Water & Wastewater Asia May/Jun 2021

FOCUS
can easily be designed and the
membrane wetting risk is low.
In the initial phase, MD was mainly
used for seawater desalination,
but with the advancement of RO
technology, it was not economically
feasible. In the recent decade,
its application is transiting from
seawater desalination to high
strength wastewater treatment to
partly replace the function of metallic
evaporator, e.g., MED and MVR, to
optimise the low-grade surplus heat
in the plant area, e.g., low-grade
steam and hot water, and reduce
the operation cost of evaporating
concentration. This effectively solve
the corrosion problems faced by
metallic evaporators in the field of
acidic wastewater treatment due to
full plastic construction materials. In
recent years, ZLD or MLD (minimum
liquid discharge) has become
increasingly attractive due to water
scarcity and environmental issues
caused by wastewater discharge,
especially in the developing countries.
The potential ZLD market in China
and India is estimated to be US$10b
and US$3bn, respectively (Gao et
al., 2017). In fact, the MD technology
has the potential to be applied in
ZLD process as a concentrating unit.
In the field of acidic waste liquid,
many metal processing enterprises,
e.g., electroplating; metal and alloy
part manufacturing; electrode
manufacturing, are producing large
amount of spent acid wastewater,
whereby evaporating methods are
used to recycle water from. Water
recycling by MED and MVR is also
facing serious equipment corrosion
issue and high OpEx. Thus, MD is able
to solve the above-mentioned issues in
ZLD of high strength wastewater and
recycling of spent acid wastewater.
However, the membrane wetting issue
limits the industrial application of MD.
In this article, the developed MD
system by Beijing CSRE, a proprietary
vacuum assisted multi-effect process
with the design of Clean & Dry in Place
and membrane wetting channels, was
first adopted in the ZLD of chloride
containing wastewater and recycling of
spent sulfuric acid wastewater to verify
the application feasibility in practical
industrial projects.
METHODS
The internal structure and flow
channels of MD modules were
described in details by Gao et al.
(2020). The developed pilot-scale
skid MD system shown in Figure 1
indicates that the two heat exchangers
provide the connection interfaces to
outer heat source and cooling source.
The developed industrial-scale MD
systems were installed in two sites:
one for re-concentrating RO brine of
chloride containing wastewater before
evaporating crystallization in ZLD
process at Chendu manufacturing
plant of Jabil Group; another for
concentrating spent sulfuric acid
wastewater containing aluminium
to recycle water at Nantong
manufacturing plant of Haistar Group.
The low-grade heat, spend steam
provided thermal energy to run the MD
system. The productivity and quality
of the MD permeate and electricity
& thermal energy consumption were
monitored in-situ to calculate the OpEx
for per ton of permeate.
1
2 3
4
Figure 1: Pilot scale
skid MD system
– 1. MD modules;
2. internal heating
system; 3. internal
cooling system;
4. connection
interfaces to outer
heat source and
cooling source
Feed
Air floatation
Sedimentation
Biological treatment
Ultrafiltration
MD
Concentrate
RO
brine
Salt mixture
Evaporating
crystallization
Membrane distilation
Softening
Reverse osmosis
Recycled
water
Figure 2: Schematic process flow diagram of ZLD project
38