VS Removal Efficiency Research Articles

Enhancement of biohythane production from palm oil mill effluent by Thermoanaerobacterium thermosaccharolyticum PSU-2 and methanogenic mixed cultures using a thermophilic two-ring bioreactor

A novel thermophilic two-ring bioreactor was employed to produce biohythane from palm oil mill effluent (POME) using Thermoanaerobacterium thermosaccharolyticum PSU-2 and thermophilic methanogenic mixed cultures. The reactor system demonstrated impressive performance, achieving hydrogen and methane yields of 113.3 ± 15.0 mL/g-VS and 473.0 ± 60.0 mL/g-VS, respectively, with a total biohythane composition of 4.3 % H2, 37.1 % CO2, and 58.6 % CH4. The process exhibited high treatment efficiency, with COD and VS removal efficiencies reaching 93.7 % and 84.3 %, respectively. Microbial community analysis revealed the crucial roles of various microorganisms in the biohythane production process. Thermoclostridium sp., Thermoanaerobacterium sp., and Anaerobranca sp. were identified as key players in hydrogen production, while Bacteroides sp. and Methanobacterium sp. were found to be essential for methane production. The optimization of operating conditions, including pH (5.0–8.0), temperature (55 °C), and hydraulic retention time (2 d for H2 and 10 d for CH4), significantly enhanced biohythane production efficiency. The techno-economic analysis demonstrated the economic viability of the thermophilic two-ring bioreactor system, with a net present value of 4,186,000 USD, an internal rate of return of 82 %, and a payback period of 1.4 years. These findings highlight the potential of this innovative technology as a sustainable and economically attractive solution for treating POME and simultaneously producing renewable energy in the form of biohythane, contributing to the sustainable development of the palm oil industry and the mitigation of greenhouse gas emissions.

Influence of pH on the performance of anaerobic piggery wastewater treatment coupled with membrane-based NH3 extraction

The influence of the pH of piggery wastewater (PWW) on both ammonia recovery and anaerobic digestion performance during PWW treatment was evaluated in a continuous stirred tank reactor coupled with a membrane-based extraction module. The anaerobic digester was operated at a hydraulic retention time of 20 days at 37 °C, while the flat sheet PTFE membrane module operated continuously at liquid recirculation rates of 250 mL min−1. The membrane module was able to gradually decrease the total ammoniacal concentration from 1.27 ± 0.01 to 0.62 ± 0.01 g L−1 after 360 days of operation. Membrane-based NH3 extraction induced a CH4 yield increase of 1.3-fold. Moreover, COD and VS removal efficiencies increased up to 1.2-fold and 1.5-fold, respectively, along with the increase in PWW pH from 7.5 to 12. Total VFAs removal efficiencies were higher at a PWW pH of 9.

Thermophilic anaerobic digestion of kitchen waste driven by microbial electrolysis cell (MEC): Digestion performances, anodic microorganisms’ distribution and current utilization efficiency

The aims of present study were to investigate the performances of kitchen waste thermophilic anaerobic digestion driven by microbial electrolysis cell (MEC), and to clarify the methanogenic metabolism pathways on the electrodes (especially on the anodic biofilm). The results showed that both 0.6 V and 1.0 V voltages supplementation could enhance methane yields and production rates, while efficient methanogenesis failed to be sustained after power outages under high organic loading rate (OLR, 4.22 gVS/L/day) condition. Voltages supplementation could also enhance VS removal efficiencies from 83.24–83.35% to 87.01–87.46%, and strengthened the hydrolysis and acidogenesis of dissolved organic matters (DOMs) on the outer layer of anodic biofilm. Voltages supplementation accelerated the β-oxidation of volatile organic acids (VFAs) on the inner layer of anodic biofilm and facilitated acetoclastic methanogenesis on the outer layer of anodic biofilm. The generated electrons on the anode were transferred to the cathode for producing hydrogen as a substrate of hydrogenotrophic methanogenesis, rather than directly generating methane by reducing carbon dioxide through direct interspecies electron transfer (DIET) pathway. On the anodic biofilm, the syntrophic relationship between Syntrophomonas and Methanosarcina was confirmed as the main contributor of methanogenesis. Analysis of current utilization efficiency proved that a little amount of energy input could lead several folds of current utilization efficiencies in return, especially under high OLRs condition, which was anticipated to be applied into practical engineering of kitchen waste thermophilic digestion for higher economic benefits.

