A butterfly valve, comprising: a valve body comprising a substantially circular flow channel a disc rotatably mounted within the flow channel of the valve body and configured to rotate between a fully open position and a fully closed position; and a valve seat comprising a plastic material and an annular ring, wherein a cross-section of the annular ring has a substantially "S" shape, wherein the substantially S-shaped cross-section comprises: a first leg positioned at and comprising a radially outermost edge of the valve seat; a first support segment extending radially inward from the first leg and substantially perpendicular thereto; a center leg extending from the first support segment in the same direction as the first leg and substantially parallel to the first leg; a second support segment extending radially inward from the center leg and substantially perpendicular thereto; a second leg extending from the second support segment in the same direction as the center leg and substantially parallel to the first leg, wherein the second leg further comprises a sealing tab extending radially inward from the second leg, and wherein the sealing tab comprises an incline at a radially innermost edge of the valve seat, characterized in that a substantially flat surface extends radially outward from the flow channel and the valve seat is positioned adjacent the substantially flat surface such that a radially inward portion of the valve seat contacts a circumferential sealing edge of the disc when the disc is in the fully closed position; and a valve seat retainer positioned adjacent the substantially flat surface and comprising an annular ring comprising at least one channel (280, 300) is configured to receive the valve seat therein. a valve body comprising a substantially circular flow channel a disc rotatably mounted within the flow channel of the valve body and configured to rotate between a fully open position and a fully closed position; and a valve seat comprising a plastic material and an annular ring, wherein a cross-section of the annular ring has a substantially "S" shape, wherein the substantially S-shaped cross-section comprises: a first leg positioned at and comprising a radially outermost edge of the valve seat; a first support segment extending radially inward from the first leg and substantially perpendicular thereto; a center leg extending from the first support segment in the same direction as the first leg and substantially parallel to the first leg; a second support segment extending radially inward from the center leg and substantially perpendicular thereto; a first leg positioned at and comprising a radially outermost edge of the valve seat; a first support segment extending radially inward from the first leg and substantially perpendicular thereto; a center leg extending from the first support segment in the same direction as the first leg and substantially parallel to the first leg; a second support segment extending radially inward from the center leg and substantially perpendicular thereto; a second leg extending from the second support segment in the same direction as the center leg and substantially parallel to the first leg, wherein the second leg further comprises a sealing tab extending radially inward from the second leg, and wherein the sealing tab comprises an incline at a radially innermost edge of the valve seat, characterized in that a substantially flat surface extends radially outward from the flow channel and the valve seat is positioned adjacent the substantially flat surface such that a radially inward portion of the valve seat contacts a circumferential sealing edge of the disc when the disc is in the fully closed position; and a valve seat retainer positioned adjacent the substantially flat surface and comprising an annular ring comprising at least one channel (280, 300) is configured to receive the valve seat therein.