Fertigation

Introduction

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 formunicipal uses often results in the reuse of wastewater for irrigation. Even though it is not recommended to use wastewater directly, due 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 Solution ‘Fertigation’

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).

Fertigation with partly treated wastewater

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.

Fertigation with urine

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 nitrogen and other nutrients and may also be used for fertigation.

Application

Generally, drip irrigation (also subsurface drip irrigation) is the most appropriate irrigation method; it is especially good for arid and drought prone areas. Surface irrigation is prone to large losses from evaporation but requires little/no infrastructure and may be appropriate in some situations. Crops such as corn, alfalfa (and other feed), fibres (cotton), trees, tobacco, fruit trees (mangos) and foods requiring processing (sugar beet) can be grown safely with treated effluent. More care should be taken when growing fruits and vegetables that may be eaten raw (e.g. tomatoes) and that could come in contact with the water. Energy crops like eucalyptus, poplar, willow, or ash trees are also suitable and can be grown in short rotation plantations (and harvested for biofuel production (EUBIA n.y.). Since the trees are not for consumption, this is a safe, efficient way of using lower quality effluent.

Drip Irrigation

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 fertiliser reaches directly the roots and you safe time, water and fertiliser. This method also lessens the potential of groundwater pollution caused by the fertiliser leaching.

Despite the advantages that 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. Water sources for irrigation that has high contents of calcium, magnesium and bicarbonates (hard waters) are undesirable, because they generate precipitates in the fertilisation tank, which leads to clogging of the drippers and/or filters. If clogging occurs 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).

Manual and Motorised Application

For small fields or single plants/trees, fertigation water or pure urine can be applied manually, bigger fields motorised. 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).

Health Aspects

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 safe use of wastewater, excreta and greywater in agriculture (WHO 2006 Vol. II and IV) recognise 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.

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 pathogen die off.

The risk of pathogen transmission during handling, transportation and reuse of diverted urine is, however, mainly based on the amount of faecal material contaminating the urine fraction. The survival of pathogens is dependent of temperature, pH value, dilution, ammonia and time. The technical design of a urine diversion system (e.g. flushing and storage procedures) may also influence pathogen persistence. The following transmission pathway have to be considered for diverted urine (WHO 2006):

1. Ingestion of urine that has not been stored: while cleaning toilets, maintaining storage tank (hand-to-mouth contact).
2. Ingestion for stored urine: during handling of stored urine, e.g. fertilising (small and large scale) with urine.
3. Inhalation of aerosols while fertilising crops with urine
4. Consumption of crops fertilised with urine

To learn more about health risks when you work with urine read more in the WHO Guidelines Volume IV (2006).

At a Glance