Article taken from Innovation Magazine
http://www.innovationmagazine.com/innovation/vol02_02/water.shtml
Singapore, a nation lacking its own freshwater resources, has been
investigating water reclamation technologies for some time.
At the National University of Singapore, the Water and Biotreatment
Group (WBG) has been actively conducting research and development
in water reclamation since the late 1980s. As the anchor R&D
group of the new Centre for Water Research (WRC) hosted by the
Department of Civil Engineering, WBG has developed several versions
of membrane-based water-reclamation systems capable of delivering
product water suitable for high-value reuse application.
"Membrane technology is an emerging technology. In probably a
couple of years' time, it will overtake the conventional method as
it is able to save on land use in the urban area," says Associate
Professor Ong Say Leong who heads the WRC. "It is one of the areas
we are now seriously working on. In the past, we focused more
effort on wastewater treatment. However, [fresh] water has become
so important that the group has decided to place a greater emphasis
on this area."
The group actually works in three major areas. The first focuses on
quality enhancement of water and treated wastewater to meet
Singapore's internal demand for both potable and non-potable water.
The second looks at desalination, using the distillation process
and membrane technology. The third centres on the biotechnology
that complements the first two areas of research.
"Another exciting research programme we are working hard at is ways
to prevent biofouling (the undesirable accumulation of
microorganisms) on membrane surface," Ong adds. "The challenge now
is to work out the operating protocol that will prevent biofouling.
Our role is to maximise the performance of the membrane."
Biofouling reduction currently constitutes a hot research topic.
When biofouling occurs, performance of the reverse osmosis (RO)
membranes deteriorates. For potable water treatment, the type of
membrane used is generally an ultra-thin polyamide layer with pore
size of less than 0.001µm. Over 98% of dissolved salts, organic
particles, microorganisms and other nutrients can be detained.
However, this process also leads to the attachment, accumulation,
and growth of microorganisms on the membrane surface. Once formed,
this biofilm acts as a trap for other particulate matter, which can
quickly build up into a dense biomass.
Planktonic water analysis and visual inspection of filters make up
two traditional methods of monitoring and detecting the development
of biofilms in RO systems. Other indicators include an increase in
pressure differential across the plant, declining membrane flux, a
drop in the percentage of salt rejection, physical evidence of a
slimy coating on the cartridge filters, and high planktonic
bacterial counts in the feed water.
"Prior to entering an RO membrane process, water is usually
pre-treated to remove possible fouling agents. The pre-treatment
methods include coagulation, sedimentation, filtration, and/or
addition of anti-foulant to the feed water. However, there is a
lack of an efficient method to assess or evaluate the effectiveness
of pre-treatment needs. The commonly used pilot-scale test would
take months or even years to produce some meaningful data over a
single scenario," explains Dr Song Lianfa, an expert in membrane
technology at WBG. "The WBG is actively pursuing the development of
a quick characterisation method for studying membrane fouling in a
full-scale RO process."
WBG is designing a modelling technique that can accurately simulate
the performance of a full-scale RO water-treatment plant (Figure
1). If successful, it will provide the basis for further
development of an innovative fouling-characterisation method, as
well as pave the way for new water quality enhancement
technologies.
"The modelling technique is able to simulate the effects of the
spatial variations of process parameters on system performance.
When the local change in membrane resistance due to fouling is
incorporated in the model, it will be able to predict the fouling
development in the full-scale RO system," elaborates Song. "By
referring to one point or a small segment of membrane in a
filtration channel, the fouling rate should be identical or similar
to that in the bench-scale (laboratory) membrane system, and
therefore, can be determined from bench-scale experiment."
WBG is also studying the action of the membrane in hostile
environments, such as extreme pH conditions and high organic
contents. "This research emphasises ways to minimise the fouling
potential - not to prevent it totally," says Ong.
Apart from the research by Song and Ong, which is nearing
completion, WBG has also achieved considerable success in its
development of the Sequencing Batch Reactor (SBR), the brainchild
of Professor Ng Wun Jern, Dean of Engineering at NUS. The SBR
records optimal performance when treating low- to medium-strength
wastewater that has mainly dissolved organic substances.
Ng's spin-off company, Biotreat International Pte Ltd, has
successfully marketed the SBR in ASEAN, Taiwan, China, and the
Middle East. The product has been implemented successfully in
plants on Jurong Island in Singapore, and at the new Kuala Lumpur
International Airport in Malaysia. The engineers designed the
processes to facilitate advanced biotreatment with nutrient
removal, as well as water reclamation at the airport.
"The SBR is now being used in more than 100 plants in Asia and the
Middle East," says Ong, who collaborates with Ng on this project.
Although the research group based the commercial version on the
first design, it is already at work on version 4 of the
reactor.
A second product ready to be launched into the marketplace has been
named the Anaerobic SBR. A joint project between NUS and the
University of Toronto, it gets its funding from both Singapore's
National Science and Technology Board and the Government of
Ontario, Canada. "It is an improvement over the Aerobic SBR because
of its lower operating costs. The system was designed with a
radical change in the bioprocess environment, which results in air
exclusion. Air inclusion, and hence its movement, is a major energy
guzzler in the Aerobic SBR," says Ng.
NUS is also developing various surface-activated granular materials
for the efficient removal of organic matter, metals, and suspended
particles (including bacteria and viruses) from water and other
liquid effluents.
For more details on work covered in this story, contact the
researchers as follows:
Assoc Prof Ong Say Leong:[email protected]
Dr Song Lianfa : [email protected]
Prof Ng Wun Jern : [email protected]
