Dynamic alpha factor prediction with operating data - a machine learning approach to model oxygen transfer dynamics in activated sludge.

Aeration is an energy-intensive process of aerobic biological wastewater treatment. An accurate model of oxygen transfer dynamics in activated sludge tanks would improve design and operation of aeration systems. Such a model should consider spatial and diurnal variation of α-factor as well as site-specific conditions that impact oxygen transfer. For this dynamic prediction a machine learning approach was used for the first time. The data-driven method was based on long-term ex-situ off-gas measurements with pilot-scale reactors (5.8 m height, 8.3 m3 vol) coupled to full-scale activated sludge tanks on the sites of two conventional and a two-stage activated sludge treatment plant. The ex-situ off-gas method allowed to quantify theoretical off-gas parameters in non-aerated zones and thus consider the whole activated sludge tank. We introduced the α0-factor to compare aerated and non-aerated zones under nonsteady-state conditions. Like the established α-factor for steady-state conditions, the α0-factor describes oxygen transfer inhibiting effects in activated sludge. α0-factor was lowest in upstream denitrification zones. This indicates an anoxic elimination of oxygen transfer inhibiting wastewater contaminants which improved oxygen transfer in subsequent aerobic zones. Random Forest models predicted α0-factor reliably in all examined activated sludge tanks even for stormwater events and seasonal variation. Model development only required online sensor data already available to operators. Our results suggest that machine learning models can dynamically predict α-factors in a variety of activated sludge processes, thus considering site-specific conditions in model training without manual calibration.

Improving aeration systems in saline water (part II): effect of different salts and diffuser type on oxygen transfer of fine-bubble aeration systems.

The objective of the present study is to investigate the different effects on the oxygen transfer of fine-bubble aeration systems in saline water. Compared to tap water, oxygen transfer increases due to the inhibition of bubble coalescence. In Part I of the present study, we investigated in laboratory-scale experiments the effect of design of diffuser membrane. The objective of Part II is the assessment of effects of different salts, diffuser type and diffuser density. We measured the concentration of various salts (MgCl2; CaCl2; Na2SO4; NaCl; KCl) above which coalescence is fully inhibited and oxygen transfer reaches its maximum (referred to as the critical coalescence concentration; CCC). For this purpose, we developed a new analytical approach, which enables investigation of the coalescence behaviour of any aeration system and (mixed) salt solution quickly and easily by evaluating the results of oxygen transfer tests. To investigate the transferability to large scale and the effect of diffuser type and density, we repeated lab-scale experiments in a 17,100 L pilot-scale test tank and carried out additional tests with tube and plate diffusers at different diffuser densities. The results show that despite the higher pressure drop, diffusers with dense slit density and smaller slits are to be recommended in order to improve efficiency of aeration systems in saline water.

Stereological study of the zona fasciculata of the adrenal cortex in stress situations (hypothermia, catabolism).

The ultrastructure of the rat fasciculata cells in stress situations, such as catabolism and hypothermia was compared descriptively and quantitatively by stereological methods with that of the nonstimulated rat fasciculata cell. The volume and surface densities are expressed per cm3 of cytoplasm, mitochondria and smooth endoplasmic reticulum. Furthermore, absolute values are given. In both stress situations the volume density of the mitochondria compared to the controls is enlarged significantly (H: 37% CA: 50%). The significant increase of the average single mitochondrium of the fasciculata cell (H: 58% CA: 58%) shows a real growth of the mitochondria in these stress situations. Also the surface density of the mitochondrial inner membranes--the mitochondrial enzymes for steroid genesis are partly located in these membranes--shows a significant increase (H: 31%, CA: 84%). Whereas the volume density of the smooth endoplasmic reticulum remains unchanged, the surface density is significantly raised (H: 44%, CA: 60%). An attempt was made to draw up a relationship between stereological and known biochemical data of steroid genesis within the fasciculata cell. An activation in both stress situations could be observed. The stereological data reflect a subcellular reaction pattern, which is similar to exogenous ACTH administration, thus indicating an endogenous ACTH liberation due to these stress conditions.