Greywater reuse, a sustainable solution for home gardening

Urban water demand has been increasing over the years due to factors like mismanagement, urbanization, agricultural and industrial intensification, and rapid population growth. Meanwhile, freshwater availability has been decreasing, and its quality has been deteriorating. In this context, using potable water for home gardening seems to be incoherent, thus, an emerging alternative solution to address water shortages, especially in the face of climate change, is greywater reuse.

What is greywater and why should we reuse it?

Greywater refers to household effluents coming from sources like washing machines, showers, and bathroom sinks. In simple terms, it is wastewater that hasn’t been in contact with food or fecal matter. Greywater can account for up to 70% of total domestic wastewater (Radingoana et al., 2020). Its reuse not only helps reduce pressure on freshwater resources but also significantly decreases the volume of wastewater, decreasing the high energy demand required for its treatment. Reducing freshwater demand is crucial for economic growth, food production, and poverty alleviation (FAO, 2003). Additionally, reducing municipal wastewater can have multiple benefits such as improving water quality in water bodies, reducing soil contamination, and enhancing public health, especially in developing countries and marginalized areas (Metcalf and Eddy, 2003).

Using Greywater for Irrigation

The most common and practical way to reuse household greywater is for irrigation. In fact, common dishwashing soaps, laundry soaps, body washes, shower gels, and bar soaps contain surfactants and builders which are made from nitrogen and phosphorus ingredients. When greywater is used for irrigation, nitrogen and phosphorus act as nutrients to the soil, which can benefit the growth of lawns, gardens, athletic fields, parks and golf courses. Moreover, with this irrigation method gardens can be watered even during drought periods. In recent years, developed countries like Australia, Germany, United States, and Japan have been at the forefront of greywater reuse for landscape irrigation. In Australia the government offers financial support for households installing greywater reuse systems, while in Tokio, greywater reuse is mandatory for large buildings (Radingoana et al., 2020).

Does greywater need to be treated before gardening?

Greywater can contain pollutants such as organic matter, pathogens, and solids. It typically includes substances like soap, shampoo, toothpaste, shaving cream, hair, lint, body oils, dirt, grease, fats, chemicals, bacteria, parasites, viruses and laundry detergents (WHO, 2006). Although these substances are not safe for human or animal consumption, the nutrients in greywater can benefit plants and act as valuable fertilizers. Therefore, if the storage time does not exceed 24 hours, greywater can be reused for irrigation with a simple mesh to catch fats and solids.

However, if greywater is to be stored for future use, further procedures are necessary. Storing greywater for more than 48 hours at temperatures between 19–26°C leads to biological degradation, resulting in malodorous compounds (Ghunmi, 2011). When raw greywater is stored in a tank, it can become septic; oxygen is depleted until the environment becomes anaerobic, causing sludge formation that may either sink or float, depending on its gas content and density (WHO, 2006). Additionally, suspended materials in untreated greywater can accumulate and cause clogs in pipelines, which can also result from biological growth (Ghunmi, 2011). To prevent these issues, a simple treatment system should be installed before the storage tank.

Additionally, households should be careful about the compounds they put into greywater pipelines. The World Health Organization’s water quality standards for irrigation set a limit on total coliforms at ≤10⁴ cfu/100 mL for restricted irrigation, which excludes crops that are eaten raw (WHO, 2001). Thermotolerant coliform concentrations in shower and bath water can range from 10² to 10⁵ cfu/100 mL, while concentrations in laundry effluents can vary from 25 to 10⁷ cfu/100 mL (WHO, 2006). Therefore, care should be taken with laundry clothes; diapers should not be included, and if the clothes are excessively dirty, it is advisable to rinse them before putting them in the washing machine.

Greywater treatments for irrigation

For home gardening purposes, a primary, low-cost treatment system such as coarse screen filters or sedimentation is recommended to remove oils, grease, and solids before irrigation (WHO, 2006). These primary treatment systems are the most economically attractive due to their low maintenance requirements (WHO, 2006). Other options include basic sand filtration, amalgamated membrane bioreactors (MBR), filters using single adsorbents like granular activated carbon with an integrated membrane (Kant et al. 2018), and combined adsorbents such as activated carbon combined with Iranian natural zeolite and stabilized nano zero-valent iron (Amiri et al., 2019). The choice of an appropriate treatment system depends on the intended use of the greywater. It’s advisable to seek expert advice when installing these technologies.

Conclusions and recommendations

Greywater reuse presents a viable solution to address water scarcity and promote water conservation, particularly for home gardening purposes such as irrigating gardens, lawns, shrubs, trees, and controlling dust. As a non-conventional water resource, greywater can help alleviate the pressure on conventional freshwater sources, thereby contributing to economic growth and improved food security (FAO, 2003). However, it is essential to consider the potential environmental and health risks associated with greywater reuse and ensure it undergoes proper treatment before being reused.

To improve greywater reuse practices, the following recommendations are suggested:

• Government Support: Financial assistance and advisory services from the government are crucial to make greywater reuse a widespread practice among households.
• Use of Plant-Friendly Products: It’s crucial to choose detergents and soaps that are biodegradable or “plant-friendly,” meaning they contain minimal amounts of salt, boron, or chlorine bleach.
• Alternative Bleach Options: Opting for hydrogen peroxide over chlorinated bleach can further reduce environmental impact.
• Safe Disposal Practices: Water used for cleaning diapers should be disposed of in the toilet to prevent fecal coliform contamination.

The ultimate goal is to create a sustainable cycle with greywater, minimizing waste and securing water supplies for future generations.

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References

Amiri, M. J., Bahrami, M., Badkouby, M., & Kalavrouziotis, I. K. (2019). Greywater treatment using single and combined adsorbents for landscape irrigation. Environmental Processes, 6, 43-63. https://doi.org/10.1007/s40710-019-00362-1

Food and Agriculture Organization of the United Nations. (2003). Water for sustainable food production, poverty alleviation and rural development in Unlocking the Water Potential of Agriculture. https://www.fao.org/4/y4525e/y4525e05.htm (accessed November 11, 2024).

Ghunmi L.A, Zeeman, G., Fayyad M., & van Lier J.B. (2011). Grey Water Treatment Systems: A Review. Critical Reviews in Environmental Science and Technology, 41:7, 657-698, DOI: 10.1080/10643380903048443

Kant, S., Jaber, F. H., & Karthikeyan, R. (2018). Evaluation of a portable in-house greywater treatment system for potential water-reuse in urban areas. Urban Water Journal, 15(4), 309–315. https://doi.org/10.1080/1573062X.2018.1457165

Metcalf & Edd. (2003). Wastewater engineering: treatment, disposal, reuse. 3rd edi tion, McGraw-Hill, New York.

Radingoana, M. P., Dube, T., & Mazvimavi, D. (2020). Progress in greywater reuse for home gardening: Opportunities, perceptions and challenges. Physics and Chemistry of the Earth, Parts A/B/C, 116, 102853. https://doi.org/10.1016/j.pce.2020.102853.

World Health Organization. (2001). Water quality: Guidelines, standards and health. https://www.who.int/publications/i/item/924154533X (accessed August 23, 2024).

World Health Organization. (2006). Overview of greywater management: Health considerations https://iris.who.int/bitstream/handle/10665/116516/dsa1203.pdf (accessed August 23, 2024).