Why should we give the greywater treatment concern and expand the areas of its use?

Greywater treatment concern and the expansion of its uses are important. Because water conservation is a major issue in arid and semi-arid regions. Groundwater is often the only available source of water, and in many places, reserves overlap.

Consumption rationalization

While wastewater is discharged into rivers and is thus lost from the local system, its use in irrigation contributes to the recharge of groundwater, thus conserving it in the system and enabling its subsequent reuse. This was the main reason for the initial spread and use of greywater treatment plants.

Prioritization

When water supplies are limited, the general tendency is to allocate water to priority uses, i.e. for drinking, domestic use and later for agriculture. This also applies to wastewater. For this reason, irrigated agriculture has received much more attention than irrigated forestry. However, health risks and cultural and aesthetic values ​​limit the agricultural use of wastewater (especially for food crops) in some places and may make it difficult for farmers to use wastewater in agriculture. Forestry is the most suitable option. In addition, in some circumstances, irrigated forests are economically competitive with irrigated agriculture and may be more profitable.

Integrating trees with irrigated agriculture in the form of windbreaks or border plantings, for example, may be the most economically attractive option in many places.

Nutrient availability for crop production

Raw wastewater and even secondary treated wastewater are rich in mineral nutrients required for plant growth (nitrogen, phosphorus, potassium and micronutrients). Experiments have repeatedly demonstrated that crop or tree yields are increased when irrigated with wastewater compared to clean water. These nutrients represent a valuable resource when compared to the equivalent cost of fertilizer. Application of wastewater at rates that balance nutrient inputs with plant uptake will promote optimal plant growth while minimising the risk of contamination. Calculations have been made in Australia on nutrient inputs and use by tree farms irrigated with wastewater.

With nitrogen contents in wastewater of 10 to 30 mg/L (average 20 mg/L). The phosphorus content is 4 to 10 mg/L (average 7 mg/L). Assuming an average annual wastewater use rate of 8,000 m3/ha, the total annual input of nitrogen (N) is 160 kg/ha and phosphorus 56 kg/ha. Small, rapidly growing farms can take up between 120 to 150 kg N/ha and about 12 kg P/ha per year. Therefore, sufficient levels of these nutrients will be available to achieve maximum growth.

Input balance

A balance is reached between the input and uptake of nitrogen. This is in the form of mobile nitrate and poses the greatest risk of leaching and groundwater contamination. Any excess phosphorus will be retained in the soil and poses no risk of contamination.

Municipal wastewater for forest irrigation

The use of municipal wastewater for forest and tree plantation irrigation is still relatively limited. While sound, well-designed and systematic research on this topic is more limited. It is possible to give some examples of research that help illustrate the benefits of wastewater-irrigated forests and the different circumstances in which this is possible.