Periodic electrolysis technique for in situ fouling control and removal with low-pressure membrane filtration
Graphical abstract
Introduction
Microfiltration is a low-pressure membrane process, which is increasingly being employed as an alternative to conventional clarification processes for the removal of microorganisms, turbidity and natural organic matter (NOM) in the water treatment process. In some situations, the microfiltration membranes are applied to obtain the ultimate treatment, while sometimes they are used as a pre-treatment for downstream advanced water treatment processes, such as reverse osmosis [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. In pressure-driven membrane processes, membrane fouling is a ubiquitous phenomenon and considered to be a major problem leading to decreased flux, potentially to below the theoretical membrane capacity without appropriate treatment [8, [12], [13], [14], [15], [16]].
Fouling is the adsorption of solute and particulates at the membrane surface or within the pores of the membrane. Mechanisms including the plugging of membrane pores, concentration polarization and cake layer formation at the membrane surface contribute to fouling build up on or within the membrane. Fouling deposits increase the required transmembrane pressure and necessitate use of chemical cleaning agents, which reduce membrane lifetime and increase operating costs [3, [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. Although the fouling term can be related to both reversible and irreversible foulant adsorption, irreversible is the most problematic as it produces a flux decline that cannot be totally recovered [8, [27], [28], [29], [30], [31]].
NOM is responsible for organic fouling and flux decline during microfiltration [32, 33]. The flux decline during water filtration results from increased resistance in the filtration system [34]. This is due to the permeability of the gel layer (surface cake) generated by colloidal material accumulation at the membrane surface, and/or to the membrane pore size reduction [18, [35], [36], [37], [38]]. Humic substances are typically classified into three categories: fulvic acids, humic acids (HA) and humin according to their solubility in water at different pH values [38, 39]. HA is an essential component of NOM and is a degradation product of biological molecules including carbohydrates, lignin, and proteins. It is commonly found in soils, and ground and surface waters in amounts varying with the seasons. It imparts a yellowish-brownish colour, as well leading to membrane fouling problems in water filtration processes [[40], [41], [42], [43]]. It is a heterogeneous of both aliphatic and, aromatic components comprising three major functional groups: carboxylic acids (COOH), phenolic alcohol (OH), and methoxy carbonyl (CO). Generally, it is more hydrophobic than other humic material. Fig. 1 shows a model structure for HA [44].
Nanofiltration and reverse osmosis processes have been broadly used to remove humic substances, since they have many advantages, such as small footprint area, high product quality, and lowered chemical reagent use. However, these membranes operate at high-pressure, which leads to high water costs [17, [45], [46], [47], [48], [49], [50]], as humic substance adsorption at the membrane surface results in increased hydraulic pressure requirements and operating efficiency losses. Therefore, pre-removal of humic substances using low-pressure membranes is an active area of research [50, 51].
Fouling models for microfiltration are used to determine the optimal set of operating conditions that minimize fouling and the frequency of backwashing and/or chemical cleaning required [31]. Previous investigations have sought to remove HA from feed water, assuming that HA is the major foulant type [29]. Yuan et al. [38] investigated filtration of 2 mg/L HA solutions through a 0.22 μm poly(vinylidene fluoride) (PVDF) microfiltration membrane for durations of 1, 5, 20 and 100 min. The initial filtration value for the membrane was 1.010 × 10−3 m/s. The authors stated that the relative flux declined to less than 10% of its initial value within the initial 20 min at a constant pressure 0.69 bar. The data obtained demonstrated that HA fouling during microfiltration is dominated by the convective deposition of a fouling component at the membrane upper surface. A model was also developed to describe the water flux behaviour as a function of time for a wide range of conditions. Lin et al. [11] studied fouling of PVDF microfiltration membranes whilst filtering HA suspensions with concentrations of 2 and 4 ppm for 100 min operation time. Ultrasound signals were used to study fouling deposition at different time intervals. They recommended that it is essential to design a proper filtration module allowing in situ cleaning of the membrane to mitigate fouling.