Effect of Mechanical Solids Separators on Potential Reduction of Methane Emissions From Dairy Manure Storage

Highlights Four mechanical manure separators were evaluated in on-farm studies. Mass balances and biochemical methane potential tests were conducted to measure on-farm separation efficiencies. Separator performance depended on environmental, material, and design factors. Over 70% solids separation and methane emission reduction potentials were achieved. Abstract. The separation efficiencies of four mechanical solid-liquid separation technologies and their effect on the reduction of methane production potential from dairy lagoons were studied using on-farm separator measurements and laboratory biomethane potential tests. The studied technologies included 1-stage sloped dual-screen, 2-stage sloped dual-screen, 1-stage horizontal scraped screen, and 1-stage sloped single-screen separators. The technologies were evaluated across various seasons on four dairies in California. On-farm sampling and measurements of the influent of flushed manure and the mass of the solids separated, as well as in-laboratory measurements of methane (CH4) production potential, were carried out. A mass balance approach was employed to determine the removal efficiencies of total and volatile solids (TS and VS) and nutrients, as well as methane emission reduction potential. The performance of the separators depended on manure characteristics, system design (e.g., screen size and orientation), separator operation and management (e.g., manure flow rate), and manure processing pit type and configuration. Among the four studied separator systems, the 2-stage sloped dual-screen separator showed the highest average TS and VS removal efficiencies, and methane emission potential reduction of 52.5%, 59.7%, and 55.8%, respectively. The 1-stage horizontal scraped screen separator had the lowest TS and VS removal efficiencies, and methane emission reduction potential of 6.3%, 9.3%, and 4.9%, respectively. The results have potential implications for nutrient and greenhouse gas management strategies and/or policies in agriculture. Keywords: Greenhouse gas, Lagoon, Manure management, Separation efficiency, Settling basin, Solid-liquid separation, Sustainable agriculture.

Performance of a percolation reactor integrated to solid-state anaerobic garage-type digesters with leachate recirculation for organic fraction of municipal solid waste treatment

Solid-state anaerobic digestion (SSAD) systems have been used for the sustainable treatment of the organic fraction of municipal solid waste (OFMSW). Unlike previous reports considering lab-scale SSAD systems as a unique system, the present study aims to deeper evaluate only the complementary percolation reactor (PR) integrated into the first semi-industrial scale SSAD system in Latin America. An innovative methodology was developed based on the probability function analysis of the monitoring data with target values reported in the literature and a linear correlation matrix to address the relation between different operating parameters. The results indicate that under a maximum organic loading rate of 1.9 kg VS.m−3.d−1, PR achieved a VS removal efficiency of 56 %, producing a highly stabilized inoculum (VS/TS ratio of 0.43). A maximum methane yield of 0.06 m3 CH4.kg VS−1removed and biogas CH4 content up to 76.5 % were reached during the long-term operation of 730 days.

