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New Advances in Aerobic Granular Sludge Technology Using Continuous Flow Reactors: Engineering and Microbiological Aspects

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New Advances in Aerobic Granular Sludge Technology Using Continuous Flow Reactors: Engineering and Microbiological Aspects water Review New Advances in Aerobic Granular Sludge Technology Using Continuous Flow Reactors: Engineering and Microbiological Aspects Aurora Rosa-Masegosa 1,2, Barbara Muñoz-Palazon 1,2,* , Alejandro Gonzalez-Martinez 1,2, Massimiliano Fenice 3 , Susanna Gorrasi 3 and Jesus Gonzalez-Lopez 1,2 1 Faculty of Pharmacy, Campus de Cartuja, University of Granada, s/n, 18071 Granada, Spain; [email protected] (A.R.-M.); [email protected] (A.G.-M.); [email protected] (J.G.-L.) 2 Department of Microbiology, Institute of Water Research, University of Granada, C/Ramón y Cajal, 4, 18071 Granada, Spain 3 Dipartimento di Ecologia e Biologia, Università degli Studi della Tuscia, Largo Università snc, 01100 Viterbo, Italy; [email protected] (M.F.); [email protected] (S.G.) * Correspondence: [email protected] Abstract: Aerobic granular sludge (AGS) comprises an aggregation of microbial cells in a tridimen- sional matrix, which is able to remove carbon, nitrogen and phosphorous as well as other pollutants in a single bioreactor under the same operational conditions. During the past decades, the feasibility of implementing AGS in wastewater treatment plants (WWTPs) for treating sewage using fundamen- tally sequential batch reactors (SBRs) has been studied. However, granular sludge technology using Citation: Rosa-Masegosa, A.; SBRs has several disadvantages. For instance, it can present certain drawbacks for the treatment Muñoz-Palazon, B.; of high flow rates; furthermore, the quantity of retained biomass is limited by volume exchange. Gonzalez-Martinez, A.; Fenice, M.; Therefore, the development of continuous flow reactors (CFRs) has come to be regarded as a more Gorrasi, S.; Gonzalez-Lopez, J. New competitive option. This is why numerous investigations have been undertaken in recent years in Advances in Aerobic Granular Sludge search of different designs of CFR systems that would enable the effective treatment of urban and Technology Using Continuous Flow industrial wastewater, keeping the stability of granular biomass. However, despite these efforts, Reactors: Engineering and satisfactory results have yet to be achieved. Consequently, it remains necessary to carry out new Microbiological Aspects. Water 2021, technical approaches that would provide more effective and efficient AGS-CFR systems. In particular, 13, 1792. https://doi.org/10.3390/ it is imperative to develop continuous flow granular systems that can both retain granular biomass w13131792 and efficiently treat wastewater, obviously with low construction, maintenance and exploitation cost. Academic Editor: Hongyu Ren In this review, we collect the most recent information on different technological approaches aimed at establishing AGS-CFR systems, making possible their upscaling to real plant conditions. We discuss Received: 26 May 2021 the advantages and disadvantages of these proposals and suggest future trends in the application Accepted: 25 June 2021 of aerobic granular systems. Accordingly, we analyze the most significant technical and biological Published: 29 June 2021 implications of this innovative technology. Publisher’s Note: MDPI stays neutral Keywords: aerobic granular sludge; continuous flow reactor; granular stability; microbial commu- with regard to jurisdictional claims in nity; reactor design published maps and institutional affil- iations. 1. Introduction Aerobic granular sludge (AGS) technology is today often regarded as a promising Copyright: © 2021 by the authors. option for biological wastewater treatment because it has demonstrated excellent physico- Licensee MDPI, Basel, Switzerland. chemical performance at the pilot scale; even the AGS full-scale has gained an optimum This article is an open access article position in the area of treatment of urban and industrial sewage [1]. Granular sludge was distributed under the terms and first discovered in upflow sludge blanket anaerobic systems in the 1980s [2]. Later, at the conditions of the Creative Commons end of the 1990s, it achieved the growth of AGS without any supporting carrier operated Attribution (CC BY) license (https:// in sequential batch reactors (SBR) [3,4]. Currently, aerobic granular sludge technology creativecommons.org/licenses/by/ remains challenging to implement for treating sewage water, several authors reporting 4.0/). Water 2021, 13, 1792. https://doi.org/10.3390/w13131792 https://www.mdpi.com/journal/water Water 2021, 13, 1792 2 of 20 the instability of granular biomass for long-term operations [5,6]. Nevertheless, several mainstream full-scale systems are being substituted by AGS technology worldwide by Nereda® for treating urban and even industrial