A moulded waste trap non-return device for use with a waste trap, the moulded waste trap non-return device comprising: a sealing portion having first and second sheet members (14, 16) biased by resilience to a sealed position in which the first and second sheet members are at least partially in surface contact; a device inlet comprising a circular flange for stiffening the device inlet; a device outlet; a transition portion providing fluid communication between the device inlet and the sealing portion, the transition portion having a bowed-out convex section of wall for allowing a build-up of fluid in the transition portion in the immediate vicinity of the sealing portion, wherein in use, fluid build-up in the bowed-out section of wall forces the first and second sheet members (14, 16) apart to an open position; wherein the sheet members (14, 16) are moveable from a sealed position, in which the members (14, 16) are at least partially in surface contact, to an open position, in which the members (14, 16) are forced apart by a flow of fluid flowing from the device inlet to the device outlet, the sheet members (14, 16) being biased by resilience to return to the sealed position from the open position such that fluid is prevented from flowing from the device outlet to the device inlet, characterised in that the non return device comprises furthermore biasing members (26, 28) joining the first and second sheet members (14, 16) together at the edges of the first and second sheet members (14, 16), wherein each biasing member (26, 28) is more than 50% thicker than the first and second sheet members (14, 16) combined, wherein the biasing members are first and second ribs, and wherein each rib has a square cross section 1 mm x 1 mm and the sheet members are 0.1 mm thick. a sealing portion having first and second sheet members (14, 16) biased by resilience to a sealed position in which the first and second sheet members are at least partially in surface contact; a device inlet comprising a circular flange for stiffening the device inlet; a device outlet; a transition portion providing fluid communication between the device inlet and the sealing portion, the transition portion having a bowed-out convex section of wall for allowing a build-up of fluid in the transition portion in the immediate vicinity of the sealing portion, wherein in use, fluid build-up in the bowed-out section of wall forces the first and second sheet members (14, 16) apart to an open position; wherein the sheet members (14, 16) are moveable from a sealed position, in which the members (14, 16) are at least partially in surface contact, to an open position, in which the members (14, 16) are forced apart by a flow of fluid flowing from the device inlet to the device outlet, the sheet members (14, 16) being biased by resilience to return to the sealed position from the open position such that fluid is prevented from flowing from the device outlet to the device inlet, characterised in that the non return device comprises furthermore biasing members (26, 28) joining the first and second sheet members (14, 16) together at the edges of the first and second sheet members (14, 16), wherein each biasing member (26, 28) is more than 50% thicker than the first and second sheet members (14, 16) combined, wherein the biasing members are first and second ribs, and wherein each rib has a square cross section 1 mm x 1 mm and the sheet members are 0.1 mm thick.