Semi-continuous Anaerobic Digestion Research Articles

Semi-continuous anaerobic digestion of the organic fraction of municipal solid waste: digester performance and microbial population dynamics

Anaerobic digestion is an attractive approach for the management of organic fraction of municipal solid waste (OFMSW) and for the recovery of energy from this waste. A semi-continuous digestion of OFMSW was conducted on stirred reactor under mesophilic condition. Three organic loading rates (OLRs) were tested throughout the anaerobic process (1.36, 2.5 and 3.5 g VS/L.d). The achieved results showed that biogas yield decreased with the increase of OLR. Thus, the highest average of biogas yield, 0.51 L/g VS, was recorded at low OLR (OLR1:1.36 gVS/L.d). Likewise, optimal VS removal, pH, VFA and alkalinity were obtained at OLR1 compared to the other tested OLR. During the anaerobic digestion, the dynamic of microbial community was also assessed by the high-throughput sequencing technology. Firmicutes, Bacteroidetes and Proteobacteria were the dominant phyla during the digestion process. This could be related to their roles in hydrolysis of different OFMSW compounds, as well as in acidogenesis and acetogenesis steps. Regarding Archaeal population, the relative abundance of the Methanoculleus genus declined, while Methanosarcina increased over time. This indicated that acetotrophic and hydrogenotrophic pathways were used for methane production.

Novel anaerobic digestion of waste activated sludge via isoelectric-point pretreatment: Ultra-short solids retention time and high methane yield

The prolonged reaction period and low methane yield have become a pivotal bottleneck in the anaerobic digestion (AD) of waste activated sludge (WAS), severely limiting its use in bioenergy. This study evaluated the long-term semi-continuous AD of WAS with isoelectric-point (pI) pretreatment in terms of the reaction period, methane yield, material resource recovery, and economic and environmental benefits. The experimental results show that after pI pretreatment, at the same solids retention time (SRT), the average methane yield and methane content in the biogas of WAS increased by 120.9% and 15.5%, respectively. The average methane yield from AD with pI pretreatment at a 5-day SRT was 1.6 times that of the AD without pI pretreatment at a 15-day SRT, indicating that pI pretreatment significantly (P < 0.001) increased the methane yield from the AD of WAS at different SRTs, even at an ultra-short SRT. Statistical analyses of the changes in the concentrations of soluble organic matter in the AD process with and without pI pretreatment confirmed that pI pretreatment enhanced the biochemical reactions related to the transformation of sludge organic matter during the whole AD process, but not the initial organic solubilisation and hydrolysis of the sludge. Furthermore, pI pretreatment recovered the NaHCO3, Na2S, and multivalent metals from the sludge, indicating that AD with pI pretreatment efficiently recovered both bioenergy and material resources from the sludge. Economic and environmental analyses of AD with and without pI pretreatment further indicated that with pI pretreatment, the annual economic benefit of AD increases by 162% and carbon emissions decreases by approximately 2.63 × 107 kg CO2/year compared with conventional AD. These findings serve as an important reference for the development of a novel AD technology for sludge that incorporates second-generation pretreatment and operates at an ultra-short SRT.

Effect of nitrite on hydrolysis-acidification, biogas production and microbial community in semi-continuous two-phase anaerobic digestion of sewage sludge

Previous study found that the pre-treatment of sewage sludge with nitrite improves the biogas production during the mono/two-phase anaerobic digestion (AD) using batch biochemical methane potential tests. In this study, the effects of nitrite on hydrolysis-acidification, biogas production, volatile solids destruction and microbial composition in semi-continuous two-phase AD of sewage sludge were investigated. The addition of nitrite promotes sludge organic matter solubilization (+484%) and VFAs production (+98.9%), and causes an increase in the VS degradation rate during the AD process (+8.7%). The comparison of biogas production from the acidogenic and methanogenic reactors with or without the addition of nitrite implies that the nitrite has no significant effect on the overall biogas production of two-phase sludge AD process. High-throughput sequencing analysis shows that the microbial communities of bacteria and archaea in two-phase AD reactors significantly changes after the addition of nitrite. Vulcanibacillus (bacteria) and Candidatus Methanofastidiosum (archaea) become the dominant genera in the acidogenic and methanogenic reactors with the nitrite respectively. These findings provide new insights about using nitrite to promote the organic matter degradation of sewage sludge in a semi-continuous two-phase AD system.

