On the other hand, in the case of PVA gel beads, COD and nitrate reductions were low as compared with those in the alginate gel beads. In particular, a large amount of nitrite accumulation (maximum 85 mg/l) was observed but reconsumption was not noted when 30g of PVA gel beads were used. It appears that PVA immobilization of cells seemed to suppress denitrification. In addition, the death of carp in the aquarium with 60g of PVA gel beads may have been caused by the accumulation of nitrite (approximately 1 .O mg/l of NOT-N) in the water.