Various treatment processes are employed in groundwater and soil remediation. Treatment processes are generally classified into physical, chemical, biological, and thermal processes. A treatment system often consists of a series of unit operations/processes, which form a process train. Each unit operation/process contains one or more reactors. A reactor can be considered as a vessel in which the processes occur. Environmental engineers are often in charge of, or at least participate in, conceptual and preliminary design of the treatment system. The conceptual and preliminary design typically includes selection of treatment processes, determination of reactor types, and sizing of the reactors.