 %, but the filter may get quickly clogged due to potentially non-existing washing-out effect. This emphasizes the importance of the elimination of leaves by upstream filters!

Left: Principe of operation of the vertical pre-tank filter VF1 and the graph of its hydraulic efficiency vs inflow of rainwater. (1)-Inlet, (2-3)-filtration chamber w/mesh screen, (4)-Outlet for filtered rainwater, (5)-Outlet for water with debris.
Right: Filtering rate (efficiency) versus water flow rate. Source: 3P Technik Filtersystem GmbH (Germany)

Everything has its price and so is “Self-cleaning” property. Someone has to clean the filter and this “someone” is a portion of rainwater. This water, (hopefully together with debris but unfortunately also clean one without any debris) will be diverted out of the main water stream feeding the tank.

In some circumstances (low annual precipitations) it may be beneficial to set-up an extra tank for collection of diverted water by pre-tank filters. Such water can be easily used for irrigation and flushing toilets. Note that Self-Cleaning Pre-Tank filters continuously divert a noticeable portion of rainwater from the main water stream. This is in contrast to First-Flush Diverters that divert only a limited, pre-defined amount of water, then they automatically ”disconnect” themselves (cut-off) from the rainwater conveyance system.

Tank’s Inlets

Inevitably, over time, rainwater tanks will accumulate sediments. Even with a well-designed and maintained 3-stage filtration system including Leaf-Screens, First-Flush Diverters, and mesh-type Pre-Tank filters, you can still expect about 0.05” to 0.1” thick annual accumulation of sediments. For the obvious reasons, the flow of rainwater into the tank should not disturb sediments accumulated at the bottom of the tank. That’s why rainwater tanks should be equipped with “quiet” (also known as ”smoothing” or “calming”) inlets. In practice, it means that an inlet pipe should reach deep down into the tank (to avoid any “waterfalls” and end with U-section discharging rainwater towards the top of the tank.

Note that such configuration of the inlet also allows for continuous replenishment of the “old” stored water by the new, fresh one containing oxygen. The presence of oxygen in lower layers of water prevents the process of anaerobic decomposition leading to “stagnant” water! (more on that in a dedicated chapter).

Concept of the Quiet Inlet. Source: Verge Permaculture (Rainwater Harvesting – Anatomy of a Rain Tank)

Commercial implementation: Graf 6” Calming Inlet. Source: Rain Harvest Systems (UK)

More sophisticated versions of quiet inlets can “smooth” usually turbulent flow of rainwater to more evenly-distributed (swirl-less) upward drift inside of the tank preventing this way disturbance of fine sediments accumulated at the bottom.

Concept and implementation: Stainless steel Flow Calmer Inlet thanks to the expanded steel mesh diffuses the flow and forces it upwards into the body of water. Source: Rain-harvesting Systems

Tank Overflow System

Each rainwater tank must be equipped with an Overflow System to handle the amount of rainwater exceeding the capacity of the tank. It’s a simple (usually DIY-type) system that basically must meet two requirements:

⦁ Convey the excess water to the safe/dedicated area (whatever it means). For example, it may be a sewer system in municipal areas, drain system in rural areas, storm-water tank (if justified), or simply a long enough pipe dispersing the water away from the container house (if allowed by local regulations).

⦁ Protect the stored rainwater in the tank from insects, rodents and any other crawling creatures

And, in specific circumstances:

⦁ If the tank does not have a dedicated Vent, the overflow system must guarantee an unobstructed airflow from or into the tank following the changes of rainwater’s level.

⦁ If the overflow is connected to the sewer system – it must include backflow and odor prevention devices.
In its simplest form, the overflow system consists of an outlet pipe connected to the hole at the upper part of the tank’s wall and a fixed, mesh-type stainless-steel screen with an aperture of about 40 mils (1mm) – commonly considered as the minimum required to keep away insects. In a more complex version, overflow screens may have the form of hinged “Flap Valve”. Under normal circumstances, the valve self-closes sealing off the tank, however, when necessary, it will open under the pressure of overflowing water to let it pass at a high flow rate that may be required during rainstorms.

Left: Standard Overflow Kit; Right: Self-closing Flap Valve Vent Screen – both with mosquito-proof 0.955 mm stainless steel mesh. Source: Tank Shop ((Australia)

De-Sludging Systems

Regardless of the quality and effectiveness of typical pre-tank flittering systems, tiny sediments will always find their way into the tank. Fortunately, due to gravitational forces, over time they will drift and subsequently accumulate at the bottom of the tank.

