Phosphorus removal and potential recovery from wastewater using aerobic granular sludge

Abstract

Phosphorus is a valuable finite resource and an essential element for any type of life. Phosphorus is currently mainly recovered by mining phosphate rocks. This is intrinsically unsustainable, and since this mined phosphorus is widely used in agriculture it also underpins the steady accumulation of phosphate in water bodies which is then discharged from wastewater, leading to water deterioration. The ability to remove phosphorus from wastewater is therefore critical both for protecting the environment, and for putting industrial phosphorus recovery on a more sustainable footing. Aerobic Granular Sludge (AGS) is an up-and-coming biological wastewater treatment technology to replace conventional activated sludge technology. Despite countless studies of AGS and initial attempts at commercialisation, however, there remains a lack of studies into utilising granules’ unique characteristics to maximise nutrient removal, especially in the context of their potential for phosphorus recovery via bioprecipitation. Bioprecipitation of phosphorus within the granules has been widely observed but its numerical contributions, and its impact on reactor performance and on granular sludge formation have not been agreed upon. This project therefore aimed to investigate the use of AGS for phosphorus removal and potential recovery by understanding how bioprecipitation could affect the overall reactor performance and how to exploit the advantages of this process for phosphorus recovery. The potential of AGS for phosphorus recovery was explored by studying nutrient release with biological fermentation enhanced by chemical addition. The first study in this project examined the formation and long-term stability of AGS for simultaneous chemical oxygen demand (COD) and nutrient removal with different start-up strategies in sequencing batch reactors (SBRs). It was found that granulation was achieved within 18 days with direct use of a short settling time strategy, which was 23 days faster than a settling strategy involving a stepwise decrease in settling times. Stable simultaneous COD, nitrogen, and phosphorus (CNP) removal could be achieved instantly when mature granular sludge was formed. Furthermore, when investigating the phosphorus removal mechanism, it was found that biologically-induced precipitation was responsible for 54–64% of total soluble P-removal. At the same time, no apparent P release and uptake were observed. Calcium phosphate was also found to be the main precipitate inside the granules, suggesting that calcium was involved in the capture of phosphorus in AGS. In this experiment, calcium supplementation was achieved through natural water hardness without additional chemical calcium and temperature, dissolved oxygen (DO), and pH adjustment. This, therefore, highlights the potential to use water hardness to support natural wastewater treatments to obtain simultaneous CNP removal with more straightforward operational control and high resource recovery potential. To investigate the effect of calcium on biological phosphorus removal in AGS further, the second experiment investigated the effects of three different calcium concentrations in wastewater in three SBRs, i.e., R1 with a calcium concentration of 100 mg/L, R2 with 50 mg/L, and R3 with 20 mg/L. The average efficiency of phosphorus removal increased concomitantly with the calcium concentration supplemented in the synthetic wastewater, with an average removal efficiency of 84% in R1, 79% in R2 and 58% in R3. The average total phosphorus fraction in the dried granular sludge was ~9.8%, ~7.3%, ~5.9%, respectively. The proportion of apatite P within the total P fraction was 80.2%, 69.9%, and 10.2%, in R1, R2 and R3, respectively. Meanwhile, it was found that the percentage of apatite was negatively correlated with the percentage of organic P retained in the sludge. Mineral spatial distribution confirmed that inorganic precipitation was observed in the interior of the granular sludge. These results indicated that phosphorus can be accumulated in the granular sludge as apatite, and that feeding more calcium into the reactor enhances the phosphorus retention capacity. Furthermore, the ratio of P release and acetate uptake (Prel/Acupt) was analysed in order to quantify the enhanced biological phosphorus removal (EBPR) capacity. It was found that the average Prel/Acupt decreased at higher calcium concentrations, with Prel/Acupt being 0, 0.22 and 0.25 in R1, R2 and R3, respectively. This experiment suggested that calcium concentration at 100 mg/L inhibited the EBPR mechanism in the granular sludge but facilitated biologically-induced phosphorus precipitation for P removal. Calcium concentrations below 50 mg/L had no detrimental effect on the EBPR mechanism. The results in this experiment imply that different calcium concentrations in the water resulted in different P removal mechanisms in AGS technology. Since the biologically-induced phosphorus precipitates in AGS are mainly Hydroxyapatite (HAP), the last experiment investigated phosphorus release from HAP-enriched granular sludge for downstream phosphorus recovery. Anaerobic fermentation was conducted in batch and continuous reactors to see if naturally-produced organic acids and low pH conditions could enhance P release from granular sludge. Three strategies were implemented to enhance phosphorus release from the sludge in the batch fermentation processes: (1) a fermentation process at an uncontrolled pH condition as a control, (2) a fermentation process controlled at pH 5, and (3) a fermentation process controlled at pH 5 with the addition of Ethylenediaminetetraacetic acid (EDTA) (mol EDTA:P = 5:1). It was found that controlling the fermentation at pH 5 could increase P-release by up to 2.2 fold compared to the control, and that coupling pH at 5 and EDTA addition during fermentation could significantly enhance P-release by up 9x higher than the control. It needs to be noted that EDTA addition higher than 0.69 gEDTA/gSS was detrimental to the fermentation process since, at this level, it inhibits volatile fatty acid (VFA) production. In addition, a continuous fermentation process was found to be associated with a 1.9 fold higher level of VFA production, but a 25% lower soluble P concentration than the control in the batch fermentation process. A leaching process was also conducted post-fermentation in order to find the best way to maximise P-release from fermented HAP-enriched sludge. The leaching process was tested in three different sludge forms, i.e., wet, dry and ash, using three different leachants (sulphuric acid, citric acid and EDTA). It was observed that both pH and leachant concentration affected the efficiency of phosphorus release, and that the highest levels of phosphorus solubilisation resulted from ash HAP-enriched sludge at pH 1.5 with sulphuric acid, resulting in ~97% P-release efficiency. The study in this chapter indicates that fermentation with the assistance of EDTA can release 40% of the P from sludge, suggesting a potential method for P recovery from HAP-enriched granular sludge. If the HAP-enriched granular sludge was then burnt as part of a sludge management protocol, P release could reach 97%. Overall, this study shows that simultaneous CNP removal can be achieved by using aerobic granular sludge in a single reactor, but the P removal mechanism in AGS is highly dependent on the concentration of calcium in the wastewater. With calcium concentrations above 50 mg/L, biologically-induced HAP precipitation is the main mechanism for P removal by AGS, leading to P content in aerobic granules as high as ~100 mg/g dry weight biomass, which is similar to the P content in typical sludge ash. The high HAP content in aerobic granular sludge means that it could be used as slow-releasing fertiliser after digestion. Alternatively, the phosphorus can be released via fermentation with the assistance of EDTA or other chemicals for making downstream products, such as struvite or pure hydroxyapatite. This project highlights the potential of AGS for phosphorus removal from wastewater and recovery via HAP, particularly in regions with high water hardness levels.

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ORCID for Yongqiang Liu: orcid.org/0000-0001-9688-1786

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