Our contribution to the Nexus of Water – Energy – Food – Raw materials

Fraunhofer IGB has been working on converting smaller sewage treatment plants from aerobic sludge stabilization to more efficient and flexible anaerobic high-load digestion producing biogas. "When sewage sludge, which consists primarily of organic carbon compounds, is treated in the absence of air in digesters with an efficient high-load process, anaerobic bacteria convert it – via acids and alcohols – into considerable amounts of biogas, a mixture of carbon dioxide and methane,” explains Dr. Marius Mohr, who manages the ”Bioprocess Engineering in Water Management and Circular Economy“ group at Fraunhofer IGB. In combined heat and power plants, biogas supplies electricity and heat that significantly reduce or in some cases more than compensate for the net energy consumption of the sewage treatment plant.

In Brazil, on the other hand, where little additional heat is needed, biogas can be used in the mobility sector. Many vehicles are equipped with a Tetrafuel engine, which can run on gasoline and ethanol as well as compressed natural gas (methane). The vehicles only need to be equipped with an additional tank. The biogas produced at the wastewater treatment plant of the city of Franca in the state of São Paulo is therefore processed according to a concept developed by IGB and can be used as biomethane. In both cases, not only the operators of the wastewater treatment plant benefit, but also the climate, as fossil fuels are saved.

About 10 years ago, Fraunhofer IGB transferred the approach of producing biogas by digesting organic substances to municipal wastewater rich in organic load. “In the DEUS 21 project, we have successfully demonstrated that the organic substances in the wastewater supply biogas if they are treated semi-decentrally in anaerobic bioreactors,” according to Mohr. The higher the concentration of organic load, the more efficiently the biogas can be produced. For this reason, biological waste generated in the household is at best “disposed of” with the wastewater.

Fraunhofer IGB and the Gesellschaft für Internationale Zusammenarbeit (GIZ) have taken up this approach in the project “The Urban Nexus”. “For ten selected Asian cities in which the development of a suitable wastewater infrastructure could not keep up with the population growth, we have developed concepts for an integrated resource management,“ Mohr explains. One example is the city of Da Nang in Vietnam. 200,000 people live on the coastline, whose wastewater is currently seeping into pits unused.

According to Mohr's concept, household wastewater is discharged via a vacuum sewer system. Nearby hotels also dispose of their kitchen waste through these pipes to increase the content of organic carbon compounds. Together with the wastewater, they land in a tank that feeds a bioreactor. In the absence of air, the bacteria digest the organic cargo into biogas, which in turn nourishes the gas flames in the kitchen in hotels and households. The purified water is used (except during the rainy season) for soil irrigation of urban agriculture that is operated in an intensive manner. This preserves groundwater reserves and reduces the risk of groundwater salinization by subsequent seawater. As the wastewater purified in the anaerobic bioreactor still contains plenty of nutrients such as phosphorus and nitrogen, the plants on the field are fertilized at the same time. Farmers can dispense with the use of further fertilizers.

After long and ultimately successful negotiations on financial support, the city of Da Nang wants to test the vacuum system on 110 plots of land. If the results are positive, it is planned to implement the overall concept by the end of 2018, which shows impressively how sustainable solutions will look when the areas of water, energy and food security are considered together.

This is exactly what the “Smart Water Future India” project, in which Fraunhofer IGB is developing a concept for sustainable water and resource management in the city of Coimbatore in southern India, is all about. The challenges of urban development are not to be considered separately according to the traditional sectors, but solutions for water supply, energy supply and food security are to be developed across the board and networked intelligently. For the requirements analysis, the project is based on the “Morgenstadt City Lab” methodology developed within the framework of the Fraunhofer City of the Future Initiative, which Mohr has already successfully applied in the Georgian capital of Tbilisi.

Regional competition for the water resource exists in many places, including in the Mediterranean regions of Europe where vegetables are grown. Since agriculture is one of the largest water consumers, new concepts and processes for water reuse are in demand. As already mentioned above, anaerobically treated wastewater still contains plenty of inorganic phosphate and ammonium salts – nutrients that are urgently needed in agriculture after the decomposition of organic compounds. Why not use wastewater for irrigation and as a source of nutrients at the same time?

“In the research project HypoWave we are investigating whether anaerobically purified municipal wastewater can also be reused for hydroponic plant production due to its nutrient content,” according to Marius Mohr. In the case of this cultivation of vegetable plants in the greenhouse, the seedlings do not need soil in their plant containers. As a result, no water seeps into the soil and it evaporates less. Initial results of a pilot experiment with lettuce plants show that only a small additional supply of nutrients is necessary for good growth – with the wastewater, the nutrients contained in it can be used again in a meaningful way.

The reuse of nutrient-rich wastewater is limited to direct, regional local use. If this is not possible, also due to legal requirements, the nutrients phosphorus and nitrogen contained in the wastewater can be recovered in solid, transportable form as fertilizers – and create the basis for a cycle-oriented agriculture.

The harvesting of plants removes nutrients from the soil, which in today's modern agriculture are primarily balanced by synthetic fertilizers. However, deposits of raw phosphates are increasingly contaminated with heavy metals. And the industrial production of nitrogen fertilizers using the Haber-Bosch process consumes enormous amounts of energy: About two percent of the world's primary energy production and five percent of the world's natural gas consumption are caused by nitrogen fertilizer production alone.

The nutrients stored in the plant are not really lost, but end up in biowaste, liquid manure and fermentation residues as well as in wastewater via the food chain. “If it is possible to close the nutrient cycle by recovering the nutrients from these waste streams and recycling them into fertilizers, natural raw material reserves and fossil energy sources can be protected,“ according to Dr. Iosif Mariakakis, Manager of the Nutrient Management Group at Fraunhofer IGB.

It is estimated that 4.3 million tonnes of phosphorus per year are lost worldwide through the sewage system alone. “With our ePhos^® process, we have developed an electrochemical process that allows nitrogen and phosphorus to be precipitated as Magnesium-Ammonium-Phosphate (struvite) from municipal wastewater without any addition of salts or alkalies,” according to the IGB expert. The energy requirement is low and can be supplied entirely from renewable sources. Struvite is a high-quality long-term fertilizer for agriculture and can be directly absorbed by the plants.

Within the scope of various projects, IGB was also able to develop a concept for recovering phosphorus as fertilizer salt from liquid manure. In a mobile, fully automated pilot plant with a throughput of one cubic meter of raw slurry per hour, more than 90 percent of the phosphorus can be separated from slurry, precipitated and crystallized (see p. 75). The combination of the precipitation step with a low-energy solid-liquid separation also produces a nutrient-poor organic fraction, which can be used as a soil conditioner, in particular to increase the moisture capacity in the soil. The process can also be used for the recovery of phosphorus from fermentation residues produced in agricultural biogas plants.

In arid and semi-arid areas, drinking water supply can often only be ensured by desalination of seawater or groundwater. However, technologies such as reverse osmosis or conventional thermal processes are energy-intensive and currently still consume large quantities of fossil fuels. IGB is working on energy-efficient and at the same time robust alternatives, which are cost-optimized in terms of the materials used and which are also affordable for poorer countries. In the case of multi-stage vacuum evaporation, for example, solar thermal energy can be used for the desalination of salt water.

In recent years, more and more electrophoretic processes for desalination have also been further developed. “Capacitive deionization (CDI) for example, requires significantly less energy than a reverse osmosis plant,” according to Siegfried Egner, Head of the Department of Physical Process Technology at IGB. In a variant of vacuum evaporation, the underlying principle is used to open up air humidity as a water reservoir.