Depending on the end-use of the stored rainwater, sooner or later accumulated sludge will have to be removed and tanks cleaned.

Typical cleaning methods:

⦁ Siphoning the sludge without emptying the tank by carefully moving across the bottom of the tank a hose ending with an inverted cone. It’s time-consuming, inefficient, and far from perfect!

⦁ Pumping-out the sludge using a suitable water pump. Same as above, although much more water-efficient than using natural siphoning effect.

⦁ Draining and cleaning the tank – in this case, the drain (de-sludging) valve located at the bottom of the tank is necessary so that the sludge can be washed-out of the tank.

TankVac Cleaning System

An interesting tank’s de-sludging method (and system) was designed and is currently offered by TankVac. It takes advantage of natural overflows happening in rainwater tanks. Typical overflow systems evacuate the upper layers of the water from the tank (which means, the freshest, richly oxygenated water). In contrast, the TankVAc-based overflow system siphons the bottom layers of the water with accumulated sediments.

TankVac and its Key Elements:

⦁ Siphoning Effect: “Purging” pipes with series of small holes along are located at the bottom of the tank. All holes face the dirtiest layers of rainwater stored in the tank. Note that the traditional Overflow Systems does not create the siphon effect because they evacuate the surface water out of the tank. It’s part of the design preventing tank’s self- emptying process.

⦁ Once the water exceeds tank’s overflow threshold, it starts to flow to the vertical exterior pipe on the side of the tank. Under normal circumstances, the outflowing water volume (flow rate) will “match” the inflow rate (in other words higher the inflow volume, higher the outflow volume. Unless the inflow is caused by a powerful rainstorm, the low-intensity outflow will not have the “purging” strength necessary to extract heavier than water sediments. That’s where the two next key components – patented Flow Generator & Overflow Hysteresis come.

Flow Generator: There are no details available, however based on visible facts one can imagine that the Flow Generator does not let the overflowing water to “leak” in small quantities (as it will happen in common Overflow Systems). Instead, it builds the larger volume of water in the exterior “downspout”-like pipe before fully opening to overflow. Such volume of water can create the powerful vacuum effect (according to the manufacturer- quoting: “ vacuum increases three-fold, drawing out sludge like a firehose-type jet at a rate of up to 2+ gallons/second (about 9 l/sec)”.

To make it more descriptive – the water will not leak via the overflow pipe. The Flow Generator will first build-up water to the full volume of the exterior pipe and then it will “open” to allow water to flow down the pipe (still in full volume). In a first approximation, the ratio of the cross-section of the external (downspout) pipe and the equivalent cross-section of all “purging holes” in the pipes deployed at the bottom of the tank makes the difference. It creates the “purging” force at the bottom of the tank!

Well, that’s the idea behind the “trick” to understand the operation of the TankVac (note that it does not need any power). But as we can expect:” the devil is in the details”

Note: For that to work, the rainwater tank must be equipped with the Air Vent. The air cannot be fed back to the tank by the overflow system, it needs its own, independent path!

The concept and scheme of the TankVac operation. Source: TankVac (Australia)

⦁ Overflow Hysteresis: Traditional Overflow System starts its operation when the water level reaches (on the way up) the overflow threshold. However, it also stops its operation when the water in the tank reaches the same level on the way down. The TankVac develops the dirt-purging siphoning effect, but it has to be stopped. Otherwise, once the siphoning effect starts, it will end only when the tank is fully emptied.

And this is a task of a small “Level-Control” hole in the upper part of the interior overflow pipe. Once the water level in the tank drops below the “Level-Control” hole, the latter will let the air-in, the water-siphoning effect will stop and with it the overflow process as well.
As simple as that!

The self-cleaning will start every time when rainwater level in the tank reaches the overflow threshold. The overflowing water is an “engine” – a propelling force necessary to purge the sediments from the bottom of the tank!

Please note, that this is an explication of physics driving the operation of the TankVac from the engineer’s point of view. It’s an attempt to make it clear that the TankVac is not a sort of Perpetual Motion Machine (Latin: “Perpetuum Mobile”). However, the author does not have any “insider” knowledge regarding the operation of the TankVac, so the above description is a sort of “Best Guess” approach.

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