An original mathematical model was applied to evaluate nutrient loads from domestic sewage directly discharged into the Venice Lagoon. The model takes into account the demographic characteristics of the area analyzed (resident population, tourism, movement of workers and students), the nutrient removal efficiency in the treatment plants, the daily nutrient unit loads of the different types of demographic classes. Monthly loads of nitrogen and phosphorus discharged into the Venice Lagoon were assessed. The model was also used to evaluate the effectiveness of hypothetical intervention plans to reduce nutrient loads.