Enhancement of swine manure anaerobic digestion using membrane-based NH3 extraction

The influence of suspended solids and pH in anaerobically digested piggery wastewater on membrane-based NH3 extraction was evaluated in batch tests. The increase in pH in the anaerobic broth from 8 to 9 resulted in an increase in NH3 removal efficiencies from 15.8 % ± 0.1 % to 20.9 % ± 0.4 % regardless of the suspended solids. The influence of membrane based NH3 extraction on piggery wastewater treatment was also assessed in a CSTR interconnected with PTFE membrane modules. The decrease in TKN concentrations mediated by membrane operation induced an increase in CH4 yield from 380.4 ± 84.9 up to 566.1 ± 7.8 NmLCH4 g VS fed−1. Likewise, COD and VS removal efficiencies significantly increased from 33.0 % ± 2.0 % and 25.7 % ± 2.3 % up to 61.8 % ± 1.3 % and 37.9 % ± 1.8 %, respectively. Interestingly, the decrease in NH3 concentration entailed a complete assimilation of VFA.

Effect of the Substrate to Inoculum Ratios on the Kinetics of Biogas Production during the Mesophilic Anaerobic Digestion of Food Waste

This study evaluates the effects of the varying substrate to inoculum ratios (S:I) of 0.5, 1, 2, 3, 4, 5, and 6 (volatile solids/VS basis) on the kinetics of biogas production during batch mesophilic (35 ± 1 °C) anaerobic digestion (AD) of simulated food waste (FW), using anaerobic digestate as the inoculum. Kinetic parameters during biogas production (scrubbed with NaOH solution) are predicted by the first-order and the modified Gompertz model. The observed average specific biogas yields are in descending order corresponding to the S:I ratios 1, 2, 4, 6, 3, 5, and 0.5, respectively, and the significant effect of the S:I ratio was observed. The tests with the S:I of 1 have the maximum average biogas production rates of 88.56 NmL/gVS.d, whereas tests with the S:I of 6 exhibited the lowest production rates (24.61 NmL/gVS.d). The maximum biogas yields, predicted by the first order and the modified Gompertz model, are 668.65 NmL/gVS (experimental 674.40 ± 29.10 NmL/gVS) and 653.17 NmL/gVS, respectively. The modified Gompertz model has been proven to be suitable in predicting biogas production from FW. VS removal efficiency is greater in higher S:I ratios, with a maximum of 78.80 % at the S:I ratio of 6, supported by the longer incubation time. Moreover, a significant effect of the S:I ratio is seen on kinetics and energy recovery from the AD of FW.

Anaerobic co-digestion of food waste, poultry litter and sewage sludge: seasonal performance under ambient condition and model evaluation

ABSTRACT With the increasing population, food waste, sewage sludge, and poultry litter management problem are scaling up even in low-income countries. The management of these wastes has therefore been challenging. Anaerobic digestion of food waste alone is not very stable due to its acidic nature and high degradability whereas sewage sludge and poultry litter have low biochemical methane potential and a high nitrogen concentration. Co-digestion of suitably selected substrate leads to enhanced biogas production potential, system stability due to synergetic effects, and resolving the problem of waste management in the vicinity in a holistic approach. However, these wastes have varying characteristics and composition, in terms of carbon-to-nitrogen (C/N) ratio, pH, and alkalinity. In addition, millions of the rural household bio-digesters operating in low-income countries are working under ambient conditions and are primarily unheated. Therefore, there is a need of research to assess the viability of biogas production of co-digestion of all above substrates in an optimal mixing ratio operating in an ambient temperature condition. In this study, food waste (FW), sewage sludge (SS), and poultry litter (PL) were co-digested at ratios (SS: PL: FW: 3:2:1, 2:1:1, 1:1:1) with 8% total solid (TS) content at ambient temperature in summer and winter seasons. Biogas yield was highest with mixing ratio of 2:1:1 with the values of 640 L/kgVS in summer while it gave extremely low biogas yield of 106 L/kgVS in winter. The 2:1:1 mixture also had the highest methane composition of nearly 65% as well as the highest VS removal efficiency of 60%, making it the most viable option for increasing biogas production. Mathematical modeling results using Gompertz model and first order model predicted well with R2 value ranging from 0.91 to 0.98, which upheld the experimentally obtained values. Findings from this study suggest that co-digestion substrates (SS:PL:FW) mixing ratio of 2:1:1 is an optimized ratio among the studied co-substrate ratio for enhanced biogas production. Furthermore, winter biogas yield is almost one sixth of summer biogas yield, which means use of temperature enhancement techniques to anaerobic digesters operating at ambient conditions would be essential to increase the biogas yield during winter.

