USE OF GRANULAR SLUDGE TO INCREASE THE CAPACITY OF SEQUENCING BATCH REACTORS

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1 USE OF GRANULAR SLUDGE TO INCREASE THE CAPACITY OF SEQUENCING BATCH REACTORS Ben van den Akker 1, Kyra Middlemiss 1, Katherine Reid 1, Joerg Krampe 2, Nirmala Dinesh 1 1. SA Water, Adelaide, SA, Australia 2. Vienna University of Technology, Karlsplatz, Vienna, Austria ABSTRACT The sequencing batch reactor (SBR) installed at Pt Pirie WWTP in South Australia is hydraulically overloaded due to significant ground water infiltration into the sewer network and requires additional treatment capacity. Modifying the operational parameters of Pt Pirie s SBRs to select for granular sludge (GS) could be a viable means of increasing capacity. Through the use of a pilot-scale SBR reactor, the aim of this study was to determine if GS can be developed and to identify the key operational parameters needed to maintain GS stability. Extensive pilot trials were conducted at two different WWTPs in South Australia which showed similar results. GS was readily established at both WWTPs where biomass settleability (as defined by the sludge volume index) improved by >90%. High removal of total nitrogen and BOD 5 were also maintained. Pilot results also showed that GS could reduce the overall SBR cycle time by up to 1h thereby increasing the hydraulic capacity of Pt Pirie s SBR by 24%, which would be an attractive, cost effective solution for a capacity upgrade. Our next step involves a full-scale trial, by modifying the operation of one existing SBR tanks. Information collected from the pilot- and full-scale trials will determine whether the operational parameters of the existing SBRs can be modified to promote GS formation and if not, determine the capital costs associated with retrofitting and upgrading the SBR to operate with GS. INTRODUCTION Granular sludge Aerobic granular sludge is a proven technology that was developed in recent years to improve the effluent quality and sludge settleability of biomass in SBRs. GS offers a number of benefits over conventional activated sludge floc-based systems and today it is widely regarded as promising technology with potential to become industry standard. Extensive research, lead largely by Delft University of Technology (TU Delft), has shown that operational parameters of SBRs can be modified to convert fluffy, slow settling activated sludge flocs into fast settling, dense microbial granules, which settle within as little as 5 minutes (Bassin et al. 2012; De Kreuk et al. 2005a). SBRs are the most favoured technology to achieve GS because they operate in batch mode (cycles of filling/ aeration/settling/decant), which can be modified to promote GS formation. Notably a rapid settling time of 2-10 minutes is used to washout slow poorly settling flocs as well as an anaerobic feed which encourages the development of dense granule-forming organisms, such as polyphosphate accumulating organisms (PAOs) (Bassin et al. 2012; Beun 2001; Beun et al. 1999). Under anaerobic conditions, PAO convert easily degradable substrates, into microbial storage polymers, and thereby gain a competitive advantage over filaments. The anaerobic feed and establishment of PAOs are important for the long-term stability of granules, especially when the reactors are required to operate under low and economical dissolved oxygen concentrations of 1-2 mg/l (de Kreuk et al. 2005b; Welles et al. 2014). Benefits of granular sludge The benefits of GS are numerous. Firstly, settling of the granular sludge is rapid, taking only a few minutes, thereby allowing shorter settling times and retainment of more active biomass. This feature alone may increase the capacity of our existing SBRs by more than 24 %. In addition, granules allow the development of an oxygen gradient through the granule depth which facilitates simultaneous nitrification and denitrification (SND) and phosphate removal (Bassin et al. 2012; Pronk et al. 2013). Subsequently, efficient nutrient removal is achieved without the need for separate reactor compartments. Furthermore, the use of an anaerobic feed eliminates the

