Effective Use of Wastewater for Irrigation

Growing industrialization and urbanization have put tremendous strain on the limited supply of freshwater resources and generated vast amounts of wastewater. The single biggest user, accounting for almost 70% of all freshwater abstraction, is agriculture. 7130 km3 of water are currently used annually to feed the world's population; if water productivity doesn't improve further or if production patterns don't change significantly, it's predicted that by 2050, 12000–135000 km3 of water will be utilized in agriculture.

In addition to agriculture, 15% of India's resources are used by the country's industries. Market forces will cause a reallocation of water resources from agricultural to urban sectors by 2050, accounting for a 30% rise in the proportion of these two sectors. According to an estimate by the central public health engineering organization, about 70-80% of these water supplies turn into wastewater. Fresh water supply for agriculture will sharply decline in the upcoming years due to rising water consumption by municipalities and industries, which will also produce more wastewater.

The ability of natural systems to absorb large amounts of wastewater is limited. Salts, pathogens, heavy metals, and other contaminants found in wastewater degrade natural resources, impair the food chain, and gravely threaten the health of people and animals. Additionally, the amount of harmful pollutants that soils can take without degrading their quality and production is restricted. These contaminants are difficult to remove once they are in the soil environment, and they may end up in groundwater, where contamination above a certain threshold is permanent. As a result, untreated wastewater discharge into surface or groundwater bodies permanently degrades the quality of these priceless natural resources.

The challenge thus is to find such low-cost, low-tech, user-friendly methods, which on the one hand avoid threatening our substantial wastewater-dependent livelihoods and on the other hand, protect degradation of our valuable natural resources. Therefore, policies and programs that address the following crucial issues for developing appropriate site-specific and waste water-specific strategies are needed for the planned, strategic, safe, and sustainable use of wastewater. These policies and programs should include low-cost decentralized wastewater treatment technologies, bio-filters, effective microbial strains, organic and inorganic amendments, appropriate crops and cropping systems, cultivation of profitable non-edible crops, and modern sewage water application methods.

1. Reduction of toxic contaminants in wastewater at the source: The disposal of industrial effluents should be strictly regulated and treated at the source.

2. Establishing a wastewater use policy: To handle or treat wastewater to the quality levels specified in the guidelines, there are two main obstacles to their adoption: first, the infrastructure, operation, and maintenance, as well as the corresponding investment and ongoing costs; and second, regulatory enforcement to guarantee compliance with required practice on the part of water authorities, those who discharge wastewater, and/or those who handle and use wastewater.

3. Crop restrictions protect consumers' health when water is unavailable for irrigation, but they don't protect farm workers and families from low-quality effluents or contaminated surface waters. Crop restriction should be part of an integrated system of control. Low-cost practices, such as hygiene education, wearing shoes, regular anti-helmintic treatment, crop growth on raised beds, and harvesting of crops above ground, can significantly reduce pathogen load.

Combine crop restriction with hygiene education to reduce cholera transmission by 90%, as has been shown to be successful in Mexico, Peru, and Chile. But only under specific circumstances is it practical: robust law enforcement, public entity oversight of waste distribution, sufficient demand for restricted crops, fair prices, and little pressure on excluded crops.

4. Climate and wastewater quality have an impact on the crops that farmers grow. It is necessary to grow salt-tolerant crops in dry environments because high evaporation rates result in saline wastewater. wastewater can be used in urban settings to produce feed, particularly dairy products. Fish species may be sensitive to changes in water quality, and livestock health and milk quality may be jeopardized.

5. Applying fertilizer: Since wastewater is a rich source of plant nutrients, soils that get wastewater irrigation have an increased nutritional content. As a result, fertilizer dosages should be modified based on crop nutrient requirements, wastewater application volume, and wastewater nutrient content. To detect nutritional imbalances or soil illness, soil testing should be done regularly.

6. lrrigation Management

a) Conjunctive Water Use:

Practically, there is no distinction between saline water irrigation and wastewater irrigation, with the exception of the existence of a high organic load. In order to address the various issues related to wastewater usage, solutions using dilution principal, such as conjunctive use in cyclic mode with fresh water sources, may be employed. Guidelines developed for saline water use may also be taken into consideration when dealing with wastewater.

b) Application Methods:

The greatest risk of salt, pathogens, and other pollutant deposits on crop surfaces is linked to spray and sprinkler irrigation systems. Additionally, adjacent communities may be exposed to bacteria and viruses through aerosols. Establishing a buffer zone, such as 50–100 meters from homes and highways, may be important when using spray irrigation with wastewater to prevent negative health effects on the surrounding community. Farm labourer and their families are also most at risk when using furrow or flood irrigation techniques. This is particularly true when moving earth by hand and wearing safety gear is not required. The risk to irrigators can be reduced if the effluent is fed into individual furrows by means of gated pipes and conveyed through pipes, thereby minimizing interaction between the polluted irrigation water and edible sections of the plant.

