Washout, definition

The washout function also provides the basis for a method of experimentally measuring RTD (Chapter 19 see also problem 13-1). Consequences of the definition of W and its relation to F and E are explored in problem 13-5. 

Typically these studies cost from 1.5 to 3 million to conduct. Based on the perceived probability of a significant QT interval effect (based on preclini-cal studies, class effects, and ECG data from phase 1 studies) a decision must be made whether to conduct the definitive QT study prior to proceeding with a proof-of-concept study in patients, or whether to delay this study until the proof of concept (POC) has been demonstrated. Of note, for many biophar-maceutical products it would not be possible nor ethical to dose to steady state in healthy volunteers, to dose at two to four times anticipated therapeutic exposures, or to use a crossover design with reasonable washout periods. Thus a QT study performed with a biopharmaceutical may need to vary from the usual design and the E14 guidance, and may present great challenges for subject or patient recruitment. 

The other obvious possibility is that materials absorbed are carried by precipitation to the surface of the Earth, that is they are definitively removed from the air. There is no intention here to discuss the formation of precipitation. We only mention that it is believed (Fletcher, 1962) that, in winter layer clouds with small liquid water content, ice crystals play an important role in precipitation formation, while in summer convective clouds the coalescence of large drops with smaller ones is the dominant process. At the same time we have to emphasize that the wet removal of trace constituents is continued by falling precipitation elements (snow crystals, raindrops) below the cloud base. This removal mechanism is called washout. 

Note that according to the definitions of (20.7) and (20.8) the relation between the wet deposition velocity uw and the washout ratio wr is... 

Most of the experimental applications of the ZLC technique have been with gaseous systems, and for these systems the technique may now be regarded as a standard method. Based on our experience it is possible to suggest some guidelines as to how the experiments should be carried out. The key parameter is L, which from its definition (Eq. 17) can be considered the ratio of the diffusional and washout time constants R /D and KVs/F. This parameter is also equal to the dimensionless adsorbed phase concentration gradient at the surface of the solid at time zero. From either of these definitions it is evident that L gives an indication of how far removed the system is from equilibrium control. This parameter is proportional to the flow rate, so it can be easily varied, and to extract a reliable time constant, it is necessary to run the experiment at at least two different flow rates. 

One can employ the definition of the dilution rate to ascertain the volumetric flow rate at which washout is observed ... 

Despite the fact that productivity of chemostat operation is similar to turbidostat operation the turbidostat is preferred especially at locations with more day-to-day variation in irradiance. Under chemostat operation biomass concentration will slowly decrease on consecutive days with cloud cover. If followed by a clear-sky day, the culture is vulnerable to photoinhibition because of its low biomass concentration. Turbidostat operation allows for a direct control of biomass concentration. Washout of a microalgae culture in a turbidostat is by definition not possible. In a chemostat this could occur in situations the dilution rate is not adjusted in time. Turbidostat operation is thus more robust and allows for rehable automation of outdoor microalgae production (Bosma et al, 2014). 

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