Global Business Environment
for desalination and water treatment projects
Plant operation modes
Plant production capacity and operation modes are 2 inseparable halves.
The clients and the designers shall be in full agreement about them as
they may define "a family car" or "a racer" differing
in price by more than 100%! For example, the seawater intake filtering
system withstanding the jellyfish inrush lasting just several days a year
is 3 times more expensive than the ordinary traveling band screen.
Constant feed pressure and flowrate(CPF)
(the designer perspective)
At this operation mode the seawater pressure before RO membranes is
always constant. All pumps are kept operated at the same operation point
around-the-clock. CPF advantages are as follows.
CPF has the following drawbacks.
Constant demand
(the client perspective)
Constant production mode is often specified by the client in cases when
information about actual water consumption and its predicted growth is
not available. The client may stay under the illusion that under the above-mentioned
circumstances this mode is the simplest to implement. But the reality
is quite the reverse. Not seldom the plant designers consider the constant
production requirement as a constant hourly production and sometimes end
up with extremely sophisticated and expensive design in terms of reliability
and maintainability. To maintain the production constant at varying seawater
temperatures and the RO membranes fouling 2 controllable parameters are
engaged: feed pressure and recovery ratio.
At winter time higher pressures and lower recoveries are used. To implement
such operation mode the seawater pumps shall be equipped with the variable
speed drives, both being oversized by 10 - 20%. The energy-recovery devices
like ERI type are susceptible to transient performance associated with
the recovery control, and extra instrumentation and standby units are
to be added.
Constant monthly demand
(what the designer hopes for)
This mode defines the monthly quantities irrespective of the daily production
rates. It allows the plant designers to relax the requirements for the
equipment availability and maintainability and to use more flexible Preventive
Maintenance Plans (PMP) as regards the pumps overhaul, CIP and the RO
membrane replacement. The designer may feel at ease about jellyfish inrush
or high TSS (total suspended solids) during stormy weather.
Varying demand
(My Murphy's Law: any constant demand eventually becomes varying one)
It is expected that the varying demand absorb seasonal variations and
the consumption growth over time. Actually most tenders treat the seasonal
variations as the basis for the design, and don't consider the plant extensibility
an important criterion of the overall design goodness. (My observation
is that any seawater desalination plant will be eventually refurbished
and extended within the first 10 years of operation.)
As a rule the news about planned future extension unexpectedly surfaces at
the end of the project when the plant common facilities are under construction
(underdrain system, chemical storage, switchgears, etc…). The figure to
the left shows extended intake station example taken from the real project.
This clumsy design contains 3 pumps each covering 50% of demand and belonging
in different compartments (!) and 2 traveling band screens each for 100%
of capacity. Despite good intensions, this design is not 100% dependable
and flexible (limited possibility of the standby pump swapping).
For Middle East countries the seasonal variation in water consumption
is within the range of 60 - 100% (The author estimate). Such turn-down
ratios pose no technical limitations on the known desalination plant layout
options - pressure center design or multiple parallel stand-alone trains.
To select the optimal strategy for partial load covering - unloading (for
parallel trains and the pressure center) or decreasing a number of trains
in operation - the impact of low loads on the RO membrane feed and product
quality shall be additionally analyzed. Fortunately, low loads usually
occur during winter time when membranes produce water of highest quality
due to the temperature effect.
Varying daily demand
(what the designer fears most)
Seawater desalination is an energy-intensive process: about 3.5 kWh is required
for 1 m3 of produced water. For the plant producing 100 million m3 of
water annually the absolute power consumption may reach 40MW. Unlike water
consumption which is relatively uniform in many cases, power demand is
undergone high-amplitude seasonal and diurnal variation with minimum load
equal to 25 - 30% of summer peak, power and water demands are not necessarily
synchronized. The typical ambient temperature and power load seasonal
profile curves of the Middle East countries are shown in the right figure.
Air conditioning load is the major contributor to the power load variation,
and lower ambient temperatures coincide with decreased power demand period.
The seawater desalination plant may be effectively engaged in the peak
shaving - the reduction of the amount of electricity drawn from a
power utility during utility designated peak time periods. In this mode
of operation the plant doesn't produce water during noon peak hours when
the electricity tariffs are the highest. The capacity of the desalination
plant operated in such a mode has to be bigger by at least 10- 15% %,
and the plant shall be designed at least for 300 startups annually. In
rare cases of mechanical outages the extra capacity mentioned may be used
as a standby reserve.
As an engineer with strong backgrounds in the energy storage and conversion
I plan to allocate more time to the peak-shaving economics of desalination
plants.