Process intensification of anaerobic digestion: Influence on mixing and process performance
Introduction
Wastewater utilities are amongst the largest users of anaerobic digestion, as it is a dominant wastewater sludge management practice. The process operates by periodically decanting sludge produced from wastewater treatment into a digester containing a slurry of anaerobic microbes and previously degraded sludge. The microbes degrade the organic fraction of the incoming sludge to produce stabilized biosolids that can safely be disposed of. Additionally, the process produces biogas, which is rich in methane and carbon dioxide, the former being valuable as a renewable energy source (Tchobanoglous et al., 2003, WEF and ASCE, 2010, WEF, 2012). Anaerobic digestion has been used for an appreciable time and is largely well understood (Appels et al., 2008). The major limitation of anaerobic digestion is the low growth rate of the methane forming microbes (Gerardi, 2003). Also, a minimum solids retention time of 10+ days is required to ensure stable digester operation (Appels et al., 2008). However, it is not uncommon to find 15+ days of solids retention time being used as prescribed by regulatory authorities.
The numerous responsibilities held by water utilities limit the opportunity and frequency of large capital expenditure projects for increasing supply and treatment capacity. This limitation has seen them increasingly turn to process intensification, which may be defined in this context as ‘processing more with less’, i.e., making better use of the existing infrastructure. As a consequence of the long retention times required, most process intensification measures designed for capacity expansion focus on increasing the solids concentration within the digester rather than increasing the process reaction rate. This is commonly achieved through increased dewatering of the incoming sludge or decoupling the solids and hydraulic retention time in a process known as recuperative thickening (Batstone et al., 2015). However, increasing the solids concentration changes the rheological properties of the sludge and makes it behave as a non-Newtonian fluid for solids concentration above 2.5% w/w when a hypothesized yield stress develops (Eshtiaghi et al., 2013). The changing rheology presents a significant issue for digester mixing, as it increases the resistance to flow thereby changing the sludge flow patterns and potentially promoting the growth of stagnant or “dead” regions. Karim et al. (2005) also reported that the influence of mixing became more apparent at higher solids loadings. Although the need for implementing new approaches to mixing thickened sludge has been recognized for a while, wastewater industry was extremely cautious and had displayed a strong desire not to be the first in adopting new technologies and processes (Speight, 2015).
Digester mixing is important for ensuring process effectiveness (Kariyama et al., 2018, Lindmark et al., 2014b), but it remains poorly understood, with a definition of adequate digester mixing yet to be found (WEF & ASCE, 2010). A review of mixing specific power input (P/V) used in various studies demonstrates the lack of standardization between experimental work (Table 1), which has limited the detailed understanding of how mixing influences the digestion process (Lindmark et al., 2014b). The outcome is that there is a significant challenge on how to investigate the influence of mixing power input on digester performance and effectively translate the results between experimental work and the full-scale digester operation.
This work aims to study how anaerobic digestion performance responds to increases in sludge total solids concentrations at various mixing specific power inputs. The intent is that the lessons learnt from this work will help to focus the scope of future research or provide a starting point for digester optimization. As such, this work used three quantitative specific mixing power inputs indicative of those used in existing wastewater treatment and two solids concentrations of feed sludge in a batch digester, whereby the sludge samples were initially fed and monitored over 21 days to see how the process unfolded.
Section snippets
Inoculum and substrate
Both the inoculum (I), digester sludge, and substrate (S), primary sludge (PS), were sourced from Melbourne Water’s Eastern Treatment Plant in Bangholme, Victoria, Australia. The plant treats 330 ML/day of wastewater mainly from municipal sources with a smaller proportion from industrial sources. The sewage entering the plant undergoes grit removal before primary sedimentation that is followed by a traditional activated sludge process for nitrogen removal. The sludge from primary sedimentation
Biogas generation
Results shown in Fig. 1 depict the typical gas production data for all the conditions tested. Based on the gas production trends, the experimental duration is broken down into four key periods, delineated as P1, P2, P3, and P4 (Fig. 1B) as described by McLeod et al. (2018).
Briefly, the first period (P1) was characterized by the consumption of readily biodegradable substances in the substrate. The biogas production rate increased sharply to a high value before falling to a minimum of 23.10
Limitations
Simplifying the design and operation of anaerobic digester mixing to a single variable has produced a few limitations that are worth noting. This relates, but is not restricted, to items such as variable feedstock rheology, experimental mode, scale, and duration as well as digester operation. Markis et al. (2016) demonstrated that even within the same treatment plant sewage sludge has varying rheology between different sources (primary, secondary, and digester). It is therefore expected that
Conclusion
Wastewater utilities are increasingly turning to process intensification to increase capacity, which given the nature of the work, requires demonstrated outcomes before implementation. This work aimed to address the limited understanding regarding the effect of varying solids concentration has on mixing power requirements for maintaining adequate digester mixing. In answering this question, the limitations of (P/V) (W/m3) as a mixing criterion were further demonstrated, and an alternative
Acknowledgments
The authors would like to express their sincere gratitude to Eddy Santos and Nick Zisis for their help with facilitating the collection of the substrate and inoculum for this work. Additionally, without the assistance of the technical staff at RMIT, Cameron Crombie and Sandro Longano, this work would not have been possible.
Declarations of interest: None. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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