First-generation ethanol plants offer successful, commercial-scale bioprocesses that can, at least partially, replace fossil fuels. They can act as platforms to integrate lignocelluloses, wastes and residuals when establishing 2nd generation ethanol. The present review gathers recent insights on the integration of intrinsic and extrinsic substrates into lot generation ethanol plants, through microbial conversion or cogeneration systems. It shows that, among different lot generation ethanol plants, sugar-based ethanol by-products, dominate integration studies characterized by strong techno-economic and life-cycle assessment components. In comparison, there are fewer studies that focus on grain-derived lignocellulosic residuals and other wastes. There is consensus that integrating second generation feedstocks into first generation plants can have positive techno-economic and environmental impacts. In addition to realizing production of ethanol from 2nd generation feedstocks, these possibilities can impact waste management by establishing relevant biorefineries and circular economy. They can also supply a wide range of renewable products. Considering the potential of this waste management strategy, further research on these and many other substrates is needed. This will shed light on the effect of the integration, the relevant types of microorganisms and pretreatments, and of other physical parameters on the effectiveness of running lot generation plants with integrated second generation feedstocks.