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Rainwater collection and filtration systems

Rainwater collection and filtration systems


The unconscious use of our natural resources for centuries has led us to a critical situation from which there is no going back. Environmentalism urges us to look for alternatives to our conventional behavior. The ecological movements around the world have shown us that there are many ways to consciously use the natural resources we have. Water is fundamental for –most kinds of– life. And yet, water is a finite source. We will start by explaining what a rainwater harvesting system is; the basic concepts and requirements for a healthy and secure scheme; and two of our projects as study cases to explain our designing and building process. Let’s go!

Our Research

Water access in the world

The WWF Water Risk Filter shows a scenario where the 100 cities, that are expected to suffer the greatest rise in water risk by 2050, are home to at least 350 million people. So the population in areas with high water risk could rise from 17% in 2020 to 51% by 2050. About 30 Indian cities have been identified as cities that will run out of water in the next few decades. [1] This is a direct result of climate change weakening the monsoon season which will only worsen as time goes on. 

South America has 31% of the world’s freshwater sources making it the most water-rich continent. [2] However there are inequalities of access and sanitation for many –particularly in poor or rural areas. In Peru, for instance, 90% of residents overall have access to safe drinking water but this drops to 70% in rural areas. [3] Inequality is also prevalent in Africa: for example In North Africa 92% of people have access to safe, clean water but this drops to 61% in Sub-Saharan Africa. [4]

More than a quarter of the population in Sub-saharan Africa travels for over half an hour to access water. Typically women are expected to gather water resulting in missed schooling and employment which compounds issues of education, sanitation and equality, disproportionately affecting women and girls. [5]

Two children are carrying water
A woman and her children are carrying water

Here in Europe, with thousands of freshwater lakes, rivers and underground water sources available, the supply of water may seem limitless. But population growth, urbanization, pollution and the effects of climate change, such as persistent droughts, are putting a huge strain on Europe’s water supplies and on its quality.

The European Environment Agency – EEA estimates that around ‘one third of the EU territory is exposed to water stress conditions, either permanently or temporarily’. For more detail on water usage in Europe check out this highly detailed resource from the EEA.

Water usage in Europe and Portugal

On average, 144 liters of water per person per day is supplied to households in Europe but this only accounts for 12% of total water usage. Agriculture has by far the highest usage at 40% followed by energy production at 28% and mining and manufacturing at 18%. [6] Therefore domestic use makes up the smallest proportion of overall water usage. However, this is the usage that we have direct control over and with collective action big changes are possible. So where to begin?

The starting point when considering water consumption is to ask: are we wasting water? 

The first step is to review our consumption habits; 

The second one is to set up systems that help us to save water and re-use it in order to improve our habits. 

The third, and the focus of this article, is to gather water from other sources, in our case, the rain. How can we go about collecting rainwater to offset our water consumption?

What is rainwater harvesting – rainwater collection system?

You may have heard about this concept before –this is not an innovative system at all– and if you haven’t, the name itself is quite self explanatory but here is a full definition: “Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from a roof-like surface and redirected to a tank, cistern, deep pit (well, shaft, or borehole), aquifer or a reservoir with percolation.” [7]

Basic concepts

A collection system can be as simple as a surface leading the rainwater to a container, but we need to find ways to do it in a really efficient and secure way. For watering our garden there is no problem just collecting the water and using it when needed, but if we want to store this water and reserve it for whatever reason, care must be taken. Even more so if we want to use that water for human or animal consumption.

There are a lot of innovative systems out there but our aim is to find methods that can be replicable in a low-tech way. From the basic scheme, surface > collector > container, we can go to a more complex system where we need to start to think about the most dangerous factor: the risk of contamination. 

Polluted rainwater is an issue we may face at some point. Currently it is safe to collect rainwater in our area –Portugal–, but water can get contaminated during the collection process and when it is stored. This is our principal concern. We aim to create systems that are safe for human –and animal– consumption.

Different types of water

The European Union has a history of over 30 years of drinking water policy. This policy ensures that water intended for human consumption can be consumed safely on a life-long basis, and this represents a high level of health protection.

In this article we will talk about different kinds of water:

  • Running water – also known as tap water, mains water, city water, town water, municipal water, sink water, etc., is water supplied to a tap from the municipal network. Its uses include drinking, washing, cooking, and the flushing of toilets.
  • Rainwater – water coming and collected from the rain, it can be stored or directly used for farming.
  • Greywater – water already used for washing or cooking
  • Wastewater/Blackwater – water unsuitable for reuse unless heavily treated or filtered through a process of phytodepuration.
Basic elements of a rainwater harvesting system
On this picture you can see a house with its garden. It explains the different parts of a rainwater collection system.


