Deep-well injection of municipal and industrial wastes, and liquid hazardous wastes is an important waste disposal practice worldwide. Performance of injection wells during the deep-well injection of liquid wastes and waste waters is critically dependent upon the physico-chemical properties of the waste, the operational parameters such as injection rates and pressures, as well as the hydrogeologic and geochemical character of the host formation. Development of theories and models that can predict the injection well performance as a function of these parameters is a vital research need. This paper presents the development and application of a well injectivity decline (WID) simulator, that can be used to model injection well performance during deep-well injection. Injectivity decline due to particulates in the injection fluid is modeled for various types of well completions. Results from the simulator are presented with an emphasis on the resulting well plugging and injectivity decline. The significant role played by injected wastewater quality, host formation properties, injection rate and pressure, well completion type, initial damage to the well/formation and the presence of gravel packs around the wellbore is discussed. The results quantitatively show that under typical injection conditions a high total suspended solids (TSS) concentration in the waste stream, low injection rate, low injection pressures, formation heterogeneity (layering), low porosity and permeability of the formation all contribute to a rapid decline in injection well performance. The simulator provides a tool for predicting well performance during waste injection as a function of the waste, formation and operational characteristics. Such simulations can be valuable during planning and operating injection wells to achieve and sustain satisfactory well performance.