Gravity de-watering is accomplished in a de-watering box. This can be a roll off type box or a self-dumping hopper. The box or hopper will have a perforated bottom plate. Typically, a 100-mesh stainless steel wire screen is over the perforated plate. A 20 mesh wire screen is also typical when a filter paper is to be used on top of the screen. Other sizes of wire mesh screen are readily available. The wire screen is typically held in a metal frame making it easy to remove for cleaning or replacement. The wire screen may be enough by itself, if not filter paper can be placed over the wire screen. There are many types of filter paper. Wilson can supply samples of up to 8 different filter papers for you to try before you buy.
An AOD (Air Operated Diaphragm) pump can also be used to pull a vacuum on the box and speed up de-watering. Some de-watering boxes have side screens in addition to the bottom screen. If your de-watering box has side screens you will not be able to use an AOD pump to assist de-watering as the air will short circuit through the upper exposed portion of the side screens.
Simple gravity de-watering boxes are by far the cheapest and easiest way to de-water sludge. The down side is that it may take longer to de-water than other types of de-watering equipment.