Separate foul sewers (also known as sanitary sewers or wastewater sewers) form an important component of many urban drainage systems. The emphasis in this chapter is on the design of such systems (Section 9.2). In particular, the distinction is made between large (Section 9.3) and small (Section 9.4) foul sewers and their different design procedures. Solids transport is introduced in Section 9.5. Analysis of existing systems using computer-based methods is covered in Chapters 19 and 20. Design of non-pipe-based (sanitation) systems is discussed in Chapter 23.