Biomethane enhancement using reduced graphene oxide in anaerobic digestion of municipal solid waste

The present research investigated the impact of reduced graphene oxide (rGO) addition on the semi-continuous anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in the range of 0.5–10 gVolatileSolid(VS)/Lreactorday organic loading rates (OLR). Adding rGO enhanced the rate and yield of biomethane production, and the maximum biomethane increment rate was obtained as 110% at an OLR of 4.0 gVS/Lreactorday. However, after increasing the OLR to 6 gVS/Lreactorday, there was a dramatic decrease in biomethane production because of volatile fatty acid (VFA) accumulation. Methanotrix is the predominant archaeal genus at OLRs lower than 6 gVS/Lreactorday in reactors (89–97%). An increment in biomethane production was associated with the higher abundance of the Methanothrix genus in the rGO-supported reactor (rG) than in the control reactor (rC).

Bacterial dynamics during the anaerobic digestion of toxic citrus fruit waste and semi-continues volatile fatty acids production in membrane bioreactors

Citrus wastes (CW) are normally toxic to anaerobic digestion (AD) because of flavors such as D-limonene. In this study, bacterial community was evaluated during volatile fatty acids (VFAs) production from CW inoculated by sludge in a membrane bioreactor (MBR) using semi-continuous AD with different organic loading rates (OLR). Four treatments including untreated CW filled with 4 and 8 g·VS·L−1d−1 OLR (UOLR4 and UOLR8), pretreated D-limonene-free CW filled with 4 and 8 g·VS·L−1d−1 OLR (POLR4 and POLR8). The initial inoculum and the CW mixture (DAY0) was used as control for comparison. There was an obviously higher bacterial diversity in raw material (66848 sequences in DAY0), while decreased after AD and higher in POLR4 and POLR8 (65239 and 63916) than UOLR4 and UOLR8 (49158 and 51936). The key bacterial associated with VFAs production mainly affiliated to Firmicutes (37.35–84.73%), Bacteroidetes (0.48–36.87%), and Actinobacteria (0.35–29.38%), and the key genus composed of Lactobacillus, Prevotella, Bacillus, Bacteroides and Olsenella which contributed in VFA generation by degradable complex organic compounds. Noticeably, methanogen completely suppressed after MBR-AD and UOLR4 has greater acid utilizing bacteria (70.09%).

Cation exchange resin pretreatment enhancing methane production from anaerobic digestion of waste activated sludge

The application of anaerobic digestion (AD) to treat waste activated sludge (WAS) still exhibits some limitations, such as low methane production. In this study, cation exchange resin (CER) pretreatment was explored to enhance the efficiency of the AD of WAS. Based on the response surface methodology, the optimal conditions for CER pretreatment were reaction time of 7.4 h, 33.8 g CER (wet weight) /g volatile solids and sludge total solids of 2.4%. Under these optimal CER pretreatment conditions, approximately 30% of metals were removed from the WAS, particularly organic-binding metals. This metal removal disrupted the structures of extracellular polymer substances and led to sludge deflocculation, thereby releasing large amounts of organic substances from the sludge solids. Batch AD experiments showed that CER pretreatment increased the maximal production of volatile fatty acids and methane by 565.7% and 80.5%, respectively. Additionally, CER pretreatment promoted each stage of AD (i.e. solubilisation, hydrolysis, acidification and methanation) and the corresponding activities of key enzymes. Experimental results for semi-continuous AD further confirmed that CER pretreatment enhanced the proportion of methane in the biogas (from 62.75 ± 2.14% to 73.96 ± 0.99%) and the production of methane. An analysis of changes in the microbial communities demonstrated that CER pretreatment enhanced the abundance of microorganisms involved in hydrolysis, acidification and acetification and changed the major methanogenic pathway from acetoclastic methanogens to methylotrophic methanogens. These findings are expected to provide a reference for developing new pretreatment methods for enhancing anaerobic biodegradability of organic matters.

