Fouling control of a membrane coupled photocatalytic process treating greywater
Introduction
A wide range of new engineered nanoparticles are becoming available for use in water and wastewater treatment (Jefferson, 2008). Recent examples include nano silver coatings on socks to inhibit microbial growth and hence odour (Ross, 2004) and zero valent iron nanoparticles for groundwater remediation (Huang et al., 2008, Ahmadimoghaddam et al., 2008). Whilst research is continuing into developing new nanoparticles actual uptake of the existing ones is rather limited in water treatment (Jefferson, 2008). The problem is a classical chemical engineering one: how to implement (scale up) a nano scale process at the meso or macro scale (Wintermantel, 1999). For context, water treatment facilities vary greatly but typically treat flows in the order of 10s–100s of ML d−1. Converted to nanoparticles, which are typically in the size range 1–100 nm this equates to 7.2 × 1026 particles and thus there is a challenge. The nanoparticles provide very large specific surface areas with which to provide high mass transfer and reaction kinetic coefficients yet fixing such a large number of very small particles so that they remain in the treatment process and do not exit with the product water is extremely challenging. Typical energy and operating costs for water treatment are in the order of 0.5 kW h m−3 and €0.2–0.5 m−3 which means solutions cannot be overly complicated or complex. Reported solutions to the problem involve either immobilisation to solid substrate (Rachel et al., 2002) or retention by filtration with membranes (Rivero et al., 2006, Chin et al., 2007).
One embodiment of this concept is the membrane chemical reactor (MCR) (Parsons et al., 2000, Jefferson et al., 2001) which utilises nano sized titanium dioxide (TiO2) particles in combination with a UV light source to generate highly reactive hydroxyl radicals which have a redox potential of 2.33 V, only surpassed by F2 (Huang et al., 1993). The TiO2 particles are retained in the system by means of a membrane filtration unit that is configured externally to the membrane but operated in an air lift, low pressure manner equivalent to that of a submerged membrane system (Le Clech et al., 2003). Long term trials for the treatment of greywater have shown it to be an effective system comparable to that of a membrane bioreactor (Pidou et al., 2008a, Pidou et al., 2008b). For instance, average effluent residuals of below 10 mg L−1 for bio-chemical oxygen demand (BOD), below 1 NTU for turbidity, below 2 mg L−1 for suspended solids (SS) and no pathogens were observed throughout the trial at a hydraulic residence time of 2 h (Pidou et al., 2008a, Pidou et al., 2008b). The observed residual levels mean that the technology is viable for treating greywater to the most stringent water quality standards available for urban reuse (Pidou et al., 2007). Consequently, it provides an alternative to biological systems such as membrane bioreactors (MBRs) where the small scale of operation, proximity to the end users and the potential for toxic shocks provides a relatively high process failure risk (Jefferson et al., 1999, Knops et al., 2007). Similar high performance of photocatalytic systems have been reported for the treatment of dyes (Molinari et al., 2002, Mozia et al., 2007), humic acid (Lee et al., 2001, Fu et al., 2006, Erdei et al., 2008), bisphenol A (Thiruvenkatachari et al., 2005, Chin et al., 2007) or pesticides (Oller et al., 2006, Lhomme et al., 2008) hence the appropriateness of the technology as a treatment solution is well established.
However, during the greywater investigation significant membrane fouling was observed. Consequently, the system could only be run for about 10 days at a flux of 5 L m−2 h−1 (LMH) before a chemical cleaning of the membrane was necessary (Pidou et al., 2008a, Pidou et al., 2008b). This was found to be contradictory to results of a previous study in which the MCR pilot plant was operated in batch mode (Rivero et al., 2006). Very little or no fouling was observed during the batch experiments for fluxes up to 120 L m−2 h−1. Such differences in operation are surprising but the results obtained during the batch operation tests can be explained by the fact that the greywater was rapidly treated and consequently for the higher fluxes the TiO2 was dispersed in fairly clean water and very little or no fouling was observed. This suggests that the fouling propensity of TiO2 changes significantly in the presence of a waste, in this case greywater.
A paucity of literature on operation of such photocatalytic hybrid membrane systems, especially for medium to high strength organic wastes, potentially limits the uptake of the technology to full scale operation. Specifically two key questions remain unanswered: (1) how to develop systems that can treat sensible flows whilst ensuring all the TiO2 in the system is active and hence degrades the organics and (2) how to ensure the membranes does not foul in systems that answer question 1.
The current paper addresses question 2 by examining the impact of different greywater products on the fouling behaviour of the system elucidating the major changes in the system when fouling occurs.
Section snippets
Filtration system
A bench-scale filtration system was used to replicate the fouling experienced when operating the membrane chemical reactor (MC-R™) (Water Innovate Limited, UK). Trials were conducted to study the influence of different parameters on titanium dioxide (TiO2) and its properties to foul membranes. This system was composed of a 9-l PVC tank in which the TiO2 and greywater slurry was placed. The slurry was pumped across the membrane module (Perspex, 28 cm × 20 cm × 8 cm) and back to the reactor at a cross
Results
Cross flow filtration of organo-TiO2 slurries containing bathroom cleaner, shampoo, hand soap or bubble bath dosed at a level of 600 mg gTiO2−1 resulted in insignificant fouling across the range of fluxes studied with a maximum fouling rate of 0.6 mbar min−1 (Fig. 1) and was not significantly different from the TiO2 system in tap water only. Consequently, the flux required to generate rapid fouling of such systems exceeded the maximum value tested in the laboratory set up and supports the previous
Discussion
The work presented in the current study demonstrates one of the barriers to implementation of hybrid membrane processes utilising photo catalysis, namely, the potential for rapid fouling due to undesirable changes to the aggregates of nano TiO2 when combined with specific chemicals. In the current case this appears to be related to the presence of polymers within some greywater products which greatly enhance the aggregation process forming very large organo-TiO2 aggregates that reduce the
Conclusions
Ultimately, whatever the mechanism of fouling, management of the organo-TiO2 aggregates is crucial for effective operation of such technologies. Given that sufficient irradiation of the organo-TiO2 complex resolved the fouling problems in the current study suggests that the key to uptake of the technology is in effective design of the UV reactor systems rather than improvements in the membrane. The challenge becomes how to ensure enough of the TIO2 surface reacts with the UV light.
Acknowledgements
This work forms part of the ‘Water Cycle Management for New Developments’ (WaND) project funded under the Engineering & Physical Science Research Council's ‘Sustainable Urban Environment’ Programme by EPSRC, UK government and industrial collaborators [www.wand.uk.net]. Permission from Water Innovate Ltd, UK, to use the MC-R™ technology is acknowledged.
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