Reuse of wastewater as irrigation water in agriculture

One of the issues of greatest concern in today’s agricultural world is water scarcity. It is no secret that the irrational use of resources, as well as the consequences of climate change, have significantly influenced the lack or excess of rainfall in several regions of the world. Therefore, in this post, we will talk about the reuse of wastewater for irrigation as an alternative to solve the problem of water shortage in crops.

In agriculture, water is an essential resource to keep crops functioning and in optimal condition. Without water, it would be almost impossible to produce food that can meet local and global demand.

We will analyze the opinion of two experts in the area, Javier Tuñón Valladares, technical director of Grupo Riegos Levante Hellín, and Mauro Quintero Rodríguez, product manager of Sotrafa’s Geosynthetics Division, who will explain the details of a project developed in Castilla La Mancha, Spain, where a serious deficit of more than 1.5M m3/year of irrigation water in the area was alleviated.

First, let’s clarify some basic questions on this topic, such as:

What is water management or water resources management?

It is the activity of developing, distributing, and directing the optimal use of water resources. In other words, it encompasses all the processes, strategies, and procedures that must be carried out to make increasingly efficient use of water.

What is water reuse?

It is the process of reusing water that has previously had another use, whether municipal or industrial. If water reuse requires means of transfer and distribution, it is called direct reuse, or if it is reused by discharging it into streams for downstream recovery, it is called indirect reuse.

What is the reality of the water scarcity problem?

As everyone knows, natural resources are finite, i.e., they don’t last forever, hence the need to make good use of them to take better advantage of the same, without this implying their depletion.

Facing this scenario, Mauro Quintero explains that historically there has been an irregular distribution of rainfall, aside from the fact that current conditions have caused a problem of water scarcity throughout the world.

However, Spain is a very particular case, since the reality is that it hasn’t stopped raining or at least it doesn’t rain less, “the data say that it rains the same. Over the last 40 years, the average rainfall data in this country is almost the same, maybe with a difference of a couple of tenths of a percent, but it remains the same. Likewise, the water storage reserve is distributed very evenly, there is the Atlantic basin which concentrates 75% of the water reserve, and the Mediterranean basin, which covers only 25% of the storage reserve since it’s where the climate is similar to the desert, so it suffers a little more.”

There are areas where there has been a lack of water, a problem that was attempted to be solved through traditional alternatives such as the creation of reservoirs, water transfers, and tactics to take advantage of storms, among others, but the reuse of wastewater for irrigation is emerging as a priority solution, as cities continue to grow exponentially, so it will be increasingly necessary to have an ace up our sleeve that will allow us to face the future.

“With high population levels come immediate consequences, as currently 10% of the world’s population, an estimated 800 million people, suffer from malnutrition. This is certainly a difficult situation that will continue to worsen, hence the urgent need to create viable solutions to ensure the cultivation of food to meet global demand,” Quintero points out.

 

 

Water use on croplands

When it comes to water use, Quintero explains that currently, 20% of the total arable land available is used for irrigated agriculture and that 20% accounts for 40% of the planet’s total food production. “If we analyze it from a national point of view, 80% of water consumption in Spain is for agricultural irrigation.”

The reuse of wastewater for agricultural irrigation is not so simple, since all the systems used require prior planning, for example, an economic study of all the infrastructures, a study of the reliability of the treatment to be applied to the water, and how it will be financed, among others.

Concerning costs, Quintero considers it necessary to analyze the particular case of the Almeria region where to be competitive, the cost per cubic meter of water used for irrigation should be around 30-euro cents per cubic meter. “The water from the salterns built in Almeria for irrigation already doubled the production cost of the optimal cost. However, the cost of using reclaimed water is between 0.05 and 0.15 euros, which leads us to wonder why more reclaimed water is not being used than what the current numbers indicate.”

What are the water treatment systems available?

