A wastewater treatment system comprising: a contact tank (110, 410) having a first inlet for receiving wastewater to be treated, a second inlet for receiving activated sludge, and an outlet, wherein the contact tank (110, 410) mixes the wastewater to be treated with the activated sludge to form a first mixed liquor; a dissolved air flotation unit (120, 420) having an inlet in fluid communication with the outlet of the contact tank (110, 410), wherein the dissolved air flotation unit (120, 420) separates suspended matter from a portion of the first mixed liquor to form a solids-lean dissolved air flotation unit effluent and floated solids, to output the floated solids through a solids outlet of the dissolved air flotation unit (120, 420), and to output the solids-lean dissolved air flotation unit (120, 420 effluent through an effluent outlet of the dissolved air flotation unit (120, 420); a biological treatment unit (130, 430) having a first inlet in fluid communication (124, 424) with the effluent outlet of the dissolved air flotation unit (120, 420) and an outlet, wherein the biological treatment unit biologically breaks down organic components of the effluent from the dissolved air flotation unit (120, 420) to form a second mixed liquor; a dissolved air flotation unit effluent conduit (124, 424) extending between the effluent outlet of the dissolved air flotation unit (120, 420) and the first inlet of the biological treatment unit (130, 430); an anaerobic digester having an inlet and an outlet; a floated solids conduit (125, 425) between the solids outlet of the dissolved air flotation unit (120, 420) and the inlet of the anaerobic digester; a first thickener having an inlet in fluid communication with the outlet of the anaerobic digester, a first outlet in fluid communication (584, 484) with the inlet of the anaerobic digester, and a second outlet wherein the first thickener receives a first portion of a digested solids stream from the outlet of the anaerobic digester, removes liquid from the portion of the digested solids stream to produce a thickened digested solids stream having an increased solids content relative to that of the portion of the digested solids stream, outputs the removed liquid from the second outlet, and returns (584, 484) the thickened digested solids stream to the inlet of the anaerobic digester; characterised in that the treatment system further includes a second thickener having an inlet in fluid communication with the outlet of the anaerobic digester, wherein the second thickner receives a second portion of the digested solids stream from the outlet of the anaerobic digester and removes liquid from said second portion of the digested solids stream to produce a dewatered digested solids stream having an increased solids content relative to that of the second portion of the digested solids stream. a contact tank (110, 410) having a first inlet for receiving wastewater to be treated, a second inlet for receiving activated sludge, and an outlet, wherein the contact tank (110, 410) mixes the wastewater to be treated with the activated sludge to form a first mixed liquor; a dissolved air flotation unit (120, 420) having an inlet in fluid communication with the outlet of the contact tank (110, 410), wherein the dissolved air flotation unit (120, 420) separates suspended matter from a portion of the first mixed liquor to form a solids-lean dissolved air flotation unit effluent and floated solids, to output the floated solids through a solids outlet of the dissolved air flotation unit (120, 420), and to output the solids-lean dissolved air flotation unit (120, 420 effluent through an effluent outlet of the dissolved air flotation unit (120, 420); a biological treatment unit (130, 430) having a first inlet in fluid communication (124, 424) with the effluent outlet of the dissolved air flotation unit (120, 420) and an outlet, wherein the biological treatment unit biologically breaks down organic components of the effluent from the dissolved air flotation unit (120, 420) to form a second mixed liquor; a dissolved air flotation unit effluent conduit (124, 424) extending between the effluent outlet of the dissolved air flotation unit (120, 420) and the first inlet of the biological treatment unit (130, 430); an anaerobic digester having an inlet and an outlet; a floated solids conduit (125, 425) between the solids outlet of the dissolved air flotation unit (120, 420) and the inlet of the anaerobic digester; a first thickener having an inlet in fluid communication with the outlet of the anaerobic digester, a first outlet in fluid communication (584, 484) with the inlet of the anaerobic digester, and a second outlet wherein the first thickener receives a first portion of a digested solids stream from the outlet of the anaerobic digester, removes liquid from the portion of the digested solids stream to produce a thickened digested solids stream having an increased solids content relative to that of the portion of the digested solids stream, outputs the removed liquid from the second outlet, and returns (584, 484) the thickened digested solids stream to the inlet of the anaerobic digester; characterised in that the treatment system further includes a second thickener having an inlet in fluid communication with the outlet of the anaerobic digester, wherein the second thickner receives a second portion of the digested solids stream from the outlet of the anaerobic digester and removes liquid from said second portion of the digested solids stream to produce a dewatered digested solids stream having an increased solids content relative to that of the second portion of the digested solids stream.