A filling apparatus (10a,10b) for a blow-fill-seal container comprising: a filling head constructed to be removably coupled to an open blow-fill-seal container, the filling head being operatively connected to a first fluid circuit for selectively delivering a purge fluid to the open blow-fill-seal container, a second fluid circuit for selectively delivering a product to the open blow-fill-seal container, and a third fluid circuit, having a vent line in fluid communication with the open blow-fill-seal container and to the ambient, for directing a bulk fluid comprised of at least the purge fluid from the open blow-fill-seal container; a first valve in fluid communication with the first fluid circuit, wherein the first valve is constructed for connection to a supply of purge fluid and wherein the first valve selectively controls the delivery of purge fluid to the first fluid circuit; and a second valve in fluid communication with the second fluid circuit, wherein the second valve is constructed for connection to a supply of product and selectively controls the delivery of product to the second fluid circuit, characterized in that the filling apparatus further comprises a monitoring device in fluid communication with the vent line, and the monitoring device is constructed to receive at least a portion of the bulk fluid and monitor the bulk fluid vented from the third fluid circuit, wherein the monitoring device is constructed to determine a size of particles in the bulk fluid and wherein the first fluid circuit is constructed to deliver purge fluid to the open blow-fill-seal container based upon the particle size determined by the monitoring device. a filling head constructed to be removably coupled to an open blow-fill-seal container, the filling head being operatively connected to a first fluid circuit for selectively delivering a purge fluid to the open blow-fill-seal container, a second fluid circuit for selectively delivering a product to the open blow-fill-seal container, and a third fluid circuit, having a vent line in fluid communication with the open blow-fill-seal container and to the ambient, for directing a bulk fluid comprised of at least the purge fluid from the open blow-fill-seal container; a first valve in fluid communication with the first fluid circuit, wherein the first valve is constructed for connection to a supply of purge fluid and wherein the first valve selectively controls the delivery of purge fluid to the first fluid circuit; and a second valve in fluid communication with the second fluid circuit, wherein the second valve is constructed for connection to a supply of product and selectively controls the delivery of product to the second fluid circuit, characterized in that the filling apparatus further comprises a monitoring device in fluid communication with the vent line, and the monitoring device is constructed to receive at least a portion of the bulk fluid and monitor the bulk fluid vented from the third fluid circuit, wherein the monitoring device is constructed to determine a size of particles in the bulk fluid and wherein the first fluid circuit is constructed to deliver purge fluid to the open blow-fill-seal container based upon the particle size determined by the monitoring device.