Nitrification and denitrification reactions occured with approximately first-order kinetics in a water-sediment system, that was wastewater from a sea-based solid waste disposal site and benthal mud taken from an estuary. Then a dynamic biological reaction model of five reaction equations was proposed. It was concluded that benthal mud plays an important role in the denitrif ication of wastewater in seabased solid waste disposal sites and the extent of nitrogen transformation of the wastewater can be calculated by this model.