Rainwater Harvesting System



This chapter provides an overview of all the components.

TODO: {Visual: overview, phases & connections}

A Rainwater Harvesting System can be divided into 3 phases, with 2 connections:

  1. Catchment: Collect rain drops from different surfaces.
  2. Inbound: Convey water to storage, from catchment.
  3. Storage: Store the water for later use.
  4. Outbound: Convey water from storage to distribution.
  5. Distribution: Extract water from the storage part to its final usage.


TODO: {Visual: examples of catchment compounds}

A catchment compound is made of the following components:

  1. Catcher: Surface catching rainwater.
  2. Funnel: Concentrate all the water to a central destination.


TODO: {Visual: examples of inbound connections}

An inbound connection is made of the following components:

  1. Rainhead: First line of filtration.
  2. First Flush Diverter: Third line of filtration.
  3. Cleanout: ??? TODO.
  4. Filtration: Second line of filtration.
  5. Inlet: The final section pouring the water into the tank.
  6. Calmed Inlet: TODO.


TODO: {Visual: examples of storage compounds}

A storage compound is made of the following components:

  1. Tank: Receptacle storing water.
  2. Air Vent: Allow air to flow in and out as needed.
  3. Access Hatch: Allow access inside the tank.


TODO: {Visual: examples of outbound connections}

An outbound connection is made of the following components:

  1. Overflow: Security to evacuate excess water.
  2. Outlet: External connection allowing to extract water.
  3. Outlet Intake: TODO.


TODO: {Visual: examples of distribution compounds}

A distribution compound is made of the following components:

  1. Pump: TODO.
  2. Pressure Bladder: TODO.
  3. Filtration System: TODO.
  4. Purification System: TODO.
  5. Consumer: Final destination of the water.


This chapter takes you on a typical journey, walking you through all the components.


  • TODO


  • Catchment Surface
  • For example the roof of a house.
  • The most important thing is that the surface is as water-repellent/nonabsorbent as possible. Meaning able to transport water with as few losses as possible.


  • For example a gutter.
  • Purpose: Concentrate all the water to a central destination (going towards the storage part).


  • First line of defense.
  • Purpose: Prevent large debris (e.g. leaves and branches) but also insects from going any further.
  • Low-tech stainless steel screen.
  • Ideally self-cleaning to reduce/avoid need for maintenance.
  • Also called “leaf eater”.


  • TODO

First Flush Diverter

  • Purpose: Trap the pollutants.
  • Working principle: (demo)
    • At the beginning, it’s empty.
    • It fills up when water starts coming in; which is the dirtiest water (as dirt must have accumulated on Catchment Surfaces).
    • Everything that is heavier than water sinks in the bottom.
    • The tube contains a light plastic ball that floats on the surface of water (i.e. where the tank inlet pipe starts). That ball seals polluted water by blocking the lower chamber.
    • From then on, water flows directly into the tank inlet pipe.
    • The lower chamber constantly empties itself slowly via a release valve (a simple hole or a flow-control valve). And therefore the ball let constantly the equivalent amount of water coming in.
    • When the water inflow stops, the lower chamber continues to empty itself.
  • It must be sized according to the Catchment Surfaces, to the dirt they can accumulate.
  • Water that comes out of the lower chamber should be put to productive use.
  • Caution: It requires regular maintenance as dirt will accumulate… being the purpose of this component!
  • If the release valve is clogged, this component will stay full of water and it will go septic.


  • TODO

??? Cleanout

  • TODO
  • (with release valve)


  • TODO


  • 30 micron
  • Can be done using a pantyhose.


  • To prevent the incoming water from disturbing the sludge layer, make the inlet pipe go down to the bottom of the tank (instead of simply falling down from the top).

