Water reuse for irrigation

The circular economy is becoming, day by day, an increasingly current concept. What was long treated as waste is beginning to be seen as a source of raw materials. This happens in agriculture as well. One of the new ideas is the use of wastewater from cities for irrigation. The advantages can be multiple. Moreover, on June 26, 2023, the provisions of a new European Regulation regarding the minimum requirements for the reuse of water for agricultural irrigation will become applicable [1,2].

Nitrogen and Phosphorus Would Come for Free

The idea of using wastewater from the city to irrigate the agricultural land in the vicinity is not new. Recently, a group of researchers proposed a project that consisted of not completely purifying the wastewater, such as to preserve the nitrogen and phosphorus in it. These two substances are essential nutrients for plants. Farmers typically spend large sums of money to procure and then distribute them to crops. By using the water that already contains them as waste, they could achieve significant savings. On the other hand, city officials would, in turn, achieve significant savings, not spending money on the energy and materials needed to extract nitrogen and phosphorus residues from water, thus, being an idea that everyone would benefit from. If the wastewater is treated properly, according to European regulations, it can represent an effective alternative for covering the water demand of the agricultural sector [3].

Thus, applying such a solution around cities could provide the water needed to irrigate tens, even hundreds of thousands of hectares of agricultural land. Added to the water would be the associated fertilization, which would mean significant energy savings and a significant reduction in pollution. Moreover, the sludges collected from sewage treatment plants are already used in agricultural practice as fertilizers and soil amendments.

According to statistics from the European Commission, the amount of treated urban wastewater that is currently recycled each year is around 1 billion cubic meters, or 2.4% of treated urban wastewater effluents and less than 0.5% of the EU's annual freshwater withdrawals. The EU's potential, however, is six times greater and is thought to be over 6 billion cubic meters. Both the northern Member States, like Belgium, Germany, and the United Kingdom, and the southern Member States, like Spain, Italy, Greece, Malta, and Cyprus, have already put in place a number of programs on water reuse for irrigation, industrial purposes, and aquifer recharge. While Greece, Italy, and Spain only recycle between 5% and 12% of their effluents, Cyprus and Malta already reuse more than 90% and 60% of their wastewater, respectively. This clearly shows the enormous potential for additional uptake.

Water Reuse – A Solution to the Looming Crisis

Water scarcity is a growing problem in Europe. This was the context in which, at the end of January 2020 the deputies of the European Parliament (EP) met with the representatives of the Commission. MEPs wanted, among other things, to secure the EU’s freshwater sources including by promoting the reuse of water in agriculture.

The demand for Europe's water resources is predicted to increase as a result of climate change. One prediction states that by 2030, water scarcity may affect half of the river basins in Europe. Members of the Environment Committee of the EP adopted an informal agreement with the Council on a proposal to reuse wastewater on January 21 in order to secure Europe's fresh water supply for years to come. To become law, the new regulations still need to be approved by the Council and the Parliament. The new regulations are designed to encourage the use of treated wastewater for irrigation, which uses around half of the water used annually in the EU. Water reuse in agriculture could be increased to assist close the deficit.

The new regulations mandate minimum water quality standards, frequent monitoring, and the creation of risk management strategies by wastewater treatment facilities in order to guarantee the protection of crops. The authorities of the Member States are responsible for authorizing treatment plants and monitoring regulations compliance.

Pollution in the Irrigation with Wastewater

The health issues related to wastewater irrigation

The wastewater discharged from the sewage systems of the cities, from various large industries as well as from animal farms and fattening, can be used for irrigation after they have been properly purified in order not to harm humans, animals, crops, or land not to pollute the environment. In Romania, at the level of 1996, there were over 60,000 ha of wastewater irrigation systems, coming from animal husbandry complexes, included in about 30 irrigation systems. Currently, they are no longer functional. It is appreciated, however, that in our country, an area of about 1,330,000 ha can be irrigated with wastewater.

The dangers generated by the use of these waters for irrigation depend on a number of factors [2-4]:

• the presence of pathogens in these waters;

• the presence of elements such as: sodium, heavy metals (Cu, Pb, Cr, Cd, Ni, etc.) in addition to other substances that can cause soil degradation and crop depreciation;

• the way of development of different organisms, located in the irrigation water;

• the viability of pathogens and their ability to survive purification treatments;

• the possibility that pathogens can survive in the soil.

