Methods and
technologies
Before waterborne sewage
systems, when sewage was
discharged raw into waterways,
a natural process of purification
took place. First, the volume of
clean water in the waterway
diluted wastes, while bacteria and
Water Sewage & Effluent November/December 2017
27
contributor
other small waterborne organisms
distributed and dissolved the
sewage and other organic matter,
converting it into new bacterial
cells, carbon dioxide and other
products.
The present higher populations
and greater volume of domestic
and industrial wastewater require
different treatments to achieve the
same result. The basic function of
wastewater treatment is therefore
to speed up the natural processes
by which water is purified.
Because wastewater is full of
contaminants, including bacteria,
chemicals and other toxins, there are
numerous ways it must be treated
to reduce the contaminants to
acceptable levels, making it safe for
discharge back into the environment –
or for human consumption, depending
on the source.
There are two wastewater
treatment methods, namely a
chemical or physical treatment
plant, and a biological treatment
particles in it can nourish plants. It
can also be used for washing and
flushing toilets.
Blackwater: is wastewater from
bathrooms and toilets that contains
faecal matter and urine. Also called
sewage or brown water, it can
carry disease-causing bacteria that
are harmful to man.
The term can also refer to
floodwater that usually comes from
overflowing bodies of water after
heavy rain, typhoons, hurricanes
or tsunamis, which combines with
sewage water that can be laden
with bacteria. Blackwater can only
be recycled as fertiliser for plants.
plant. While biological treatment systems
are perfect for treating wastewater from
households and business premises,
physical wastewater treatment plants are
typically used to treat wastewater from
industries, factories and manufacturing
firms. This is because most of the
wastewater from these industries contains
chemicals and other toxins that can largely
harm the environment.
Physical waste treatment plants use
chemical reactions as well as physical
processes to treat wastewater. There are
five basic steps:
Step 1 - Preliminary treatment and bar
screening: screen the wastewater as it
enters the wastewater treatment plant
to filter and remove large items from the
sewage. The goal of this step is to remove
debris.
Step 2 - Primary treatment, settling tank:
let all debris sink to bottom of tank and
discharge from a tube in the bottom of the
tank.
Step 3 - Secondary treatment, aeration
and sedimentation: remove grit from the
wastewater by forcing the sewage through
a grit chamber. Forcing the wastewater
quickly through the chamber prevents
organic waste from settling and aerates
the mix.
Step 4 - Advanced treatment filtration:
skim off grease and other oils that have
separated from the water to the surface.
Step 5 - Disinfection, chlorine or
ultraviolet: disinfection can be achieved
by any method that destroys pathogens.
A variety of physical or chemical methods
can destroy microorganisms under
certain conditions. Physical methods
might include, for example, heating to
boiling, or incineration or irradiation with
X-rays or ultraviolet (UV) rays. Chemical
methods might hypothetically include the
use of strong acids, alcohols, or a variety
of oxidising chemicals or surface-active
agents (such as special detergents).
UV disinfection is now a standard final
treatment stage in most wastewater
treatment plants worldwide. It is widely
favoured because of its environmentally
friendly attributes, including its chemical-
free ability to provide high-log reductions
of all known microorganisms, including
chlorine-resistant strains such as
cryptosporidium.
As growing environmental awareness and
stricter regulations drive effluent discharge
standards and water re-use initiatives
to achieve greater performance, UV
disinfection is fast becoming the treatment of
choice on many wastewater streams.
Biological waste treatment plants use
organic matter and bacteria to break down
waste matter.
Com pani es us e a pr oces s wher e
microorganisms are grown on a submersed
fixed-film media, where they feed on organic