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The effect of shear stress on the viability of Chaetoceros muelleri was studied using a combination of a rheometer and dedicated shearing devices. Different levels of shear stress were applied by varying the shear rates and the medium viscosities. It was possible to quantify the effect of shear stress over a wide range, whilst preserving laminar flow conditions through the use of a thickening agent. The threshold value at which the viability of algae was negatively influenced was between 1 and 1.3 Pa. Beyond the threshold value the viability decreased suddenly to values between 52 and 66%. The effect of shear stress was almost time independent compared to normal microalgae cultivation times. The main shear stress effect was obtained within 1 min, with a secondary effect of up to 8 min.

Many chemical, physical and biological transformations of wastewater in sewer systems may take place and cause significant changes in the composition during transportation. This paper focuses on the methods for measuring changes in wastewater composition (organic matter, dissolved oxygen, nitrogen and sulfur compounds), characteristics for changes in wastewater composition and empirical equations for oxygen uptake rates, sulfide production rates and BOD removal rates in both pressure mains and gravity sewers. Simple and more complicated biofilm models are evaluated for use in sewer systems, but so far no suitable advanced model for sewer biofilms exists, due to a lack of quantitative information about the transformations in such high-loaded multispecies biofilms.

Under conditions of high organic surface load and high hydraulic shear the influence of bulk water dissolved oxygen (DO) concentration on biofilm oxygen consumption, biofilm growth and sulfate reduction in biofilms was investigated using both domestic wastewater and synthetic glucose - yeast extract medium. The biofilm thickness apparently did not approach a steady-state value; instead biofilm thickness increased linearly until the entire biofilm eventually sloughed. Potential activity of sulfate-reducing bacteria was highest when the biofilms were grown at low DO levels in bulk water. Potential sulfide production measured per biofilm volume remained fairly constant during the growth of the biofilms. Only at oxygen levels close to 0 ppm was sulfide production into bulk water under aerobic conditions evident; at higher oxygen levels, complete reoxidation of sulfide occurred within the biofilm. For biofilms growing at bulk oxygen conditions below 1 ppm, the sulfate reduction (and thus sulfide production) was of significance for the processes in the biofilm system in terms of oxygen consumption and COD removal.