Attached algae settlement is frequently observed in effluents of wastewater treatment plants at locations with sufficient sunlight. For their growth they incorporate nutrients and the surface of the algal biofilm accumulates suspended solids from the clarified wastewater. During the photosynthesis process of algal biofilms oxygen is produced while dissolved carbon dioxide is consumed. This led to an increasing pH due to the change of the carbon dioxide equilibrium in water. The high pH causes precipitation of dissolved phosphates. Furthermore an extensive removal of faecal bacteria was observed in the presence of algae, which may be caused by the activity of algae. The experimental results indicate the high potential of these attached algae for polishing secondary effluent of wastewater treatment plants. Especially for small wastewater treatment plants a post connected stage for nutrient removal and bacteria reduction can be developed with the aid of an algal biofilm.

Experiments with a falling film reactor were conducted to examine the potential of the freeze concentration to reuse municipal waste water. The waste water was recycled in the falling film reactor while ice films were frozen on the cooled inner walls. Pure water was removed from the solution by crystallization, and the liquor was concentrated. Finally, the separated clear ice was melted. Various parameters such as conduction, TOC, COD and NH4-N of the melted ice and waste water concentrate were measured. A clearance efficiency up to 99% was observed for municipal waste water and 91% for high concentrated waste water. In addition, particulate solutions were removed from the aqueous phase as efficiently as the organic and inorganic solutes.

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In a cylindrical laboratory reactor, in which a biofilm was grown on a gas-permeable silicone membrane tubing through which oxygen was supplied, the removal of xylene from the bulk fluid was investigated. Two days after starting the experiment 98 % of xylene was degraded and was no longer transferred into the gas phase. Using polarographic microelectrodes the thickness of the biofilm and the boundary layer as well as the oxygen profiles in both layers have been measured. The fluid velocity had three major influences: it affected the boundary layer thickness, the biofilm density and the sloughing of the biofilm. At higher fluid velocities (Reynolds numbers) high oxygen consumption within the biofilm could be quantified. At these higher fluid velocities the biofilm was grown with a higher density and adhered better to the membrane. By application of higher oxygen partial pressures in the gas phase and higher fluid velocities in the liquid phase, the mean degradation efficiency was increased from 38 to 96 %. A computer simulation showed good correspondence with the experimental investigations and allowed a total process analysis. Membrane-biofilm reactors are preferred for technical applications as, e.g., treatment of landfill leachates with high contents of volatile organics.

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