According to Quintero, the systems can be divided into three parts:

• Purification phase (primary, secondary, and tertiary)
• Pollutant removal means (biological, physical, and chemical)
• Cost of water exploitation

In the primary purification phase, certain organisms and residues are prevented from entering the purification process. The secondary purification phase is more biological in nature since aerated lagoons, peat beds, and other processes come into play. It then moves on to the tertiary phase, which is biological in nature as well as physicochemical, where slightly more complex processes are performed such as phosphorus removal, ozonation, and even the use of ultraviolet rays for water purification.

The operating cost of water treatments can be divided into low-cost processes, also known as extensive white methods, and high-cost processes, which are currently the vast majority.

Low-cost processes are characterized mainly by their ease of operation since no specialized laborers are required, but they require a large surface area to be used, have a very low energy cost, and have a very good integration into the environment. This type of procedure refers to special lagoons and peat beds, among others.

High-cost procedures are also known as conventional cost procedures and are the opposite of the ones we already talked about, as they are intensive and require very little space to be performed. These processes require a lot of energy, which results in high energy costs, as well as specialized work-force to execute them due to their importance, but in the end, they achieve excellent results in terms of purification.

Advantages of using wastewater for agricultural irrigation

Although you may initially think that using water from a wastewater treatment plant can bring nothing but negative consequences, the truth is that it has many benefits, such as the following:

• Having a continuous supply of water
• No need to seek additional water supply
• Obtaining water with a high nutrient content
• Significantly reduce the consumption of water from natural water resources
• Possibility of having a resource that can help preserve the environment
• Reducing water stress on riverbanks and tributary streams

Risks of using wastewater for agricultural irrigation

Just as the reuse of wastewater for irrigation has its benefits, it is necessary to keep in mind that there are risks that may arise, especially when talking about the management of the resource, for example:

• Negligence in the maintenance of a wastewater treatment plant
• Failure to follow the analysis schedule for all samples, which must be carried out rigorously and meticulously
• Failures in the continuous evaluation of salts and suspended contents in treated water

What are the regulations for the use of reclaimed water?

When it comes to water management and subsequent reuse, the matter cannot be taken lightly. In the case of Europe, Spain is a pioneer in the area, and all processes are regulated.

“The European Union (EU), in its fight against the consequences of climate change and for the protection of the circular economy, established quality systems and uses that are governed by EU Regulation 2020/741 of May 25, on minimum requirements for water reuse,” Quintero explains.

The purpose of this regulation is not only to ensure that reclaimed water is safe for irrigation, but also to protect human and animal health, and multiply the use of reused water.

In addition to the EU regulatory framework, Spain has its own regulation focused on ensuring that reused water is of high quality. Thanks to this regulation, Spain has become the European country with the most reused water. “We are almost at 11% when the average for European countries is approximately 2.4%. The vast majority of reused water is used in agriculture, specifically 70%, while only 17% is used for urban purposes.”

In the specific case of Almeria, Quintero explains that currently, of the 16 cubic hectometers consumed in the region, 14 are recovered and treated to facilitate a total of 3,200 hectares of irrigation (approx. 7907,37 acres), specifically in the areas of Vega de Almeria and Bajo Andarax. This is possible through highly advanced processes of purification, ozonation, and microfiltration. “If regeneration were added to purification so that the use of reclaimed water would be viable for all crops, we could have an extra 50 cubic hectometers that could irrigate approximately 10,000 hectares of crops with plastic covers (approx. 24710,54 acres), this would be of great help, not only to multiply the number of crops but also for all users who face the lack of water.”

What parameters should be followed to achieve optimal water?

Wastewater must meet certain quality characteristics and sanitary, agronomic, and microbiological standards to be used not only for agricultural irrigation but also for water used for other purposes.

The quality of the water will depend on many factors, for example, the origin of the water, the treatment given to it, the distribution systems, as well as the purification, and desalination machinery used.

Achieving optimal water is relatively straightforward, but the real challenge is maintaining water quality throughout the distribution process, which can sometimes be a lengthy process.

