Advances in Water Treatment and Enviromental Management

118 WATER TREATMENTFor the aeration of water, the mass transfer rate must be sufficient to satisfy theoxygen demand, both chemical and biological, over the required treatment period.Equation 1 identifies the main controlling variables in the process:1. The mass transfer coefficient (K ). This is affected to some degree by theLlevel of turbulence in the liquid,but more significantly by the liquid physicaland chemical properties. Thus K will depend on the effluent composition.L2. Interfacial area (a). This will depend on the degree of turbulence in thesystem and the fluid properties. The physical and chemical properties ofthe liquid medium influence bubble coalescence behaviour and hencebubble size distribution.3. Concentration driving force ().For the calculation of k a the interfaciallconcentration is assumed to be the liquid concentration which is inequilibrium with the bulk gas concentration. The concentration drivingforce can be increased either by increasing the percentage of oxygen inthe gas (eg use of pure oxygen rather than air) or by pressurising the gas(eg by increasing tank depth).To cope with the uncertainties of 1 and 2, many manufacturers quote a so called“a-factor”, which is the ratio of oxygen transfer rate in the actual effluent to that inclean water (where most development work has been carried out). K and a areLdifficult to measure independently so the vast majority of literature data are reportedas the product k a.lIn addition to the primary purpose of supplying oxygen, mixing systems often haveto satisfy a number of additional functions.(i) Good liquid mixing to prevent ‘dead zones’,(ii) Suspend solids,(iii) Strip out Carbon Dioxide produced by the bacteria.3. The Differences between Water and Chemical Process RequirementsRequirementsBefore looking at the techniques employed in the two industries, it is worth identifyingsome significant differences between the requirements. These, in general terms,are as follows:1. The value of the product. Water is probably the ultimate “low value”product. As such, running and capital cost are the key considerations inthe design. For higher value chemical, and particularly pharmaceutical,products cost is of lower importance. If a system can be designed, forexample, to increase product yield, this is very likely to overwhelm anyincrease in development or capital/running costs.