CREATING CLEAN CARBON ZERO MANUFACTURING
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, pre-processing 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.
The effective design of any ZLD
system is therefore dependent on
the correct analysis of the water/
waste stream, making it essential
to have an accurate analysis of
composition, flow rates, chemistry,
etc. Without this, any designed
solution will fail to deliver the
required results, if it works at all.
Energy-efficient evaporation
Vapour compression evaporation
is a common method used in ZLD
as evaporation can recover up to
95% wastewater as distillate. Any
remaining concentrate is further
treated physically or chemically to
produce solid residues (such as
crystals) and water. By running the
evaporators at lower pressures,
the boiling point of the liquid being
treated is reduced. This means
that multi-effect evaporation can
be made possible; that is, steam
from a previous evaporation
stage is used as thermal energy
in the next stage which works at
a lower boiling point. This way,
multiple evaporation stages are
combined, generating significant
energy savings. For many
components, crystal precipitation
is favoured at lower temperatures,
therefore lowering evaporation
temperatures helps to increase
the solids yield.
The role of heat exchangers
ZLD is certainly not the only waste
treatment solution available: it will
not be suitable for all situations
and it is not a panacea for the
treatment of hazardous wastes.
However, in the right situation it
can play a key role in reducing
waste volumes or preparing waste
streams for final, safer, disposal. In
the case of India’s textile industry,
ZLD has helped to clean up the
area around the city of Tirupur and
the Noyyal River, saving the area’s
textile industry at the same time.1
In Europe HRS Heat Exchangers
have installed a ZLD system for an
industrial client, which uses proven
technology. Heat exchangers
play a crucial role in reducing the
running costs of a ZLD system by
utilising heat from process water
and other existing sources, and
also recapturing heat at the end
of the process and reusing it to
boost the energy efficiency of the
overall ZLD system. Where there
is a hazardous liquid waste stream
to deal with, then the potential
to utilise ZLD techniques as part
of the overall treatment solution
should definitely be investigated.
HRS Unicus Series scraped-surface evaporators are used to maintain thermal efficiency and remove fouling during evaporation in ZLD installations
26 November 2019