Fertigation

Growing competition between agricultural, industrial and domestic water use particularly in arid, semi-arid and densely populated regions increases the pressure on freshwater resources. It is estimated that within the next 50 years, more than 40% of the world’s population will live in countries facing water stress or water scarcity (HINRICHES et al. 1998). More fresh water is abstracted and used in agriculture in arid and semi-arid countries than for any other purpose (i.e. for domestic uses and industrial uses combined). Water scarcity in agriculture, together with large increases in water demand for municipal uses often results in the reuse of wastewater for irrigation. Even though it is not recommended to use wastewater directly, due to the related risk of chemical and biological pollution, the combined use of the water and the nutrients contained in wastewater is a promising option to increase sustainability in water use and agriculture.

The combined irrigation and fertilisation is called fertigation. There are different ways how fertigation can be done:    

• Mixing chemical fertiliser with irrigation water;
• Using partly treated wastewater which still contains nutrients for irrigation;
• Mixing urine as a nutrients source to the irrigation water.

To use wastewater that has been (partly) treated and contains sufficient nutrients to be used in fertigation has several advantages: no freshwater is required; the wastewater requires only to be treated partly (as the nutrients will further be removed by the plants); it provides an extra source of nutrients instead of buying expensive and non-renewable industrial fertiliser. Using urine mixed with fertigation water as a source of nutrients is another way avoiding the dependence on mineral fertiliser (see also peak phosphorus).

The use of wastewater in agriculture and aquaculture (animal or plants) can also act as a low-cost secondary treatment method that increases food production to supply growing urban and periurban populations with fresh produce. While plants take up the fertigation water and some of it infiltrates into the soils, most nutrients and biological oxygen demand are removed and pathogens die off.

Wastewater used for fertigation water, depending on its initial composition, should have undergone both physical treatment to prevent clogging of the irrigation system, and biological treatment to prevent health risks. A combined physical settling and biological treatment can be achieved (e.g. in septic tanks, biogas settler, waste stabilisation ponds, etc.) to reduce pathogens and to limit the risk of crop contamination and the health risk to workers and end consumers. Greywater may be used directly after removal of coarse particles.

Urine contains large amounts of nutrients and is generally pathogen free. In order to bring the nutrients (mainly P and N) contained in the urine to the plants, the urine can be added to the irrigation water. For this technique, urine needs to be collected separately (see also urine diversion components, waterless urinals, urine diversion dehydration toilet, urine diversion flush toilets). A common pre-treatment method for urine, in order to make sure, that there are no pathogens is storage. However this pre-treatment is only optional (see also JOENSSON et al. 2004) and can also lead to precipitation of some of the phosphorus at the bottom of the storage tank. Struvite production from urine is a way to recover the phosphorus in powder form. The effluent from struvite precipitation still contains a lot of nitrogen and other nutrients and may also be used for fertigation.

There are two kinds of irrigation technologies appropriate for treated wastewater:

1) Drip irrigation above or below ground, where the water is slowly dripped on or near the root area; and

2) Surface water irrigation where water is routed overland in a series of dug channels or furrows.

To minimize evaporation and contact with pathogens, spray irrigation should be avoided. Surface irrigation is prone to large losses from evaporation but requires little/no infrastructure and may be appropriate in some situation.

Properly treated wastewater can significantly reduce dependence on fresh water, and/or improve crop yields by supplying increased water and nutrients to plants. Raw sewage or untreated blackwater should not be used, and even well-treated water should be used with caution. Long-term use of poorly or improperly treated water may cause long-term damage to the soil structure and its ability to hold water.

The application rate must be appropriate for the soil, crop and climate, or it could be damaging. To increase the nutrient value, urine can be dosed into irrigation water; this is called “fertigation” (i.e., fertilization + irrigation). The dilution ratio has to be adapted to the special needs and resistance of the crop. 

Drip- or subsurface drip irrigation is about applying the nutrients more directly to the wetted root volume, where the active roots are concentrated. With this method, a maximum of water and fertilizer reaches directly the roots and you safe time, water and fertiliser. This method also decreases the potential of groundwater pollution caused by the fertiliser leaching. Despite the advantages of drip irrigation combined with fertigation, this system is very prone to clogging. Therefore the removal of solids from the wastewater, from the urine mixed with irrigation water or the effluent from struvite production before application is critical. In drip irrigation systems care should be taken to ensure that there is sufficient head (i.e., pressure) and maintenance to reduce the potential for clogging (especially, with urine from which struvite will spontaneously precipitate). Water sources for irrigation that have high contents of calcium, magnesium and bicarbonates (hard waters) are undesirable, because they generate precipitates in the fertilization tank, which leads to clogging of the drippers and/or filters. To avoid clogging periodic injection of acid in the fertigation system is recommended in order to dissolve the precipitates and to unclog the drippers (adapted from KASHEKYA 2009).

For small fields or single plants/trees, fertigation water or pure urine can be applied manually, for bigger fields motorized. It should not be applied on leaves or other parts of plants, as this can cause foliar burning due to high concentrations of ammonia and salts when drying as well as hygiene considerations (RICHERT et al. 2010).

The health risks associated with the use of greywater in agriculture are considered to be lower than those for wastewater. Greywater generally has lower concentrations of pathogens than wastewater, but it may still contain some pathogens, which are introduced into the greywater e.g. from washing babies’ diapers, laundry, personal hygiene or other sources. Wastewater should only be used after appropriate treatment. The WHO Guidelines for the safe use of wastewater excreta and greywater Volume II and IV recognize the potential of using wastewater, excreta and greywater in agriculture and promote a flexible multi-barrier approach for managing the health risks associated with the use of wastewater in agriculture. This multi-barrier concept comprises of a series of measures/barriers (from waste generation to consumption) where each of the barriers has a certain potential to reduce health risks associated with the wastewater use and it is recommended by WHO to put in place several of these barriers if needed in order to reduce the health risk to an acceptable minimum.

Appropriate treatment (i.e., adequate pathogen reduction) should precede any irrigation scheme to limit health risks to those who come in contact with the water. Furthermore, it may still be contaminated with the different chemicals that are discharged into the system depending on the degree of treatment the effluent has undergone. When effluent is used for irrigation, households and industries connected to the system should be made aware of the products that are and are not appropriate to discharge into the system. Drip irrigation is the only type of irrigation that should be used with edible crops, and even then, care should be taken to prevent workers and harvested crops from coming in contact with the treated effluent. The WHO Guidelines for the safe use of wastewater excreta and greywater Volume II should be consulted for detailed information and specific guidance.

Drip irrigation systems must be periodically flushed to avoid biofilm growth and clogging from all types of solids. Pipes should be checked for leaks as they are prone to damage from rodents and humans. Drip irrigation is more costly than conventional irrigation, but offers improved yields and decreased water/operating costs.

Workers should wear appropriate protective clothing.