Impact of AnMBR operating conditions on anaerobic digestion of waste activated sludge.

The impact of solids retention time (SRT) and hydraulic retention time (HRT) on anaerobic digestion of thickened waste activated sludge (TWAS) in a pilot-scale anaerobic membrane bioreactor (AnMBR) was compared with that achieved in conventional anaerobic digestions (CD). The AnMBR was able to successfully digest municipal TWAS at HRTs ranging from 7 to 15days and SRTs ranging from 15 to 30days. Increasing SRT in the AnMBR resulted in a significant improvement in COD and VS removal efficiency when compared against CD operating at the same HRT. The VS and COD destructions (35%-50%) observed in the AnMBR were similar to those observed in CD operating at the same SRT but longer HRTs. Operation at elevated ratios of SRT/HRT resulted in the production of a thickened biosolid (2%-3% TS). Specific methane production values for AnMBR operating at HRT-SRT ratios of 15-30, 7-30, and 7-15 were 0.19, 0.19, and 0.14m3 CH4 /kg of COD fed, respectively, showing a 25% increase in methane production with SRT. A model based upon describing hydrolysis of biodegradable solids using first-order kinetics was able to describe VS destruction as a function of SRT. PRACTITIONER POINTS: The AnMBR process was able to successfully digest waste activated sludge at a shorter seven-day HRTs Operation at elevated ratios of SRT/HRT resulted in enhanced biogas and thickened biosolid (2%-3% TS) production requiring reduced downstream processing The AnMBR process produces a particle-free permeate that might be suitable for side stream nutrient recovery A model developed by considering hydrolysis as a limiting process can be used to determine design SRTs.

Mesophilic methane fermentation performance and ammonia inhibition of fish processing wastewater treatment using a self-agitated anaerobic baffled reactor

The performance of the self-agitated anaerobic baffled reactor (SA-ABR) was investigated by increasing the organic loading rates (OLRs) from 0.46 to 9.50 g-COD/L/d. A good performance was achieved by the SA-ABR for the treatment of fish processing wastewater (FPW). The maximum OLR was 6.77 g-COD/L/d and the biogas production rate reached 2.16 L/L-reactor/d with a methane content of 69% at this OLR. The COD, carbohydrate, protein, lipid and VS removal efficiencies were as high as 64, 65, 68, 78 and 79%, respectively. Ammonia inhibition was assumed with inhibition concentrations of 10% (IC10) and 20% (IC20) at 4140 and 5780 mg/L. However, it was found that the reactor could tolerate ammonia at a high concentration range of 4500–6373 mg/L after a long-term continuous experiment. Ammonia inhibition was addressed by diluting the substrate and the sludge in the reactor with tap water.

Enhanced volatile fatty acid degradation and methane production efficiency by biochar addition in food waste-sludge co-digestion: A step towards increased organic loading efficiency in co-digestion

This work investigated the effect of biochar addition to mitigate VFA accumulation and enhance methane production in mesophilic food waste/sludge co-digestion. Different types of biochar derived from agricultural and forestry residues at two pyrolysis temperatures were tested. Results showed that wheat straw biochar 550 °C supported the highest specific methane yield of 381.9 LCH4/kg VSadded and VS removal efficiency of 41.62% among all treatments. Degradation of propionic acid and long-chain fatty acids such as valeric, caproic and isovaleric acids was observed. This also corresponded to an increase in methanogenic favorable substrates including acetic acid (>40%) and butyric acid (~20%) over the control. Consequently, a 24% increase in overall methane production was obtained as compared to control. This demonstrated that biochar addition had positive effects on VFA degradation and methane production which could be a useful strategy to increase the organic loading in co-digestions without the fear of process failure.

