By far my favorite is "Sewer Mains and Laterals," with thick, colored-pencil stripes in brown, blue, and yellow showing the locations of different sizes of underground sewer pipes--starting from 6-inch diameters in neighborhoods like mine to the largest mains back then, 24 inches. What I love about the map is the outfalls--at Crabtree Creek north of town and Walnut Creek to the south (safely downstream from the pump that brings drinking water to the city), the colored-pencil stripes simply stop. That's where the sewage goes: into the river.
Those days seem almost absurdly quaint now, but they're not so bygone after all. In 1940, in some of the largest cities in the United States--Boston, Pittsburgh, Cincinnati, St. Louis, Kansas City--every drop of whatever you flushed down the toilet was dumped untreated into a nearby harbor, river, or lake. New York City in 1940 treated approximately one quarter of its sewage, and it reached 100 percent only in 1986. Until then, had you visited your Aunt Louise on the Upper West Side, all your business would have flowed directly into the Hudson.
Historians estimate that before indoor plumbing became widespread, the average person used less than 5 gallons of water per day; nowadays a good round (and low) estimate of American at-home water consumption is 100 gallons per day, per person. Some of that gets sprinkled on lawns and a bit washes cars and pets, but overall we use that water either for cleaning ourselves and our dishes and clothes, in which cases it ends up going down the drain, or for drinking, in which cases it ends up going down the toilet. Every day each one of us turns 100 gallons or so of water into sewage. That's a lot of sewage, requiring a lot of treatment--and very little of it is poop.
At least now we do treat it. Though people have been piping sewage for thousands of years, actual sewage treatment is barely a century old. People had to figure out first that human waste was not just noisome but actually unhealthy and then how it was unhealthy before they could begin figuring out what to do with it. Once they did, they got busy in a hurry; you can all but drink most of the water that comes out of Western treatment plants, and most of the biosolids removed during the process are used to fertilize crops and treat soil. The system is not flawless--biosolids sometimes contaminate water; grease clogs cause sewage spills or system failures; heavy metals, pharmaceuticals, and personal care products build up in biosolids--but overall it works splendidly.
Perhaps the first written sanitation instructions come from the Bible, which, written by and for a nomadic people, takes a small-is-beautiful approach: Deuteronomy urges you to dig a hole and "cover that which cometh from thee." By about 3000 BC, inhabitants of the Orkney Islands had invented toilets: Existing stone hut walls from that period have little niches with holes that drained to underground channels. The sewer historian Jon Schladweiler says that by a thousand years later, civilizations throughout the ancient Mediterranean and Middle East were using pipes to transmit both stormwater and human wastewater away from homes and cities and, usually, into waterways. By about 1500 BC the Cretan palace of Knossos had an actual flush toilet--a seat, a pan, and a slave to pour water to sluice what disposable-diaper companies call "the insult" to a drain in the floor. Cretan techniques for channeling water and wastewater spread throughout Greece, and by the 5th century BC, Athenians were piping wastewater and stormwater to a reservoir outside of town and using it to irrigate crops.
The Romans improved on even that: After considering Rome's many accomplishments, Pliny the Elder called the sewers "the greatest accomplishment of all." (The word "sewer" comes from the Latin exaquare, "to carry away water.") The constant flow of water coming into the city from the aqueducts supplied public fountains and baths, and Romans figured out that public bathwater ought to be changed a couple of times a day. "They built public latrine buildings immediately adjacent to the baths," Schladweiler says, and flushed the latrines by routing the used bathwater under them. The majority of human waste, though, was simply thrown into the streets; aqueduct water was used to wash the streets and sweep that waste into the drains. Because Roman sewers lacked ventilation, the only egress for sewer gas was those same drains and latrines. On the plus side, Romans also invented portable toilets, setting urns by the side of the road near the entrances to the city (vendors would rent you what Schladweiler calls "a modesty cape"). Further, the 1st-century emperor Vespasian had workers collect the contents of urinals, which he then taxed and sold to fullers, tradesmen who cleaned and dyed the Romans' clothing--they had figured out that the ammonia in urine had cleaning powers.