Most of us don’t think about poo and what happens to it until it appears where it isn’t wanted – on our beaches or pouring into rivers. But what does ‘dealing’ with poo actually entail? How do you process these enormous quantities of offensive and hazardous organic waste and dirty water that are flushed into the sewers? There is a kind of biological magic involved in transforming dirty sewage water into (relatively) clean water to be returned to the rivers, lakes or the sea, that can be drunk over and over. It is imperative that our rivers are kept clean as a source of drinking water but also for wildlife and recreational activities. Although not normally a source of drinking water, the seas and lakes equally provide these other services and are also vulnerable to the damage by untreated/inadequately treated sewage. Therefore, what happens to poo on its journey from toilet to effluent from sewage treatment is an important one.  As well as explaining the history behind the development of the UK’s sewage system and its impact on the health of the nation, The Science of Sewage looks at what happens when things go wrong, or the system can’t cope. How is the aquatic ecosystem affected and how does it respond when raw sewage flows into the river, lake or sea? Julian Doberski shines a light on the biologically fascinating process and clarifies the complexities of the treatment processes involved and why the current sewage infrastructure doesn’t always return clean water and looks at the challenges required to eliminate the current inadequacies of the sewage treatment infrastructure.