Effects of aging of ferric-based drinking water sludge on its reactivity for sulfide and phosphate removal

Highlights

• Akaganeite is the main iron oxide species in ferric DWS

• Akaganeite transforms from amorphous to more crystalline form during DWS aging

• Sulfide removal capacity of ferric DWS decreases with increased sludge age

• Aeration during activated sludge treatment breaks the crystalline form of akaganeite

• Enhanced P removal in WWTPs can be achieved by using ferric DWS

Abstract

Recent studies demonstrated the practical potential of multiple beneficial reuse of ferric-rich drinking water sludge (ferric DWS) for sulfide and phosphate removal in wastewater applications. In practice, ferric DWS is often stored on-site for periods ranging from days to several weeks (or even months), which may affect its reuse potential through changes in iron speciation and morphology. In this study, we investigated for the first time the impact of ferric DWS ‘aging’ time on the iron speciation and morphology and its subsequent impact on its reactivity and overall sulfide and phosphate removal capacity. A series of coagulation tests were conducted to generate ferric DWS of a practically relevant composition by using raw influent water from a full-scale drinking water treatment plant (DWTP). A comparison with ferric DWS from 8 full-scale DWTPs confirmed the similitude. The presence of akaganeite (β-FeOOH) was detected in ferric DWS (through XRD analyses), independent of the DWS storage time. However, the morphology of akaganeite changed over time from a predominant poorly-crystalline phase in ‘fresh’ DWS (8 ± 0.1% of total Fe) to a highly crystalline phase (76 ± 3% of total Fe) at a sludge aging time of 30 days which was confirmed by means of Rietveld refinement in XRD analyses (n = 3). Subsequent batch tests showed that its sulfide removal capacity decreased significantly from 1.30 ± 0.02 mmol S/mmol Fe (day 1) to 0.60 ± 0.01 (day 30), a decrease of 54 % (p < 0.05). The level of crystallinity however had no impact on sulfide removal kinetics, most sulfide being removed within 10 minutes. Upon aeration of sulfide-loaded ferric DWS in activate sludge, amorphous iron oxides species were formed independent of the initial DWS crystallinity which resulted in efficient P removal at capacities similar to that of conventional FeCl₃ dosing.

Introduction

The majority of drinking water treatment plants (DWTPs) rely on coagulation and flocculation for the removal of turbidity, colour, natural organic matter (NOM) and pathogens from raw water (Bratby, 2016; Matilainen et al., 2010; Okour et al., 2009). Amongst the various coagulants used at DWTPs, the most commonly used are aluminium sulfate (often refered to as alum) and ferric salts (i.e. either in the form of ferric sulfate or ferric chloride) (Pikaar et al., 2014). An unavoidable by-product of coagulation-flocculation is the generation of large amounts of drinking water sludge (DWS) rich in aluminium or iron, depending on the type of coagulant used (Babatunde and Zhao, 2007). As examples showing the enormous amounts produced, DWS generated in the United Kingdom and The Netherlands exceeds 130,000 and 29,700 wet tonnes, respectively per year (Aquaminerals, 2018; Binnie et al., 2018).

Management of DWS incurs large costs and often comprises a substantial fraction of the operational expenditure of DWTPs, with landfilling often used as ultimate disposal route (Frias et al., 2013; Keeley et al., 2016). Therefore, significant research efforts have been made focussing on coagulant recovery, purification and direct reuse within the drinking water treatment process. The benefits of such an approach are twofold as it results in a reduced chemical demand in terms of ‘fresh’ coagulant as well as in a reduced DWS production (Keeley et al., 2014; Keeley et al., 2016). While the practical feasibility of various approaches including Donnan dialysis (Prakash and Sengupta, 2003), liquid ion exchange (Sthapak et al., 2008) and ion exchange with cation resin (Petruzzelli et al., 2000) has been successfully demonstrated, the relatively low coagulant prices make selective recovery and purification approaches economically challenging (Keeley et al., 2012). Moreover, direct reuse within the drinking water treatment process comes with certain technical challenges as the purification process needs to adhere to stringent regulatory requirements in terms of product quality in order to safeguard human health (Keeley et al., 2016).

Considering the above described limitations of direct reuse within the drinking water treatment process, there is a general interest in low-cost and low risk coagulant reuse approaches. In this context, the reuse of ferric based DWS in a sewer context is of special interest. Iron salts are the most commonly used chemicals to combat hydrogen sulfide induced sewer corrosion, a notorious and costly problem for utilities globally (Pikaar et al., 2014). Considering the high iron content of ferric based DWS, it has the potential to be reused in sewers for sulfide control. Indeed, the effective reuse of ferric based DWS for efficient sulfide control in laboratory scale rising main sewer reactors was demonstrated previously (Sun et al., 2015). Importantly, in a very recent study, the feasibility of the multiple reuse of iron-rich DWS for sulfide control in sewers, followed by phosphate removal in wastewater treatment and sulfide control during anaerobic digestion at the downstream wastewater treatment plant (WWTP) was demonstrated through long-term continuous experiments using a laboratory scale reactor system mimicking the urban wastewater system (Rebosura et al., 2020). It was found that DWS achieved similar treatment performance compared with FeCl₃ dosing in sewers in terms of sulfide control and phosphate removal (Rebosura et al., 2020; Rebosura et al., 2018).

While the above described studies clearly highlight the potential of beneficial reuse of ferric DWS in sewers and downstream WWTPs, the detailed characterization and potential transformation of iron species prior to reuse was not investigated in detail. Such information is essential as iron chemistry is complex with potential transformations in iron speciation and morphology that may occur over time from more amorphous and reactive species that are thermodynamically unstable (e.g. ferrihydrite and akaganeite) to more crystalline and less reactive species such as goethite and hematite (Atkinson et al., 1977; Baltpurvins et al., 1996; Cornell et al., 1989). The ultimate iron speciation and rate of transformation is complex and depends on several factors including solution pH, the anions present in solution, presence of oxygen and the storage temperature. To our best knowledge, no studies have been conducted investigating the aging of Fe-DWS under the prevalent anaerobic conditions occurring in real-life application. As in a practical situation DWS is often stored on-site from days up to several weeks, such transformation may thus occur, with a potentially negative impact on the reuse ability of DWS. Therefore, this study aimed to determine the impact of the storage time on the physicochemical changes of ferric DWS, and subsequently its reactivity and capacity in sulfide removal in sewers and in phosphorus removal in the downstream wastewater treatment plant. For this purpose, a series of laboratory-scale jar tests (i.e. coagulation experiments) were conducted to generate Fe-DWS using real influent from a local water treatment plant (in South-East Queensland) and subsequently stored under anaerobic conditions to mimic the storage conditions of Fe-DWS encountered in real-life applications. Importantly, to confirm that the produced ferric DWS was of a similar composition with that obtained in real-life applications, we conducted an industry survey of ferric DWS originating from 8 full-scale DWTPs (with ferric chloride as coagulant in their treatment process).

The produced Fe-DWS was characterized with XRD (combined semi-quantitative) and SEM-EDS analyses in order to investigate and quantify any changes in iron speciation and morphology in the DWS at increasing sludge aging times over a period of 30 days. The impact on sulfide removal in sewers and phosphate removal in activated sludge tanks was assessed through comprehensive batch sulfide and phosphate removal experiments using the produced ferric DWS at different sludge aging times.