Calmed Inlet

  • To prevent the incoming water from disturbing the sludge layer, add a “calmed inlet” to the tank inlet pipe.
  • “A Calmed Inlet at the bottom of your water tank will ensure that the water flowing into your tank will not disturb the settled sediment. The shape of the Calmed Inlet means that water is pushed upward into the tank. The upward flow of the water into the tank oxygenates the stored water. This oxygen-rich water prevents anaerobic reducing conditions forming in the storage tank and ensures that the water stays fresh and keeps your rainwater clean.”
  • Another advantage, it’s quiet.


  • Level the tank.
  • Ensure it has a solid foundation capable of supporting the weight of a full tank.
    • Remove all the topsoil below and replace it with non compactable material (e.g. gravel, concrete).
    • 1L of water = 1kg, so total weight = weight of the tank + total capacity in liters.

Air Vent

  • The tank is never void, it is always full of air and/or water.
  • The air vent allows air exchange as the water level rises or falls (i.e air goes out when water comes in, and vice versa). Also if the state of the water changes, between liquid and gas/vapor.
  • Without that air regulation, there is a risk that a vacuum is created inside the tank making the tank collapse on itself.


  • Purpose: Evacuate excessive water from the tank.
  • Working principle: Located on top of the tank (or at an upper limit), when the tank fills in to that level, water flows naturally through that pipe.
  • That pipe must be of the same size or bigger than the inlet pipe (to evacuate at least as fast as water flows in).
  • That water must then be properly evacuated somewhere safe. It can go to another tank.
  • Overflow connected to the outlet. (source)
    • Important: A hole on top prevents the tank from automatically siphoning all out in one go.

Outlet Intake

  • TODO


  • The outlet should not be placed at the very bottom of the tank but above that sludge layer.
    • …or, another school of thought, place it at the very bottom so that the sludge will always incrementally come out instead of building up.
    • “When you harvest rainwater, it is important to filter the water before it reaches the water storage tank. However, very small particles such as pollen and oil will often go through your filter mesh and into your storage tank. This causes your water to sour and the particles in the tank to build up over time. Particles heavier than water (such as pollen) will fall to the bottom of the storage tank and form a layer of sediment.”
  • The intake can be:
    • A L-shape pipe on the inside, taking the water in from above. Usually 100~200mm off the bottom of the tank.
    • A flexible hose with a buoy that floats on the surface, drawing water from the central column {TODO: ?!? clarify that}.
  • The intake should have a filter.


  • TODO


  • TODO

Pressure Bladder

  • TODO

Filtration System

  • TODO

Purification System

  • TODO

Consuming Endpoint

  • TODO


Access Hatch

  • TODO

Level Gauge

  • TODO


Potable Water

  • Use all potable/food grade materials.
  • Be careful that the water is not impregnated with chemicals (e.g. fungicides, pesticides, herbicides).
    • So avoid catching from asphalt shingle roofs.
    • Preferred materials for roof: enameled steel, galvalume (galvanized aluminum alloy), or galvanized.
  • Test water for pathogens, pH, heavy metals, herbicides, pesticides.

Cold Climates

  • “When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%.”
  • Harvesting snow is different from rain.
  • Collect on South-oriented surfaces to reduce freezing.
  • Pipes insulation.
  • A diverter valve can allow to not harvest water when there are risks to damage the system (cf. freezing).
    • If so, ensure that the water is directed to an appropriate location. Ensure that it is at least 3 meters/10 feet from a house foundation.


  • The water tank must be cleaned.
  • Screens and filters have to be emptied, and cleaned.
  • The bottom of the first flush diverter has to be emptied, and cleaned.



  • Has all the necessary nutrients for plants and agriculture.
  • Is an excellent source of water for vegetable gardens with no chemicals and dissolved salts, and free from all minerals.
  • Relatively simple and easy to install and operate.
  • Promotes water and energy conservation.
  • Decreases the demand for water.
  • Improves the quality and quantity of groundwater level.
  • Reduces soil erosion, stormwater runoff, and flooding.
  • Reduces pollution of surface water with fertilizers, pesticides, metals, and sediments.
  • Reduces water bill.
  • Reduces electricity bill for pumping water up from a borewell.

