The idea of growing microalgae in wastewaters emerges from the idea of resource conservation and the recovery of nutrients. In fact, microalgae are able to take up nitrogen, phosphorus and carbon from wastewaters, even adsorb metals, and in many cases, can be co‐cultivated with various bacteria that are prevailing in municipal wastewater treatment plants. The cultivation of microalgae in municipal wastewater has been known for about half a century and investigated accordingly. Despite this long history, there are still many questions to answer before this technology will be ready for implementation in large‐scale projects. In this review, recent developments are presented. One crucial point in developing a viable process out of wastewater grown algae is the downstream processing of the accumulated algal biomass. The authors decided to focus on hydrothermal carbonization (HTC) as a processing strategy. HTC uses wet biomass and relatively mild process conditions to produce an energy‐rich biochar and a liquid fraction that can be further processed to higher‐value substances. The latest findings in the carbonization of microalgae are highlighted in the second part of this article.

The occurrence of multiple steady states in a toluene biodegrading, diffusion-limited biofilm under aerobic conditions was investigated by computer models: one steady-state, and one nonsteadystate. Two stable and one unstable intermediate steady-state were identified in a narrow set of combinations of parameters values. The nonsteady-state model predicts conditions that evolve to a steady state that is within 0.02-1% of the solution of the steady-state model, depending on the number of grid points used, confirming the algorithms are valid. Multiple steady states occur if, (1) a biofilm is exposed to a constant gas-phase pollution concentration, which exceeds or undershoots a certain threshold, (2) in a narrow range of parameter values and (3) provided that the pollutant degradation follows Haldane kinetics. Such a biofilm displays half-saturation (i.e., Michaelis-Menten)like apparent ("falsified") kinetics from a concentration range starting at zero up to the occurrence of a second steady state. Multiple steady states and falsified kinetics can negatively affect a biofilter and the experimental determination of kinetic parameters, respectively.Implications: The occurrence of multiple steady states in a VOC treating biofilm, shows the significant impact of degradation kinetics and diffusion limitation on the biofilm behavior. Moreover, the implied possible sudden drop of removal efficiency of a biofilter, based on the occurrence of multiple steady states lead to possible bottle-necks in biofilter application and operation.

Ein Hauptgebiet des Umweltschutzes sind chemische und elektrochemische Oberflächenbehandlungsprozesse wie das Galvanisieren, Beizen, Ätzen, Anodisieren u. a. Derartige Oberflächenbehandlungen verursachen häufig einen hohen Chemikalienverbrauch sowie große Mengen Abwasser und schwermetallhaltige Abfälle. Eine stoffverlustminimierte Prozesstechnik ist das Resultat einer Systemoptimierung zwischen interner Stoffkreislaufschließung und externer Abfallverwertung. Kostensenkungen, Prozesssicherheit, Qualitätssicherung und Umweltverträglichkeit sind Ziele der komplexen Systemoptimierung. Prozesseinheiten, bestehend aus Prozessstufe für die Oberflächenbehandlung (Prozesslösung), Spültechnik, Konzentrator und Regenerator stehen im Mittelpunkt der Systemoptimierung. In diesem Beitrag wird der Zusammenhang verschiedener Prozesskriterien diskutiert. Der optimale interne Rückführgrad für Spülwasserkonzentrate, die Prozesschemikalien von der Oberflächenbehandlung enthalten, ist ein Schlüsselkriterium bei der Systemoptimierung.

Different steady states in a toluene biodegrading biofilter were explored experimentally and numerically. Experimental results showed that a gradual increase of the toluene inlet concentration over several weeks leads to a consistently low exit concentration, with a drastic increase at an inlet concentration change from 7.7 to 8.5 g m−3, indicating an alteration in steady state. A significant and sudden drop in the removal efficiency from 88 to 46% was observed. A model that includes nitrogen and biomass dynamics predicted results matching the experimental biofilter performance well, but the timing of the concentration jump was not reproduced exactly. A model that assumes a gradual increase of toluene inlet concentration of 0.272 g m−3 per day, accurately reproduced the experimental relationship between inlet and outlet concentration. Although there was variation between experimental and simulated results, a clear confirmation of the jump from one steady state to another was found.