NOVELTY - A wastewater treatment system (100) comprises: a contact tank (110) configured to mix wastewater to be treated with activated sludge to form first mixed liquor; a dissolved air flotation unit (120) having an inlet, a first outlet, a second outlet, and a gas inlet; a biological treatment unit (130) configured to biologically break down organic components of the first mixed liquor and of an effluent from the dissolved air flotation unit; and a clarifier (140) configured to output a clarified effluent through the effluent outlet and a return activated sludge though the return activated sludge outlet.    USE - For treating wastewater (claimed).    ADVANTAGE - The dissolved air flotation unit removes 60-100% of suspended solids in the first portion of the mixed liquor as waste biosolids; removes 40-80% of biological oxygen demand in the first portion of the mixed liquor; and removes a significant amount of soluble biological oxygen demand from wastewater prior to the wastewater entering the biological treatment portion of the wastewater treatment system. This provides for a reduction in the size of the biological treatment portion of the wastewater treatment system for a given wastewater stream as compared to a conventional wastewater treatment system and a commensurate reduced capital cost for the overall system. Utilization of the dissolved air flotation system also reduces the requirement for aeration in the biological treatment portion of the treatment system to effect oxidation of the biological oxygen demand of the wastewater, reducing operating costs. The amount of waste sludge generated by the biological treatment portion of the treatment system is also reduced. The reduction of COD and/or BOD in the anaerobic treatment unit may also provide for a reduction in size of the stabilization tank or other aerobic treatment unit in the wastewater treatment system as compared to systems not utilizing the anaerobic digester. The material removed from the wastewater in the dissolved air flotation system is utilized to produce energy, e.g. in the form of biogas in a downstream anaerobic digestion system. The biogas is used to provide salable energy through combustion or through use in, e.g. fuel cells.    DETAILED DESCRIPTION - A wastewater treatment system (100) comprises: a contact tank (110) having an first inlet to receive wastewater to be treated, a second inlet and an outlet, and configured to mix the wastewater to be treated with activated sludge to form a first mixed liquor; a dissolved air flotation unit (120) having an inlet in fluid communication with the outlet of the contact tank, a first outlet, a second outlet, and a gas inlet configured to introduce gas into the dissolved air flotation unit to facilitate the flotation of suspended matter from the first mixed liquor and the removal of the suspended matter from the dissolved air flotation unit; a biological treatment unit (130) configured to biologically break down organic components of the first mixed liquor and of an effluent from the dissolved air flotation unit to form a second mixed liquor; and a clarifier (140) configured to output a clarified effluent through the effluent outlet and a return activated sludge though the return activated sludge outlet. The biological treatment unit has a first inlet in fluid communication with the outlet of the contact tank, a second inlet, a third inlet in fluid communication with the first outlet of the dissolved air flotation unit, and an outlet. The clarifier has an inlet in fluid communication with the outlet of the biological treatment unit, an effluent outlet, and a return activated sludge outlet in fluid communication with the second inlet of the contact tank and with the second inlet of the biological treatment unit. The biological treatment unit includes: an aerated anoxic region having a first inlet in fluid communication with the outlet of the contact tank, a second inlet, and an outlet; and an aerobic region having a first inlet in fluid communication with the outlet of the aerated anoxic region, a second inlet in fluid communication with the first outlet of the dissolved air flotation unit, and an outlet. The system further comprises an anaerobic digester having an inlet in fluid communication with the second outlet of the dissolved air flotation unit and an outlet; a thickener having an inlet in fluid communication with a solids-rich outlet of the primary clarifier and an outlet in fluid communication with the anaerobic digester. INDEPENDENT CLAIMS are included for the following:    (1) facilitating increased operating efficiency of a wastewater treatment system involving: providing the dissolved air flotation unit in the wastewater treatment system in fluid communication between the contact tank and the biological treatment unit; and providing for a solids-liquid separation unit in fluid communication downstream of the biological treatment unit to recycle a return activated sludge formed from a mixed liquor output from the biological treatment unit to the contact tank; and    (2) treating wastewater involving: introducing the wastewater into a contact tank including an activated sludge; mixing the wastewater with activated sludge in the contact tank to form a mixed liquor; directing a first portion of the mixed liquor to a dissolved air flotation unit; separating the first portion of the mixed liquor in the dissolved air flotation unit to form a dissolved air flotation unit effluent and waste biosolids; directing a second portion of the mixed liquor to a biological treatment unit; directing the dissolved air flotation unit effluent to the biological treatment unit; biologically treating the mixed liquor and the dissolved air flotation unit effluent in the biological treatment unit to form a biologically treated mixed liquor; directing the biologically treated mixed liquor to a clarifier; separating the biologically treated mixed liquor in the clarifier to form a clarified effluent and a return activated sludge; recycling a first portion of the return activated sludge to the contact tank; recycling a second portion of the return activated sludge to the biological treatment unit; and directing the clarified effluent to a treated wastewater outlet.    DESCRIPTION OF DRAWING(S) - The figure shows a block flow diagram of a wastewater treatment system.    Wastewater treatment system (100)    Source of wastewater (105)    Contact tank (110)    Dissolved air flotation system (120)    Biological treatment unit (130)    Clarifier (140)