Most of the "nasties" in old sand will be as particles lodged between the sand grains. The actual porosity of sand is fairly low and very little material will be absorbed into the sand grains themselves.
While it would be preferable to start with new sand, seeded with a cup or two of old, "live" sand, economics cannot be ignored.
If it were my sand, and I wanted to re-use as much as possible, this is what I would do:
1) put about a three-inch layer of sand in a 5- (or 6-) gallon bucket.
2) add clean tap water (this will use so much water that I would not try to use RODI)
3) stir like mad to get all of the sand into suspension, and then finish by stirring round and round to create a whirlpool (homebrewers - you know what I am talking about if you do grain brewing)
4) allow the sand to settle and the water to completely stop moving
5) siphon out the "crud" and really fine sand which collected in the middle, as well as the top 1/8 to 1/4" of sand all over. Also siphon out the remaining water.
This sand will now be quite clean, but will be missng some of the finer particles. That is the sacrifice you will have to make in order to clean the sand by the above method.
Note - the logic behind the above procedure is that detritus is lighter (in specific gravity) than sand, and so it settles more slowly. The larger sand particles settle first, followed by the smaller sand particles and finally by the detritus and finest sand particles. The above procedure separates out the detritus for easier removal.
This procedure will probably eliminate a lot of the desirable bacteria as well as desirable critters. If you think there is anything desirable still alive in the sand before you start the above procedure, set aside a cup or two of the sand before you start.