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As mammals, we follow one of the basic life processes; excretion. This
includes bringing out waste from the body, such as faeces or urine. When we
defecate into a toilet and ush it away, we are using a signi cant amount of
water; each ush takes about 1.28 - 1.6 gallons of water. On average, a
human being is said to ush about 5 times a day, meaning 7.2 gallons of
water is consumed in one day for ushing alone. That means 50.4 gallons a
week and 2620.8 gallons per year for one person. For comparison, the
average person drinks 182.5 gallons per year.
The waste we ush is sewage and there are three main types; Domestic
sewage from homes, industrial sewage from factories and storm sewage,
such as precipitation run o . This sewage is ushed into pipes which carry
the water to treatment plants which remove the waste and clean the water.
This allows the water to be returned to natural water sources without causing
eutrophication. These facilities use physical, chemical and biological
processes to clean the water. These processes are categorised in
preliminary, primary, secondary and tertiary.
The preliminary and primary stages remove rags and suspended solids by
having the sewage ow through a screen. These materials are removed and
burned.
The secondary processes mainly remove dissolved and suspended
organic matter. The grit and heavy particles settle to the bottom as the
sewage is passed through channels. The grit is later returned to the land.
The tertiary process achieves polishing of the water and further removal of
waste. The channels lead the sewage to sedimentation tanks. This is where
disinfection occurs, which is when all the pathogens in the water are nally
destroyed. The solid material settles on bottom and the e uent (the sludge
and liquid) remains on top. This allows for them to be separated as the
sludge is pumped into tanks to be decomposed by anaerobic bacteria like
listeria. On the other hand, the e uent is treated with aerobic bacteria so all
organic waste can be removed and then chlorine is used to kill the bacteria.
This means that the water is clean enough and can be returned to our homes
and factories, restarting the cycle.
Sometimes, this sewage can be processed a second time to be used as
drinking water, though if it is not processed e ciently it can lead to sickness.