by Jiangyong HU, How Yong NG, and Say Leong ONG
ater, essential to life, is drying up in many parts of the world. The United Nations’ report on world water resource in 2006 has shown that more than 10 billion of the world’s population lack enough safe water to support basic needs and 40% of the people have no access to basic hygiene infrastructure.
An urgent need thus exists to look into tapping water from non-traditional sources to supplement limited freshwater. Progress in water reclamation technology has made the production of high-quality water from such sources possible. Because membranes plays a key role in water reclamation, ensuring the advancement of the technology and the expansion of new applications in water reclamation require further research and development efforts.
To resolve the issue of insufficient water for sustainable development, countries recognise that membrane processes will play an increasingly significant role as the dominant technology in water purification. The types of membrane processes commonly used include microfiltration, ultrafiltration, and nanofiltration for water and wastewater treatment and reverse osmosis (RO) for desalination and water reclamation. Water reclamation typically employs microfiltration/ultrafiltration to pretreat biologically treated wastewater before the RO process.
Another approach to water reclamation combines wastewater biological treatment with membrane pretreatment in a single process, known as a membrane bioreactor (MBR). This method uses microfiltration or ultrafiltration membranes in place of gravity sedimentation for clearing up treated wastewater and filtering out biomass or organic substances. MBR configuration comes either as a side-stream system, in which the membrane unit connects to the bioreactor externally, or a submerged system, in which the membrane modules go directly into the bioreactor. In either case, the membranes provide a complete barrier to biomass, producing an effluent with very low turbidity and organic concentrations. This can be tapped for non-potable use or channelled as feed water in RO for indirect potable use.
MBR offers many advantages over the traditional activated sludge system, such as superior effluent quality and a smaller footprint than that needed to treat the same amount of wastewater. MBR installations implemented worldwide are growing rapidly, particularly in response to the tightening regulatory environment and adoption of water reuse.
Although market opportunities for MBRs are rising, two key challenges inhibit widespread adoption — membrane fouling by microorganisms and the elevated energy the process requires. To address fouling, a side-stream reactor needs an increased pumping rate to achieve effective scouring of membrane surfaces at a steep cross-flow velocity, resulting in greater cost. A submerged MBR uses intensive aeration to agitate and scour the hollow fibres or flat-sheet membranes to delay fouling, leading to more cost. In addition, all MBRs require on-site chemical cleaning for flux recovery after the membranes become fouled.
Researchers at the Division of Environmental Science and Engineering, National University of Singapore (NUS), have shown that biomass characteristics — affected by operating conditions such as the production of extracellular polymeric substances or soluble microbial by-products rather than biomass concentration — contribute significantly to membrane fouling. Thus, operating MBRs in a way that minimises these substances will help delay fouling.
Possible areas of improvement include membrane materials, modules, and systems with effective scouring and a low membrane-fouling rate. New reactor concepts that minimise aeration can also reduce energy consumption.
RO will continue to play a leading role in water reclamation and desalination. High-productivity and low-cost antifouling polyamide thin-film composite membranes currently make desalination and water reclamation a viable option.
Investigators have made progress in understanding transport and membrane-fouling mechanisms, pretreatment of feed water and brine control, and chemical cleaning of fouled membranes. The characterisation of fouling potential of feed water for water reclamation requires further R&D. The silt-density index (SDI) or modified SDI does not suffice to predict the fouling potential of pretreated secondary effluent used as RO feed water. Characterising the fouling tendency of RO feed water urgently needs a better index.
The 0.2m-diameter RO element has been the standard size used in full-scale RO systems for both desalination and water reclamation for years. Recently, larger diameter RO elements that require fewer membrane elements for similar surface area have started to emerge (Figure 1). Two pilot studies in the US, one by the Department of the Interior, Bureau of Reclamation, found that such fibres promise substantial cost savings for maintenance and operation.
A larger-diameter RO membrane element needs a greater feed water flow rate to achieve good feed water distribution. Owing to this constraint, both earlier reports showed significant cost savings (20 to 27%) only in infrastructure and auxiliary equipment and piping costs, but not in process performance in the 0.4m RO system compared to a conventional 0.2m system. An independent study conducted by GrahamTek Singapore Pte Ltd shows that using 0.4m spiral-wound RO elements with a perforated flow distributor and an electromagnetic field could offer benefits such as increased productivity, reduced membrane-fouling rate, and enhanced product-water quality.
Emerging membrane processes such as membrane distillation and forward osmosis (FO), now in the research and pilot-testing stage, present potential technologies for reducing overall energy requirements in desalination and water reuse. In the presence of a temperature gradient, membrane distillation, which uses a hydrophobic membrane with micrometer pore size, offers an attractive alternative in the presence of a waste heat source.
