Water on the Planet

Proper water resource management is essential in a world where freshwater, vital for human survival, is a limited resource. To understand the magnitude of this situation, we can begin by examining the global distribution of water forms.

97.5% of the Earth’s water is saltwater, leaving only 2.5% as freshwater. Of this percentage, 69.5% is found in Antarctica, Greenland, and other frozen areas, such as glaciers or permafrost. The remaining 30.1% constitutes groundwater.

In summary, only 0.4% of the planet’s water is fresh surface water, and this is distributed among soil moisture (67.4%), natural wetlands (20.7%), and the atmosphere (9.5%). Surprisingly, only 1.6% of this fraction is contributed by rivers, representing a mere 0.1% of the planet’s total water. [1]

Potable water, essential for daily life, is becoming increasingly scarce. In Spain, the average consumption is 133 liters per inhabitant per day, according to the latest INE data from 2020, a figure that can vary significantly in different regions of the world. [2] The WHO estimates that 100 liters of water per person per day are necessary for comfortable living.

Given this scenario, responsible management of water resources is necessary, and this involves two fundamental objectives:

– Reduce consumption, reuse as much as possible, minimize pollution, and utilize resources that have not been considered until now.

– Ensure higher quality and healthier water for human consumption.

The Problem of Wastewater

Untreated wastewater can cause serious environmental and health problems, such as bacterial infections (typhoid fever, salmonellosis, cholera, gastroenteritis), viral infections (gastroenteritis, hepatitis A), or parasitic diseases (coccidiosis, amoebiasis, hydatidosis, cysticercosis, nematodiasis).

From an environmental perspective, water pollution not only eliminates a significant portion of native aquatic vegetation and fauna but also causes widespread imbalances throughout the terrestrial ecosystem that depends on these resources.

Excess organic matter and nutrients in water (nitrogen and phosphorus) lead to eutrophication, meaning oxygen depletion and the death of most living organisms, while the presence of heavy metals and other toxic compounds cause poisoning and bioaccumulation.

There are countless known systems for returning organic matter to the environment; some philosophies are based on minimizing blackwater or reusing it for the growth of plant species of interest to humans.

Another approach involves taking the concept of water separation to the extreme, potentially having 5 distinct water circuits: rainwater, mains water, yellow water, greywater, and blackwater.

Additionally, systems that eliminate wastewater generation can be implemented, such as the so-called dry toilet, which converts feces into compost through mixing with ash or sawdust.

Bacteria in Wastewater

Bacteria are used as indicators of fecal contamination in water, with Escherichia coli being the most commonly used. An adult can excrete up to 2,000,000,000 coliform bacteria daily. The presence of salmonella and Vibrio Cholerae, which causes cholera, is also notable. Finally, protozoa and parasites such as worms also appear.

Wastewater Treatment Systems

Due to these wastewater challenges, urban areas require imperative water treatment, which is carried out using conventional systems, commonly known as WWTPs (Wastewater Treatment Plants).

These facilities allow for the efficient treatment of large volumes of water using relatively small land areas, albeit at the expense of high energy consumption, the costs of which usually fall on local residents. However, this necessary active energy input typically comes from renewable sources, starting with the fact that 30-40% of consumption comes from the biogas produced during the treatment process, and the rest can consist of renewable electrical energy (wind, hydro, photovoltaic…).

Rural populations have not traditionally adopted wastewater treatment practices, as historically this process occurred naturally in streams or rivers bordering their settlements due to much lower population density and less waste resulting from a different lifestyle.

Unlike the high density of urban areas, rural areas have ample land, which has allowed for the development of a series of “soft systems” for wastewater treatment in small communities.

These so-called soft treatment systems are methods that are generally less costly and sophisticated in terms of operation and maintenance than conventional ones, with relatively low energy consumption.

Among these systems, “phytosystems” stand out for their main characteristic: the utilization of solar energy through natural biological processes, such as photosynthesis. This methodology replaces the active energy required by WWTP treatment systems in large populations, thus providing a more sustainable and suitable alternative for rural environments.

What is phytoremediation?

Phytoremediation is a natural, efficient, and economical method for treating wastewater that mimics the natural mechanism of vegetation filtering and absorbing wastewater constituents. Thus, it recreates the same function that natural wetlands have in nature, which, as we mentioned, constitute 20.7% of the planet’s 0.4% water, through the creation of an aquatic environment with floating macrophytes, a series of aquatic plants such as water spikes, water lilies… These plants, through biological and physicochemical processes, progressively purify the wastewater until it reaches the optimal level of purification for subsequent discharge into the environment.

Thus, there are a series of phytosystems with low conventional energy consumption and therefore low cost, but which require a large land area per inhabitant to adequately utilize solar energy through algae or aquatic plants, which produce the oxygen necessary for the growth of the microbial population that will degrade a large part of the organic matter. These systems include lagoons (algae and bacteria suspended in water), green filters based on herbaceous or woody species, and artificial wetlands, which we will detail throughout this article.

Phytosystems for Wastewater Treatment in Small Communities

In the search for sustainable and efficient solutions for wastewater treatment in small communities, phytosystems have emerged as a promising alternative.

