AFM media works on the weak bond principle. This means that it holds the positively and negatively charged ions to the AFM media in a so called weak trap. This then is released in backwash because the flow has a high enough LV (lineal velocity) to tear the trapped ions from the weak bond. This means the Service flow has to be half that of the backwash flow in order to maintain those ions in the weak bond. This means there is less water in a AFM media Backwash because the backwash is more of a shunt than a steady cleansing.