Residual municipal solid waste as co-substrate at wastewater treatment plants: An assessment of methane yield, dewatering potential and microbial diversity

Impact of impeller design on anaerobic digestion: Assessment of mixing dynamics, methane yield, microbial communities and digestate dewaterability

The efficiency of anaerobic digestion (AD) processes is intricately tied to mixing quality. This research investigates the influence of two impeller types, namely a helical ribbon impeller (HRI) and a pitched-blade impeller (PBI), on key aspects of AD. The investigation encompassed mixing dynamics, methane production, microbial communities, and the previously unexplored impact on digestate dewaterability. Results show that agitation with the PBI exhibited stratification, with bottom layer total solids (TS) values of 3.1% for the PBI and 2.6% for the HRI. Nevertheless, methane yield remained unchanged, averaging 286 LN/kg volatile solids (VS)added. Slower mixing with the HRI achieved more uniform mixing and reduced energy requirements. Additionally, impeller type significantly affected digestate dewaterability, leading to a 3.8% increase in TS of the dewatered sludge when using the PBI. These findings highlight the importance of considering mixing not only for methane production and reduced maintenance but also for achieving optimal digestate dewaterability.

Effect of FeCl3 concentration in chemically enhanced primary treatment on the performance of a conventional wastewater treatment plant. A case study

The effect of coagulant dosage in a chemically enhanced primary treatment (CEPT) on the performance of a conventional wastewater treatment plant (WWTP) has been investigated. Lab-scale experiments simulations were carried out in order to evaluate the effect of coagulant addition on the primary settling performance. In these experiments, FeCl3 was used as coagulant. Later, the WWTP was theoretically simulated using a commercial software (WEST®) to evaluate the effect of coagulation/flocculation on the global system, based on the results obtained at lab-scale. According to these results, the CEPT modifies the organic matter balance in the WWTP, decreasing the contribution of readily (SS) and slowly (XS) biodegradable fractions of COD to the aerobic biological process up to 27.3% and 80.8%, respectively, for a dosage of FeCl3 of 24 mg L-1. Consequently, total suspended solids in the aerobic reactor and the secondary purged sludge decreased up to 33% and 13%, respectively. However, the influence on effluent quality was negligible. On the contrary, suspended solids concentration in the sludge to be treated by anaerobic digestion increased, mainly regarding the Ss and Xs fractions, which caused an 8.1% increase in biogas production potential, with approximately 60% of CH4 concentration.

Supplemental Sewage Scum and Organic Municipal Solid Waste Addition to the Anaerobic Digestion of Thickened Waste Activated Sludge: Biomethane Potential and Microbiome Analysis

Sewage scum (SS) is collected from sedimentation tanks in wastewater treatment plants (WWTPs). Despite its huge biogas potential, there is limited information on its potential as a co-substrate and microbial ecology, especially during anaerobic co-digestion (ACo-D) of the organic fraction of municipal solid waste (OFMSW) and thickened waste activated sludge (TWAS). In this biomethane potential (BMP) study, the bioenergy yield achieved by the supplemental addition of SS and OFMSW to TWAS was investigated, along with the microbial ecology. Compared with the digestion of TWAS alone, which produced 184.6 mLCH4 gVS−1, biomethane yield was enhanced by as much as 32.4–121.6% in trinary mixtures with SS and OFMSW, mainly due to the positive synergistic effect. Furthermore, a mixture of 40%SS + 10%TWAS + 50%OFMSW produced the highest biogas yield of 407 mLCH4 gVS−1, which is proof that existing WWTPs can produce additional energy by incorporating external bioresources, thereby reducing greenhouse gas emissions. Modified Gompertz and logistic function estimates showed that methane production rate improved by as much as 60% in a trinary mixture compared with the digestion of TWAS alone. The genus Methanosaeta, capable of generating methane by the acetoclastic methanogenic pathway among all the archaeal communities, was the most prominent, followed by hydrogenotrophic methanogen Methanospirillum.

Enhancing methane production in anaerobic co-digestion of sewage sludge and food waste by regulating organic loading rate

This study presented mechanistic insights into the long-term effects of stepwise-increasing organic loading rates (OLRs) on anaerobic co-digestion (AcoD) of sewage sludge and food waste. The maximum methane (CH₄) yield of 500.0 ± 10.5 mL CH₄/g VS_fed was achieved at medium OLR of 3.5 g VS/L/d. This excellent performance was associated with the high hydrolysis efficiency (78.4%), three-fold enhancement in the acidogenesis enzyme activity, and 87.0% enhanced methanogen activity. Soluble intermediates (carbohydrates and proteins) were largely degraded (>98.5%), especially tyrosine-like and tryptophan-like aromatic proteins. The particulates were effectively decomposed from macromolecules to micromolecules, and the crystallinity of cellulosic substances decreased by 24.5%. The newly-shaped combined syntrophic acetate oxidation-hydrogenotrophic methanogenesis pathway dominated enhanced CH₄ production. Energy balance analysis based on medium OLR demonstrated the high energy recovery potential in full-scale AcoD. These findings suggest the optimal medium OLR can facilitate the bioconversion of organics to CH₄ through a new metabolic pathway.