Delivering cost-effective and efficient wastewater treatment is a global challenge and a key part of addressing water scarcity. Biological treatment forms the foundation of modern wastewater treatment systems, valued for its effectiveness and cost efficiency. The two most widely used and proven processes are trickling filters and activated sludge. This chapter introduces an integrated approach that combines both. Combined trickling filter/activated sludge processes, also known as dual biological processes, operate trickling filters and activated sludge reactors in series to leverage the benefits of both systems. Trickling filters are known for their higher resistance to shock loads and lower energy consumption, whereas activated sludge processes typically yield higher effluent quality with lower effluent BOD and TSS concentrations due to the bio-flocculation of suspended biomass. The most used combined processes are the trickling filter/solids contact (TF/SC) process and the roughing filter/activated sludge (RF/AS) process. In both processes, effluent from the trickling filters is directed to downstream activated sludge basins without clarification. Returned activated sludge (RAS) from the secondary clarifiers is recirculated back to the activated sludge aeration basins to maintain proper MLSS concentration. Combined trickling filter and activated sludge processes are widely used in the U.S. and offer flexibility for upgrading trickling filter plants.