Enhanced methane generation and biodegradation efficiencies of goose manure by thermal-sonication pretreatment and organic loading management in CSTR

The current research investigated the effects of thermal-sonication pretreatment on the anaerobic digestion of goose manure. The CSTRs were run at three different organic loading rates: 1.4 g-VS/L.day, 2.9 g-VS/L.day and 4.4 g-VS/L.day, consequently termed as Phase-1, Phase-2 and Phase-3, respectively, and five different sonication pretreatments levels: (25 min, 45 min, 60 min, 90 min, and 120 min) at 28 KHz that were employed to optimize the anaerobic digestion process for goose manure to enhance methane production. The second phase was proved to be the optimum phase with maximum methane enhancement capability, CODs stabilization rate, and VS removal efficiency. Owing to thermal-sonication and organic loading management, methane enhancement up to 40.76% was evaluated for reactor C during the second phase of experiment. Furthermore, the biochemistry of the anaerobic digestion process was deeply interpreted at each three-day interval and the techno-economic analysis was performed to evaluate the pretreatment feasibility.

Improved Mixing System for Anaerobic Sequencing Batch Reactors

HighlightsA novel single-jet mixing system was designed for ASBR digesters.Mixing energy was reduced to the point that solids were only partially suspended in the reactor vessel.The partial mixing system increased effluent quality as measured by suspended solids content.The partial mixing system increased solids retention, allowing hydraulic retention time (HRT) to be reduced to at least 7.5 days while maintaining solids retention time (SRT) above 100 days.The partial mixing system did not reduce biogas production rate nor biogas yield.Abstract. An anaerobic sequencing batch reactor (ASBR) is a high-rate anaerobic digestion system ideally suited for the treatment of liquids with high organic strength and low solids content. Biota are retained in an ASBR by settling solids prior to decanting effluent from the top of the reactor. Solids retention time (SRT) can be managed separately from hydraulic retention time (HRT) in an ASBR. One problem encountered with ASBRs is poor solids retention due to inefficient solids settling. A novel mixing system in which solids are only partially mixed in the reactor prior to decanting was investigated in a series of three experiments. A battery of six 30 L ASBR reactors were fed a mixture of dilute swine manure (0.30% TS, 0.20% VS) and raw glycerol. In a side-by-side comparison of two reactors operated at an organic loading rate (OLR) of 0.30 g COD L-1 d-1 with 15-day HRT and two feeding cycles per day, the partially mixed reactor outperformed the fully mixed reactor as measured by effluent quality (130 vs. 350 mg VSS L-1), SRT (354 vs. 52 days), and VS removal efficiency (88% vs. 79%). In a replicated study of five reactors operated at 0.31 g COD L-1 d-1 OLR, 15-day HRT, and two feeding cycles per day before and after switching from full to partial mixing, the partially mixed reactors showed significantly (p = 0.05) better performance as measured by effluent quality (100 vs. 382 mg VSS L-1), SRT (760 vs. 72 days), and VS removal efficiency (85% vs. 71%). Biogas production did not significantly change with the change from full to partial mixing in the five replicated reactors, i.e., average biogas yield was 0.81 and 0.77 L biogas g-1 COD with partial and full mixing, respectively. Effluent quality, SRT, VS removal efficiency, and biogas yield did not significantly change when the OLR was increased from 0.31 to 0.62 g COD L-1 d-1 and HRT was reduced from 15 to 7.5 days in a replicated study of six partially mixed reactors. A mass balance of COD across the six partially mixed reactors showed that endogenous respiration of retained biomass accounted for approximately 50% of the biogas produced by an ASBR with SRT exceeding 400 days. Keywords: Anaerobic digestion, Anaerobic sequencing batch reactor, ASBR, Biogas, Glycerol, Hydraulic retention time, Mixing, Operation, Performance, Solids retention time, Swine manure.

