A new, model-based design method for denitrification using methanol was tested and found to be accurate in four full-scale plants: the Blue Plains Advanced Wastewater Treatment Plant, Washington DC., U.S.A., the Alexandria Advanced Wastewater Treatment Plant, Alexandria, Virginia, U.S.A., the Western Branch Wastewater Treatment Plant, Upper Marlboro, Maryland, U.S.A. and the Rotorua Wastewater Treatment Plant, Rotorua, New Zealand.The method, implemented in a process model, can be used to calculate design parameters (anoxic HRT and SRT, methanol/N ratio, etc.), as well as investigate such difficult issues as optimal operation during cold temperature and startup conditions. Important contributions to modeling and design include: 1. Slower than expected growth rate of methanol utilizers suggesting larger anoxic tank volumes are required 2. Low decay rates of methanol utilizers, suggesting faster startup and recovery after process failure conditions 3. Facultative growth of methanol utilizers under both aerobic and anoxic conditions 4. High Arrhenius constant indicating greater sensitivity to winter conditions and larger anoxic volume requirement for cold weather operation