8 Parameters for optimal water:

1. Salinity. Excess salt can significantly reduce productivity, and even cause the failure of a crop. The levels of salt content can be controlled by the electrical conductivity of the water.
2. Amount of sodium. An extremely high concentration of sodium in water, relative to that of magnesium and calcium (a ratio of three to one), can reduce soil permeability. When water doesn’t reach the conditions and quantities that it should, it can lead to the formation of carbonates and bicarbonates that can damage the crops and the soil.
3. Trace heavy metals. Wastewater can contain high concentrations of many metals such as nickel, copper, or lead, which can be toxic to plants and the animals that consume them, creating the perfect scenario for spreading chaos to the rest of the food chain.
4. Amount of chlorine. Chlorine concentrations of more than 5 millimeters can damage most plants. Chlorine is a minimum fundamental element to make the first purification of any type of water.
5. PH. It is very important since it affects the solubility of metals that may be dissolved in the wastewater and may alter the soil equilibrium.
6. Pathogens. Wastewater may contain many pathogens, which can be defined as the presence of viruses of human origin that can be transmitted through the products and the food chain, affecting food and plants, thus incorporating themselves back into the food chain, which would be detrimental.
7. Suspended solids. They are basic and are usually seen in the primary phase of purification. These solids can cause clogging of irrigation and water distribution systems, which would increase costs, depending on the use of this type of resource.
8. Biodegradable organic material. The water contains proteins, carbohydrates, and fats that will generate a need for dissolved oxygen, and that will require certain control measures to be taken. One of them is the chemical oxygen demand, which indicates the amount of pollutants in the water that can be oxidized or eliminated utilizing the oxygen impulse. There’s also the biological oxygen demand, which is the amount of dissolved oxygen required during a specific time for the biological degradation of the organic material contained in the wastewater.

Quintero stresses the importance of reusing wastewater for irrigation and emphasizes that this is not a mere utopia, but something that is already happening, for example, in regions of the world such as Israel where 90% of wastewater is already used in any type of crop.

“There is a project called Reutivar, which is a sustainable irrigation model that contemplated the optimization of reclaimed water in the olive grove of Jaén, where 6 irrigation communities received 4.1 hectometers of reclaimed water. In this way, they were able to take care of three thousand hectares of olive groves (approximately 7413 acres) with highly efficient localized irrigation, together with a precision fertigation system that has given excellent results.”

What happens once the water is ready for reuse?

According to Quintero, once the water is ready it must be redirected as many kilometers as necessary until it reaches a point where a reservoir of considerable dimensions must have been created to contain as much regenerated water as possible.

The reservoir must be waterproofed to guarantee the service to a large number of community members who are waiting for that water supply to carry out their agricultural work.

 

 

A success story: Water reprovisioning and irrigation pond installations on SAT Regadíos de Cancarix, Castilla La Mancha, Spain

Javier Tuñón, Technical Director of Grupo Riegos Levante Hellín, explains that a major project that required a large investment was recently completed. The purpose of this project was to respond to the urgent need to meet the demand for water by irrigators in the area, especially in the summer months and in years when the drought was at its worst.

“We realized that there was not enough water stored for the crops that were underway, which were often vegetables and, as we know, these types of crops have very little margin to withstand water stress. Therefore, we realized there was a pressing need to build a new irrigation reservoir in the area,” said Tuñón.

When wastewater is used for irrigation, the storage volume is a crucial aspect, so the volume of the ponds must always be larger. Compared to river water, where you can play with the flow rates and pumping hours depending on the months/seasons, wastewater doesn’t stop and cannot be regulated, since it is produced from the beginning throughout the year without stopping. This allows us to have a good reserve of water when the summer arrives, and the levels drop.

Has there been a breakthrough in irrigation water management?

