Low cost decentralized water purifier - a block of wood

This simple low cost design using a slice of a tree trunk could provide low cost purified water to millions of people.

It’s a brilliant biomimetic idea - wood is one of nature’s best drinking straw…developed over millions of years. Tree’s suck up water from the ground and send the water up to the leaves to evapo-transpire in the atmosphere.

This design is based on the brilliant research of Mingwei Zhu, Yiju Li, Guang Chen and Feng Jiang of Nanjing U, Harbin Engineering U, U of Maryland and U of British Columbia respectively, in their 2017 paper: Tree-inspired Design for High-Efficiency Water Extraction.
The great thing about this technology is that it is advanced science but extremely low tech. It can yield an astounding 11 litres of water / hour in the lab and can be used not only to purify water, but extract water right from the earth! For this reason, I have designed two different models to test.

10.1002adma.201704107.pdf (2.5 MB)

Designs covered are:

  • unit for manually desalinating seawater or brakish / gray water
  • unit for extracting water right from the soil in the ground.
  • smallholdings agricutlural application: long aqueduct and channel ocean water to circulate inland. The aqueduct can also be lined with many slices of tree trunks and covered with poly plastic with a channel on the inside to drain the potable water into the surrounding ground for drip irrigation. In drought regions of South Africa near the ocean, pumping ocean water inland a short distance into coastal land can irrigate it.
  • simplest low tech emergency model: the simplest model is to put the ocean or brakish water in a tub and place a piece of poly plastic covering with channels for the potable water.
  • These units could also be an excellent solution for residential water purification. We will experiment with this in our own ecohome design.

If Day Zero should ever strike, these units could be used in an emergency situation to allow people to draw potable water right from the ground their home is built on. If we can reach theoretical limits of 11 litres of water / m2 / day, that would be more than enough to provide for a family.

In South Africa, the natural species to try is Bluegum. Tests would be required to test Bluegum on land to see what kind of potable water rates we can achieve. Such units could prevent many people from having to go to queues. Experiments need to be conducted to determine rates of water purification for different trees, ring thicknesses and water quality for each country. The bonus for invasive species is that it is exploiting the very property of invasive species to solve the water scarcity problem. So we can solve two problems at once, controlling invasive species, but repurposing the property that makes them so damaging to benefit us instead.

Sketch of unit for purifiying Ocean Water / Gray Water

Sketch of design of unit for extracting potable water directly from soil

Such simple technology may have a profound impact for millions of marginalized people around the globe who do not have access to clean water. Different groups can test different species around the globe to find the optimal solution for their respective country. @RicardoRug can try species in South America, @Qing can suggest colleagues to try different species in China, Innovators in the Middle East, India, Australia, any water stressed city, can experiment to find the optimal sustainably harvested species.

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Here some of the ideas in Bogotá we think can be developed with this tech.

his proposal can be installed in parks to extract water from the floor and be used for drinking fountains or the irrigation systems of the same park, to reduce the amount of water that is used for this, without the need to connect to the aqueduct.

Can also be installed in public spaces connected to urban furniture cone chairs, which can contain the mechanisms inside and this water is used for drinking or to have water for animals.

Some households are located in areas that do not have connections to urban aqueduct networks, which limits their access to potable water, in this case it can be installed in areas of several homes where several families can access to consumption.

Another option is to connect them to gray water systems and pipes to filter and recover it to reach the environment, evaporating in the air, helping in the cleaning of these structures and reducing the impact of gray water in the city.

In the poor neighborhoods of the city, or in other public areas, they can be installed next to stairs, so that they can recover the water, be moved to a collection system that takes them to the homes or is taken to shared spaces for access by all population. These structures can serve to structure and strengthen the terrain, improving the quality of life of people.

With this same objective, it can become structures that allow improving public spaces impacting on the beautification of areas, connection with urban crops, improving security, repopulating green areas, and providing public water for all.

Great design ideas Ricardo! I’m actually building a prototype this week. I charred the log slice and will build a proof-of-concept unit with very primitive materials. What tree species do you have that you can sustainably harvest in Bogata?

You can couple a low cost hand pump to it to pump the water up. It would be good for various innovators to try it in different parts of the world and see what kinds of tree species are optimal in their part of the world, and perhaps make a simple google spreadsheet to keep track of them all. I can make that spreadsheet.

We should be using this as well as working on the long term solution, for instance as @omaes72 Justdiggit project to restore the hydrological cycle in water stressed places like Cape Town and Bogata. These work on the larger scale of the watershed, but are simple interventions. We just need to mobilize people to do it.

Hey Ricardo, do we have any colleagues in Brazil on OSCEdays or in your network we can bring into our team so that they can test in Brazil? @William and @Cindy and I were chatting on email and William sent this newspaper clipping:

I think we should bring arborists with expertise in the various respective countries we want to try the pilots in, onto this project to help us identify the best species we can harvest sustainably for this project…preferrably fast growing and alien species. I found a database of invasive trees for various parts of the world. Here is Brazil:

I absolutely love the block of wood “solution” - this has legs!

