In 2003, Singapore’s national water agency
launched an unprecedented program. Using two new facilities, they planned to provide more than 50%
of their nation’s water supply by recycling wastewater. And yes, we do mean that wastewater. While this might seem
like a desperate decision, this program had been
planned for decades to ensure the island nation
never ran out of clean water. And today, as climate change
increases the frequency and duration of droughts worldwide, more and more regions
are facing this problem. But is it really safe to reuse anything
we flush down the toilet? To answer this, we have to understand exactly
what’s inside this cloudy cocktail. Wastewater is classified
into several types, but the primary three are: gray water used
in sinks, bathing, and laundry; yellow water containing just urine; and black water which has
come into contact with feces. Globally, we generate enough wastewater to fill about 400,000 Olympic-sized
swimming pools every day. In cities and towns with sewage systems, this wastewater combines
in underground pipes, which actually aren’t filled with feces. The average 4,000 liters
of sewage contains only a single liter
of solid fecal material. But sewage is still rife
with dangerous contaminants, including billions of pathogens
and microorganisms, trace chemicals,
and excess inorganic nutrients that can pollute rivers and lakes. So even if we aren’t planning
to drink this concoction, we still need to clean it; which is why sewer systems typically
run to wastewater treatment plants. Most plants remove major contaminants such as feces, pathogens,
and excess nitrogen from all the water they process. And this involves a ton of biological,
chemical, and physical interventions. Some of the most important include
settling tanks to remove large particles, biological reaction tanks where microbes
eat unwanted materials, and chemical disinfection processes
that kill pathogens. After these procedures,
typical treated wastewater in the US is already cleaner than most
natural bodies of water, making it safe to discharge
into rivers and lakes. If we plan on reusing the water
for non-potable purposes, such as irrigation or washing cars, it gets even further disinfected
to prevent bacteria from growing during storage. But if we want it clean enough to drink, there's much more treatment to be done. One common process
includes microfiltration, where membranes with pores
one millionth of a meter across filter out small particles
and larger microorganisms. Next, the water passes through an even
finer reverse osmosis membrane, which can remove particles as small
as a tenth of a billionth of a meter. This membrane is semi-permeable,
allowing water to pass through, but stopping things like salt, viruses,
or unwanted chemicals. After this stage, UV lamps are
plunged into the water, emitting radiation that
permanently damages the genetic material
of any lingering life forms. Sometimes UV disinfection is then combined
with further disinfection processes that use chemicals like hydrogen peroxide to handle a wide range
of microorganisms and micropollutants. At this point, the treated wastewater
is tested rigorously. And if it passes, it can safely enter
the typical pipeline for drinking water, going through the standard
treatment procedures before joining the municipal supply. This approach is called
direct potable reuse, but even though it’s perfectly healthy, there’s still some concern
with such a direct system. Instead, most places opt
for indirect potable reuse, where the treated wastewater is discharged
to an environmental buffer, such as a reservoir, lake, wetland,
or groundwater aquifer. After some time in this environment, any lingering chemicals
from the treatment process will diffuse and degrade. Then, the water can be extracted
and enter the drinking water pipeline. Indirect potable reuse is the
process used in Singapore, and it's become an increasingly common
lifeline for arid regions in the US. But this system is only feasible in places
with centralized sewer systems and infrastructure for pumping
water into people's homes. This means it can’t help communities dealing with the most serious
sanitation issues, where access to clean water
is a daily struggle. Researchers are investigating smaller
scale technologies to recycle sewage into potable water on site. But helping these communities
in the long term will require us to take a closer look
at all the water we’ve been wasting.