According to Tuñón, the last decades have brought significant advances in terms of irrigation practices, moving from tactics such as gravity irrigation, for example, which today are obsolete and inefficient because they generate water consumption without any savings, to what in this case is the reuse of wastewater for irrigation.

Today, it is evident how irrigation systems have been modernized, aid has been given to irrigation communities to modernize their irrigation systems and to make water management increasingly efficient, thus reducing water losses. Today we have managed to provide less water but with greater efficiency.

Installing technicians or staff has also become modern, changing the way they manufacture certain materials and equipment, seeking to generate savings and maximum efficiency. “Those of us who have been on the installation or facility design side also feel the need to adopt solutions that lead to more efficient water management,” says Tuñón.

What is the average cost per cubic meter to convert wastewater into water suitable for irrigation?

Tuñón explains that in the case of projects such as Cancarix, the cost is zero because although there is a cost to generate or treat the water that is part of the urban water cycle, the water generated at the outlet/end of a treatment plant is not an expense for the farmer.

“In this specific case, there has been no need to boost the water, since it arrives by gravity to the irrigation area. There would be a cost in the case of any intermediate boosting point, hence the difference between certain areas where desalinated water can be worth up to 60-euro cents, where it would be wise to analyze whether it’s viable or if it’s impossible to do,” Tuñón indicates.

The good thing about having a reservoir where the water is pumped by gravity is that it isn’t necessary to use any type of energy to pump it from the treatment plant to the pond.

Also, Tuñón comments that “at the beginning, it was simple, because the wastewater treatment plant is 50 meters above the irrigation area, so we knew that the water could be conveyed by gravity since there is a 50-meter difference in level, otherwise it would have been a failure.”

Tuñón continues to explain that the complexity arose from the fact that the irrigation area was very far away, an estimated 30 kilometers from the treatment plant. “We had to cross a large part of the municipal area, so studies were needed to determine a route that would have minimal effects and, above all, to ensure that we avoided areas of ascent or slopes that could compromise the passage of water, thus guaranteeing a gravity-driven process.”

When it comes to this topic, we recommend reading our article on how to improve water consumption in crops.

 

 

What material is used to waterproof the pond?

Bearing in mind that a reservoir was created to hold one million cubic meters of water, Tuñón indicates that on this occasion the material chosen was high-density polyethylene geomembranes, one and a half millimeters thick, manufactured with materials subjected to high levels of quality control, not only to protect the reservoir and prevent it from breaking but also to protect the surrounding environment.

According to Tuñón, “One of the fundamental premises has been to use very safe materials to minimize the risk of breakage or collateral damage.”

Several premises requested by the environmental agency had to be complied with to obtain authorization for the construction of the pond, such as, for example, the care of the small fauna in the area, since there was a possibility that it could slip through the enclosure of the coronation corridor and end up inside the pond without being able to escape.

As Tuñón comments, “Facing this scenario, we decided to place a rough Sotrafa sheet in interlayer panels as evacuation ramps for the fauna in the area. This made us pioneers so that future constructions will probably require the use of these rough sheets for the benefit of the local fauna.”

As Quintero states, in this sense, there are many types of roughened sheets, and it’s necessary to analyze the needs of each particular case to offer the most effective solution.

How can geosynthetics manufacturers help to optimize the design and installation of these solutions in hydraulic works projects?

According to Tuñón, manufacturers are always making improvements to their materials, considering lengths and thicknesses, durability, and appearance, especially to meet the specific needs of each area where a project will be developed.

One of the options available on the market is Sotrafa’s geosynthetic solutions for hydraulic works.

“The size of the polyethylene roll has been improved so that installation technicians have to make fewer welds, which translates into greater safety of the material and, therefore lower costs. Likewise, the type of rough sheeting that didn’t exist until a few years ago has been improved, which makes it possible to solve important problems,” Tuñón points out.

Concerning the colors of the plastic sheets, there have also been significant improvements, since not only black sheets are available, but also white, green, brown, or gray geomembranes can now be manufactured, depending on the aesthetic needs of each region or project, thus generating less visual impact. Everything will depend on the requirements and the available budget.

