German researchers recovering high-quality products from municipal wastewater

In Germany, sewage treatment plants do more than just clean wastewater. They are also sources of raw materials. In the KoalAplan project, researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB are working with partners to recover high-quality products from municipal wastewater. These include ammonium, hydrogen and polyhydroxyalkanoates (PHAs), which can be used to produce bio-based and biodegradable plastics.

One of the methods established in the project, the primary stream method, involves wastewater treatment following prior solid separation during primary clarification. In a traditional sewage treatment plant, the nitrogen in the wastewater is removed biologically. Microorganisms convert the nitrogen compounds into gaseous nitrogen (green house gases), which escapes unused into the atmosphere. In the KoalAplan project, by contrast, the nitrogen is not lost. Instead, it is recovered as a raw material by physically removing ammonium using a zeolite filter or ion exchange system. After that, the nitrogen is recovered by regenerating the zeolite filter, producing a concentrated ammonium solution that can be used in agriculture as a nitrogen fertilizer.

In a traditional municipal sewage treatment plant, a large portion of the solids present in the wastewater is separated through sedimentation during the primary clarification. This “primary sludge” is fermented in a digestion tower, producing methane. In the biorefinery concept, it undergoes dark fermentation instead, with the degradation process being halted at the stage involving production of short-chain organic acids. After two-stage solid separation, a particle-free hydrolysate is produced. This hydrolysate is rich in short-chain organic acids and can be used for a variety of purposes. KoalAplan project explores these pontentials, for eg. microbial electrolysis for production of hydrogen and microbial production of PHAs.