1. Ductile Cast Iron R&D Dept. Kubota Corporation, 26 Ohamacho 2-chome, Amagasaki-city, Hyogo 660, Japan
2. Fukui Construction Office, Japan Sewage Works Agency, 2F-Tsutae-biru, 1905 Wada 2-chome, Fukui-city, Fukui 910, Japan
3. Wastewater system division, Public Works Research Institute of Ministry of Construction, Asahi 1, Tsukuba-city, Ibaraki 305, Japan