Elsevier

Bioresource Technology

Volume 267, November 2018, Pages 778-781
Bioresource Technology

Short Communication
Novel system configuration with activated sludge like-geometry to develop aerobic granular biomass under continuous flow

https://doi.org/10.1016/j.biortech.2018.07.146Get rights and content

Highlights

  • Granular biomass was obtained in a system composed by two CSRTs in series.

  • This configuration could be easily implemented in current activated sludge systems.

  • The tank 1/tank 2 vol ratio is determined by the feast/famine length ratio required.

  • Equivalent cycle time is controlled by the biomass recirculation rate.

Abstract

A novel continuous flow system with “flat geometry” composed by two completely mixed aerobic tanks in series and a settler was used to promote the formation of aerobic granular sludge. Making similarities of this system with a typical sequencing batch reactor (SBR), for aerobic granules cultivation, the value of the tank 1/tank 2 vol ratio and the biomass recirculation rate would correspond with the feast/famine length ratio and the length of the operational cycle, respectively, while the settler upflow liquid velocity imposed would be related to the settling time. From the three experiments performed the best results were obtained when the tank 1/tank 2 vol ratio was of 0.28, the sludge recycling ratio of 0.25 and the settler upflow velocity of 2.5 m/h. At these conditions the aggregates had settling velocities between 29 and 113 m/h, sludge volume index at 10 min (SVI10) of 70 mL/g TSS and diameters between 1.0 and 5.0 mm.

Introduction

In the last years, research focused on obtaining stable aerobic granular sludge under continuous flow conditions is gaining attention. This is due to the fact that existing wastewater treatment plants operate normally in continuous mode and as a consequence upgrades based on aerobic granular biomass are easily applicable if they are developed in continuous systems too (Sarma and Tay, 2018). Most of the attempts carried out to develop granular biomass or maintain its stability under a continuous flow regime were done using airlift reactors or columns type reactors (Kent et al., 2018). At laboratory scale these reactors are characterized by a height to diameter ratio (H/D) greater than 6–8, which helps to maximize the hydraulic shear forces. Nevertheless, until now, only few studies have reported on the achievement of aerobic granulation using systems with “flat geometries” (H/D ≤ 1), similar to those of conventional activated sludge systems (Morales et al., 2012, Devlin and Oleszkiewicz, 2018).

Already, “accidental granulation” has been reported in continuous flow activated sludge systems but the operational conditions which promote granulation phenomena are still unknown (Bruce et al., 2014). As a first step to understand accidental granulation, a recent survey was conducted in North America, to help to identify full-scale facilities achieving very low sludge volume index (SVI) values, which may indicate the presence of granular biomass or the potential for granulation (Martin et al., 2016).

An interesting system to maintain granular sludge with continuous flow was proposed by Li et al. (2016). These authors used a two reactors configuration (anaerobic/aerobic) to keep granular sludge for biological phosphorus removal. However, they used as inoculum already formed granules and the granulation process was not studied from the beginning. Furthermore, as these authors stated, the granules stability for biological phosphorus removal is easier to maintain than for the traditional aerobic granular sludge comprising organic matter removal.

Based on these observations, the aim of the present research work is to define a novel reactor configuration with two serial tanks to promote aerobic granulation, from conventional activated sludge, using a continuous flow system with “flat geometry” for organic matter removal.

Section snippets

System description and operational conditions

In order to obtain the formation of aerobic granular sludge a continuous flow system composed of two completely stirred tanks in series, with an overall height/width/length ratio of 1/0.8/1, was used (Fig. 1). The purpose of the first tank (tank 1, useful volume of 0.6 L) was to allow the contact between biomass and substrate during a short period of time to promote its conversion into intracellular storage compounds (feast period) while, in the second tank (tank 2), substrate concentration

Configuration change from anoxic/aerobic (Stage I) to aerobic/aerobic (Stage II)

During Stages I and II the tank 1/tank 2 vol ratio was of 0.15 and the upflow velocity imposed in the settler of 1.4 m/h. In the first stage the tank 1 was operated under anoxic conditions and although filamentous biomass and small flocs were predominant, on day 56, some granule-like particles appeared. As the organic matter removal efficiency in tank 1 was low (around 36%) and the most part was removed in the tank 2, the feast/famine regime being not fulfilled.

Therefore, in Stage II the tank 1

Conclusions

It is possible to form aerobic aggregates (SVI10 of 70 mL/g TSS and settling velocities of 29–113 m/h), for organic matter removal in continuous flow systems, operating under hydrodynamic conditions that simulate those obtained in a discontinuous reactor. The novel configuration of the proposed system is very similar to the “flat geometry” of the activated sludge systems usually used, so it would be relatively easy to convert them into aerobic granular systems.

Pictures of biomass morphology

Acknowledgements

This work was funded by the Chilean Government through the project FONDECYT 1180650, CORFO/14ENI2-26865 and CONICYT/FONDAP/15130015 and by the Spanish Government through TREASURE (CTQ2017-83225-C2-1-R) and GRANDSEA (CTM2014-55397-JIN) projects. J.L. Campos belongs to Center UAI Earth and the authors from the USC belong to CRETUS (AGRUP2015/02) and the Galician Competitive Research Group (GRC ED431C 2017/29). All these programs are co-funded by FEDER, Spain.

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