wastewaters [7–10]. Undoubtedly, AGS technology has numerous advantages in comparison with the most common technology implemented: conventional activated sludge (CAS). Conventional technology needs longer hydraulic and sludge retention times, more time to treat and recycle sludge and a larger surface area for implementation. For these reasons, AGS has become increasingly popular among the scientific and engineering communities. AGS technology has been employed for treating highly toxic and recalcitrant com- pounds in wastewater at full scale, including particulate matter, nitrogen, phosphorous, pharmaceutical compounds, phenols, nuclear waste and heavy metals [11–14]. However, it is important to mention that most recently applications are being focused on the denitrifica- tion of groundwater contaminated with pesticides and nitrates, implemented at full scale and thereby providing biosafety and high-quality drinking water [15]. Thus, nowadays the technology continues innovate in the water biological treatment area [16]. Granules are defined as compact and dense spherical biofilms with a solid structure, conformed by aggregates of prokaryotic and eukaryotic microorganisms embedded in extracellular polymeric substances, constituting a tridimensional matrix without any sup- porting material and encouraged by self-immobilization [17]. The excellent properties of granular biomass encouraged the high settleability, and consequently, the technology is able to remove organic matter and nutrients in a compact design reactor. Moreover, the microbial relationships found in the granular sludge encourage several metabolic pathways to degrade pollutants in syntrophic association, such as reported by Kasina et al. [18] These factors promote a technology with a low footprint, a small surface in need of implementation and low energy costs [16]. 2. Characterization of Aerobic Granular Sludge Biomass 2.1. Physicochemical Characterization (a) Regular shape Granular sludge is defined as a biofilm with a high degree of sphericity and a smooth surface, owing to the hydrodynamic pressure exerted by the circular and continuous motion within the bioreactor. Viruses, bacteria, archaea, protozoa, fungi and metazoan microorganisms, among others, are joined through the excretion of extracellular polymeric substances, which act as glue among cells. The spherical conformation is produced without the presence of any supporting material for promoting growth, as seen in other biofilm technologies [19,20]. (b) Excellent settleability Settleability is a significant advantage of AGS relative to CAS. In wastewater treatment plants (WWTPs) with CAS systems, it is essential to separate the liquid phase from the solid fraction. In this sense, CAS systems require a larger land surface for building a WWTP due to floccules’ lack of settleability. However, the incomparable granular decanting ability allows for easy and fast solid-liquid separation because the granules are in different phases. This fact is driven by the negative cell surface charge of granules and the hydrophobic compounds they produce, which are mainly constituted by proteins [21]; the feast–famine periods produced changes on cell surface charges that intensified the better settleability. This settling capacity allows granules to reach a decanting velocity of up to 138 m h−1, as reported by Muñoz-Palazon et al. [17], whereas floccules of CAS show velocities below 10 m h−1 [11]. Such impressive settleability allows operations to be conducted in shorter time periods, ranging from 2 to 10 min. The recurrence of this action enables the selection of the most compact granules, while fluffy and filamentous flocs are washed out [22] in sequential batch reactors. The separation of solid–liquid phases takes place in the same reactor; hence, secondary clarification after biological treatment is not required. All these Water 2021, 13, x FOR PEER REVIEW 3 of 21 Water 2021, 13, 1792 3 of 20 same reactor; hence, secondary clarification after biological treatment is not required. All characteristicsthese characteristics reduce bothreduce the both surface the required surface forrequired implementation for implementation and the investment and the forin- thevestment build, asfor also the givenbuild, byas thealso vertical given by design the vertical of the technology. design of the technology. (c)(c) DenseDense structure structure DenseDense andand compactcompact granulesgranules areare selectedselected duedue toto theirtheir excellentexcellent settleability,settleability, asas men-men- tionedtioned in in the the previous previous paragraph. paragraph. However, However, this this structure structure is also is also important important for conferringfor confer- onering of one the of most the importantmost important characteristics characteristics and advantages and advantages of granular of granular sludge: sludge: the dense the layersdense promotelayers promote the existence the
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