Dry batch anaerobic digestion of food waste in a box-type reactor system: Inoculum preparation and reactor performance

Abstract A box-type reactor system with liquid inoculum has been studied for the dry anaerobic digestion of food waste. The food waste was processed without any pre-treatment to remove physical impurities, neither water addition to dilute and slurry the feedstock. The experiment was carried out with inoculum to substrate ratios of 1:1 (w/w) and 0.08:1 (VS basis). Previous acclimation of liquid inoculum enhanced the process, assuring a fast start up of the box digester and preventing from process failure by volatile fatty acids accumulation. The percolate recirculation strategy was shown to have a relevant effect on the progress of the process. The results suggest that the process can be optimized by providing low percolate recirculation rate during the start-up of the box digester followed by an increase in the percolate recirculation rate when volatile fatty acids decrease and methane content in the biogas increases. The methane yield obtained in the box digester from the food waste was in the range 460–477 L CH4 kg−1 VS, being the VS removal efficiency between 91.1 and 91.4%. Globally, the process operated at an organic loading rate of 2.5 kg VS m−3 d−1 and yielded a volumetric methane production rate of 1.0 m3 CH4 m3 d−1. These results show the high potential of food waste for its conversion in renewable energy by using the dry batch anaerobic technology.

Evaluation of Posidonia oceanica residues as feedstock for anaerobic digestion

The suitability of Posidonia oceanica as feedstock for anaerobic digestion process was evaluated carrying out several biochemical methane potential tests. No clear evidence of process inhibition were observed during tests. Nevertheless, the very high lignin content of Posidonia did not allow the organic matter solubilization by microbial lithic enzymes, thus limiting the effectiveness of the anaerobic digestion process. Therefore, Posidonia was subjected to thermal (132 °C) and thermo-chemical pre-treatment in order to increase its anaerobic digestibility. The opportunity of reducing biomass stress due to the high salinity of Posidonia suspension (performing a washing operation before anaerobic digestion) was also evaluated. Thermal treatment and salt removal allowed only a limited increase in anaerobic digestion process efficiency. Whilst thermo-chemical pre-treatment at 132 °C in presence of HCl 0.40% resulted in a relevant increase of Posidonia biodegradability leading to a specific methane production higher than 90 mL/gVS and a VS removal efficiency of about 50%.

Sludge-based biochar-assisted thermophilic anaerobic digestion of waste-activated sludge in microbial electrolysis cell for methane production

The development of microbial electrolysis cells (MECs) for methane production from waste activated sludge (WAS) is arrested due to the limited methane yield and fragile system stability. This study proposed a strategy to accelerate and stabilize MEC via 1.0 g/g DM (dry matter) sludge-based biochar (BC). The results showed that BC clearly accelerated methane production by 24.7% and enhanced VS removal efficiency by 17.9%, compared to control group. Variations of SCOD, proteins, carbohydrates and VFAs indicated biochar promoted hydrolysis and acidogenesis process. Cyclic voltammetry (CV) curves and coulombic efficiency (CE) suggested organic matters degradation and electron generation on anode were enhanced with supplement of biochar. Microbial community analyses revealed that biochar addition could both promote DIET through substituting exoelectrogen (e.g., Thermincola) on anode and enrich hydrogenotrophic methanogens (e.g., Methanothermobacter) on cathode, which is beneficial to development of MEC as to methane recovery from organic matters.