Roof // usually, the easiest way to collect rainwater is from our roofs.

Gutters // a trough along the eaves to catch and carry off rainwater.

Drainage // drainage in pathways, under houses, can help collect impressive amounts of water.

Containers // receptacles –such as tanks, cisterns or pits– for storing the rainwater.

Filters // devices or materials for suppressing elements or particles that are not suitable for human or animal consumption. These can be chemicals or physicals. An example for this is the leaf screen and the first flush. 

Rainwater collection system

The best roof material for collecting rainwater is a metal roof. Tile and wood shingle roofs are good materials as well, but gaps between tiles are susceptible for crud to build up. If you have a roof with asphalt shingles you should not use this to collect water for the first few years but after that they are fine to use as well. Also, it is important to calculate the gutter in a big enough size, so the water is not splashing over it. 

This picture shows all the parts of a rainwater collection system from the roof to the pit

From the gutter the water is flushing through a leaf filter, which sits just below it. The leaves will fall off on the angle while water is coming through the grid to the collection system. The water runs through the pipes and the first dirty flush will be collected by the first flush divider. It has an outlet with a fentil at the bottom so it empties automatically till the next rain. This part needs the most maintenance, because you have to clean the dirt out regularly. 

The clean water goes to the tank which could either be a concrete, plastic, metal or wood one. The tank has different components. There should be a vent which is letting the air out if new water is entering the tank. Also the tank needs an overflow which should be the same size or bigger than the incoming pipe. If there is a heavy rain the water must be able to leave the tank. This overflow can also be connected to more tanks. Finally, there is the tap where you can use your stored water. You can either use the water to irrigate your garden or pressure it with a pump to use it, for instance, to flush the toilet or the laundry machine. The tank needs to be totally insect, rodant and amphibian proofed, otherwise animals can live and die inside. In addition the tank should be dark so no light is shining through to prevent the growth of algae. Over time a layer of mud will build up at the bottom of the tank. The best way to avoid getting it into your water system is to not install the tap on the very bottom of your tank. Lift it a little bit up, so the mud stays inside until you clean it. Another thing to consider is to make sure that the inlet pipe goes down all the way to the bottom of the tank, so it does not disturb the mud by flushing in. [8]

Rainwater Filtration System

The following filtration system, which we found on the website of Aqueous Solutions, is a pretty easy example of how to make rainwater drinkable. It is made out of four connected 200 liter tanks which can filter up to 300 liters of rainwater per day.

This picture shows the different barrels of a rainwater filtration system.

The first tank is the gravel pre- filtration. It removes sand, organic matter and some microorganisms from the water. The water flows slowly upward through the gravel. So sand and other gravel settle to the bottom of the tank. Due to that it is important to open the valve at the bottom of the tank to flush out the accumulated solids. 

The second tank is the slow sand biofiltration tank. It removes fine particles, biodegradable dissolved organic matter and microbial pathogens. The water flows down through a bed of fine sand which removes small particles by physical straining. Over time a natural biofilm of beneficial microorganisms built up at the top of the sand. This removes microbial pathogens and biodegradable dissolved organic matters including some synthetic chemicals. If the flow rate of the filtration system gets worse you should remove some of the natural biofilm at the top of the sand. 

The third tank is the biochar adsorption tank. It removes all the organic and chemical contaminants which are left in the water. The water flows down through a bed of crushed biochar. All chemical contaminants are drawn into the fine micro-pores of the char and stick there. To make sure the water can be cleaned properly by the biochar, you should replace it once a year. The fourth tank is the storage tank where the drinkable water can be saved ready for use. [9]

Our Approach

When Critical Concrete started working on refurbishing social housing in 2016, one of our main objectives was to do it in a more ecological way. Although we saw the emissions leveled off in 2013, in the last five years building and construction sector haven’t stopped increasing their emissions: “buildings and construction sector accounted for 36% of final energy use and 37% of energy and process-related carbon dioxide (CO2) emissions in 2021, 10% of which resulted from manufacturing building materials and products such as steel, cement and glass” [10]

Fueled by our dedication to reverse this trend, we’ve been researching on how to implement more sustainable systems in our buildings. In the case of rainwater collection systems, we focused on trying to use less polluting materials, vernacular techniques and specific schemes for any case in order to be more environmentally friendly in our building process and our daily life.

