Biological process architecture in continuous‐flow activated sludge by gravimetry: Controlling densified biomass form and function in a hybrid granule–floc process at Dijon WRRF, France

Abstract

Full-scale demonstration of activated sludge conversion into a granule-floc hybrid process was implemented in Dijon (France) water resource recovery facility (WRRF). Biomass densification was achieved based on external gravimetric selection using hydrocyclones within continuous-flow anaerobicanoxic-oxic (A 2 O) biological nutrient removal (BNR) bioreactor. The goal was to optimize settleability of biological sludge by lowering and stabilizing sludge volume index (SVI) to improve process robustness and resiliency. Process proved to stabilize operation and to uncouple the total solids residence time (SRT) between floc and granule morphologies. The densified biomass initially produced stable SVI < 100 ml/g for a period of 4 months and thereafter a steady state year-round SVI below 50 ml/g, including the winter period during which the control train SVI expansion >200 ml/g. The densified biomass successfully broke the vicious cycle of interannual bulking. Form and function interrelationship is proposed for the densified biomass (hybrid floc-granule).The concept of biological architecture is proposed as the purposeful control of granule and floc proportions, with a proposed "form factor" ratio of 1:2 granule to floc, that produce a "SRT uncoupling function factor" ratio of 4:1 granule to floc, further resulting in very stable settling and effluent functionalities. Practitioner Points• Controlling granule-floc proportions allows for sludge volume index (SVI) operational adjustment, which further allows for increased clarified design accuracy.• One-third aggregates dramatically improved settling characteristics: 20% and 35% of AGS ensures SVIs below 100 and 50 ml/g, respectively.Sudhir Murthy is a WEF member/fellow. Bernhard Wett is a WEF member.

Coupling physical selection with biological selection for the startup of a pilot-scale, continuous flow, aerobic granular sludge reactor without treatment interruption

“…If this more conventionally applied bioflocs-to-AGS transition setup were applied, the external physical selector would be the only driving force to transform flocculent sludge to AGS, and thus a much more radical biofloc washout type of short SRT would have to be employed ( Sun et al., 2019 ). Even with the sidestream hydrocyclone installation, only 5–7% of the RAS can be subjective to physical selection ( Avila et al., 2021 ; Partin, 2019 ; Regmi et al., 2022 ), leaving majority of the bioflocs and AGS unseparated in the RAS returned to the feast zone where bioflocs gain more growth advantage over AGS, which explains the slow startup period, e.g., one year, needed to observe sludge settleability improvement ( Ford et al., 2016 ; Kent et al., 2018 ; Roche et al., 2022 ; Welling et al., 2015 ; Wett et al., 2015 ).…”

“…This low strength selection pressure plus the high suspended solids content in RAS, e.g., 5000 mg L − 1 , compromised the physical selection efficiency of hydrocyclones for AGS ( Ford et al., 2016 ). As a result, a prolonged startup phase perhaps as long as a year was required to observe significant sludge settleability improvements in full-scale continuous flow reactors in which AGS formation is still a challenge ( Ford et al., 2016 ; Kent et al., 2018 ; Roche et al., 2022 ; Welling et al., 2015 ; Wett et al., 2015 ). For this reason, an alternative strategy should be developed to provide sufficient selection strength to promote successful continuous flow AGS formation while maintaining the startup treatment performance required for large full-scale applications.…”

Densification des boues activées par hydrocyclones – impact de la granulation partielle sur les performances de traitement

“…Le suivi de cette file, en parallèle avec une file témoin, a confirmé la réduction et le maintien d'un indice de boues très faible, y compris pendant les périodes de foisonnement filamenteux (ROCHE et al, 2020). L'indice de boues mesuré en diluant les boues à 2 g/L est directement corrélé au pourcentage de boues de taille supérieure à 200 µm (ROCHE et al, 2022).…”

“…Cet article complète les résultats exposés dans ROCHE et al (2022) en analysant finement le fonctionnement du dispositif et son évolution dans le temps. L'augmentation de la taille des particules est décrite, de même que la comparaison des distributions de taille des flocs obtenues par granulométrie.…”

“…Parmi eux, l'installation d'hydrocyclones sur la boucle d'extraction des boues a montré son aptitude à stabiliser des indices de boues à des valeurs très faibles (REGMI et al, 2019;WILLOUGHBY et al, 2020;AVILA et al, 2021;ROCHE et al, 2022). Ce procédé consiste à imposer une pression de sélection gravimétrique pour retenir dans le système les flocs et granules les plus denses.…”