Biojardineras or how to purify water with stones, plants and microorganisms

When we were told that it was possible to purify wastewater by growing plants on a bed of gravel, we had a hard time believing it. But we were even more surprised to learn first-hand about the case of the community of El Rodeo, in El Salvador, where 60 bio-gardens have recently been installed to treat the gray water of more than 80 families and reuse it for irrigation.

Gray water is the wastewater that comes from the shower, sink and bathtub, and differs from black water, which comes from the toilet.

According to data from the United Nations, more than 80% of wastewater is dumped into rivers, lakes or seas without prior treatment, which causes contamination of water resources.

The agricultural sector accounts for approximately 70% of global water consumption, which is why interest in reusing gray water for irrigation is growing, especially in communities in the global south.

But how can the installation of wastewater treatment systems be promoted in communities with limited financial and material resources?

The development of appropriate alternative technologies is essential to ensure that all people have an adequate sanitation system and thus contribute to achieving some of the goals established in the sixth axis of the United Nations Sustainable Development Goals.

Among other options, bio-gardens or artificial wetlands attract a lot of attention for being accessible, economical, efficient and simple to install and maintain systems.

Biojardinera installed in the community of El Rodeo (El Salvador).
Author provided

How does a biogarden work?

These sanitation systems seek to replicate and even improve the conditions that exist in natural wetlands in order to take advantage of their purification capacity. They combine physical, chemical and biological processes.

At the entrance of the system, the water is introduced into one or several pretreatment tanks, also called grease traps, where oils and greases are separated by flotation, and where heavy solids are collected at the bottom of the tank.

The water then passes into a gravel-filled basin. This is where the magic (or rather, the science) happens. All the elements used in these systems are combined to ensure that the water comes out clean and ready to be reused.

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Basin filled with gravel to purify the water.
Author provided

Coarser stones are placed in the first section of the pond with the mission of retaining the coarse suspended solids that have not been retained in the pretreatment.

The fine gravel, which occupies most of the pond, serves to favor the growth and fixation of microorganisms: small unicellular organisms that are responsible for the digestion of organic matter suspended in the water, the main contaminant in water domestic waste. These microorganisms are present in the soil and in the water, and they proliferate naturally when the right conditions are met, which is what is intended to be replicated in the gravel bed of the bio-garden.

Lastly, the roots of the plants help to retain small suspended solids and, above all, absorb the nutrients generated by the microorganisms in the digestion of the organic matter.

Therefore, when we build a bio-garden we have to choose some plants that adapt to the climate and can live with their roots permanently flooded (local aquatic plants) and a gravel that allows the passage of water and the fixation of microorganisms. And if we make sure the microbes don’t go hungry, nature will take care of the rest.

Also a decorative element

Although the main achievement of the bio-garden is to guarantee a sufficient level of gray water purification so that it can be reused or discharged without altering the balance of natural water systems, this system has other advantages.

The aerial part of the plants is decorative and has a positive visual impact, which facilitates its integration into the landscape in any location.

In addition, due to the high level of organic load elimination, the water that comes out of the bioplanter can be used to irrigate gardens, fruit trees or to ensure horticultural production during the dry season in tropical regions.

In most cases, bioplanters are used to treat gray water from homes, so they adapt very well to domestic environments. However, they have been successfully installed on a larger scale, such as for the treatment of water from the dining rooms of the University of El Salvador.

The Science of Constructed Wetlands

Although interest in artificial wetlands as a natural solution for water treatment began in the 1980s, it was not until a few years ago that these systems attracted the attention of the scientific community.

In the last decade, the need to delve into the knowledge of purification mechanisms to optimize the design and take advantage of the full potential of these systems has aroused interest in this type of natural solutions.

Recently, members of the Department of Chemical Engineering at the University of the Basque Country have studied the versatility of these systems through mathematical modeling and simulation of their operation. We have shown that they are capable of withstanding changes in flow and composition of greywater and even seasonal variations in tropical regions.

A bio-garden in her garden

One of the advantages that makes the implantation of artificial wetlands more attractive is the ease of construction and maintenance of the installation during its useful life. That is why many communities in the global south are betting on this system for the treatment of domestic greywater, such as in El Salvador and Costa Rica.

If there are so many advantages of bio-planters, why not install this system in our garden?

Numerous manuals on how to build and maintain bio-gardens are now available. So if we have land and can stock up on some simple building materials (such as plastic pipes and drums), in a few weeks we can have our own sustainable domestic wastewater treatment system.

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