Solids removed from primary clarification and secondary treatment are thickened prior to anaerobic digestion. Settled primary solids are conveyed to the gravity thickening process. The solids concentration is increased from approximately 1% total solids to at least 4% total solids. Thickened solids are pumped to the anaerobic digestion process and removed liquid from both processes are reintroduced towards the beginning of the treatment plant for liquids treatment. The gravity thickening resembles and operates much like a primary or secondary clarifier, in which the solids are settled by the force of gravity to the bottom of the tank, forming a concentrated solids blanket. Gravity thickening is one of the most common sludge concentration processes due to the relatively inexpensive operation and maintenance of the system. Floatable primary solids and secondary biological solids are conveyed to the dissolved air flotation thickening process. Solids concentration within the dissolved air flotation thickener is accomplished by forcing the solids to float to the top of the thickening unit and removing the relatively clear water from below the solids. Floating of the solids is achieved by injection of air, which lifts the sludge particles as it bubbles to the surface of the liquid. Secondary sludge solids are slightly denser than water, and therefore float easily with attached air bubbles.