When vacuuming fine, silty dirt or debris, you may notice a cloudy stream of dirt coming back into the pool via the return. This can continue slowly, long after you stop vacuuming, and can create a frustrating cycle for the pool janitor at your pool. More common in sand filters than in other types, the dirt can be pushed right through the filter, especially one which may need a sand replacement. Indeed, this situation may indicate internal filter problems. It may also indicate a problem with the filter control valve. Old, loose multiport or push-pull valves can allow water to bypass the filter and return to the pool unfiltered. For more info on valves, click here.
Another possibility is that the pump is oversized for the filter, and is pushing the water so hard, it pushes dirt right through the filter medium. A sand filter actually works a little better when it's a little dirty; the added dirt helps to trap more dirt, so don't backwash prior to vacuuming a pool with a sand filter. You may also use filter aids, added through the skimmer, which provide a gelatinous layer on top of the sand bed to help trap dirt. Another tip is to vacuum to waste, especially if the debris looks especially filter clogging. To do this, fill the pool very full first and set the multiport valve to the Drain/Vacumn to Waste position. Roll out the backwash hose, and vacuum the dirt (and water) right out of the pool...to waste.