Assimilative Capacity Modelling of Collingwood Harbour: A Case Study on the Selection of Effluent Limits for Municipal Wastewater Treatment Plant Expansion

An assimilative capacity analysis of Collingwood Harbour was conducted to identify appropriate effluent limits for proposed expansion of the Collingwood Wastewater Treatment Plant, especially with respect to total phosphorus and nitrogen discharges. Two hydrodynamic modelling packages, CORMIX and GEMSS, were used to define the predicted water quality impacts. The modelling approach included outfall relocation and the design of a new discharge diffuser to meet regulatory policies and water quality objectives. Risk assessment and the opportunity to provide an enhanced degree of environmental protection led to the resolution to upgrade to tertiary treatment for the proposed expansion. The project is an example of the integration of environmental values to move beyond the minimum effluent quality requirements dictated by technical and policy considerations.