These systems harness the natural ability of plants to purify water, transforming organic waste into essential nutrients for plant growth. The application of phytosystems in a specific project requires careful consideration of several key factors such as consumption, rainfall, land availability, regulations, etc.

Water Consumption: The first step in implementing a phytosystem is to assess the community’s water consumption. This analysis provides essential information on the required system size and treatment capacity. Proper design ensures that the phytosystem can efficiently handle the wastewater load, ensuring optimal results.

Rainfall: The amount of rainfall in the region is a crucial factor in determining the effectiveness of a phytoremediation system. Rainfall directly affects the availability of water for system treatment and regeneration. In areas with prolonged dry seasons, water storage and management solutions must be incorporated to ensure continuous operation.

Land Availability: Topography and soil quality are determining factors in selecting the site for installing a phytosystem, as the land layout must allow for adequate wastewater flow through the system.

Pretreatment Systems

Screening: Screening, an essential phase of phytosystems, involves a specific type of mechanical filtration designed to remove large particles and coarse solids present in wastewater. This process acts as the first barrier, preparing the water for subsequent treatment stages.

Grit Chambers: Grit chambers are key components in pretreatment, designed to remove finer sand and sediments. By removing these particles, they prevent obstruction or damage to subsequent stages of the phytoremediation system.

Grease Traps: Designed to separate and remove fats and oils present in wastewater, grease traps are essential in preparing the water before it enters the next level of treatment. This ensures efficient operation and prolongs the lifespan of the phytosystem.

Primary Treatment Systems

Sedimentation or Decantation: In the primary treatment phase, sedimentation and decantation are crucial processes. Sedimentation allows heavier particles to settle at the bottom, while decantation facilitates further separation of settleable solids.

Flotation: Facilitates the removal of lighter materials on the water’s surface. This step significantly contributes to water clarification before it enters the phytoremediation system.

Homogenization: Water arrives with varying loads throughout the day, making it necessary for it to be mixed during the primary phase, which lasts approximately 8 to 12 hours, so that the outflow is as homogeneous as possible.

This process is carried out in a clarifier, septic tank, multi-purpose tank, or Imhoff tank, all of which are devices whose function is to settle/decant and retain floating matter, improving the overall quality of the water leaving the system with the dissolved pollutant load that will be treated in the next process.

Wastewater Treatment Phytosystems

Lagoon Systems

This involves discharging wastewater into a series of lagoons where purification occurs. In this phytoremediation system, elements such as the aqueous medium, solar radiation (ultraviolet and visible), organic matter, bacteria and microorganisms, oxygen content (primarily depending on depth), depth (influences the reach of light radiation), and inorganic compounds participate.

Depending on their depth, lagoons can be classified as anaerobic (more than 2.5 m), facultative (between 1.2 and 2.5 m), and maturation (between 0.3 and 0.6 m). Each zone within a lagoon plays specific roles, from an aerobic surface layer rich in oxygen, light, and algae to a deep anoxic zone where anaerobic digestion by bacteria takes place. Each type of lagoon carries out specific processes that contribute to water purification through the action of algae and bacteria.

Anaerobic Lagoons: These are deep lagoons (2.5-6 m) with a predominant anaerobic environment due to the lack of oxygen in the lower layers, used to reduce organic load through anaerobic digestion by bacteria, although they have the drawback of producing foul odors due to hydrogen sulfide production.

Facultative Lagoons: These are shallower lagoons (1.2-2.5 m) with both aerobic and anaerobic zones, achieving combined purification through the action of algae and bacteria. These lagoons significantly remove nutrients, thus preventing water eutrophication.

Aerobic or Maturation Lagoons: These are shallow lagoons (0.3-0.6 m) with aerobic conditions throughout the profile, where nutrient reduction and oxygenation processes take place, although they are typically used after other types of lagoons due to their reduced purification capacity.

Wastewater treatment lagoons are particularly suitable for small rural communities. The system’s effectiveness can be adapted according to population size and organic load, making it most appropriate for communities with moderate wastewater treatment demands.

Meteorology plays a crucial role in the performance of treatment lagoons. Factors such as solar radiation, water temperature, and winds directly impact processes like photosynthesis, aeration, and sedimentation, thus affecting the overall efficiency of the system.

Maintenance requirements include regular monitoring of organic load, oxygen conditions, and water quality. Additionally, it is essential to maintain aquatic vegetation and control sludge accumulation in the deep zone of the lagoon. Proper maintenance ensures optimal performance and long-term sustainability of the treatment system.

Green filters

Green filters consist of areas of land with or without vegetation onto which wastewater is discharged. The main treatment elements are the soil and the plants’ rhizosphere—that is, the area of soil near plant roots where microbial life develops.

However, they have various limitations and, in some cases, can be counterproductive, negatively impacting nearby aquifers. Limitations include land availability, climate influence, hydrology, wastewater characteristics, and the soil’s effective retention of bacteria, with crops for human consumption being particularly discouraged.

