The earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. Before the 19th century it was commonplace to empty human waste receptacles, e.g., chamberpots, into town and city streets and slaughter animals in open street "shambles," while the use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement, while paving systems pre-Macadam were mostly porous, so that precipitation could soak away and not run-off and urban rooftop rainwater was often saved in rainwater tanks. Open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century. In the majority of developed countries, large efforts were made during the late 19th and early 20th centuries to cover the formerly open sewers, converting them to closed systems with cast iron, steel, or concrete pipes, masonry, and concrete arches, while streets and footpaths were increasingly covered with impermeable paving systems. Most sewage collection systems of the 19th and early to mid 20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs (to the extent that relatively clean rooftop rainwater was not saved in butts and cisterns for drinking and washing.) This type of collection system is referred to as a combined sewer system. The rationale for combining the two was that it would be cheaper to build just a single system. Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" (UK terminology) or "storm sewer" (US terminology) system. Moreover, runoff was, pre-automobile, likely to be typically highly contaminated with animal waste, while until the mid-late 19th century, the frequent use of shambles (open street slaughterhouses) contributed more waste. The widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, construction of municipal slaughterhouses, and provision of mains water in the 20th century changed the nature and volume of urban runoff to be initially cleaner, include water that formerly soaked away and previously saved rooftop rainwater after combined sewers were already widely adopted.