Another important aspect is the drainage system of the pond. To understand all the aspects to consider in this regard, we invite you to watch the webinar on wastewater recovery and what was the drainage solution designed for this particular project in Castilla La Mancha.

What is the estimated reservoir volume required per irrigated hectare or acre?

According to Tuñón, it’s impossible to know exactly, first because irrigating a hectare (approx. 2,471 acres) of pistachio almonds, for example, will never be the same as irrigating a hectare of broccoli or tomatoes. The requirements and the annual distribution of needs are not the same for all crops.

We should also begin with the most important thing, which is the amount of water available, since a supply of 100 liters per second is not the same as a supply of 50 or 20 liters per second, among other things.

As Tuñón comments, “The calculation of the volume of a reservoir is something that must be carefully done, which implies determining a monthly balance of consumption needs and a monthly deficit that will lead to the annual volume needed.”

We also recommend reading our article on poly-irrigation: what it is, advantages, disadvantages, methods used, and benefits of using plastic pipes.

 

 

How sensitive is the geomembrane to solar radiation? What type of maintenance does this material require?

On this particular aspect, Quintero explains that geomembranes should be seen or understood as human skin, so they must be protected as they are affected by the same things such as ultraviolet radiation, oxygen, the passage of time, and the diverse climatic factors present daily.

“Based on the geomembranes we produce in Sotrafa, in our case the Alvatech 5002, although this applies to the rest of the varieties produced in the company. Returning to this particular example, we are talking about a geomembrane that is taken care of from the beginning, that is, we incorporate nitrogen into the raw material through the pipes through which the polyethylene pellets travel, this is done to displace the oxygen, so that time doesn’t elapse until the geomembrane comes out through the head of the sheet. We make two antioxidant packages, a primary one that is used to protect the geomembrane during the production process, and a secondary one that begins to act once the geomembrane is exposed.”

Quintero also points out that carbon-black is generally used as a UV protector since it is more common, but other protectors can be used, such as those used in white or colored geomembranes where carbon-black cannot be used. Although they are sensitive films, they are very durable and last longer than the warranty period that is commercially given to this type of geomembrane.

How is the cleaning of sediments in the pond performed?

For the Cancarix project, there are two systems. On the one hand, a five-meter concrete ramp was built to allow the passage of any trailer or truck along a slope, and, on the other hand, we placed an exclusive cleaning exit where less exhaustive cleaning can be done.

In the case of the concrete ramp at a time when a major cleaning is required, which is rare, since the water intake is floating and has floating perforated pipes that allow the water to be collected from the surface and never from the bottom, the vehicles can enter the bottom of the pond to perform the procedure without inconveniences.

“As the impurities remain below, cleaning will be done only when a level of impurities is reached or when the storage capacity is being lost, that is, it’s not necessary to clean it regularly, but when it’s needed, we have this cleaning ramp,” Tuñón said.

How durable is the geomembrane?

According to Mauro Quintero, the durability of the geomembrane will depend on several factors, such as the production process, the type of raw material used, how it is treated, and the use given to the geomembrane.

“In the case of the one produced by Sotrafa, I can say that, by implementing an adequate process, using top-quality raw material, and thanks to the relationship we have with the best petrochemical companies in the world, we obtain high-quality geomembranes with an average durability between 10 or 15 years approximately, everything will depend on the climatic elements to which the geomembrane is subjected and where it is placed,” Quintero points out.

He also explains that once the geomembranes exceed their warranty period, tests are carried out and samples are taken from the reservoirs to control the geomembrane’s useful life and thus know, among other aspects, the percentage of additivation package remaining, “at that point, it is possible to determine if it’s still in good condition and has useful life left, or if it’s necessary to eliminate it.”

To learn more about this topic, we recommend watching our webinar on wastewater recovery as irrigation water in agriculture.