Long-term stability of aerobic granular sludge for the treatment of very low-strength real domestic wastewater
Graphical abstract
Introduction
Aerobic granular sludge (AGS) is considered one of the most promising new biotechnologies for wastewater treatment due to important saving in terms of energy usage (20–50%) and footprint (25–75%), and a rapidly increasing number of municipal and industrial plants (WWTPs) are currently operating this bioprocess (Pronk et al., 2017). AGS technology has been developed and is to date mostly applied for the treatment of municipal wastewaters characterized by high organics concentration (Chemical Oxygen Demand, COD), mainly originating from separate sewage systems (Giesen et al., 2013).
In the Italian scenario and in other developed countries, drainage systems are often combined, where wastewaters are diluted by meteoric precipitations. It is also not infrequent the steady intrusion of extraneous waters in sewer systems (Broadhead et al., 2013). In some cases, wastewaters are pre-treated by septic tanks before sewer inlet with a further concentration reduction of organic compounds entering the sewage treatment plants. Combined sewers, as well as some diluted industrial effluents, correspond to wastewaters chemically and physically very different from those for which granular aerobic technology has been developed, specifically regarding COD concentration, significantly lower.
Most of the studies reporting the successful granulation of activated sludge treating real municipal wastewaters were performed by using influents with high COD concentrations, as the mixture of municipal and industrial wastewaters reported by Giesen et al. (2013) and Liu et al. (2010); municipal wastewater with the addition of external carbon sources (Coma et al., 2012); real wastewater originating from separate sewerage systems (Wagner and da Costa, 2013).
Recent studies focused on the influence of low influent COD concentration values on aerobic granules stability. Long-term stability of aerobic granules is still a challenging issue and has been regarded as the major restriction for practical application of this technology (Zhang et al., 2016).
The first attempt to achieve the granulation of conventional activated sludge with a real low-strength municipal wastewater (<200 mg COD L−1) was carried out by Ni et al. (2009). To achieve 85% granulation a long start-up period of about 300 d was required together with intensive physical selection based on settling velocity which was indicated as key for the granulation process. That study did not evaluate important aspects such as long-term granules stability, anaerobic COD storage efficiency, biological phosphorus removal and the selection for slow-growing organisms.
Derlon et al. (2016) tested low strength municipal wastewater applying the lowest organic loading rate (OLR) reported (0.4 ± 0.2 kg COD m−3 d−1). Successful granulation of activated sludge was reached in 3 months of operation, by applying a selective utilization of organic carbon and the selective withdrawal of slow-settling sludge (<16 m h−1). In this study granule size was rather small (0.25 < d < 0.63 mm) with a significant fraction of flocs present in the sludge (about 30%) affecting the simultaneous nitrification and denitrification (SND) which was indeed not observed.
Zhang et al. (2011) reported that AGS operated with an average influent COD concentration of 200 ± 20 mg L−1 and an OLR as low as 0.58 kg COD m−3 d−1 presented a loose, porous and hollow structure and became unstable when their diameter increased above 1 mm. These results are confirmed by Peyong et al. (2012), who achieved small granules with a synthetic influent COD concentration of 400 mg L−1 and reported their disintegration when raw domestic wastewater with influent COD values fluctuating within 42–180 mg L−1 (0.13–0.54 kg COD m−3 d−1) was used as feed throughout two months of operations. The authors suggested that minimum influent COD concentration of 200 mg L−1 and an OLR of 0.6 kg m−3 d−1 are required in order to maintain the integrity of aerobic granules. These values are considerably lower than the OLR and influent COD concentration previously used to develop aerobic granules in low strength wastewater (Wang et al., 2008), but still represent a limitation for the treatment of mixed sewage systems and diluted industrial effluents.
In the case of municipal low-strength wastewater, granular long-term stability and long start-up phases currently hamper the development of the aerobic granular sludge process. While the latter issue can be overcome by the temporary addition of readily biodegradable organic substrates and/or the inoculation with granular sludge taken from operating full-scale installations when available, the comprehension of the key factors affecting granules stability is still lacking, representing the main drawback for AGS application in the long run. AGS reactors are operated at high sludge retention time (SRT) values and biomass retention inside the system relies on the excellent settling properties of mature and intact granules. Granules long-term stability is therefore fundamental to avoid biomass wash-out from the reactor and to guarantee the efficacy of bioconversion processes. This study aimed at evaluating the feasibility of cultivating stable AGS at influent soluble COD load of 0.4 kg COD m−3 d−1 - the lowest reported together with Derlon et al. (2016) - with real very low-strength domestic wastewater as influent (115 ± 23 mg COD L−1) - the lowest reported together with Ni et al. (2009) - by applying a novel operational strategy solely based on a strict metabolic selective pressure. This new strategy is based on the maximization of the anaerobic biodegradable COD (bCOD) uptake by slow-growing organisms (i.e. polyphosphate-/glycogen-accumulating microorganisms clades, PAOs/GAOs clades) by dynamic control of the duration of an anaerobic mixed phase applied after the anaerobic feeding phase, thus preventing the competition with conventional aerobic heterotrophic biomass and flocs formation. The novelty of this study is that the most common selective pressure based on the hydraulic selection of fast settling biomass, previously indicated as key for aerobic granulation, was in here not applied on purpose. The applied minimum settling velocity of 1.9 m h−1 is much lower than the values reported by Ni et al. (2009) (16.8 m h−1) and Derlon et al. (2016) (16 m h−1) and is the lowest value ever reported in the literature for the cultivation of AGS using very low-strength wastewater.
By identifying the key factors to preserve biomass long-term stability and functionality, the application of aerobic granular sludge technology in the context of combined sewers systems could be improved and the retrofitting of existing wastewater treatment plants facilitated.
Section snippets
Reactor set-up and operations
A sequencing batch reactor (SBR) was inoculated with 1 L of conventional floccular activated sludge mixed liquor (8.0 g VSS L-1), collected from the full-scale municipal WWTP of Florence (San Colombano, Italy), and operated for 175 d. The reactor run was divided in two different stages comprising a start-up phase (Stage A hereafter, 1–50 d) followed by a prolonged cultivation phase (Stage B hereafter, 51–175 d) which main operational parameters are reported in Table 1. The reactor was a
Formation and long-term stability of aerobic granules
The aim of the start-up phase (Stage A) was to develop a completely granulated sludge bed using as inoculum conventional flocculent activated sludge, characterized by a fluffy and loose morphology and an SVI of about 100 mL g−1; during Stage B, the dosage of sodium acetate was stopped to evaluate the long-term stability of AGS cultivated with very low-strength real domestic wastewater as the only source of carbon.
The morphological evolution of the activated seed sludge at different granulation
Conclusions
In the present study a novel strategy was successfully applied for the cultivation of long-term stable aerobic granules treating real very low-strength wastewater originating from a municipal combined sewage system. AGS achieved at an average influent COD concentration of 290 ± 44 mg L−1, through the application of a feast/famine regime and the hydraulic washout of slow settling biomass, was subsequently cultivated for more than 4 months at influent COD concentration as low as 115 ± 23 mg L−1
Acknowledgements
The authors wish to thank Francesca Cecconi, Francesca Casagli, Serena Falcioni, Claudio Zuffi, Elisabetta Fedeli and Arianna Nardi for their passionate contribution to the presented research. Finally, we would like to express a warm thanks to Publiacqua S.p.A. and all the staff members of San Colombano WWTP (Florence, Italy) for their valuable and constant help.
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