Depending on what water inlet you have, if you have the Crystal 2, then just above where the pump plugs into the van is a clear round screw in "plug", the water filter is there.
If it does not have this filler system, then as Parks says, it may be under a seat where the water comes into the van, or it may be under the sink, attached to the cold water pipe.
Personally we always use the water from th evan, or aquaroll, and have never had any ill effects, neither do we have any filter apart from the grit filter built into the pump.
Unless you cahnge filters regularly, then they can become more of a health risk than not having one, as you end up with stagnant water sitting in the filter all the time the van is not in use.
As you have granules coming out of the taps, then it is probable that the pipes have the granules in them and are being held in the pipes by the strainers on the tap spout.
If possible, remove the strainers (they should unscrew)and pump water through until all the granules are cleared.
Sterilising the system requires removing the filter, using an aquaroll with the corerct dilution of Milton, filling the system until you know the fluid is coming out of the taps( you should smell it)and leaving for no more than 2 hours, then fresh water in the aquaroll, and flush at least two full containers of water through.
Then replace the filter with a new one, or dispense with the filter completely.
If it is the Crystal 2 , then the filter just pulls off the clear screw cap and use the cap as a blanking plug.
If the filter is in line on one of the pipes, just replace th efilter with a length of the same pipe.