Content of Rainwater


  • Oxygen, sulfur, potash minerals.
  • Beneficial microbes that help build the soil.
  • Bacteria, heavy metals, bird poop, and all sorts of nasty stuff.
  • Compared to tap water:
    • No chlorine.
    • Microorganisms are able to proliferate {TODO: Is it a good thing?}


  • Garden irrigation, agriculture irrigation, domestic use, drinking water, groundwater recharge.
  • Rainwater makes the best compost tea.
    • {TODO: Before applying to plants, dilute the tea with rainwater or dechlorinated water (ratio 10:1).} (source)


  • Fluid dynamics.
  • Without pump, take gravity and water level equalization into account (cf. levels, slopes, inlets/outlets).
  • Having the screen tilted at 45° makes any debris fall down. Therefore no maintenance required.
  • Flow is limited by the diameter of the smallest pipe.


  • Water flowing in the tank must be as clean as possible to reduce dirt getting in the tank; therefore requiring cleaning.
    • Dirt adds up. The more clean each component remains, the less dirt will accumulate.
  • Appropriate slopes ensure efficient water flow.
    • Stagnant water gets dirty and sludge forms.
    • Sludge and accumulated dirt reduce water flow.
  • The tank needs to be completely vermin and insect proof. All openings must be protected as needed.
  • To prevent algae growth: (source)
    • Don’t let sunlight get into the water. Use completely opaque water storage tanks.
    • Add 1/4 teaspoon of bleach to every gallon of water you store.
    • Add 4 parts of chlorine to every 1,000,000 parts water.
  • Even with all the protections, there will still be a layer of sludge/sediment that will build up at the bottom of the tank. Do not disturb the anaerobic sediment layer.
    • A sediment layer, actually interestingly enough, is a very important part of the treatment train within the rain tank. Research paper from professor Coombs (Australia) about both biofilms that form on the inside of rain tanks as well as the sediment that forms at the bottom of the tank. These sediments and biofilms have been shown to be part of what’s called an ancillary or additional kind of incidental treatment train. They’ve proven that the sediment (sludge) at the bottom of the tank and the biofilms (slime) on the sides of the tank is actually hyperaccumulating heavy metals (e.g. lead, cadmium, aluminum), happening for about 2-3-4 weeks. So basically, you want your water to sit in this tank for that period of time for that cleaning mechanism to take place. Because of that, we don’t want that sludge layer to be disturbed (cf. tank calmed inlet and outlet). (source)

Electricity Generation

{Is it worth it?}

  • Hydropower or hydroelectricity is the conversion of mechanical energy from flowing water into electrical energy.
  • The fall and movement of water is part of the water cycle; a continuous natural cycle.
  • Water gains potential energy just before it flows downhill. This potential energy is converted into kinetic energy as water flows.
  • The water flow is used to turn the blades of a turbine, which coupled with a dynamo, generates electricity.

Rain Intensity

Based on precipitation rate. Wikipedia.

  • Light: < 2.5 mm (0.098 in) per hour
  • Moderate: between 2.5 mm (0.098 in) – 7.6 mm (0.30 in) or 10 mm (0.39 in) per hour
  • Heavy: > 7.6 mm (0.30 in) per hour, or between 10 mm (0.39 in) and 50 mm (2.0 in) per hour
  • Violent: > 50 mm (2.0 in) per hour

Harvest Calculator

  • Catchment Capacity (L) = Rainfall (mm) × Catchment Area (m²)
    • 1mm rain over 1m² roof = 1L
    • Imperial: gal = in × ft² × 0.623
  • Estimated Catchment (L) = Catchment capacity (L) × System Efficiency (%)

Example: (source)

  • Monthly rainfall: 79mm
  • Roof area: 395m² (85×50ft / 25.9×15.25m)
  • System efficiency: 50% {TODO: How can we guestimate that?}
  • Estimate harvest for that month: ±15.600L (=> annually: 171.000L)



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