2 need for anoxic selectors and return streams, which reduces space and energy requirements associated with the operation of conventional SBRs. Application of Granular Sludge The 4 ML/d SBR plant installed at Pt Pirie WWTP in South Australia operates under challenging conditions as it is subject to extremely high and variable inflows, and very high salinity caused by substantial ground water intrusion into an aging sewer network. This in turn has adverse effects on treatment performance leading to carryover of suspended solids (SS) from bulking sludge and intermittent loss of nitrification. The existing SBR plant needs to be operated more efficiently to increase the much needed hydraulic capacity, however, at the same time meet tightening nutrient and suspended solids discharge limits. Modifying the SBR s operating parameters to select for GS may be a cost effective solution to achieve these goals and may be accomplished without significant capital investment. Accordingly, the aim of this study was to confirm whether GS can be developed and maintained in SBRs treating municipal wastewater and to identify the window of operation critical for GS formation. Pilot trials were conducted at two WWTPs in parallel to full-scale conventional floc-based SBRs for side-by-side comparison. METHODOLOGY Study sites A pilot-scale granular sludge reactor was operated at two WWTPs, with each trial lasting over 140 days. The first trial was conducted nearby at Bolivar High Salinity (HS) WWTP which treats high-saline municipal sewage within the range of 5,800 to 7,000 mg/l of TDS. Pilot trials were then repeated at Pt Pirie WWTP which treats extremely high-saline municipal sewage in the range of 20,000 to 28,000 mg/l of TDS. Pilot-plant description The pilot-plant GS SBR used in this study is presented in Figure 1. The pilot was comprised of a 60L Perspex reactor that was 1m high x 0.3m diameter. SBR cycle times were controlled using a programmable logic controller (PLC) to allow the cycle times to model both our full-scale SBRs and those needed for GS development (see Table 1). Pilot trials were performed in parallel to the fullscale SBRs to allow side-by-side comparison. To encourage GS development, the pilot was feed anaerobically from the bottom of the SBR in a plug-flow fashion using a peristaltic pump. This also eliminated the need for an anoxic selector. In addition, the pilot SBR employed a rapid settling time of 2 to 10 minutes to ensure the washout out of slow settling flocs. Following settling, the treated water was extracted under gravity via a decant pipe situated within the reactor. A Figure 1: GS pilot plant showing (A) front control panel; and (B) side profile of the SBR. Monitoring parameters Reactor performance was monitored, by closely monitoring changes in ammonia-n, nitrite-n, nitrate-n, phosphate-p and BOD 5 concentrations in the sewage and decanted effluent. Mixed liquor samples underwent regular analysis for mixed liquor suspended solids (MLSS), and sludge settleability was characterised using the 5 and 30 minute Sludge Volume Index (SVI). Table 1: Typical SBR cycle times of the granular sludge pilot-plant and full-scale SBRs at Bolivar and Pt Pirie WWTPs Fill (min) GS pilotplant 60 (anaerobic) Bolivar HS SBR 108 (aerobic) Aerate (min) Settle (min) Decant (min) Cycle time (min) B Pt Pirie SBR (aerobic)

3 24/08/ /09/2013 3/10/ /10/ /11/2013 2/12/ /12/ min SVI (ml/g) RESULTS & DISCUSSION Pilot start-up At the beginning of each trial, the pilot plant was initially seeded with MLSS harvested from the neighbouring full-scale SBR. In comparison to the full-scale SBRs, significant improvement in the sludge settleability of the granular sludge pilot were seen early, where the SVI had decreased from 240 to 45 ml/g at Bolivar HS WWTP (e.g. Figure 2) and from 123 to 24 ml/g at Pt Pirie. Start-up times at Bolivar and Pt Pirie were similar despite the large difference in sewage salinities. Within 4-5 weeks, the ratio of SVI measured at 5 min/30 min reached values as low as 1.1 at Bolivar and 1.04 at Pt Pirie, which confirmed that the granulation of the biomass had occurred (Liu et al. 2010). The settling performance of the granular sludge mixed liquor was superior to that of mixed liquor sampled from the full-scale SBRs, given that all of the settling was typically completed within the first 2.5 to 10 minutes of the settlers test. 350 A B 5cm Full-scale SBR Pilot granular sludge C 0 Figure 2: Comparison of granular sludge and fullscale SBR MLSS settleability defined as the 30 min SVI during start-up at Bolivar WWTP. MLSS morphology Changes in the activated sludge morphology were obvious with the naked eye (Figure 3). After 100 days of operation, the MLSS had undergone a complete transformation from exhibiting a fluffy, floc-like structure to dense microbial granules that were approximately 2 to 5 mm in size. In contrast to the seeded floc, only very few filaments were seen under the microscope. Figure 3: Mixed liquor sampled from the granular sludge pilot plant contained within a 10cm diameter dish on: (A) floc on day 1 of start-up; (B) mature granules seen after day 100 at Bolivar HS WWTP; and (C) granules seen after day 100 at Pt Pirie WWTP. Nutrient removal performance Despite the pronounced transformation in mixed liquor morphology, the nitrogen and BOD 5 removal performance of the granular sludge pilot were unaffected. A summary of the pilot-plants performance is presented in Table 1. Simultaneous removal of ammonia, total nitrogen, BOD 5 and phosphate were observed during both trials at optimised DO concentrations between 1-3 mg/l. The

