DIGEST-UP, or How to Leverage Wastewater to Optimize Biogas Production

Wastewater treatment is one of the critical parts in the transition towards a zero-waste circular economy. Once the water is purified, one of the challenges is the disposal of excess organic matter, known as sludge. These sludges are treated by thickening, dehydration, and sanitation to use them as farming fertilizers or forest soil regeneration. In some cases, incineration is used. However, there are ways to optimize their performance, especially in the field of biogas. One of them is the R+D project, DIGEST-UP, which seeks to take advantage of substances present in the sludge, such as ammonium, to accelerate the metabolism of the most irreducible sludge, as is the case with secondary sludge.

The DIGEST-UP technology approach opts for a pre-treatment of this secondary sludge utilizing a current rich in ammonia generated in the plant itself for improved environmental, energy, and economic efficiency. In addition to reducing the total mass of sludge, this process generates a greater production of biogas.

The pilot plant designed within the project is currently housed in the Wastewater Treatment Plant (WWTP) of Arroyo Culebro Cuenca Media-Alta in Pinto (Madrid). The experiment offers some encouraging results. The daily production of biogas increased between 22% and 44%, depending on the initial concentration of ammonium, and the production of methane also increased by 32%.

Biogas, a raw material with multiple applications

The resulting gas can be used as fuel for electrical cogeneration, in other words, it allows wastewater treatment plants to generate part of the energy they consume and boost their sustainability. Moreover, if purification processes are applied, this biogas can be used as natural gas for household or industrial purposes and automotive fuel. Ultimately, its use, whether purified or in its original state, will contribute to reducing carbon dioxide emissions and provide yet another tool in the fight against global warming.

DIGEST-UP, an ACCIONA initiative financed by the Center for the Development of Industrial Technology (CDTI), from the Ministry of Science, Innovation, and Universities, is a technology that is versatile enough to work in all types of WWTPs that have anaerobic sludge digestion. The technology can be applied both in existing plants and in new generation plants.

Other solutions for the wastewater challenge

In addition to DIGEST-UP, other alternatives are being explored to optimize wastewater treatment. One example is the Life CELSIUS initiative, which uses anammox bacteria to convert the nitrogen dissolved in the water into nitrogen gas, releasing it into the atmosphere. This project reduces the need for aeration of wastewater, an energy-intensive process.

Although it may come as a surprise, to make the whole process more sustainable globally and obtain other resources from the purification of this water, there are alternatives for obtaining fertilizers from ammonium. This system has been optimized in recent years to reduce the energy consumption of treatment plants. Thus, researchers from the University of Aalto in Finland have developed a method to purify urine. Their approach is based on calcium hydroxide that raises the water's PH to obtain gaseous ammonia, a raw material used in fertilizers' manufacture. In the same process, employing calcium salts, phosphorus is precipitated and then used as a fertilizer too.

The next years' main goal will be the transition to renewable energies and the obtaining of useful and non-polluting byproducts for an eco-friendly water cycle.

Source: ACCIONA