The wastewater treatment unit processes described to this point remove solids and biochemical oxygen demand (BOD) from the wastestream before the liquid effluent is discharged to its receiving waters. What remains to be disposed of is a mixture of solids and wastes called process residuals, more commonly referred to as sludge or biosolids. The most costly and complex aspect of wastewater treatment can be the collection, processing, and disposal of sludge because the quantity of sludge produced may be as high as 2% of the original volume of wastewater, depending somewhat on the treatment process being used. Because sludge can be as much as 97% water content and because the cost of disposal will be related to the volume of sludge being processed, one of the primary purposes or goals of sludge treatment (along with stabilizing it so it is no longer objectionable or environmentally damaging) is to separate as much of the water from the solids as possible. Sludge treatment methods may be designed to accomplish both of these purposes.