Semi-Continuous Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Digester Performance and Microbial Population Dynamics

Semi-Continuous Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Digester Performance and Microbial Population Dynamics

Chronic Effects of Benzalkonium Chlorides on Short Chain Fatty Acids and Methane Production in Semi-Continuous Anaerobic Digestion of Waste Activated Sludge

Chronic Effects of Benzalkonium Chlorides on Short Chain Fatty Acids and Methane Production in Semi-Continuous Anaerobic Digestion of Waste Activated Sludge

Bioaugmentation with a propionate-degrading methanogenic culture to improve methane production from chicken manure

Volatile fatty acid (VFA) accumulation caused by high ammonia concentrations is often encountered during the anaerobic digestion (AD) of ammonia-rich substrates. In this study, propionate-degrading methanogenic cultures were introduced to augment the semi-continuous AD of chicken manure under high ammonia levels. Introduction of a methanogenic culture enhanced the methane yield in the bioaugmented digester by 17–26% when the organic loading rate (OLR) was 2–4 g L-1d-1 compared to that in the control. When the OLR was further increased from 4.0 L-1d-1 to 5.0 g L-1d-1, and bioaugmentation ceased, methane yield improved by 15–18% under a high total ammonia nitrogen level of 5.0–8.4 g NH4+-N/L. Moreover, bioaugmentation reconstructed the methanogenic community in the digester, promoting the dominance of hydrogenotrophic Methanobacterium and slightly increasing the abundance of aceticlastic Methanothrix and the syntrophic propionate-oxidizing bacteria Syntrophobacter, which were the key contributors to the improved AD under high ammonia concentrations.

Hydrothermal Pretreatment of Rice Straw with Alkaline Addition for Enhancing Biogas Production in Semicontinuous Anaerobic Digester

This study presents the results of hydrothermal pretreatment of rice straw on its ability to improve the biogas production process in anaerobic digesters. Hydrothermal treatment on rice straw biomass was carried out with the addition of 0%, 3% and 5% NaOH (w/w rice straw) for one hour at a temperature of 140 °C. This study showed that hydrothermal and alkaline hydrothermal pretreatments were able to increase organic degradation of rice straw as indicated by an increase in the dissolution of lignin and hemicellulose from rice straw. Temperature and NaOH worked synergistically to dissolve lignocellulose in the hydrothermal pretreatment process. In the semicontinuous digester fed with pretreated rice straw, NaOH content in the pretreatment stage was found to give significant effect in enhancing biogas production. Average daily biogas production by the untreated rice straw, hydrothermal pretreated without NaOH addition, hydrothermally treated rice straw with 3% NaOH and 5% NaOH was 23.9, 57.1, 95.8 and 108.8 L/kg rice straw, respectively.

Enhancing anaerobic digestion using free nitrous acid: Identifying the optimal pre-treatment condition in continuous operation

