As described in Section I, adsorption is a two-step process (movement from the bulk of the solution to a surface by diffusion, followed by weak chemical bonding at the surface), and that takes time, which requires a minimum contact time between the water and the adsorbent to be effective. Contact time can be increased by using more adsorbent or by reducing the flow rate. The empty bed contact time (EBCT) is an important engineering property of adsorbers, calculated by dividing the volume of the filter bed by the flow rate (e.g., liters divided by liters per minute yields minutes of EBCT. Calculations involving it apply mostly to designing large municipal filters with hundreds of cubic feet of media, and not so much to the small cartridge filters used for point-of-use treatment. Scale-up calculations for these are limited to simple, direct proportions with a maximum, multiplier or divisor of three. (e.g., if one liter of media lasts for 1000 L of test water, then three liters of media should last for 3000 L. But large amounts should be tested again.) The granular adsorption media used by this industry are limited to bed depths of 30 inches, or about a meter, because of the pressure drop they produce at useful flow rates. Thus, there is a limit to the number small filter cartridges that can be plumbed in series to achieve better adsorption performance.