Drying may take several weeks. Should it rain before the sludge is dry, not much harm is done because water tightly held in a gel behaves differently than loosely adhering rain water. Rain water drains quickly and drying resumes. The dry cake is collected for burial or may be used as soil conditioner. Sewage sludge is good for composting because it provides water, organic matter, and microorganisms.
Filtration is an alternative to drying, but costs are relatively high.
The most usual reason for using filtration is avoiding the land requirement for drying on a sand bed. Some large cities have only very expensive land available for waste treatment plants so the compactness of filtration units is a deciding factor.