Free Nitrous Acid (FNA) pre-treatment is a promising technology demonstrated effective in improving waste activated sludge degradability and anaerobic digestion (AD) performance. Pre-treatment conditions including FNA concentration and treatment duration determine operational and capital cost of full-scale implementation, which have not been studied in long-term experiments. The knowledge of FNA pre-treatment conditions improving the AD performance is urgently required to determine suitable conditions for the technology implementation. In this work, five different FNA concentrations (2.2, 4.4, 7.2, 12 mgN/L and nitrite only without pH adjustment) and three treatment durations (8, 24 and 48 h) were studied in four lab-scale semi-continuous AD reactors for over 300 days. FNA pre-treatment was shown under all tested conditions effective in enhancing AD performances, while its effectiveness and resulted benefits varied substantially amongst different pre-treatment conditions. The long-term experiment demonstrated that the methane production, sludge reduction and digested sludge viscosity of AD are positively correlated with the FNA concentration and durations, until an optimal condition is reached, which was identified in this work to be FNA concentration of 7.2 mgN/L and treatment duration of 24 h. Microbial community changes supported the apparent observation of enhanced sludge degradation at elevating FNA concentrations applied during pre-treatment. The short-term sludge solubilization results were inconsistent with the long-term AD performance, which was potentially caused by inhibitions from stringent FNA pre-treatment conditions applied (FNA = 12 mgN/L with 24-hour treatment & FNA = 7.2 mgN/L with 48-hour treatment). Overall, results suggested FNA pre-treatment at the optimized condition is highly beneficial to WWTPs and competitive with other pre-treatment technologies, e.g., thermal hydrolysis pre-treatment. This work comprehensively evaluated the key design parameters of FNA pre-treatment process, reached a major milestone in the development and applications of FNA technologies.

Improved energy balance at a municipal wastewater treatment plant through waste activated sludge low-temperature alkaline pretreatment

Sludge management has a significant impact on operating costs in municipal wastewater treatment plants (WWTPs). The current trend is to optimize the sludge management in order to enhance the energy independence from external energy suppliers, while reducing the operating costs of the WWTP. One of the methods is through the pretreatment of waste activated sludge (WAS). In this work, low-temperature alkaline pretreatment of WAS was examined as a method of improving the economic balance of the WWTP. In the initial pretreatment tests, the impacts of NaOH dose, temperature and reaction time on COD release as well as the degree of sludge disintegration were examined. Semi-continuous anaerobic digestion (AD) trials were then performed for pretreated WAS in selected conditions with low NaOH dosage for which no pH adjustment was required (16 g of NaOH/kgTS of WAS, 60 °C, 60 min). The tests showed an increase in methane yield from 210.4 ± 10.9 mL CH4/gVS to 248.8 ± 13.1 mL CH4/gVS and 267.1 ± 14.3 mL CH4/gVS for HRT 15 d and HRT 21.5 d, respectively. Energy balance and economic calculations for a full-scale WWTP were conducted based on the obtained results. It was concluded that it would be possible to increase electricity production from methane originated from AD of pretreated WAS up to 27% as well as to reduce the amount of dewatered sludge up to 8.7%. Energy balance and cost-benefit analyses indicated a possibility to reduce the operating cost by over 300,000 EUR/year in the case of a 1,200,000 people-equivalent WWTP.

Experimental investigation and mathematical modelling of batch and semi-continuous anaerobic digestion of cellulose at high concentrations and long residence times

In the context of the anaerobic digestion of slowly biodegradable substrates for energy and chemicals production, this study investigated the anaerobic digestion of cellulose without any chemical pre-treatments using open (undefined) mixed microbial cultures. The anaerobic conversion of cellulose was investigated in extended-length (run length in the range 518–734 days) batch and semi-continuous runs (residence time 20–80 days), at high cellulose concentration (20–40 g L−1), at temperatures of 25 and 35 °C. The maximum cellulose removal was 77% in batch (after 412 days) and 60% (at 80 days residence time) in semi-continuous experiments. In semi-continuous experiments, cellulose removal increased as the residence time increased however the cellulose removal rate showed a maximum (0.17 g L−1 day−1) at residence time 40–60 days. Both cellulose removal and removal rate decreased when cellulose concentration in the feed was increased from 20 to 40 g L−1. Liquid-phase products (ethanol and short chain organic acids) were only observed under transient conditions but not at the steady state of semi-continuous runs. Most of the observed results were well described by a mathematical model which included cellulose hydrolysis and growth on the produced glucose. The model provided insight into the physical phenomena behind the observed results.

