An oil/grease separator (10; 110) to separate a heavier liquid from a mixture of the heavier liquid and a lighter liquid with which the heavier liquid is immiscible comprising: a housing (12; 112; 156) having an inner volume, (18, 32, 42; 134,146,147; 158, 160, 174), an inlet (16; 114; 170) into the inner volume to receive an influent flow of a mixture of the heavier liquid and the lighter liquid with which the heavier liquid is immiscible, wherein said lighter liquid rises to the top of the mixture by gravity, a first outlet (28; 120; 186) from the volume to discharge the heavier liquid positioned such that the outlet (28; 120; 186) defines a static liquid level in the housing (112; 112; 156) and a second outlet (36; 122; 172) positioned in the housing (12; 112, 156) so substantially all of the lighter liquid held in the volume can be discharged from the second outlet (36; 122; 156), together with heavier liquid, a storage vessel (46; 126) to receive the liquid from the second outlet (36; 122; 172), and a return path (36; 148) from a low portion of the storage vessel (46; 126) to the housing (12; 112; 156) to return heavier liquid from the storage vessel (46; 126) to the housing (12; 112; 156), characterized in that said inner volume comprises multiple chambers with varying heights from the inlet (16; 114; 170) to the first outlet (28; 120; 186) to promote a significant rise and fall flow of liquid level, said multiple chambers including a separation chamber (32; 146; 174) to receive the influent flow of the mixture and wherein said lighter liquid rises to the top of the mixture by gravity, and an effluent discharge chamber (42; 147; 160) through which the heavier liquid below said lighter liquid is discharged to said first outlet (28; 120; 186); and in that said second outlet is positioned in the separation chamber (32; 146; 174) above the static liquid level in the, separation chamber (32; 146; 174) so that surges of influent flow received in the housing (12; 112; 156) and temporarily raising the liquid level above the static liquid level in the separation chamber (32; 146; 174) cause lighter liquid on top of the heavier liquid to flow to, and be discharged through said second outlet (36; 122; 172) into said storage vessel (46; 126). a housing (12; 112; 156) having an inner volume, (18, 32, 42; 134,146,147; 158, 160, 174), an inlet (16; 114; 170) into the inner volume to receive an influent flow of a mixture of the heavier liquid and the lighter liquid with which the heavier liquid is immiscible, wherein said lighter liquid rises to the top of the mixture by gravity, a first outlet (28; 120; 186) from the volume to discharge the heavier liquid positioned such that the outlet (28; 120; 186) defines a static liquid level in the housing (112; 112; 156) and a second outlet (36; 122; 172) positioned in the housing (12; 112, 156) so substantially all of the lighter liquid held in the volume can be discharged from the second outlet (36; 122; 156), together with heavier liquid, a storage vessel (46; 126) to receive the liquid from the second outlet (36; 122; 172), and a return path (36; 148) from a low portion of the storage vessel (46; 126) to the housing (12; 112; 156) to return heavier liquid from the storage vessel (46; 126) to the housing (12; 112; 156), characterized in that said inner volume comprises multiple chambers with varying heights from the inlet (16; 114; 170) to the first outlet (28; 120; 186) to promote a significant rise and fall flow of liquid level, said multiple chambers including a separation chamber (32; 146; 174) to receive the influent flow of the mixture and wherein said lighter liquid rises to the top of the mixture by gravity, and an effluent discharge chamber (42; 147; 160) through which the heavier liquid below said lighter liquid is discharged to said first outlet (28; 120; 186); and in that said second outlet is positioned in the separation chamber (32; 146; 174) above the static liquid level in the, separation chamber (32; 146; 174) so that surges of influent flow received in the housing (12; 112; 156) and temporarily raising the liquid level above the static liquid level in the separation chamber (32; 146; 174) cause lighter liquid on top of the heavier liquid to flow to, and be discharged through said second outlet (36; 122; 172) into said storage vessel (46; 126).