As long as the faucet turns on and the toilet flushes, the world moves along at an unbroken pace. When we wake up and there is no water pressure or the toilet is backed up, the first thing that happens is a bad mood. Drag that out for a few days and it gets worse. After a month, cities would be busting at the seams with waste that has nowhere to go, and local plumbers would be out of the job.
Hurricane Katrina was a fiasco with fingers of blame pointing everywhere. The media provided round-the-clock footage of what was happening with those who remained behind. Storm refugees that tried to take shelter at what was left of the Louisiana Superdome had no running water in the pipes to flush the commodes. Images of the toilets in the stalls showed them overflowing with fecal matter.
Though the answer varies from one source to the next, the consensus seems to be that a person creates half-a-pound of waste (poop) a day. The US Census of July 2011 indicates that there were 8,244,910 people living in New York City. If only eight million of them were regular, that means four million pounds of poop gets flushed down toilets every single day in the Big Apple. Well, except for the few who may be going behind trees in Central Park. The thing is that if disaster comes and the grid goes down, all that waste will begin to be dumped outside in the streets.
London, England had about 150,000 residents in the mid 1500s. That is about 75,000 pounds of human waste, not including urine, being dumped per day. Some neighborhoods literally had it flying out the windows. Those on upper floors would empty filled chamber pots right out the window to the streets below. An old legend speaks of men walking with women down the street, keeping the women closest to the wall. This was because those flinging the waste were aiming for the street making it more likely to hit the men.
Local water sources become contaminated in proportion to the population being supported by the source. No modern city has enough fresh water without working water and sewer treatment facilities. Savvy individuals may rely on stored water for drinking and contaminated water for toilet flushing for a time. However, there is just no feasible way to support a city the size of New York without a steady supply of fresh water and a working sewer system. Disease and death would become pandemic in a short period of time.