Abstract It is usual to design and evaluate the performance of field waste stabilization ponds (WSPs) by using physical models. However, the actual performance of field ponds often deviates from that dictated by the corresponding physical models. This paper attributes this deviation, among other factors, to differences in environmental operating conditions, pond geometry and flow conditions. Thus, it has been suggested that results from physical models be modified by scale-up factors and factors of safety to account for the above differences before transferring them to full waste stabilization ponds. It might also be helpful to ensure that hydraulic similarity exists between the model and the prototype.