Biological and Physical Selection for Continuous-flow Sludge Densification

Abstract
Biological and physical selectors used together for the accumulation and use of aerobic granules, and densified biological flocs (collectively referred to as AGS, herein) in continuously flowing WRRF (referred to as CF, herein) processes is a topic of leading-edge research and practice. This study focuses on the application and design guidelines of biological selectors to promote the growth of densified biological flocs and its interaction with physical selectors to provide selective pressure for the formation and retention of aerobic granular sludge in CF systems.
Introduction
Biological treatment for carbon and nutrient control is key for the sustainability and future of WRRF. Advancing biological technologies that allow for unlocking treatment capacity, while minimizing capital and operating costs and improving effluent quality is therefore critical. Processes that intensify CF WRRF through the controlled accumulations of AGS are a focus of research and practice. These processes intensify CF WRRF through biological and physical selection to promote the growth and retention of a biological-aggregate that seek to achieve process intensification via improvements to sludge settling velocity and SRT de-coupling. The key for the design and implementation of sludge densification can be based on the biological architecture concept, defined as form and function (or morphology and physiology). For this study the authors present three function levels of physiology/function desired for CF-AGS as presented in Figure 1. Methods Experimental and performance data presented in this study was collected from several full-scale WRRF for over two years to study biological and physical selection for CF-AGS.
Results and Discussion
Biological Selection Biological selection is defined as the accumulation of specific bacteria in aerobic granules, and biological flocs through bacteria feast and famine. Selector design and applications for bulking control in full-scale activated sludge plants have become common practice in recent years and are still worldwide the most applied engineering tool for prevention of bulking sludge in biological processes. The term biological selection is employed to describe processes where natural selection governs or limits the growth of undesirable bacterial species. The specific objective of biological selectors for process intensification is to provide the adequate feast and famine conditions to maximize the production of EPS for biological aggregation while preventing the excessive growth of filamentous organisms. Figure 2 (left) shows the impact of biological selector's F/M, as a condition to express feast, on densified biological flocs. The high F/M in the biological selector reduces diffusion-limited growth by promoting substrate penetration into the biological aggregate core, supporting the uptake of readily biodegradable substrate and creating desired microenvironments within the floc structure, which is critical to form densified biological flocs. The high feast conditions in the selector causes elevated production of EPS resulting in an increase floc diameter and density which are key to sludge densification and AGS formation.
Protein-based EPS predominantly comprise aerobic granules and are considered to be more adhesive than carbohydrate-based EPS. Therefore, selecting environmental conditions to promote the production of protein-rich EPS is expected to enhance biological-floc densification. As depicted in Figure 2 (right), the high F/M conditions resulted in higher concentration of protein-rich EPS compared to the concentration of carbohydrates in the EPS matrix resulting in lower SVI and larger floc diameter for densified biological flocs. A goal of biological selection is to promote storage-product accumulating heterotrophic bacteria growth, which seems to be the key to sludge densification and ultimately, AGS formation. These bacteria condense excess organic substrate as internal storage products and produces EPS. Storage-product accumulating heterotrophic bacteria have a competitive advantage over ordinary heterotrophic bacteria in WRRF processes that promote bacteria feast and famine. During feast conditions, storage-product accumulating heterotrophic bacteria consumes an organic substrate at a rate that is greater than that of their growth; hence, bacteria can condense excess organic substrate as internal storage products that can be hydrolyzed during famine conditions. Therefore, it can be argued that storage-product heterotrophic bacteria can used their stored carbon as electron donor for denitrification during low carbon conditions, which is a topic of interest for researchers and practitioners. Figure 3 presents specific denitrification rate results, with no external carbon addition, from mixed liquor samples at different biological selector F/M conditions expressing higher denitrification rates for the samples exposed to higher feast conditions indicating higher storage-product. PHA data at different biological selector F/M conditions will be provided with the full manuscript.
Physical Selection Physical selection occurs through devices such as rotary-drum screens, hydrocyclones and surface wasting devices that are, typically, incorporated with waste solids strategies. It is well understood that washing out smaller and lighter flocculent particles can aid to AGS formation. However, physical selectors to proper function as such, need the aid of biological selection for optimum process intensification results. Figure 4 (left) presents the impact of F/M on the particle size distribution for biological selection only and biological and physical selections. Two main points can be concluded from these figures: 1. F/M impacts the particles size distribution in systems with biological selection only and biological and physical selection. 2. Despite of the F/M, biological selection seems to be the key for densified biological floc formation; more so than physical selectors. Sieve analysis from a hydrocyclone, presented in Figure 4 (right), show the particle sizes selection in the underflow and overflow with greater the fraction of 200 μm particles being retained in the system via the underflow; resulting in lower SVI and better settleability. Figure 5 shows the hydrocyclones can allow for SRT de-coupling providing an ecological niche for slow-growing bacteria (e.g., storage-product accumulating heterotrophic bacteria and autotrophic nitrifying bacteria) where, as presented in Figure 4, a higher nitrifier (SNR) and phosphorus release and uptake rates (PRR and PUR) activity in the underflow (containing 70% granules) of the hydrocyclone compared to the waste activated sludge overflow (containing 35% granules) allowing clear opportunities to de-couple SRT for densified biological flocs and AGS.
Conclusions
Keys for biological architecture for CF-AGS can be summarized as:
- Granule Generation. Biological selection strategies including selector design, feast-famine conditions and SRT control.
- Granule Retention. Physical selectors, such as hydrocyclones and screens, for the controlled retentions of densified biological flocs and AGS in BNR systems.
- Granule Management. Management of densified sludge to increase biologically active fraction and reaction rates of BNR organisms.
The key to engineering CF-AGS is the control of the biological architecture and thereby build engineering approaches based on functional levels.