FO for water application capitalises on the natural phenomenon of osmosis by exploiting an osmotic-pressure gradient generated by a concentrated solution (known as “draw” solution) to allow water to diffuse through a semi-permeable membrane from saline feed water with lower concentration. Consequently, it produces a less concentrated draw solution, which may be further treated to extract freshwater.
FO is comparable to RO; in both processes water moves through a semi-permeable membrane while the membrane retains salts. However, the concentration differences between the feed and the draw solutions across the membrane, in contrast to the high pressure applied in the RO process, naturally creates the driving force in the FO process. Thus, FO requires less energy.
Nevertheless, the FO process for freshwater production faces two major challenges — the lack of an ideal membrane that can produce a high flux comparable to the RO membrane and a draw solution easily separated to produce freshwater.
With escalating environmental pollution, an urgent need exists to study issues relating to emerging and unidentified contaminants. They have significant implications on the levels of treatment, monitoring, and testing in order to safeguard the safety of consumers. Compounds that mimic the hormone oestrogen can adversely affect endocrine and reproductive systems in laboratory rodents and probably those of humans and wildlife as well. Pathogens could infect humans at low doses and cause fatal diseases in some immuno-compromised populations. Thus, researchers have to develop new methods for monitoring and removing potential contaminants from source waters.
Evaluating membrane systems’ ability to remove oestrogenic compounds requires establishment of detection protocols for quantifying endocrine-disrupting chemicals (EDCs) and their corresponding activity. Common methods include the E-Screen assay and the yeast-based oestrogen screen assay for detecting EDCs present in water. Scientists have applied these approaches successfully to identify selected xenoestrogens and oestrogencity in treated wastewater as well as reclaimed water from membrane systems. A recent study by the NUS team suggested that MBR systems offer relatively good performance in oestrogenic compounds removal. However, after treatment by MBR systems, substantial amounts of oestrogens and their conjugates still pass through treatment systems and enter the aquatic environment.
The researchers at NUS pursue approaches such as the simultaneous detection of a group of phenolic compounds, constituting weak oestrogens, as well as EDCs at sub-cellular levels. Using two-dimensional gel electrophoresis, complemented by software analysis, they distinguished protein profiles from normal cells and MCF-7 cells (an oestrogen-responsive breast adenocarcinoma cell line) treated with EDCs. This technique enables obtaining a differential protein expression pattern, a technique that may lead to the development of a rapid and sensitive biomarker for oestrogen detection.
Membrane processes employing nanofiltration (NF) and RO present powerful options for ridding water of those micropollutants. Researchers have studied the operational characteristics of the membrane system for removing oestrogenic compounds using the flat-sheet membrane system for three EDCs — steroid hormones estrone (E1), 17a-estradiol (E2), and 17a-ethinylestradiol (EE2). It is found that RO membranes proved the most effective removal for EDCs, with a 90% rejection rate after 24-hour operation.
Studies have shown adsorption and physical sieving instrumental to maintaining good retention of hydrophobic EDCs during NF/RO processes. However, scientists know little about the removal mechanism of low-concentration pollutants in a water matrix in the presence of other dissolved organic matter (DOM). The NUS team recently found that the results of E1 removal correlated with the structural characteristics of DOM. The addition of humic acid with elevated aromaticity significantly improved membrane adsorption of E1 whereas the enhancement effect on E1 rejection was limited.
Microbial contaminants represent a major concern in water reclamation. Besides providing physical disinfection, membranes can remove microorganisms using several mechanisms. However, limitations persist such as the membrane’s inability to filter out viruses and imperfections that allow leakage.
The NUS researchers investigated removal of MS2 bacteriophage (a virus that infects bacteria) using different membranes and materials under different operating pressures in the laboratory. RO polyamide membrane proved the most effective and yielded the greatest removal. The team believes size exclusion and charge repulsion constitute the mechanisms for such removal. The members then investigated the long-term performance of a spiral-wound RO polyamide membrane on MS2 removal in a site study. Results showed a higher removal rate of MS2 than those obtained in bench-scale study. Additional removal mechanisms contributed to virus removal during long operational periods include cake-layer formation and irreversible fouling.
Increasing global implementation of water reclamation from marginal water resources augments limited and shrinking water supplies, especially in water-scarce regions. Not withstanding the various challenges and concerns, membrane processes will certainly continue to play a key role in ensuring sufficient and superior water to meet the ever-growing demand for freshwater. With advances in membrane technology addressing emerging contaminants, water reclamation will become more cost effective as a competitive option for meeting water demand.
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