First thought - without reading in any depth, is - how do you know when to change the block?

No idea William! I’m going to digest the research paper some more to find out. I’ll probably contact the scientists after I’ve accumulated a bunch of questions.This is such a low tech solution…find the invasive species of the country, the ones that have the same characteristics as American Basswood, and test it out. We are using bluegum.

I just torched my bluegum block yesterday:

I just have to make a polysheet cover with channels to drain the fresh water and try my experiment to see if I can get a large amount of evaporation and condensation. I haven’t compared bluegum to basswood yet but I assume we are looking for trees which are efficient at sucking up water. It’s simple enough to try.

i have found some that are locally grown for wood use are the following, and that are allowed for this.

pinus patula
Brosimum utile
Humiriastrum procerum
acacia mangnium
tectona grandis
pinus oocarpa
pinus tecunumanii
pinus patula
gmelina arborea

I am looking for physical characteristics to analyze with which we could do tests.

Another quick thought - some woods are poisonous. As a sometime woodworker I know this only to well - I made a Yew mantelpiece which was seriously dangerous to work on - beautiful wood but very toxic (it’s a well known source of a couple of chemotherapy drugs - https://www.independent.co.uk/life-style/health-and-families/features/the-clippings-of-a-300-year-old-150ft-yew-hedge-can-save-the-lives-of-cancer-patients-9655500.html).

The best list I can find is on http://www.wood-database.com/wood-articles/wood-allergies-and-toxicity/
There’s also http://www.hse.gov.uk/pubns/wis30.pdf - I haven’t compared the two.

That’s definitely good to know William! Remember to apply that toxicity test to any species you come across, @RicardoRug. We call Eucalyptus Bluegum here in South Africa.

Maybe best to stick with the Latin names :slight_smile:
Poor Linnaeus did the best he could with the knowledge he had at the time.
Eucalyptus globulus - https://en.wikipedia.org/wiki/Eucalyptus_globulus
It has plenty of chemicals in it but they actually look really positive - https://en.wikipedia.org/wiki/Eucalyptus_oil - eucalyptus flavoured water? I can think of a lot worse.
Toxic yes, but to microbes not humans!
It would be a really interesting experiment to compare e coli levels in tainted water filtered through different woods. The Pine/fir family also have plenty of phenols - also excellent bactericides - http://wgbis.ces.iisc.ernet.in/energy/HC270799/HDL/ENV/enven/vol357.htm
This might be positive or not - it’s probably about levels. First flush the wood through maybe?

Hey @RicardoRug, I think what we need to do is, aside from insuring it’s not a toxic tree species, find out how the tree species performs against American Basswood. I have bolded below the relevant features of basswood that any other comparable species must have to perform like basswood:

The efficient solar steam generation from the bilayer basswood arises from its unique hierarchical structure, which facilitates both light/thermal management and fluidic transport
(Figure 4).”


