Long-term stability of aerobic granular sludge for the treatment of very low-strength real domestic wastewater

Highlights

• Granules formation and long-term stability with influent soluble COD of 115 ± 23 mg L⁻¹.

• Metabolic selective pressure allows granules formation and long-term stability.

• Selective pressure based on settleability is not required for granules formation.

• Anaerobic mixing time control for complete COD uptake is key to limit flocs formation.

Abstract

Aim of the present study was to assess the feasibility of cultivating aerobic granular sludge for the treatment of real very low-strength (COD <120 mg L⁻¹) municipal wastewater by the application of a strict metabolic selective pressure. The feasibility was evaluated in terms of long-term physical stability of the granular biomass as well as COD and phosphate removal efficiencies. A laboratory-scale sequencing batch reactor was inoculated with conventional activated sludge and operated for 175 d. Complete granulation of conventional activated sludge (SVI₅/SVI₃₀ = 1) was achieved within 45 d at an average influent COD concentration of 290 ± 44 mg L⁻¹ obtained by the addition of an external source of acetate. At day 51, acetate dosage was stopped and the reactor was then operated for other four months with only real wastewater as carbon source, resulting in an influent COD concentration as low as 115 ± 23 mg L⁻¹. Besides the decrease of the influent COD concentration and of the organic load, the long term stability of the granular sludge was not affected and granular size continuously increased until it reached a maturation phase with an average diameter of 1.5 mm, with more than 30% of the aggregates being larger than 2 mm and marginal presence of floccular biomass (about 5%). The applied metabolic selective pressure based on the complete biodegradable COD uptake under anaerobic conditions (85 ± 7% biodegradable COD removal efficiency obtained) was shown to be able to stimulate the formation of stable and large granules, indicating the other commonly applied selective pressure based on settling velocity (commonly > 8 m h⁻¹ versus 1.9 m h⁻¹ applied in this study), as not strictly required. The identification of the key factors to preserve biomass long-term stability and functionality, poses the basis to allow the application of aerobic granular sludge technology in the context of combined sewers systems and facilitate the retrofitting of existing wastewater treatment plants excluding the need for a plug-flow distribution system for the influent wastewater.

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.