Semi-continuous anaerobic digestion of secondary sludge with free ammonia pretreatment: Focusing on volatile solids destruction, dewaterability, pathogen removal and its implications

Our previous work has reported the pretreatment of secondary sludge with free ammonia (NH3, FA) enhanced the methane production in batch biochemical methane potential tests. However, the batch biochemical methane potential test could only provide conservative results compared to continuous/semi-continuous anaerobic digestion. Also, the impacts of FA pretreatment on the key anaerobic digestion parameters, including volatile solids (VS) destruction, sludge dewaterability and pathogen removal, are still unknown. This study for the first time investigated these impacts using semi-continuous anaerobic digestion systems for 130 days. Pretreatment of secondary sludge for 24 h at an FA concentration of 560 mg NH3-N/L improved VS destruction by 26.4% (from 22.0 to 27.8%), supported by a similar increase of 28.6% in methane production (from 126.7 to 162.9 ml CH4/g VSfed). Model based analysis revealed that FA pretreatment improved the sludge degradability extent, which may be the reason for the enhanced VS destruction. Equally importantly, the dewaterability of the digested sludge with FA pretreatment was also enhanced by 9.2% (from 12.0 to 13.1% in solids content of the dewatered digested sludge), which could be partly attributed to the increased zeta potential from -16.7 to -14.5 mV. Anaerobic digestion with FA pretreatment enhanced the removals of Fecal Coliform and E. Coli by 1.3 and 1.4 log MPN/g TS (MPN: Most Probable Number; TS: Total Solids), indicating FA pretreatment was effective in enhancing pathogen removal. With inorganic solids representing 21% of the sludge used, the volume of dewatered sludge to be disposed of was reduced by 14.5% via FA pretreatment. This will substantially decrease the cost as evaluated by economic analysis. In brief, this study provides a promising strategy to enhance sludge reduction in anaerobic digestion and is of great significance in promoting the application of FA pretreatment strategy in the real world.

Semi-continuous anaerobic digestion of mixed wastewater sludge with biochar addition

This work analysed the effects of Biochar (BC) addition to the Anaerobic digestion (AD) of wastewater Mixed sludge (MS) in semi-continuous mode. A 3 L digester was operated at 37 °C for 100 days, feeding MS collected every three weeks in the same wastewater treatment plant, and 10 g L−1 of BC. The average performance of MS digestion (biogas 188 NmL d−1, 68% methane) improved in presence of BC (biogas 244 NmL d−1, 69% methane). According to the results of the multiple linear regression analysis performed on the experimental data, the 79% variation of the soluble COD in the MS was the driving factor for the 38% increase of biogas and methane yields. In conclusion, in the considered experimental conditions, the variability of the substrate’s composition was the key factor driving the performances of the AD of MS, independently of the addition of BC.

Specialized trace elements and volatile fatty acids interactions for enhanced methane production and biomethanization process stability during high organic loading rate

Volatile fatty acids (VFAs) and trace elements (TEs) interactions (VFAs*TEs) during biomethanization have effects that could be exploited to enhance anaerobic digestion (AD) of biomass. The goal of this study was to validate biocatalytic effects of specialized VFAs*TEs identified from a batch-derived Optimum TEs Configuration (or simply ‘Optimum’) on high organic loading rate (OLR) involving mixed fruit residue (MFR) fed in semi-continuous AD operation. The specialized VFAs*TEs were formulated as Variants of the Optimum and included Optimum –Cobalt (Co) for specialized VFAs*Co effects, and Optimum +Selenium (Se) for specialized VFAs*Se effects. Four duplicate AD reactors were treated with formulations reflecting the Optimum and the Variants. Each duplicate reactor was semi-continuously fed with MFR at varying OLR until instability occurred. Methane production, total volatile organic acidity (FOS) / total alkalinity (TAC) and VFAs fingerprints were measured as main responses. The results showed that reactors of the Optimum and its Variants were unstable at OLR of 8g oDM/L/d, but stability was restored in the Optimum –Co (FOS/TAC values of 0.6 compared to 1.51 and 1.67 for Optimum and Optimum +Se respectively). The average specific CH4 production (Nml/g oDM) of the Optimum and its Variants were Control: 431±36; Optimum: 553±16; Optimum –Co: 580±12; and Optimum +Se: 545±13. Optimum –Co also had the lowest acetic acid and butyric acid accumulation, but had higher propionic acid concentration (0.7 g/L) compared to the Optimum (0.3 g/L) and Optimum +Se (0.4 g/L).

