Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste in Plug-Flow Reactors: Focus on Bacterial Community Metabolic Pathways

High-solid digestion - A comparison of completely stirred and plug-flow reactor systems

High-solid digestion (HSD) for biogas production is a resource-efficient and sustainable method to treat organic wastes with high total solids content and obtain renewable energy and an organic fertiliser, using a lower dilution rate than in the more common wet digestion process. This study examined the effect of reactor type on the performance of an HSD process, comparing plug-flow (PFR) type reactors developed for continuous HSD processes, and completely stirred-tank reactors (CSTRs) commonly used for wet digestion. The HSD process was operated in thermophilic conditions (52 °C), with a mixture of household waste, garden waste and agricultural residues (total solids content 27-28 %). The PFRs showed slightly better performance, with higher specific methane production and nitrogen mineralisation than the CSTRs, while the reduction of volatile solids was the same in both reactor types. Results from 16S rRNA gene sequencing showed a significant difference in the microbial population, potentially related to large differences in stirring speed between the reactor types (1 rpm in PFRs and 70-150 rpm in CSTRs, respectively). The bacterial community was dominated by the genus Defluviitoga in the PFRs and order MBA03 in the CSTRs. For the archaeal community, there was a predominance of the genus Methanoculleus in the PFRs, and of the genera Methanosarcina and Methanothermobacter in the CSTRs. Despite these shifts in microbiology, the results showed that stable digestion of substrates with high total solids content can be achieved in both reactor types, indicating flexibility in the choice of technique for HSD processes.

Spatial monitoring of hydrolysis in a plug-flow bioreactor: a support for flexible operation?

Hydrolysis at changing hydraulic retention time, recirculation, bedding straw content in the feed, bioaugmentation and the impact of those changes on gradient formation in the liquid phase in plug-flow reactors (PFRs) was examined. The pH-value, conductivity and oxidation-reduction potential (ORP) were monitored at three spots along the PFRs to study potential correlations to process performance during a total process time of 123 weeks. The on-line monitoring showed good correlations to acidogenesis: namely, the pH and ORP to the acidification, to butyric (and lactic) acid concentration and to the acid yield. The ORP (measured at the inlet) showed the most stable correlation to acidogenesis under dynamic operation, while the conductivity (at the outlet) correlated to the acid concentration in dependence on the feedstock. Multiple measurement spots as used in this study allow to gain more information about acidogenic fermentation than a single spot, simplifying process control and automation attempts with recalcitrant feedstock.

Volatile fatty acids bio-production using extracellular polymeric substances disengaged from sludge for carbon source recycling

Excessive waste-activated sludge (WAS) and insufficient carbon source (CS) for biological nitrogen removal (BNR) often coexist in municipal sewage treatment. Although the production of volatile fatty acids (VFAs) from WAS has been recognized as a promising solution, the development is limited by low VFAs production efficiency and dewatering deterioration of sludge. This study extracted the extracellular polymeric substances (EPS) from sludge by low-temperature thermal-hydrolysis (LTH) and high-speed hydro-cyclone (HSHC) pretreatment and recovered it for high-quality VFAs bio-production in thermophilic fermentation. Microbial mechanism analysis disclosed that interspecific interaction networks composed of functional flora, which accumulate VFAs by bio-converting EPS primarily and supplemented by EPS synthesis, guaranteed the efficient bio-production of VFAs. This process scheme shows promise in providing alternative denitrification CSs and avoiding deterioration of sludge dewaterability.

Dry Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Biogas Production Optimization by Reducing Ammonia Inhibition

The aim of this work is to optimize biogas production from thermophilic dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) by comparing various operational strategies to reduce ammonia inhibition. A pilot-scale plug flow reactor (PFR) operated semi-continuously for 170 days. Three scenarios with different feedstock, namely solely OFMSW, OFMSW supplemented with structural material, and OFMSW altered to have an optimal carbon-to-nitrogen (C/N) ratio, were tested. Specific biogas production (SGP), specific methane production (SMP), the biogas production rate (GPR), and bioenergy recovery were evaluated to assess the process performance. In addition, process stability was monitored to highlight process problems, and digestate was characterized for utilization as fertilizer. The OFMSW and the structural material revealed an unbalanced content of C and N. The ammonia concentration decreased when the optimal C/N ratio was tested and was reduced by 72% if compared with feeding solely OFMSW. In such conditions, optimal biogas production was obtained, operating with an organic loading rate (OLR) equal to 12.7 gVS/(L d). In particular, the SGP result was 361.27 ± 30.52 NLbiogas/kgVS, the GPR was 5.11 NLbiogas/(Lr d), and the potential energy recovery was 8.21 ± 0.9 MJ/kgVS. Nevertheless, the digestate showed an accumulation of heavy metals and low aerobic stability.

Multilinear Regression Model for Biogas Production Prediction from Dry Anaerobic Digestion of OFMSW

The aim of this study was to develop a multiple linear regression (MLR) model to predict the specific methane production (SMP) from dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW). A data set from an experimental test on a pilot-scale plug-flow reactor (PFR) including 332 observations was used to build the model. Pearson′s correlation matrix and principal component analysis (PCA) examined the relationships between variables. Six parameters, namely total volatile solid (TVSin), organic loading rate (OLR), hydraulic retention time (HRT), C/N ratio, lignin content and total volatile fatty acids (VFAs), had a significant correlation with SMP. Based on these outcomes, a simple and three multiple linear regression models (MLRs) were developed and validated. The simple linear regression model did not properly describe the data (R2 = 0.3). In turn, the MLR including all factors showed the optimal fitting ability (R2 = 0.91). Finally, the MLR including four uncorrelated explanatory variables of feedstock characteristics and operating parameters (e.g., TVSin, OLR, C/N ratio, and lignin content), resulted in the best compromise in terms of number of explanatory variables, model fitting and predictive ability (R2 = 0.87).