Irrigation over grassy surfaces: Irrigation over grassy surfaces is carried out using classic methods such as sprinklers, furrows, or gravity. Modern methods such as micro-sprinklers or drip irrigation can face clogging issues, making both pretreatment and primary treatment systems very important. Treatment is achieved through the water passing through the soil and plant uptake—for example, irrigating recreational areas.

Green filters with woody species: These filters use fast-growing species adapted to wet soils. Poplars and willows, especially basket willows, are used for treatment and for producing wood or biomass for energy purposes—an energy source considered renewable.

Runoff over vegetation cover: Runoff over vegetation cover involves application using fixed sprinkler systems. Treatment is carried out through physical, chemical, and biological systems as the water passes through vegetation on a slope. Although it offers advantages such as low installation cost, it requires ample land and, if poorly implemented, could entail health risks.

Infiltration: The infiltration system uses various irrigation methods, with flooding and furrows being the most effective. Treatment occurs through biological, physical, and chemical processes during filtration through the soil. However, there is a high risk if it is located near an aquifer, making this a virtually obsolete method.

These systems are better suited to small or rural communities, where land availability and the lower wastewater load can be managed effectively. For larger populations, more complex systems should be considered.

These systems have positive impacts by offering sustainable solutions for wastewater treatment, but potential health risks must be taken into account and they require careful management to avoid associated issues, such as aquifer contamination.

Maintenance involves monitoring water quality, ensuring compliance with regulations, and, in the case of green filters with vegetation, may include vegetation management and preventing clogging in irrigation systems. In addition, continuous monitoring is required to ensure proper operation and avoid potential environmental and health risks.

Wetlands

Wetlands are waterlogged areas where a characteristic aquatic vegetation proliferates, perfectly adapted to having all or part of its organs submerged in the aquatic environment. They can be natural or artificial, and flooding can be temporary or permanent. Wetlands used as a phytotreatment system are artificial and use species with roots anchored in the bottom or substrate of the wetland (emergent macrophytes) or free in the water (floating macrophytes). Constructed wetlands are divided into three main types, depending on their water flow.

Surface-flow wetlands: Designed for visible water flow on the surface. Vegetation adapts to shallow, constant water conditions, with common plants such as rushes and cattails contributing to phytotreatment.

Subsurface-flow wetlands: Water flow occurs below the soil surface. Plants adapt to saturated soils but are not completely submerged. Species such as bur-reed and common reed can thrive, participating in water treatment.

Aquatic systems: Designed to host a variety of aquatic plants, mimicking the diversity of natural aquatic environments. They include plants such as water lilies, duckweed, and water hyacinths, which perform specific functions in water treatment.

Constructed wetlands as an ecosystem:

Following the principles of regeneration, constructed wetlands replicate the dynamics of natural ones and are delicate ecosystems. They result from the interaction of physical, chemical, and biological processes in an environment created and managed by humans. Although they share similarities with natural wetlands, the key difference lies in the level of control that can be exercised over the processes involved.

– Vegetation in constructed wetlands: Vegetation in wetlands, natural or artificial, plays a crucial role thanks to its roots and rhizomes. As photoautotrophs, plants use solar energy to convert inorganic carbon into organic compounds and transfer oxygen from the atmosphere through leaves and stems, fostering aerobic regions for microbial activity. Emergent plants in wetlands perform essential functions, such as stabilizing the substrate, absorbing nutrients, and facilitating gas transfer. In constructed wetlands, plants such as rushes act as natural filters, contributing to phytotreatment by absorbing nutrients and promoting sedimentation. In addition, these plants provide habitats for wildlife, enriching biodiversity in these built environments.

– Microorganisms in constructed wetlands: In these environments, microorganisms—mainly bacteria, yeasts, fungi, and protozoa—play a fundamental role in biological treatment. In the upper zone of the wetland, where oxygen released by plant roots and oxygen from the atmosphere are present, colonies of aerobic microorganisms form. In the remaining part of the granular bed, anaerobic microorganisms predominate. These microorganisms carry out key processes such as the degradation of organic matter, nutrient removal, and disinfection. In addition, thanks to biological activity, many pollutants are converted into gases and released into the atmosphere.

– Algae in constructed wetlands: Algae are essential components in constructed wetlands, especially in aquatic systems. These microscopic plants carry out photosynthesis, contributing to water oxygenation and competing with undesirable algae. Their balanced presence is crucial to maintaining a healthy aquatic environment.

– Fauna in constructed wetlands: Fauna in these ecosystems is diverse and plays a crucial role in the ecosystem’s balance. It includes various species of water birds, insects, amphibians, and fish adapted to the particular conditions of these wetlands. The interaction between flora and fauna contributes to the success of phytotreatment and to maintaining water quality in these built ecosystems.

CASE STUDY: LA TORRE DEL CODINA

An example of applying phytotreatment systems in a detached home is Torre del Codina, a rural tourism farmhouse located in the municipality of Taladell, in Lleida, which operates as a birdwatching observatory. The project strongly supports biodiversity preservation through the management of the complex’s wastewater, which consists of a rectangular tank circuit. Next come two gravel ponds through which the water flows subsurface, and finally an open-air constructed wetland with plants and amphibians, which birds use as a drinking spot.