Impact of rheological properties of substrate on anaerobic digestion and digestate dewaterability: New insights through rheological and physico-chemical interaction

Mesophilic batch anaerobic digesters fed by different substrates were set up to identify the role of substrate rheology in anaerobic digestion performance while operating below the toxic level. Five substrates of different rheological behaviour but at the same amount of organic matters were prepared by addition of different amount of an inert material (0, 0.03, 0.07, 0.11, and 0.20 g) per g of waste activated sludge (WAS). To gain a comprehensive insight, the interactive relationship between substrate rheology, physico-chemical properties and biogas production as well as digestate dewaterability was investigated. The results proved that better access of microorganisms to organic matters improved the digester performance and led to 19.29% and 12.5% increase in biogas yield and VS removal efficiency, respectively. Moreover, the statistical analysis showed that consistency index and loss modulus of sludge could be employed as promising indications for biogas yield while yield stress could predict dewaterability of digestate as far as the other physico-chemical properties remained unchanged. During digestion measurement of consistency index and loss modulus of digestate could be performed as a reliable tool to monitor biogas production.

Food waste treatment by anaerobic co-digestion with saline sludge and its implications for energy recovery in Hong Kong

Potential of methane production by co-digestion of food waste with saline sludge produced from sewage receiving seawater toilet flushing was investigated to determine its suitability for food waste management in Hong Kong by making use of excess design capacity of sludge digesters. High salinity of saline sludge (12.8 mS/cm) affected degradation of organic compounds resulting in an increase in sCOD by 135% as compared to an increase by 283% in treatments with non-saline sludge (4.2 mS/cm) co-digestion. This inhibitory effect was also evident by lower VS removal efficiency of 32.65% for saline versus 54.23% for non-saline sludge based co-digestion. Furthermore, non-saline sludge gave a 3.4-fold higher methane yield than saline sludge co-digestion. It is concluded that co-digestion of food waste with both sludges could be adopted as a potential strategy to make use of excess digestion capacity of existing wastewater treatment facilities but is more viable for non-saline sludge.

Co-treatment of sewage sludge and mature landfill leachate by anaerobic digestion

In this study, sewage sludge was co-digested with different amounts of mature landfill leachate (0, 5, 10, 20, and 40% v/v) to evaluate the yield capacities of biogas and CH4. The highest biogas yield was obtained from the treatment with 10% leachate dose (T2) and control with 0% leachate dose, while the lowest biogas was produced from the treatment with 40% leachate dose. The highest CH4 production (maximum yield per kg VS consumed) was also obtained from T2, which was about 36% (v/v) higher than the control. This result was confirmed by the first-order kinetic model. It was also observed that the COD, TS, and VS removal efficiencies significantly decreased with increasing the leachate dose. However, the increased dose of leachate did not inhibit TN removal efficiency. This study demonstrates that mature landfill leachate could be treated via anaerobic co-digestion process with sewage sludge to enhance biogas production at low doses (up to 20% v/v).

Effect of ultrasonic pretreatment on the semicontinuous anaerobic digestion of waste activated sludge with increasing loading rates

Anaerobic digestion (AD) is the most widely used method to stabilize and recover energy from waste activated sludge (WAS). However, AD of sludge results in a low biogas yield. Ultrasonic pretreatment (USp) is the most effective sludge disintegration technology and has elicited interest to increase the biodegradability and improve the efficiency of the AD. This study investigated the influence of USp on the semicontinuous AD of WAS as a strategy to increase its performance. During an experiment lasting 90 days, 0.8-L digesters were semicontinuously fed under different conditions: raw sludge and sonicated at 15000, 25000 and 35000 kJ/kg TS (total solids); and an increase of the organic loading rate (OLR) from 1 to 3 kgVS/m3·d, for 30 days each. The maximum biogas rate (0.218 L/d) was reached at 25000 kJ/kg TS and 3 kgVS/m3·d. The highest VS removal efficiency (23.7%) was observed at the highest USp and the lowest OLR. The reactors fed with raw sludge had a lower performance compared to the sonicated WAS in terms of VS reduction and biogas production. In contrast, the USp allowed the bioreactors to be operated at a high OLR (3 kgVS/m3·d) without inhibition. Finally, a kinetic analysis of accumulated biogas was performed.