There are some wastewaters that have a high content of nitrogen, phosphorus, potassium, and other nutrients (such as those from the fertilizer production industry, alcohol, etc. presented in Table 1) stimulating plant development. They have the ability to extract about 75% of the nitrogen in the irrigation water, phosphorus up to 100%, potassium 60-10%, etc. It is estimated that 50-100 m³ of wastewater can contain as many nutrients as a ton of manure, and especially corn, wheat, soybeans, potatoes, alfalfa, and clover absorb significant amounts of nutrients from irrigation water.

Research has shown that using wastewater does not involve major health issues. On a depth of 0.9 to 1.2 m, all the bacteria are filtered. The soil retains on the first centimeter of depth about 90% of the total microbial load, and on the first millimeter 62 to 64%. Intestinal bacteria that reach in soil along with the wastewater are mostly adsorbed by soil colloids and destroyed by saprophytic bacteria [5]. Wastewater that was naturally filtered by the soil reaches the phreatic layer free of microbiological pathogens. In the case of wastewater irrigation, the most important microorganisms are listed in Table 2.

Table 1. Characterization of some industrial wastewater according to CBO5 size and nitrogen content

Table 2. Most important pathogens found in wastewater.

The soil “gets tired” if wastewater irrigation is used for a long time, a reason for which is recommended that the water should be treated before being used. Due to the very diverse chemical content, wastewater treatment should be processed to avoid destroying crops, soil salinity, etc. Improving the quality of reused wastewater is recommended through dilution with clean water, decreasing the content of sodium, reducing the salt from animal diets, adding gypsum to substitute sodium carbonate, and so on. The existing research indicates that the reuse of wastewater for irrigation lead to an increased in yield, mainly due to the increased soil moisture and fertilization.

For soils with good permeability, sub-surface irrigation methods are preferred, due to the reduced and very efficient water use, as well as due to the protection of the surrounding areas especially when wastewater are used. Cleaned wastewater can be also used in the fertigation of crops grown in greenhouses, through sub-surface irrigation. They are also recommended for technical plants (such as sugar beet), pastures, vegetable crops that are consumed after thermal preparation (potatoes), ornamental plants, and flowers, with the condition that the irrigation is stopped a month before harvest.

Negative effects of wastewater irrigation

Wastewater irrigation requires serious management and control of soil properties such as to avoid the deterioration of soil. It is mostly recommended for soils with good permeability for water and air, favoring the mineralization of the organic matter and pathogen destruction. Soil self-purging depends mostly on the microbiological load of the soil (bacteria, actinomyces, fungi, protozoa, larvae) and consists of:

• The mineralization of the organic matter in anaerobic conditions – the sugars are decomposed in methane, water, and carbon dioxide, proteins are transformed into aminoacids, and the fat is reduced to glycerin and fatty acids; as a result of fermentation and decomposition, some strong-smelling compounds are produced such as ammonia, hydrogen sulfide, methane.

• The nitrification – autotrophic bacteria and archaea produce the oxidation of some inorganic compounds leading to the formation of nitrite. It is an aerobic process.

Applying wastewater irrigation in an uncontrolled manner can lead to some drawbacks such as [2-4]:

• Soil fatigue – can appear after years of wastewater irrigation and it is evidenced by increased concentrations of microelements (e.g., Zn, Cu), increased concentrations of harmful compounds (e.g., chloride, sulfate, fatty acids), levigation of potassium and iron at high depts, degradation of the soil structure, insufficient aeration. As a consequence, the plants will exhibit chlorosis, and the development process of some plants will be affected. It was proved that the main cause of these drawbacks is related to the increased irrigation due to population increase and water consumption, with 60.000 – 70.000 cubic meters per hectare as compared to the optimum values of around 3000 cubic meters per hectare, as well as the use of unprocessed wastewater and unsuitable soil management.

• Soil clogging – is due to the increase of organic matter usually accompanied by the increase of nitrogen content which partially stops the plant development. When the wastewater has high nitrogen content, good results have been obtained by keeping the water in oxidation ponds - a very good treatment because the organic matter is decomposed by microorganisms in the presence of oxygen. The method is used in many countries such as Australia, North America, and Israel.

• Secondary soil salinization – is produced especially on clay soils, with low permeability, due to an increased in soluble salt concentrations in soil above the critical value. The presence of sodium and other cations in large quantities leads to the destruction of structural aggregates of soil and reduced water permeability.

• Soil acidification – intensifies the solubility of calcium and magnesium salts, leading to deficiencies in plant nutrition; this phenomenon takes place when the wastewaters have a pH lower than 6.5.

