There are two types of filters in common use in fuel trucks. first are filter water separators which consist of a multiple in to out microfilters and teflon screens which cause the water to coalesce and collect in the bottom of the filter vessel (sump). The other type are filter monitors, this is also a combined dirt and water filter which contains super absorbent polymer (same stuff as in nappies). Drawback with this type is when the fuel contains a lot of water, the DP of the filter will rapidly rise which then requires frequent filter changes. This type of filter is being withdrawn as it was found under high DP and flow rates, the SAP can release into the fuel causing IFSD or other related issues. If you want any more info PM me.