THE ROLE OF ZERO LIQUID
DISCHARGE IN REDUCING HAZARDOUS WASTES
Thanks to tighter environmental
regulations and greater public
awareness, companies are
increasingly looking to reduce
or eliminate the waste that they
produce. In recent years, Zero Liquid
Discharge (ZLD) has become an
important waste reduction technique,
but its potential in dealing with
hazardous waste streams has not
been fully appreciated until now.
Hazardous waste is waste which is
dangerous or potentially harmful to
the environment or human health.
It can come in any form: solid,
gaseous, sludge or liquid. Many vital
materials such as cleaning products,
pesticides and industrial chemicals
are, by their nature, hazardous
and therefore present disposal
challenges.
Traditionally, hazardous wastes
have been treated by a number of
different physical, thermal, chemical
and biological methods, including
precipitation, high temperature
incineration and even burial in
specialist secure sites. However,
in parts of the world, particular
industries have become associated
with environmental pollution through
the inappropriate disposal of
hazardous wastes, with the textiles
industry in India and South East Asia
being such an example; one which
has attracted NGO interest and the
development of new cleaner waste
disposal schemes.
One of the advantages of ZLD
over other treatment techniques
is its theoretical ability to separate
unwanted materials from water,
CREATE CLEAN
whether they are benign, hazardous
or toxic. The resulting solid residue
is often more stable, making it
suitable for recycling or landfill. A
well-designed ZLD system should
minimise or even eliminate liquid
waste streams, resulting in clean
water for reuse or environmentallyfriendly
discharge, and a solid
residue suitable for further
processing (often to recover valuable
components for use elsewhere) or
for safe disposal.
Correct analysis is crucial
The composition of wastewater
streams varies greatly, even where
the same basic processes are
involved. Certain wastewater
sources, such as power plants and
boilers with wet gas scrubbing,
often contain salts which may be
hazardous or valuable, or even both.
Environmental regulation usually
means that treatment is required
to reduce or remove such toxic
compounds before wastewater can
be discharged. Where the initial
wastewater is relatively dilute, preprocessing
can also be required
prior to the evaporation phase, often
using common water treatment
techniques such as reverse osmosis.
Other sources, such as wet flue gas
desulfurisation, may contain highly
soluble calcium and aluminium salts,
as well as heavy metals, which are
not easily crystallised by evaporation.
Such sources therefore need
significant pre-treatment, often using
lime or soda ash to introduce sodium
ions so that a crystalline solid can be
produced by the evaporation stage.
36 NOVEMBER 2019