Process and methodology

For our RWC, we use the following basic methodology: 

  1. Analysis of the environment, local context and situation: weather and rainfall statistics; differences in height on the plot of land and distances.
  2. Analysis of inhabitants’ water consumption habits: quantity of consumed water per day and month and different kinds of usage.
  3. Technical characteristics of the place where we want to implement the system: roof measurements and free space for reserves and suitable equipment.
  4. Scheme design

We will explain the development of this basic process applied to one case study of a rainwater collection system in our headquarters adapted to the drainage and plumbing system that already exists.

Case Study

Our case study is based in the north of Portugal. In 2019 the Portugal Nature Association (ANP) in collaboration with the international World Wide Fund for Nature (WWF) issued a report on “Portugal’s Vulnerability to Drought and Scarcity”. In it, they state that healthy aquatic ecosystems are the best defense against rising temperatures and reducing rainfall. They also advocate for wastewater reuse amidst the increasing risk of droughts. [11]

Our project is part of the fight against water scarcity and is designed to be adaptable to many different circumstances.

On this picture you can see the headquarters of Critical Concrete in Porto.
Critical Concrete headquarters, Porto

Critical Concrete’s headquarters are located in the outskirts of Porto city, in the north region of Portugal. 

The building has three floors. On the ground floor you can find the kitchen, dining room, bathroom and the workshop. The first floor is mainly used for the office. There are some additional rooms for visitors who stay at the headquarters overnight. In addition there are two people living permanently under the roof on the third floor. We’ve been refurbishing the building with low-tech methodologies during the last years. The main building got a renovated roof with new tiles on it. Next to it there is the workshop on the ground floor which is covered by a green roof. On the back side of the building there is a terrace which can be accessed by the first floor.

1. Analysis of the environment

Porto has a warm summer Mediterranean climate with an influence of an oceanic climate. It has warm, dry summers and mild, rainy winters. But also the summer can be interrupted by cool and rainy days which normally may last a few days. Porto is one of the wettest major cities in Europe due to its high annual precipitation. Even in the rainiest months it is possible to get warmer days with a lot of sun. [12] During the last six years the average annual rainfall in Porto was about 1285mm/year. [13] As a reference, Paris averages 634.3mm/year [14], Mumbai 2213.4mm/year [15], Johannesburg 713mm/year [16], São Paulo 1,658.3mm/year [17] and on the other extreme Cairo 24.7mm/year [18].

2. Analysis of inhabitants’ water consumption habits

There are currently two people living at the headquarter of Critical Concrete. In addition there are approximately 15 people at the office during the working days, including preparing the lunch for everyone.

In the following graphic you can see the total average collectable rainwater we can get from all the surfaces we have in each month in blue. On the other side you can see the total average water consumption per month in red. The yellow line shows the tank level at the end of every month. So as a conclusion we can see that only in August, September and October we would need to get extra water from the city. All the other months we could only use the collected rainwater. However this is in an idealized system where we could collect, filter and store 100% of the rainwater. In reality there will always be a lost, but it will still be a significant amount to collect and use.

Rainwater Collection CC [19]

In this calculation we used the actual water consumption which you can normally find on your water bills. In case you do not have these numbers you can also use the following system to calculate your approximate water consumption.

What?Consumption per dayConsumption per month
80 l/day per person
2 persons= 160 l4800 l
10 l/cycle
1 cycle per day = 10 l300 l
Toilet Flush:  
5 l/flush
working 17 persons, 2 flushes each = 170 l
weekend 2 persons, 3 flushes each = 30 l
3640 l
50 l/cycle
3 cycles per week = 21 l630 l
7 l/day per person
working days 17 persons = 119 l
weekend 2 persons = 14 l
2492 l
2 l/day per person
working days 17 persons = 34 l
weekend 2 persons = 4 l
412 l
12.274 liters
Water consumption calculation [20]

So from this calculation we can see that the approximate water consumption of the Critical Concrete headquarters is around 12.300 liters per month. If we compare this to the average water consumption of the water bills which is 19.400 l per month, we see that around 7.100 l are missing. This is because we use a lot of water to irrigate the plants on the green roof and also we are using some water in the daily workshop tasks.