Localized irrigation techniques like bubbler and drip trickle offer health protection and lower crop contamination. However, clogging of emitters is a major issue in drip systems. Tertiary treatment and chlorination can reduce clogging caused by bacteria and algae. Filtering can also help reduce clogging by preventing suspended particles from entering the system. Studies in India show that gravel media filters, screens, and disk filters are superior to screen filters in filtration efficiency, discharge rate, and E. coli population reduction. Low-cost drip irrigation techniques like backet kits have shown potential for use in low-income countries, with bucket drip kits showing higher contamination reduction.

c) No irrigation before harvest:

Stopping irrigation 1-2 weeks before harvest can reduce crop contamination, but is challenging for leafy vegetables, but may be possible for some fodder crops not harvested at peak freshness.

7. Alternate land uses:

a) High rate transpiring plantations:

In contaminated land, alternative land uses like manmade forests with high-value trees and green belts can overcome health hazards. These plants can transpire water higher than soil matrix, reduce N and P leaching, and improve groundwater quality. The biochemical oxygen demand removal efficiency of tree plantations ranges from 80.0 to 94.3%. To avoid heavy metal accumulation, crop removal is suggested. High-rate transpiration systems can be developed based on varying water demand in different seasons.

b) Agro-forestry:

Wastewater irrigated agro-forestry in Hubli, Karnataka could reduce irrigation requirements, reduce farmers' exposure to wastewater, and increase income. Key tree species include sapota, guava, coconut, mango, areca nut, and teak, with field crops grown in dry and kharif seasons.

C) Selection of crops as per toxic constituents of wastewater:

Crop tolerance to soil concentrations of heavy metals varies. Their differences also extend to the accumulation of absorbed heavy metals in various plant sections and their metal affinities. Therefore, crops should be chosen so that they can grow in parts of the plant that are either unimportant or not consumed and can withstand the specific hazardous components of wastewater.

8. Phyto-remediation:

Phyto-remediation techniques like rhizo- filtration, photo-degradation, phyto-stabilization, and phyto-volatization can be used to remove harmful metals from soil. Species like Tilaspi caerulescens and B. juncea have shown effectiveness. Soil amendments like chelators and organic acids improve metal extraction.

9. Bio-remediation:

Microorganisms like algae, fungi, and bacteria are used in bio-remediation to reduce pollutants in aquatic systems. They lower BOD and COD levels, break down organic materials, and eliminate heavy metals from wastewater.

10. Use of chemical amendments:

It has been noted that applying sewage and industrial effluents can increase agricultural soil's accessible metal status by a factor of two to one hundred. The majority of these metal accumulations are found on the surface soil itself, and their contents have a negative correlation with soil pH and a positive correlation with soil organic carbon and clay content. Lime and other additions can raise the pH of the soil, which can make the metals unavailable. Applying phosphate, kaolin/ zeolite, and Fe-Al oxides to soils also lowers the amount of harmful elements that are available.

11. Use of Bio-absorbents:

Low-cost, low-tech removal methods like activated charcoal, press mud, rice husk, and sawdust effectively reduce metal prevalence in wastewater through sorption principles.

12. Treatment Systems

13. Construction of weirs:

The construction of irrigation infrastructure, particularly weir, improves river system water quality through sedimentation, dilution, aeration, natural die-off, and UV-light exposure, reducing helminthes, E.coli, and biological oxygen demand.

14. Research should be promoted in areas like nanotechnology to find out ways and means to build cheaper wastewater management plants.

15. Indigenous technical knowledge, local knowledge and traditional knowledge should be properly documented for safe and sustainable wastewater use.

16. Socioeconomic studies: Like caste, class, ethnicity, gender and land tenure influence the type of wastewater-dependent livelihood activities.

To ensure sustainable wastewater management, it is crucial to consider the economic niche and user perceptions, while also considering the advantages and disadvantages of wastewater irrigation.