Ultrafiltration and nanofiltration hybrid systems in wastewater treatment and reuse
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Wastewater reuse is increasingly seen as an essential strategy for making better use of limited freshwater resources, and as a means of preventing deterioration of the aquatic environment from wastewater disposal. Membrane processes are now being successfully used to obtain water of recyclable quality. However, membrane fouling is a critical limitation on the application of membranes to wastewater reuse. Pretreatment of biologically treated sewage effluent (BTSE) prior to membrane processes will reduce organic deposition and subsequent biogrowth on membranes due to dissolved organic matter. Pretreatment also reduces the need for frequent chemical cleaning, which is a major factor that impacts on membrane life. From these perspectives, pretreatment offers significant potential for improving the efficiency of membrane processes. The main objectives in this study are i) to evaluate different pretreatment methods of removing effluent organic matter (EfOM) from BTSE and in reducing membrane fouling, ii) to investigate the variation in the ultrafiltration (UF) and nanofiltration (NF) membrane foulant characteristics in terms of molecular weight (MW) distribution of foulants and the characteristics of fouled membrane, iii) to examine the effect of semi flocculation and semi adsorption (with partial doses of flocculants and adsorbents, respectively) on the membrane filtration, iv) to study the phenomena of membrane filtration and pretreatment using different fractions (hydrophobic (HP), transphilic (TP) and hydrophilic (HL)) of BTSE, v) to assess the effect of hybrid hydrodynamic cleaning with high rate crossflow and relaxation modes in comparison with pretreatment to membrane, vi) to evaluate the merits/demerits photocatalysis hybrid system in comparison with NF and UF with pretreatment and vii) to develop different flux decline models to quantitatively compare different pretreatments. The highest removal of organic matter was observed when flocculation followed by adsorption was used as pretreatment. The flocculation and adsorption removed 68.5% and 71.4% of hydrophobic organic matter. After the flocculation pretreatment, the majority of large MW EfOM was removed. The pretreatment of the flocculation followed by adsorption led to very high removal of both small and large organic matter. Further, this pretreatment led to practically no filtration flux decline. The weight averaged MW (Mw) of the organics in the foulant on the membrane surface was 510 daltons (UF) and 190 (NF) without pretreatment and 350 (UF) and 180 (NF) after pretreatment with flocculation followed by adsorption, respectively. The flux decline with the HP fraction was high compared with the TP and HL fractions. It was observed that a particular amount of flocculant and adsorbent to UF was necessary below which the UF membrane became heavily fouled. The detailed analysis of Mw indicated that the Mw values of organic matter in the synthetic wastewater and in the flocculated effluent were 29800 daltons (initial), > 25000 (after flocculation with 40 mg/L FeCl₃ or less) and < 1000 (after flocculation with 50 mg/L FeCl₃ or more). The Mw values suggested the reason why the permeate flux was decreased with 40 mg/L FeCl₃ semi flocculation followed by semi adsorption due to the remaining large Mw. A detailed investigation of the utilisation of two automated cleaning techniques to reduce fouling problems was explored. The two cleaning techniques studied were periodic membrane relaxation and a periodic high rate cross-flow. The study found that an optimised usage of these two de-clogging techniques, with a 1 hour production period followed by a 1 minute relaxation period and then a 1 minute high cross-flow rate period resulted in a net productivity increase of 14.8%. Three different semi-empirical mathematical models were investigated to partially quantify the effects of different pressures and pretreatments. The three different models used were i) empirical flux decline (EFD) model, ii) series resistance flux decline (SRFD) model and iii) modified series resistance flux decline (MSRFD) model. The flux decline coefficient values determined from the EFD and SRFD models can be used as an index to assess flux decline and compare different operating conditions and pretreatments. With the MSRFD model, when flocculation of 21 mg-Fe/L was used as a pretreatment at a pressure of 300 kPa, the values of the bulk concentration (Cb), the concentration on the membrane surface (Cm) and adsorption resistance (Ra) significantly decreased by 4.4, 3.1 and 12.9 times, respectively. After 0.1 g/L adsorption as a pretreatment, the values decreased by 2.2, 2.0 and 1.8 times, respectively. Thus, pretreatment can significantly decrease membrane fouling. Although pretreatment reduces flux decline caused by membrane fouling, it cannot completely prevent membrane fouling. Further, as time proceeds, membrane fouling by organic matter is converted into biofouling and the concentration from the retentate constantly increases. To resolve these problems, this study recommends three near-zero fouling systems with an integrated photocatalysis membrane hybrid system.
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