Other Water Related Research

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CURRENT PROJECTS

Since reverse osmosis membranes are very sensitive to foulants such as colloids, inorganic scale and biofouling, proper pre-treatment process therefore becomes a critical factor for a successful long-run seawater reverse osmosis (SWRO) plants. Recently, low pressure membrane has been successfully used in the pre-treatment for wastewater reclamation by reverse osmosis (RO). This is because membrane pre-treatment offers several advantages such as smaller plant footprint, better quality of feed water for RO unit and less chemical consumption. As a result, the use of low pressure membrane is now being considered as a viable solution for pre-treatment to SWRO plants but further improvements in membrane configurations and operations need to be investigated to reduce fouling and energy consumption.

One of the aims of this study is to investigate the efficiency of pre-treatment using MF and UF submerged hollow fibre system by varying the operation parameters. All experiments were operated on a bench scale rig which can automatically control the filtration, backwash cycle. The influence of different operation parameters, such as filtration time, backwash duration, backwash strength, air scouring during backwash is investigated. The following information is used to evaluate the efficiency of the different pre-treatment: the cycle time, the net flux, TMP profile, the permeate composition and the newly developed membrane filtration index (MFI) value for the permeate. The cake deposition and removal will also be investigated. A new MFI was developed to predict better predict fouling propensity of feed waters, particularly where small particles are present. A cross-flow sampler was used to simulate the hydrodynamic conditions close to RO membrane in terms of particle capture. The fouling resistance of the captured was then assessed by dead-end ultrafiltration. Comparison between fractionated particles and bulk solutions were compared with model solutions of mixed silica particles. Further validation of this new fouling index (CFS-MFI) will be carried out during reverse osmosis filtration. Other aspects of the project include optimal design of spacers to reduce pumping costs in SWRO membrane modules using computational fluid dynamics.

 

Photocatalysis using UV-illuminated suspended TiO₂ has been shown to efficiently mineralize humic acids (HA) contained in surface waters. However, complete recovery of the sub-micron TiO₂ aggregates from the treated water remains a major obstacle in the application of slurry photocatalysis. In order to achieve the aforementioned recovery, hybrid process coupling photocatalysis with submerged membrane filtration is proposed in this study. Unlike previous photocatalytic research which emphasized on HA degradation kinetics, this project focussed on investigating the hydraulic performance of membrane separation when operated simultaneously within a photocatalytic system.

In a new experimental setup, the submerged membrane photocatalysis reactor (SMPR) was externally illuminated by 8 x 8W blacklight blue lamps (peak emission at 365nm). Polypropylene hollow fibre membranes (from Memcor) with an average pore size of 0.22 µm were operated under constant flux (100L/m²/hr) and each experiment lasted 2.5 hours. TiO₂ catalyst used was Degussa P25.

The relative effect of individual components featuring in the SMPR (i.e. filtration, UV degradation and TiO2 adsorption) was assessed and compared to the efficiency of the hybrid process. The highest TOC removal (65%) occurred when all three components (UV + TiO₂ + membrane) were combined together, confirming the synergy effect of the hybrid process for HA mineralization. The significant fouling layer observed for TiO₂ adsorption only (29.9 x 1011m^-1) was explained by the dense coating layer created by the TiO₂-HA-laden particles. In the other hand, photocatalysis (UV + TiO₂) resulted in negligible membrane fouling, indicating its effectiveness in removing HA constituents, usually prone to heavily foul the membrane. Optimum TiO₂ concentration was found to be 0.6 g/L, when removal efficiency reached 73%. At lower TiO2 concentration, increasing TiO₂ loading led to higher surface area for HA degradation and improved removal efficiency. However, above 0.6g/L, such effect was counteracted by the increased turbidity due to excess TiO₂, resulting in a low UV penetration. When high initial concentrations of HA were tested (up to 50 mg/L), the removal efficiency remained relatively constant (around 50%). As expected, higher HA concentration caused greater fouling layer forming on the membrane surface. Such results highlighted the potential application of the hybrid process for treatment of wastewaters with high organic concentration.