Elucidating the key factors in semicontinuous anaerobic digestion of urban biowaste: The crucial role of sludge addition in process stability, microbial community enrichment and methane production

Anaerobic digestion, a virtuous process to reduce and reuse biowaste to obtain renewable bioenergy, is often dramatically subject to process failure during long timescale operation. This study aimed to evaluate the process robustness and reliability by elucidating the composition of microbial populations and the link with process parameters under a wide range of different operating conditions in long-term semicontinuous reactors. Mesophilic semipilot digesters were fed with real urban biowaste, namely, food waste alone or food waste with activated sludge, to explore the impact of feedstock composition and organic loading rate (OLR: 0.8–3.5 kgVSfed m−3d−1) on volatile fatty acid (VFA) composition and pattern, biomethane production, and core microbiome dynamics. The major bacterial phyla were Bacteroidetes (basically more abundant when the feedstock was only food waste), followed by Chloroflexi and Firmicutes, while Euryarcheota hydrogenotrophic methanogens, mainly represented by members of the Methanomicrobiales family, prevailed in all systems. During monodigestion of food waste, however, independent of the OLR, methanogenic conversion of fermentative end-products decreased progressively after 1 HRT. In particular, the observed propionate and butyrate accumulations seemed to derive from thermodynamic limitations due to the increase in hydrogen levels, indicative of progressively lower H2 consumption rates. However, the induced process recovery assured a large degradation of fatty acids with higher carbon chain lengths together with methane production, suggesting the upswing of hydrogenotrophic methanogen activity emerging after stressful conditions. The addition of a minimal amount of sludge boosted the methane production rates (up to 0.29 ± 0.1 Nm3CH4 kg−1VSfed) along with excellent process stability, also at a high OLR, creating a robust methanogen community shaped by high biodiversity. Methanospirillum and Candidatus Methanofastidiosum, strictly related to the codigestion systems, have a large H2 consumption capacity, thus preventing thermodynamic bottlenecks and process failure.

Determination of Nitrogen and Sulphur Mineralization in Batch and Semi-Continuous Anaerobic Digestion Using an Artificial Fiber Bag Technique

In the biogas industry, feedstock plans are used to estimate methane production and nutrient content in the digestate, however, these predictions do not consider the mineralized nitrogen fraction of the feedstock, which is useful when determining the quality of the digestate. In this study, the artificial fiber bag technique, which is commonly used to study feedstock degradation in ruminants, was implemented in anaerobic digestion to quantify mineralization of N and S. The artificial fiber bags were used to enclose substrates but with access to inoculum because of small pores in the bags, thereby enabling digestion. The content of the bags was analyzed before and after digestion to quantify residual mass as well as N and S concentration in the substrate. The method was validated through batch anaerobic digestion of a single substrate with and without bags, where the bags showed little influence on methane production and degradation. Semi-continuous anaerobic digestion experiments showed higher substrate degradation and higher N and S release at thermophilic conditions using four different types of feedstocks and proved useful for solid feedstocks but less so for semi-solid feedstock. For N, most of the mineralization occurred during the first 15 days over a trial of 30 days.

Effect of Prestage Hydrothermal Treatment on the Formation of Struvite vs Vivianite during Semicontinuous Anaerobic Digestion of Sewage Sludge

Effect of Prestage Hydrothermal Treatment on the Formation of Struvite vs Vivianite during Semicontinuous Anaerobic Digestion of Sewage Sludge

Steady state of semi-continuous anaerobic digestion of cattle manure under the stress of adding iron and titanium oxide nanoparticles

Steady state of semi-continuous anaerobic digestion of cattle manure under the stress of adding iron and titanium oxide nanoparticles