That is what was discussed at the event ‘Irrigating Forests with wastewater: Natural and effective water treatment?’ held during the World Water Week 2017 in Stockholm. The event was organized in collaboration between FAO, CIFOR, SIANI, Forest Trends, IUFRO, Focali, The Swedish Forest Agency, SLU, and SIWI. The event discussed the pros and cons of using wastewater for irrigation of trees and highlighted successful cases from South Africa, North Africa and Sweden.

The pros and cons

Wastewater use in forestry and agroforestry has many benefits. Trees sequester carbon, cool terrestrial surfaces, regulate fresh water flows, prevent erosion and maintain soil fertility. In drylands, where water is scarce, wastewater can be used in landscape restoration helping to combat desertification.

“Wastewater has a huge untapped potential for resource recovery and reuse!” - emphasized Akiça Bahri (National Agricultural Institute of Tunisia, INAT). “Imagine if we could use all of the globally generated wastewater to irrigate land, provide fertilizer, biogas as well as electricity?” she continued. In fact, the UN 2017 World Water Development Report was called “Wastewater: The Untapped Resource.”

Even though, some may say that from the financial point of view reusing water in forestry is not as viable as in agriculture, the benefits are much higher if we consider the total economic value of forests with all environmental services they provide.

All that said, using wastewater in forestry and agroforestry raises concerns about public and environmental health as well as about food safety. Eduardo Mansur (FAO) pointed out that, if not done properly, irrigating with wastewater can increase soil salinity and cause pollution of ground and surface water. “It is true that there are concerns, but the risks are much lower compared with using wastewater for irrigating agricultural crops. Yet, wastewater irrigation is mostly restricted to agricultural land, while regulations for application in forestry are missing”, highlighted Bahri.

“Dealing with the risks and applying wastewater safely is essential. The gains are worth the risks, especially in water scarce regions,” stressed Mansur.

Experimenting with fecal sludge in the outskirts of Durban

Cities in the Global South are growing at a very high pace and water infrastructure does not often keep up. So, many people opt for latrines. Sludge from pit latrines is a valuable source of nutrients, but sorting waste is rarely possible. That is why it may contain paper, plastic, chemicals and pathogens, which restricts its use in agriculture and industry. However, there is a way to overcome this challenge.

Jay Baghwan (Water Research Comission) presented recent trials from Durban, South Africa, where fecal sludge from pit latrines was buried using deep row entrenchment. After that, trees were planted over the area, leaving about a meter of soil between the sludge and the surface.

The first trials were small-scale, fruit trees were planted over the buried sludge. Results from those trials suggest that sludge improved tree growth and health. More recently, larger-scale trials were held with fast growing eucalyptus. The experiment showed similar results: tree growth on sites where sludge was applied was better. What is more, monitoring has not detected groundwater pollution and has shown a complete pathogen die-off after 3-4 years. 

Using sludge from pit latrines for planted forests and tree-based landscape restoration is promising. Naturally, there are risks, like contamination of groundwater. So, it is important that the distance to groundwater bodies is sufficiently far enough and that the soils are not too porous. Therefore, if applied properly, this can be a safe sludge treatment option which is simple, cost-effective, and offers a great alternative to mineral fertilizers.

Greening the desert with treated wastewater in North Africa

In drylands, where people’s livelihoods are hindered by water scarcity and environmental degradation is high, tree planting can enhance ecosystem services. FAO has been promoting the use of treated wastewater to irrigate forests and trees since the 90s and is currently working on several tree-based landscape restoration projects in Algeria, Egypt, Morocco and Tunisia. In Algeria, constructed wetlands were used to filter wastewater and to irrigate planted forests.

According to Elaine Springgay (FAO), irrigating trees with treated wastewater improves soil quality and health. This way conditions for agriculture are better, so farmers can improve their incomes and food security. In addition, using treated wastewater for irrigation reduces the burden on freshwater - a big bonus in drylands.

Finally, this approach can also improve water quality and pollution in the area of application since wastewater goes through treatment and is not discharged directly into water bodies or the desert. Landscape restoration activities are vastly promoted on the international level through frameworks like the Bonn Challenge and AFR100. Using treated wastewater can help to achieve the goals of these policies.

Replacing mineral fertilizers with digestate in Sweden

Digestate, a by-product from anaerobic digestion of biodegradable waste, can be produced from sewage sludge and is a nutrient-rich substance often used as soil conditioner. Ensuring it is free from pathogens to avoid water and soil pollution is tricky, but it can be done. One solution is a certification standard. 

Anders Finnson (Swedish Water & Wastewater Association) presented an example of the Swedish certification system called REVAQ designed to ensure digestate is free from pathogens and heavy metals. According to Finnson, today more than half of the Swedish population is connected to a wastewater treatment plant certified through REVAQ.

Development and performance of the REVAQ system is stimulated through a set of goals, which are: avoid unacceptable accumulation of metals or undesired organic substances on agricultural land in the long term, have no accumulation of cadmium from 2025, and reduce the accumulation of non-essential substances to a maximum of 0.2% per year from 2025. The framework is forward looking so the users can improve gradually.

REVAQ certified digestate contains a lot of phosphorous, nitrogen, micronutrients and organic matter and is thus an excellent alternative to regular mineral fertilizers. “If we connect the entire Swedish population to REVAQ certified wastewater plants and improve acceptance of digestate in agriculture we can cut the use of mineral fertilizers by 50%”, highlighted Finnson. 

Towards a more sustainable future

These cases show that applying wastewater in the irrigation of forests would allow us to shift from waste disposal to production. Doing that will require overcoming some issues about waste, like social perceptions of the “yuck factor”. And a lot will have to be done in infrastructure development too. However, the concept has already started to take off leading to an emerging challenge: competition between all the potential users, like agriculture and energy. All in all, there is plenty of room for innovation with wastewater recycling, especially in terms of resource efficiency, which is an essential tool if we are to achieve the Sustainable Development Goals and reduce the effects of climate change.

This blogpost was originally published at SIANI.se