 

Trace chemicals, such as endocrine disrupting compounds (EDCs), pharmaceutically active compounds (PhACs) and personal care products (PCPs), present in wastewater effluents are known to potentially cause detrimental effects to human health and to the biotic environment if not removed during the treatment process. High-pressure membrane processes such as nanofiltration (NF) can be used efficiently in applications where a high water quality is required. Previous research indicated that the fouling layer formed on the membrane surface during filtration could significantly affect the rejection of trace chemicals and could either improve or jeopardize the quality of the treated water. Conflicting results on the exact effect of fouling on rejection have indeed been reported and the mechanisms and physicochemical interactions occurring during the rejection of the trace chemicals by fouled NF membrane are, so far, limited.

Accelerated organic fouling was achieved on a NF270 membrane (from DOW) by using a variety of natural organic matter (NOM) fractions ranging from humic acids, extracted from river water and from soil, surface water, protein (bovine serum albumin) solution, and wastewater effluent from a tertiary treatment process (membrane bioreactor). Different concentrations of NOM and operating modes (such as constant flux and constant pressure operation) were considered. A mixture of 18 trace chemicals representing a wide range of different physicochemical properties was added at the nanogram per liter (ng/L) range to the different feed water qualities and their level of rejection was assessed by a gas chromatography-mass spectrometry (GC-MS). According to their characteristics, the trace chemicals were grouped into three categories: (1) hydrophilic non-ionic, (2) hydrophilic ionic, and (3) hydrophobic non-ionic. Variations in hydraulic resistance, membrane surface charge, roughness and relative hydrophobicity were measured for each experiment. Preliminary results indicated that the feed matrices and operational modes were the major factors governing the trace chemicals rejection. Under constant flux operation, it was found that the rejection of contaminants increased after fouling, as compared to those obtained under constant transmembrane pressure.

Changes of the membrane surface characteristics due to the formation of an organic fouling layer were clearly confirmed by the observed increased hydrophobicity and decreased surface charge, which could explain the rejection mechanisms of compounds detected in this study: (1) the main rejection mechanism for the hydrophilic non-ionic compounds was size exclusion, as their rejection remains relatively constant throughout the experiments. (2) In the case of the hydrophilic ionic chemicals, the initial rejection mechanism was electrostatic exclusion, which became offset by size exclusion as fouling occurred. This was explained by the rejection performances of small compounds declining by 10%, while rejection of larger chemicals decreased only by 2%. (3) Hydrophobic non-ionic compounds presented high initial rejection due to their adsorption on the membrane surface. During long-term operation, their rejection decreased as they were able to diffuse through the hydrophobic fouling layer.

 

The pulp mill industry consumes 10-40 liters of water per kilograms of produced paper and ranks as third (99,238 ML) in overall water consumption for industry in Australia (Australian Bureau of Statistics 2007). There are 22 mills across Australia. Disposal of wastewater from pulp and paper mills provide a significant challenge, particularly for inland plants. Recycling however requires improved treatment processes to restore the quality of the water to a standard comparable with the fresh water. In order to produce high quality water from the pulp and paper mill effluent, reverse osmosis (RO) needs to be used due to the high level of soluble contaminants. However the presence of large amount of organics in paper mill effluent will decrease the performance of RO. By introducing an appropriate pre treatment method prior to RO can minimize the pollutant level and increase the efficiency of RO. The aim of this study is to focus on nanofiltration as a designated pre-treatment option for RO and predict performance of RO with nanofiltration permeate. Also challenges on using RO such as silica fouling will be compared with alternative discharge and treatment options using cost modeling.

 

Recently, centre researchers have demonstrated that a subsurface drip irrigation line manufactured from a commercially available reverse osmosis membrane was capable of providing sufficient water for the cultivation of tomato plants on brackish water while preventing the accumulation of salts in the soil. These preliminary experiments demonstrated that brackish and saline groundwater that would normally be unsuitable for use in irrigation systems for the cultivation of food crops, can be used to grow plants, such as tomatoes without allowing the build up of salts in the soil. In these experiments, the plants grew by creating suction in the root zone (sub-surface) which provided sufficient force to draw water across the reverse osmosis membrane, however, the RO membrane prevented the transport of salts which can damage the soil. The membrane technology, developed by Professor Leslie and the University of Sydney’s Professor Bruce Sutton, has been patented by UNSW commercial arm, NewSouth Innovations and commercialization partners are being sought.

 

Greg Leslie examining tomato plants grown with brackish water treated with in-situ membrane based drip irrigation technology.

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