4 extremely high salinity concentrations of Pt Pirie s sewage (20,000 to 35,000 mg TDS/L) had no adverse impact on granule formation, stability or nitrogen removal performance. For both study sites, biological phosphate removal which is deemed important for the stability of GS systems was lower than expected for GS systems and other SBRs that operate under feast-famine conditions (e.g. De Kreuk et al. 2007; de Kreuk et al. 2005b; Pronk et al. 2013). This may have been attributed to the uniquely high saline conditions, which can inhibit PAO activity (Pronk et al. 2013). Nevertheless the level of activity seen in our trial appeared sufficient to facilitate granule formation and impede the development of filaments that often result in bulking. Table 1: Pilot granular sludge SBR performance achieved after 100 days of start-up (mean values). Parameter Bolivar pilot trial Pt Pirie pilot trial 30 min SVI (ml/g) MLSS (g/l) 6 12 Sludge age (d) NH 4 removal (%) Total N removal (%) BOD 5 removal (%) PO 4 removal (%) Full-scale considerations Our findings confirmed that the formation of granular sludge can be easily achieved and were maintained for the entire duration of the trial. Based on these findings, there appears potential for retrofitting existing SBRs to select for granular sludge to increase plant capacity. In particular, modifications to the inlet design and the decant weir operation would be required to ensure plug flow conditions. Furthermore, the rapid rate of decant used in this pilot study (2.5 minutes) is not possible at full-scale. Typically, the full-scale SBRs decant over a 60 min period which can be reduced to 35 min for high flow / storm mode. If this is not sufficient, this problem may overcome by simultaneously feeding and decanting the reactor, which would be far more economical, provided plug flow is maintained (De Kreuk 2006; de Kreuk et al. 2005b). Given the success of our pilot trials, our next step will involve a full-scale trial, by modifying the operation of one of two SBRs at Pt Pirie WWTP. Preliminary findings from this full-scale trial will be presented. CONCLUSIONS Our results showed that granular sludge was readily established from flocculating sludge, by employing an anaerobic feed and rapid settling time of only 2.5 to 10 minutes (during stable operation). These modifications improved sludge settleability and have potential to eliminate bulking which commonly plague activated sludge. SA Water owns and operates five WWTPs that operate using batch fill and draw cycles, and two of which are operating close to their design capacity. Our results showed that improving the sludge settleability using granular sludge would reduce total cycle time by up to 1 hour, thereby increasing plant capacity by 24%. If granulation of the sludge could be achieved with modifications to sewage distribution during filling and the decant weir operation, this would be an attractive, cost effective solution for a capacity upgrade, even if only 10 to 20% of extra capacity was gained. ACKNOWLEDGMENTS The authors acknowledge the South Australian Water Corporation for funding this research. We are also grateful to Berri Water Engineering Technologies (WET) who constructed the pilot-plant. We also thank Michael Corena and Rowan Steele of SA Water Corporation, and the staff at the Bolivar High Salinity WWTP and Pt Pirie WWTP for their support. REFERENCES Bassin JP, Kleerebezem R, Dezotti M, van Loosdrecht MCM Simultaneous nitrogen and phosphate removal in aerobic granular sludge reactors operated at different temperatures. Water Research 46(12): Beun JJ PHB metabolism and N-removal in sequencing batch granular sludge reactors. Delft: Delft University of Technology. 155 p. Beun JJ, Hendriks A, van Loosdrecht MCM, Morgenroth E, Wilderer PA, Heijnen JJ Aerobic granulation in a sequencing batch reactor. Water Research 33(10): De Kreuk MK Aerobic Granular Sludge: Scaling up a new technology. Delft: Delft University of Technology. 199 p. De Kreuk MK, Heijnen JJ, Van Loosdrecht MCM. 2005a. Simultaneous COD, nitrogen, and

5 phosphate removal by aerobic granular sludge. Biotechnology and Bioengineering 90(6): De Kreuk MK, Picioreanu C, Hosseini M, Xavier JB, Van Loosdrecht MCM Kinetic model of a granular sludge SBR: Influences on nutrient removal. Biotechnology and Bioengineering 97(4): de Kreuk MK, Pronk M, van Loosdrecht MCM. 2005b. Formation of aerobic granules and conversion processes in an aerobic granular sludge reactor at moderate and low temperatures. Water Research 39(18): Liu Y-Q, Moy B, Kong Y-H, Tay J-H Formation, physical characteristics and microbial community structure of aerobic granules in a pilot-scale sequencing batch reactor for real wastewater treatment. Enzyme and Microbial Technology 46(6): Pronk M, Bassin JP, Kreuk MK, Kleerebezem R, Loosdrecht MCM Evaluating the main and side effects of high salinity on aerobic granular sludge. Applied Microbiology and Biotechnology:1-10. Welles L, Lopez-Vazquez CM, Hooijmans CM, Van Loosdrecht MCM, Brdjanovic D Impact of salinity on the anaerobic metabolism of phosphate-accumulating organisms (PAO) and glycogenaccumulating organisms (GAO). Applied Microbiology and Biotechnology 98(17):

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