Abstract
Aerobic granular sludge (AGS) comprises an aggregation of microbial cells in a tridimensional 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 fundamentally sequential batch reactors (SBRs) has been studied. However, granular sludge technology using SBRs has several disadvantages. For instance, it can present certain drawbacks for the treatment of high flow rates; furthermore, the quantity of retained biomass is limited by volume exchange. Therefore, the development of continuous flow reactors (CFRs) has come to be regarded as a more competitive option. This is why numerous investigations have been undertaken in recent years in search of different designs of CFR systems that would enable the effective treatment of urban and industrial wastewater, keeping the stability of granular biomass. However, despite these efforts, satisfactory results have yet to be achieved. Consequently, it remains necessary to carry out new technical approaches that would provide more effective and efficient AGS-CFR systems. In particular, it is imperative to develop continuous flow granular systems that can both retain granular biomass and efficiently treat wastewater, obviously with low construction, maintenance and exploitation cost. 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 the advantages and disadvantages of these proposals and suggest future trends in the application of aerobic granular systems. Accordingly, we analyze the most significant technical and biological implications of this innovative technology.
Highlights
Aerobic granular sludge (AGS) technology is today often regarded as a promising option for biological wastewater treatment because it has demonstrated excellent physicochemical performance at the pilot scale; even the AGS full-scale has gained an optimum position in the area of treatment of urban and industrial sewage [1]
Aerobic granular sludge technology remains challenging to implement for treating sewage water, several authors reporting the instability of granular biomass for long-term operations [5,6]
Long et al [55] reported a continuous flow reactors (CFRs) system with a double column cyclic aerobic granular reactor (DCCAGR), which consisted in two equal bioreactors composed by a column and a settling tank connected by an inclined tube (Figure 5)
Summary
Aerobic granular sludge (AGS) technology is today often regarded as a promising option for biological wastewater treatment because it has demonstrated excellent physicochemical performance at the pilot scale; even the AGS full-scale has gained an optimum position in the area of treatment of urban and industrial sewage [1]. 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. Due to the high specific surface and biomass retention capacity of granules, the technology can handle high organic loading ratios in comparison with conventional activated sludge [27]. AGS has a high biomass retention capacity, making it possible to treat large amounts of inflow in a smaller bioreactor relative to other technologies such as CAS because, as mentioned before, the surface in contact with raw water is greater [11]. Redox conditions and diffusional resistance towards external molecules and high tolerance to toxicity make granules different from the flocs found in CAS or membrane bioreactors (MBRs) [31]
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