Applied electric force is a powerful means to decrease the membrane fouling caused by negatively charged organic pollutants, is an environmentally benign technique, with significant milestones achieved commercially and scientifically so far [26]. This method is sometimes known as electro-filtration [52]. In an electrochemical water treatment set-up, the conductive substrate performs as the anode, causing direct oxidation of foulants [53], or as a cathode, where foulants are removed via generation of tiny bubbles at the conductive surface [54]. This is the principal mechanism on which electrochemical membrane fouling mitigation is based [52]. However, it has recently been suggested that whilst bubble formation via electro-reduction is an efficient mechanism for in situ cleaning of membranes, the oxidation method may damage the membrane itself [[55], [56], [57], [58], [59]]. Bubbles offer a promising option for a clean, inexpensive and environmentally friendly technique appropriate for in situ cleaning of conducting substrates. However, the use of the bubbles in new technology is yet to be investigated and is challenging to implement [60].
When modifying membranes several studies have found that the thin coatings are eroded due to water flow. As a result, much attention has turned to examination of modification of feed spacers, which can have thicker and more durable coatings applied without affecting membrane transport [59]. As a result, several studies have highlighted the use of an electrically conductive feed spacer to prevent biological and organic fouling. Noticeable flux recovery was detected, which was related to the electrostatic repulsion between the foulant and the feed spacer strand [59]. Baek et al. [61] employed a lab scale cross-flow system with a titanium feed spacer. The feed spacer was activated via application of positive, negative and alternating potential for 30 min to de-foul P. aeruginosa PA01 GFP biofilm layers. Consequently, permeate flux recovery was achieved. In our previous study [55] we investigated the ability of the application of an electrically conductive feed spacer to function as a means for limiting organic fouling in a lab scale cross-flow system. An aqueous suspension of 20 ppm sodium alginate was employed (as model organic foulant), which acted as the electrolyte solution. When an electrical potential was applied, in situ fouling mitigation was observed with noticeable flux enhancement without any obvious damage to the membrane surface.
The objective of the current research was to study the effects of applied periodic electrolysis on water flux and flux recovery after fouling, through a combined use of electrically conductive feed spacer with microfiltration membranes. These investigations were performed with HA filtration at concentrations of 8, 12, 16 and 20 ppm for three intervals of 30, 45 and 60 min duration, employing two configurations of feed spacers.
Section snippets
Materials
Polypropylene feed spacer with two different configuration, mesh A with aperture size 2 × 2 mm (diamond shape) and mesh B with aperture size 3 × 3 mm (GE, USA) were employed as a feed spacer in the filtration system; both of the meshes were coated with a carbon-based ink comprised of graphene nanoplates (GNPs) using a dipped coating method, which has been reported elsewhere [55]. PVDF membranes (Millipore, pore size 0.22 mm: see Table 2), sodium chloride, HA (HA), sodium dodecylbenzene
Results and discussion
The coatings of the surface feed spacers were characterized by SEM imaging, with representative images shown in Fig. 4-a and b. The durability of the surface coating prior to and after applied potential, was observed, as shown in Fig. 4-c and d, with the overall structures of the before and after surface appearing similar. As such, no apparent damage to the structure of the feed spacer surface coating due to the applied potential was identified.
The electrochemical behaviour of the feed spacers
Conclusion
In this study, an in situ cleaning method using periodic electrolysis was investigated for two configurations electrically conductive coated feed spacers. An obvious improvement in water flux recovery was seen due to this cleaning process. The in situ membrane cleaning effect was evaluated by filtration of a HA suspension. The self-cleaning process was carried out in electrochemical cross-flow set-up. Applying periodic electrolysis lead to the formation of tiny bubbles on the feed spacer
Nomenclatures
- NOM
natural organic matter
- HA
humic acid
- PVDF
poly(vinylidene fluoride)
- COOH
carboxylic acids
- OH
phenolic alcohol
- CO
methoxy carbonyl
- SDBS
sodium dodecylbenzene sulfonate
- HCl
hydrochloride acid
- NaOH
sodium hydroxide
- DIW
deionized water
- LSV
linear sweep voltammetry
- CV
cyclic voltammetry
- KCl
potassium chloride
- SEM
scanning electron microscope
- Zp
zeta potential
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