In sewage-source heat pump systems, the properties of sewage flow and heat transfer in heat exchange pipes and the growth characteristic of fouling resistance are very important design parameters of sewage heat exchangers. A test system is set up to investigate the convection and heat transfer properties of original sewage in the heat exchanger tube in practical engineering. The growth process, growth related influencing factors and the growth law of fouling in the sewage-source heat pump system are obtained by analyzing the test data. By analyzing the experimental data, the fouling resistance based model for the heat transfer from sewage flowing in a horizontal pipe is built. The model shows that the fouling resistance is distributed asymptotically and the asymptotic time is inversely proportional to the square of the initial flow velocity. In addition, the asymptotic value showed a linear and decreasing relationship with initial flow velocity. This mathematical model for fouling resistance provides an important theoretical basis for the optimal design of sewage heat exchangers. (C) 2017 Elsevier B.V. All rights reserved.