Dissolved air flotation pretreatment for low-pressure membranes in water treatment

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Dissolved air flotation pretreatment for low-pressure membranes in water treatment

Dissolved air flotation pretreatment for low-pressure membranes in water treatment: A review of fouling mitigation and product water quality

https://doi.org/10.1016/j.jwpe.2023.104391Get rights and content

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Abstract

Coagulation/flocculation/dissolved air flotation (CF-DAF) has garnered increased interest in recent years as pretreatment for low-pressure membranes (LPM) due to its ability to treat colored and algal-laden waters. These challenging water types are now more frequently encountered due to eutrophication and the effects of climate change, which necessitates pretreatment to prevent severe LPM fouling. In this study, pertinent literature regarding CF-DAF as pretreatment for LPMs in water treatment applications is reviewed. Specifically, the paper presents a brief overview of CF-DAF, compares CF-DAF pretreatment performance with other coagulation alternatives, outlines relationships between CF-DAF treated water quality and LPM fouling, reviews the treated water quality from the integrated system (i.e., CF-DAF-LPM), and finally discusses cost considerations of CF-DAF implementation as LPM pretreatment. When comparing CF-DAF with inline coagulation and coagulation/flocculation/sedimentation as pretreatment strategies for LPM fouling mitigation, it was found that the relative performance is influenced by both the feed water quality and the membrane configuration/type. CF-DAF pretreatment improved removal of hydrophobic organics (as measured by UV254 and SUVA) and for the limited waters tested resulted in reduced irreversible fouling measures, which are critical to long-term economic operation. According to the knowledge gaps identified throughout the study, the manuscript concludes by outlining guidance on potential foci of future research.

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Introduction

Low-pressure membranes (LPM) have become a well-established technology in drinking water treatment (DWT) applications, with rapid growth over the past two decades [[1], [2], [3], [4]]. The popularity of LPMs may be attributed to several factors including: (1) reliable operation with a superior quality product water, even in the face of changing feed water quality [5,6]; (2) modularity, which results in a compact process that can be operated with a high degree of automation [7,8]; (3) excellent removal of microorganisms, pathogens, and protozoa [[9], [10], [11], [12]]; (4) increasingly stringent treated water quality regulations [[13], [14], [15]]; (5) decreasing membrane costs due to economies of scale [13,16]; and (6) the use of lower-quality freshwater sources in response to stress from climate change, industrialization, and population growth [17]. Despite the numerous advantages of LPMs compared to conventional treatment using dual media deep bed filtration, problems associated with membrane fouling remain a crucial impediment preventing a more extensive usage of LPMs for DWT [[18], [19], [20], [21]]. LPM fouling is characterized by a loss of productivity over time resulting from the deposition of feed water constituents on the membrane surface or within the membrane pores [12,19,22,23]. For LPMs operated with a constant filtration flux, this productivity loss is manifested through greater pressure drops, which results in increased energy costs [[24], [25], [26]].

Natural organic matter (NOM), which is ubiquitous in natural waters, has been identified by many researchers as the consensus principal foulant of LPMs [[27], [28], [29]]. Besides NOM, it has also been reported that severe LPM fouling can result from the filtration of algal-laden surface waters [30,31]. With climate change yielding an increase in the total NOM content [[32], [33], [34]] and color [35] of natural waters globally, as well as resulting in increased frequency and intensity of algal blooms [[36], [37], [38], [39]], LPM feed water pretreatment strategies are needed as climate change adaptation measures to achieve improved economy of operation. Coagulation prior to LPM filtration is the most commonly applied feed water pretreatment strategy [40,41] as it is reported to be an efficient and cost effective solution to simultaneously mitigate LPM fouling as well as produce a superior-quality product water [9,16]. Although there has been recent interest in inline coagulation (C-IN) pretreatment (without particle separation prior to LPM filtration) due to its reduced process complexity and shorter retention times [42], several studies have found it unsuitable for the control of LPM fouling by water supplies with elevated NOM content (i.e., >5 mg L−1) [43,44], color [45], or those that are algal impacted [46,47]. Compared to conventional technologies involving coagulation/flocculation and clarification by gravity sedimentation (CF-SED), coagulation/flocculation/dissolved air flotation (CF-DAF) is a high-rate process with a small footprint that compliments the compactness of membrane installations well [45]. Furthermore, for highly colored or algal impacted waters, CF-DAF provides a superior performance compared to CF-SED due to the low densities of NOM and algae flocs, which makes them more amenable to removal by flotation [[48], [49], [50]].

Several detailed reviews on alternative pretreatment strategies for LPMs in water treatment [9,16,51] were presented over a decade ago. Since this time, Malkoske et al. [52] have offered an updated perspective on coagulation pretreatment for LPMs in a recent paper which surveys both C-IN and conventional (i.e., CF-SED) approaches. However, despite the increasing research interest and number of publications on CF-DAF pretreatment of LPMs in water treatment (Fig. 1), none of these existing studies have reviewed this research area. Accordingly, the current work aims at reviewing pertinent literature to provide a comprehensive perspective of CF-DAF pretreatment for LPM technologies with a specific focus on DWT applications. This report will be organized into four principal sections. The first section briefly presents an overview of CF-DAF practice and discusses its integration with LPMs for application in drinking water and wastewater treatment. In the next section, the performance of CF-DAF for LPM fouling mitigation is compared with CF-SED and C-IN. Furthermore, analysis and discussion of the impacts of key coagulation, flotation, and water quality parameters on LPM fouling is provided. In the third section, the reported quality of CF-DAF-LPM product water is reviewed. The final section provides a discussion of some of the practical economic considerations associated with implementing CF-DAF for LPM pretreatment. The report concludes by summarizing critical knowledge gaps and recommending directions for future research.

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