“The bilayer wood with a carbonized surface treatment exhibits a broadband light absorption approaching 99% over almost the entire measured wavelength range. Notably, the light absorption is much higher than the natural wood, especially in the visible range. While the carbonized layer enhances light absorption for solar steam generation, the bottom natural wood layer provides excellent thermal isolation and rapid water transport when inserted into the ground or seawater. To illustrate this, the temperature distribution of the bilayer wood under different illumination intensities was monitored with an infrared radiation (IR) camera (Figure 4D; Figure S12, Supporting Information). Stable surface temperatures of 28.3, 34.7, 42.3, and 57.4 °C, were measured at 1, 2, 3, and 5 suns, respectively. These results indicate that the bilayer wood can attain a high solar steam generation efficiency at relatively low surface temperatures. Note that the natural wood has a low thermal conductivity of 0.2 W m−1 K−1, which mitigates thermal dissipation from the top carbonized wood surface when exposed to solar illumination. The wood microstructures provide unique water transport characteristics. As shown in the SEM image in Figure 4E, each vessel (with an average size dv ≈ 50 μm) of the basswood is surrounded by fiber tracheids (with an average size dt ≈ 5–15 μm).The cell walls of both the vessels and fiber tracheids are composed of cellulose microfibrils embedded within the lignocellulosic matrices. This ensures that the walls are nearly impenetrable and can confine the water transport within the vascular lumens.[30] However, the vessels and fiber tracheids remain connected via pits (dp ≈ 2 μm), which are located throughout the cell walls.[31] A unique ability of the wood is the rapid transport of water through the structure when in contact with the water underneath. This water transport capability stems from the mesoporous and hydrophilic nature of the basswood itself. To understand how water transports through basswood (Figure 4E), computational fluid dynamics (CFD) simulations were conducted on one repeating block unit (75 × 75 μm2, as shown in Figure 4F; Figures S13 andS14, Supporting Information) using SOLIDWORKS. In the present study, the height of the wood block (≈3 cm) is much smaller than the maximum height (35 cm) attainable by the capillary rise of water. Note that this maximum height is obtained by balancing the forces between surface tension and the weight of the rising water (i.e., the Jurin height; Static Height Calculation, Supporting Information).[33,34] Consequently, a thin water film (≈25 μm) is apparent[35] on the top wood surface (≈3 cm from the base water surface) since the water evaporated (average evaporation rate of 3 μm s−1 under 10 suns) is continuously replenished by capillary-driven water ascension (filling rate of 5 mm s−1 at 3 cm above the water surface) (Calculation of the Filling Velocity; Figures S15 and S16, Supporting Information). The derived water trajectories depicted in Figure 4G (CFD simulation) indicate that the vessels provide the dominant pathways for water to ascend the wood microstructure. The velocity contours at various heights show the water flux/transport through the open-ended vessels located in the middle of the wood structure. On the contrary, one witnesses a very low water flux inside the tracheids with smaller lumen size and intermittent closed ends. In this scenario, the tracheids act as obstacles that limit water transport, despite the fact that these structures are connected to the middle vessel via massive pits. The 3D CFD simulation also demonstrates that the pits have little involvement regarding water transport along the vessels. Consequently, the effective capillary size for evaporation-driven water transport through the basswood is predominantly determined by the lumen size of the open-ended vessels (see the Supporting Information for more discussion on flow physics).
The carbonized wood is stable in air, which is ideal to avoid material degradation under highly concentrated solar steam conditions. The two wood-based layers are seamlessly integrated, which avoids the typical degradation effects plaguing conventional water extraction and solar steam materials (e.g., oxidation, detachment, and agglomeration, especially with nanoparticles). The bilayer wood operates in a continuous and stable manner for 100 h under 5 suns in seawater (Figure 5A). Note that salt deposition is only apparent at illumination intensities ≥5 suns. This indicates that at higher illumination intensities, the surface evaporated water cannot be readily replenished by the water that refills the channels. In this case, the amount of salt that deposits on the wood surface increases to a level that can be observed. Note that salt accumulation is not an issue for
ambient solar irradiation (1 sun) since the relatively slow evaporation rates at lower illumination intensities hinder the salt’s ability to reach its crystallization concentration. Nevertheless, the salt that accumulates after 10 h of operation under 5 suns does not noticeably reduce the steam generation performance.”

So the basswood has unique properties. We have to talk to wood physiology scientists in our respective parts of the world to see if eucalyptus, and other invasive species have close enough properties.Of course, we can simply do an empirical test first to see as well…and that’s what I intend to do.

Also note upon reading it, that the 11 Kg/m2/hr flow rate is with 10 suns, which means you would need a lens system of some kind to achieve that rate:

Under 10 suns, the E.R. for the generated solar steam exceeded 11.2 kg m−2 h−1. The E.R. versus optical concentration (Copt) is shown in Figure 3C.”

In figure 3, the Evaporation Rate (Kg/m2/hr) is measured against the Copt - which is the Intensity of Incident Illumination, measured as a ratio of suns ranging from 1 sun (normal) to 10 suns (10x magnification). So with NO magnification, we would achieve 1 litre/m2/hour. This means 1 square meter would generate 1 litre per hour. If we want 11 litres/hr, we would need 10 square meters. It’s also interesting to note that the evaporation rate is not much different between placing it directly on sea water or placing it on wet soil. The grains of sand physically obstruct the pores in the trees only slightly.

If we want to concentrate the sunlight, we need to use low cost magnification system. @MarkusKruger and I were looking at all kinds of concentrated solar systems for our Stop Reset Go ecohome design last year. Maybe we can apply some of that design knowledge here.

This nature paper uses micro lenslet array fabricated by 3D printer:

but it uses a top lens layer and bottom reflecting one, with the Concentrate solar PV in between. This wouldn’t work for us obviously because our wood is at the bottom. But the idea of micro fresnel lens array is intriguing to concentrate sunlight onto the surface. The normal fresnel lens concentrates everything to one point with intense energy. A fresnel lens array would distribute a less concentrated pattern over a wider area…which is what we would want.

We can print optics using new generation 3D printers: https://www.azom.com/article.aspx?ArticleID=11129

So we could print micro fresnel lens array like this for more efficient models:

haha…first flush! Interesting how new technologies create new processes!

Here’s a fellow I’ll get in touch with here in Cape Town:

Dr. Luvuyo Tyhoda
Wood chemistry, anatomy and composite materials​
He looks like he would fit the profile of what knowledge we need here.