• Soil contamination – can appear due to irrigation that uses industrial wastewater contaminated with harmful chemicals such as heavy metals (e.g., As, Cu, Ba, Ni, Cr, Zn, Co), phenols, different salts, and detergents. In a very small concentration, they can trigger the stimulation of plant growth but over some limit, they can inhibit the development. Moreover, they could be absorbed by the plants entering the food circuit and posing threats to animal and human consumption. It is not recommended to use for irrigation water that has arsenic over 1 mg/L, copper, over 7 mg/L, nickel and cobalt, over 2 mg/L, sodium, 80 mg/L, SO4 over 250 mg/L. The presence of tensio-active substances (detergents) affects nitrification, also influencing respiration and photosynthesis due to the formation of a pellicle over the surface of the leaves.

• Soil infestation with pathogens – can happen when there is an abusive use of wastewater without any prior treatment.

Other disadvantages of using wastewater in irrigation are:

• the fertilizers could lead in some conditions to excessive vegetative development, detrimental for the yield (fruits, grains, etc.), for the microbiological activity of the soil

• when silt from wastes is used as organic fertilizer, there might be a reduction of soil water content

• wastewater irrigation is a problem during the winter.

Advantages of using wastewater in agriculture

Rational use of wastewater in agriculture has many economic advantages among which the most important are:

• Elimination of the issues related to wastewater evacuation

• Reduced expenses to ensure the water for irrigation in the areas where other water sources are scarce

• Partially adding to the underground water resources, lifting up their level

• Is a technical and economical solution for the biological cleaning of wastewater

• Allows the extension of the irrigated areas, even in the zones with fewer water sources

• Contributes to increased yield

• It is a good way to fertilize the soil

• Make accessible to the plants useful elements found in this water

• Offers the possibility of reducing the use of expensive fertilizers; the wastewater from animal farms is a very good nitrogen-potassium-phosphor fertilizer

• The elements are easily assimilated by the plants

• On unstructured soils, irrigation using wastewater from the cellulose industry, rich in sulfites, improves the soil structure, thus, increasing the water permeability

Soil protection during wastewater irrigation

The measures for the protection of soil irrigated with wastewater impose that:

• the transportation and distribution of water should be carefully done

• the quantities of water used to be rationally calculated

• the wastewater treatment should be done previous to its use in irrigation

• the contents of toxic chemicals and pathogens should be under the recommended limits

• the concentrations of salts should be within normal limits, such as they don’t have a negative effect on the physical and chemical properties of the soil, plant growth, and yield

• the pH should be between 6.5 and 8.5

• the irrigation with wastewater should consider the overall quantity of substances in the water, the physical and chemical properties of the soil, the irrigation regime, the phreatic level, the irrigation schedule, and the harvesting moment

• perform regular drainage of the irrigated fields

• the fields used for wastewater irrigation should have small slopes, with good soil permeability, without soils with fractured rocks and deposits or in the vicinity of a water source

• to avoid secondary salinity, the irrigation should be done once every other year, the wastewater should be diluted with clean water in a ratio of 1/2 to 1/5, and small quantities of water should be used

References

1. Singh, A. A review of wastewater irrigation: Environmental implications, Resources, Conservation and Recycling, 168, 2021, 105454, https://doi.org/10.1016/j.resconrec.2021.105454

2. Qadir, M., Wichelns, D., Raschid-Sally, L., McCornick, P.G., Drechsel, P., Bahri, A., Minhas, P.S. The challenges of wastewater irrigation in developing countries, Agricultural Water Management, 97, 2010, 561-568, https://doi.org/10.1016/j.agwat.2008.11.004.

3. Hashem, M.S.; Qi, X. Treated Wastewater Irrigation—A Review. Water 2021, 13, 1527, https://doi.org/10.3390/w13111527.

4. Al Omron, A.M.; El-Maghraby, S.E.; Nadeem, M.E.A.; El-Eter, A.M.; Al-Mohani, H. Long term effect of irrigation with the treated sewage effluent on some soil properties of Al-Hassa Governorate, Saudi Arabia. Journal of the Saudi Society of Agricultural Sciences, 2012, 11, 15–18, https://doi.org/10.1016/j.jssas.2011.04.004.

5. Li, B.; Cao, Y.; Guan, X.; Li, Y.; Hao, Z.; Hu, W.; Chen, L. Microbial assessments of soil with a 40-year history of reclaimed wastewater irrigation. Science of the Total Environment, 2019, 651, 696–705, https://doi.org/10.1016/j.scitotenv.2018.09.193.