Which solids treatment method uses chemicals to mix the recycled flow with the process residual flow in a reactor?

The correct answer to the question regarding the solids treatment method that uses chemicals to mix the recycled flow with the process residual flow in a reactor is lime stabilization. Lime stabilization is a widely used process in wastewater treatment that involves adding calcium hydroxide (lime) to the residual solids. This chemical process serves multiple purposes: it helps to raise the pH of the sludge, which promotes the stabilization of organic matter, and it also facilitates the reduction of pathogens.

Through this chemical addition, lime interacts with the other materials present in the residual solids and aids in binding them, while also helping to produce a more stable and manageable final product. The use of lime enhances the dewatering process and results in a decrease in odor and volume of the sludge, making subsequent handling and disposal easier.

Other methods such as chlorine oxidation primarily focus on the disinfection aspect of waste treatment rather than the stabilization of solids. Gravity thickening and composting are physical processes or biological treatments respectively, which do not involve chemical mixing in the same reactive manner as lime stabilization does.