3. Technical characteristics of the place

There are four areas at the headquarters of Critical Concrete where it is possible to collect water from. The first and biggest area is the roof of the main building with 207 m². In addition there is the green roof with 128 m², the terrace with 38 m² and the roof of the courtyard with 77 m². This makes a total of 450 m² surface to use for rainwater collection. With the average annual rainfall in Porto of 1285mm/year, it could be possible to save up to 265.000 liters of rainwater per year.

4. Scheme design

The idea is to save as much rainwater as possible from the roofs, so we can run everything which doesn’t need drinkable water with the saved rainwater. For example the toilet flush, the dishwasher, the washing machine or the plant watering. 

In addition to that we are planning to build a rainwater filtration system as mentioned above. This will also allow us to use the rainwater for things where you need to have drinkable water, for example for showering or cooking.

Tank size

Footprint of roof (m²) x drainage coefficient x annual rainfall x 0.05 = recommended tank size

There is an easy way to calculate the tank size you need by yourself. You multiply the footprint of the roof with the drainage coefficient (which takes care of the water which will splash from the roof not landing in the tank) and the annual rainfall in mm. From all this you take 5% as your recommended tank size. [21]
So in our case the roof area is 206,50 m², a normal drainage coefficient is 80% and the annual rainfall is 1285 mm/year.

206,5 m² x 0,8 x 1285 mm/year x 0,05 = 10.614 l

The recommended tank size for our headquarters is around 10.000 l. With this number in our head we can go on to the next step of finding the good location for the tank in our headquarters.


When you think about the location of your storage tank you have to consider a few things. First you need to find a spot which is big enough for your tank. In some cases it could be smart to split the tank into more then one or also dig it under the earth so you can still use the space on top. Also you have to consider that the location is easily accessible for the pipes coming from the roof. In addition if you decide to put your tank on the ground floor or even under the earth, you should keep in mind that you will need a pump for later using it on a higher level.

In our case we have an empty room on the ground floor, which was a toilet before, where we would like to put our tanks. Due to the limited room in there we need to split the tanks into separate smaller ones. Also we would start with 6000 liters, because of limited room. Here you can see a floor plan of our design proposal. In our case, since we also want to use the water in the upper floors, we would need a pump to get the water up as well.

This is a floor plan of the ground floor of Critical Concretes headquarters and the proposal of the collection tanks.
Ground floor CC headquarters


This case study shows your first steps you should think about, if you are planning your own rainwater collection and filtration system. As with all sustainable solutions you have to assess what works for your environment, usage and ambitions. So let’s get started and build your own rainwater collection and filtration system. Already a small system can help to save a lot of water and is really easy to do by yourself. Follow Critical Concrete on social media for updates on how these projects progress. 


[01] https://www.wwfindia.org/?19602/Cities-across-the-globe-face-an-alarming-rise-in-water-risks

[02] https://www.worldbank.org/en/news/feature/2012/08/30/agua-saneamiento-america-latina

[03] https://borgenproject.org/access-to-safe-drinking-water-in-south-america/

[04] https://www.un.org/waterforlifedecade/africa.shtml

[05] https://www.unicef.org/press-releases/unicef-collecting-water-often-colossal-waste-time-women-and-girls

[06] https://www.eea.europa.eu/signals/signals-2018-content-list/articles/water-use-in-europe-2014#:~:text=On%20average%2C%20144%20litres%20of,differs%20from%20region%20to%20region

[07] https://en.wikipedia.org/wiki/Rainwater_harvesting

[08] https://www.youtube.com/watch?v=DhEaKdmHeCk

[09] http://www.aqsolutions.org/images/2016/02/blue-barrel-system-manual-English.pdf

[10] https://globalabc.org/resources/publications/2021-global-status-report-buildings-and-construction

[11] https://www.theportugalnews.com/news/portugal-continues-to-consume-too-much-water/51465

[12] https://en.wikipedia.org/wiki/Porto#Climate

[13] https://en.climate-data.org/europe/portugal/porto/porto-161/

[14] https://en.wikipedia.org/wiki/Paris#Climate

[15] https://en.wikipedia.org/wiki/Mumbai#Climate

[16] https://en.wikipedia.org/wiki/Johannesburg#Climate

[17] https://en.wikipedia.org/wiki/S%C3%A3o_Paulo#Climate

[18] https://en.wikipedia.org/wiki/Cairo#Climate

[19] https://en.climate-data.org/europe/portugal/porto/porto-161/

[20] https://www.ccwater.org.uk/households/using-water-wisely/averagewateruse/

[21] https://www.owlshall.co.uk/rainwater-harvesting/knowledge-base/what-size-rainwater-tank/

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