Simultaneous Toilet Flushing
Simultaneous Toilet Flushing also known as The Big Flush at certain sporting events is the general name given to a theoretical period in which large amounts of people flush toilets at the same time. This has also been referred to as the "flushpocalypse". This could cause some overflows or breakages of the sewage systems in certain areas. The Big Flush implies not only simultaneous flushing but also breakages and a septic system overflow. This topic is widely controversial and its effects are widely debated. The Big Flush is mainly relevant in North America, where the vast majority of toilets are flush toilets using water as an odour seal.
The average water flushing toilet uses around 6 to 13.6 litres (or 1.6 to 3.6 gallons) per flush. The underlying theory behind The Big Flush is that if everyone were to flush their toilets at the same time, it could fill the sewage system. It has been theorized that if the system is unable to handle the volumetric flow rate of the event, catastrophic failures could occur. Rumours of breakages due to The Big Flush go as far back as the 1930's. Simultaneous flushing of toilets has been questioned by multiple authors.
General City Scenario
In the United States, the average household has 2.45 people and over one third of households have 3 toilets. The estimated number of toilets in the united states, is around 350 million, meaning that on average, there is around 1 toilet per person in the United states. The concern arises if everyone in a city were to flush a toilet at the same time, there could be millions of litres of water all entering the wastewater treatment system at the same time. This could have the potential to increase pressure and create backups in the system that could break pipes and pipe connections. One wastewater treatment facility operator, Steve Cox, mentioned in an interview that although the outcome was not known "It would be ugly". Residences are typically only fitted with 4-inch pipes and connect to a pipe normally 12-inches in diameter. To overcome changes in elevation, sewer systems use lift stations, wastewater plants that push sewage uphill toward its final treatment destination. These stations would likely be the first overwhelmed by unanimous flushing.
One of the things to consider is that all the effects of a simultaneous flush would be very localized within the city that performs such an event. In some towns, the biggest problem faced would be a shortage of water for refilling all the toilets at the same time. According to Michael Johnson, a civil engineer at the Utah Water Research Laboratory who models fluid flow in sewer systems, the consequences of simultaneous flushing could range from having negligible effects to pipe explosions depending on where you are. In mornings, when many people in a city are taking showers, using the washroom and getting ready to go to work, it is typical to see a peak water usage that is nearly double the average water usage for the day. Furthermore, as some cities such as New York have shown, water usage and peak water usage has significantly reduced since the time of the wastewater treatment system's construction. This is mainly due to better efficiency toilets that require less water. This means that the sewer system is able to handle much more water than is normally flowing through the pipes today.
High Rise Scenario
In buildings with many storeys, as the pipes fill, the problem is that the water is then backed up in a vertical manner. Because pressure increases as a function of height of a fluid in a given column and not quantity or volume, every ~10.33 m of water that is backed up, means that 1 atmosphere of differential pressure is created in the pipes at the bottom of the system. This increases linearly so that for example, 20.66 m of water backed up would mean 2 atmospheres of differential pressured in the pipes. In the most common scenario, using PVC pipes in the sewage system, the PVC pipes are able to withstand large amounts of pressure before cracking. However, the seals between the pipes, pipe angles and other metal pipes are often weak spots that consist of glue/primer. These connections are particularly vulnerable to pressure differences and likely to fail with extraordinary strain.
Flushing all the toilets of an upper level at the same time could cause lots of strain on lower levels' water evacuation systems. However, this is mainly relevant to older buildings.
Cities plan on every resident using around 80-100 gallons (300-380 litres) per day. This means that 1 flush per toilet (assumed to be about 1 flush per person) is only about 2.9% of the water usage of a person in a given day. Furthermore, when water is flushed in a city, it is not instantly sent to the water-mains. A consideration to be made in that cities are often spread out over large areas. Sewage system flow is not extremely fast. Most sewage systems plan for wastewater to flow at around 2 feet per second (FPS) and this means that slopes of the pipes can range from less than 0.15% to over 1%. This means that there could be a few hours delay between the average time for one house's "flush" to reach a water plant when compared to another house's "flush". It should also be considered that omnipresent pump stations.
As a result of this, if everyone were to flush a toilet at the same time in a city, it would stagger the water's arrival time at key places in the septic system. To cause a Big Flush with failures, there would be more likelihood to succeed by staggering flushes based on the distance from the wastewater treatment plants. This still unlikely to cause many problems because flushing 1 toilet is not a large portion of household water usage.
For tall buildings, it is quite unlikely that such an event known as The Big Flush would happen. When building, volume is taken into account in the pipe size, meaning that pipes increase in diameter nearing the base of the structure. Unlike underground pipes that attempt to be straight when possible to minimize travel distance, in order to slow down water in tall buildings and not cause water to fall down the pipes at terminal velocity, the pipes are often slanted or zig-zagging to prevent free fall. Water systems in tall buildings are often divided into subsections and use multiple pumps to regulate water pressure and transportation. This would slow down and cause the water to not reach the same section of pipe at the same time. Similarly to the city logic, to cause a Big Flush with failures, there would be more likelihood to succeed by staggering flushes based on the distance from the wastewater treatment plants. This still unlikely to cause many problems because flushing 1 toilet is not a large portion of household water usage.
The Big Flush and sports events
Super Bowl Sunday
It is a myth that during Super Bowl Sundays, due to the large gatherings of people drinking, that the volume of persons using a toilet during/shortly after the event increases largely. Although this could be true, the overall amount of flushing around that time of the day is probably very similar on a normal day as it is on a Super Bowl Sunday. It must be considered that the time for the water to reach water mains and sewage treatment plants varies largely depending on the distance of the location, ie. household, from such water hubs. Some houses are much closer and some houses are much further, meaning that the water does not reach the same pipe at the time given time. There could be a few hours delay between the average time for one house's "flush" to reach a water plant when compared to another house's "flush". The probability of there being a surge that the sewage system can handle is quite small. On a normal day, there is a typical increase in water consumption and flushing in the morning, and this surge is easily handled by sewage systems.
The myth is believed to originate from the 1984 sewage main break during the Super Bowl. The breakage was deemed to be simply a coincidence, due to the common breakages that are "normal".
Hockey Nights in Canada
It is also a myth that between hockey periods in Canada, there is a surge of flushing that causes sewage system problems. While it is most likely true that, in specific short amounts of time the flushing rates go up, the amount of flushing around that time of the night is probably around the same on a usual night as it would be on a hockey night. The same rationale exists as the Super Bowl Sunday explained above.
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