where the y-axis represents the contaminant concentration (ppm or mg/L) and x-axis displays the relatives contact area for mass transfer Mass transfer takes place from the contaminant-rich process stream to the water stream and the vertical concentration gap represents the driving force for this water-using operation Consequently as the two streams contact along the pathway of the operation the contaminant concentration in the process stream representing the operation must be of greater than that in the water stream for mass transfer to occur for convenience we can replace to contact area for mass transfer which may be difficult to determine in practice with the mass load of the contaminant transferred (Kg/hr) as show in fig 2.4 Each point on the x-axis of fig 2.4 corresponds to a point on fig 2.4