Volatile Solids Removal Efficiency Research Articles

Mesophilic anaerobic co-digestion of two-phase olive-mill waste and cattle manure: Optimization of semi-continuous process

Anaerobic co-digestion of organic wastes is an effective technology for the management of two or more substrates with different characteristics. In this context, the main objective of this work was the optimization of biogas production for the treatment of a mixture of two-phase olive-mill waste (2POMW) and cattle manure (CM) (60:40 w/w) at mesophilic temperature range (35 °C). The effect of hydraulic retention time (HRT) on the performance and stability of the digestion process was studied. A decreasing series of HRTs in the range of 40–12 days was analyzed. The corresponding organic loading rates (OLR) were in the range 2.01–6.07 gVS/LR·d. Pseudo steady-state operation of the reactor was established for HRTs between 40 and 15 days. For 15-days HRT, the maximum values of methane productivity (0.94 LCH4/LR·d) and specific methane yield (0.52 LCH4/gVSremoved) were obtained while total acidity (measured as acetic acid) in the effluent were<150 mg/L, verifying process stability. In addition, the removal efficiencies of volatile solids (VS) and dissolved organic carbon (DOC) were 38 and 67 %, respectively. At 12-days HRT, decreases in methane production and organic matter removal efficiencies were observed, with values of 27 and 47 % for VS and DOC removal, respectively. Therefore, 12-days HRT was considered as inadequate for the anaerobic co-digestion of 2POMW and CM because a clear increase in volatile fatty acids was observed at the end of this period, leading to process destabilization and a decrease in biogas production.

Anaerobic co-digestion of food waste and sewage sludge in anaerobic sequencing batch reactors with application of co-hydrothermal pretreatment of sewage sludge and biogas residue

The effect of pretreatment technologies and reactor types on conversion efficiency and operating costs of anaerobic co-digestion of food waste and sewage sludge were investigated by 300-day continuous experiments. The volatile solids (VS) removal efficiency increased from 61% to 77% with the application of co-hydrothermal pretreatment of sewage sludge and biogas residue. Deep dewatering reduced the volume of hydrothermally pretreated biogas residue by 85%. When continuous stirred tank reactors (CSTRs) were converted to anaerobic sequencing batch reactors (ASBRs), vS removal efficiencies increased by 6%, attributed to a 1.4–1.6-fold increase in solids retention time (SRT). The bottom drainage of mineralized sludge every 40 days increased ASBR stability. Firmicutes and Methanosphaera dominated the bacterial and archaeal communities, respectively. Operating costs decreased by 14.9 US$/metric ton feedstock by applying ASBRs. Compared to CSTRs, ASBRs achieved higher organic matter conversion efficiency, smaller volume of biogas residue, and lower operating costs.

Accelerated stabilization of high solid sludge by thermal hydrolysis pretreatment in autothermal thermophilic aerobic digestion (ATAD) process.

Autothermal thermophilic aerobic digestion (ATAD) is a rapid biological treatment technology for sludge stabilization. To improve digestion efficiency and shorten stabilization time, thermal hydrolysis pretreatment was employed before ATAD of high solid sludge. The results showed that accelerated stabilization of high solid sludge (total solid=10.1%) was achieved by thermal hydrolysis pretreatment with volatile solid removal efficiency of 40.3% after 8 days of ATAD, 11 days earlier than unpretreated sludge. The enhanced release and hydrolysis of intracellular organics resulted in a solubilization degree of 45.3%. The reduced sludge viscosity and improved fluidity after thermal hydrolysis facilitated mixing, aeration and organics degradation during ATAD. Excitation emission matrix analysis indicated that the fluorescence intensity of soluble microbial byproduct and tyrosine-like protein increased markedly after thermal hydrolysis and decreased after ATAD. The proportion of high molecular weight (MW>10kDa) substances in the supernatant increased significantly after thermal hydrolysis, while the low MW (MW<1kDa) substances decreased after ATAD. The significant difference in microbial composition between the pretreatment and control groups elucidated the accelerated sludge stabilization under thermal hydrolysis. This work provides an efficient and practical strategy to achieve rapid stabilization of high solid sludge.

Deciphering the role of microplastic size on anaerobic sludge digestion: Changes of dissolved organic matter, leaching compounds and microbial community

Here the role of microplastic size on dissolved organic matter, leaching compounds and microbial community during anaerobic sludge digestion was evaluated. Compared to that without the addition of polyvinyl chloride (PVC), during the 30 days' incubation, the anaerobic sludge digestion by adding PVC at the size of 75 μm and the concentration of 2.4 g/g volatile solids (VS) showed a 8.5% lower cumulative methane production, while a 17.9% higher cumulative methane production was noted by adding PVC at the size of 3000 μm and the concentration of 2.4 g/g VS. A long-term fed-batch laboratory-scale fermenter test for 147 days further testified, that higher removal efficiencies of total solids, volatile solids, and total chemical oxygen demand, and higher methane production were noted by adding PVC (2.4 g/g VS, 3000 μm) into the fermenter. More interestingly, higher concentrations of proteins, polysaccharides, volatile fatty acids, and soluble microbial by-products component were noted in the liquid phase of sludge drawn from the fermenter added with PVC since the biomass therein showed higher efficiencies of solubilization, hydrolysis, acidification, and methanogenesis. Moreover, as identified from the fermenter added with PVC, dibutyl phthalate (DBP) was the most predominant leaching phthalates compound, although the biomass therein showed a 93.4% anaerobic biodegradability of DBP. The leaching of DBP drove the predominance of microbial community towards Synergistota and Methanosaeta. More irregular elliptical shallow dimples were noted on the PVC surface after 147 days' incubation, accompanied with abundances of Proteobacteria, Actinobacteriota, Chloroflexi, Methanosaeta and Methanobacterium. The results from this study showed that the size of microplastic was a crucial factor in evaluating its impact on anaerobic sludge digestion.

Co-production of Biohydrogen and Biomethane from Chicken Manure and Food Waste in a Two-Stage Anaerobic Fermentation Process.

Batch experiments were performed to evaluate the biohydrogen and biomethane production by co-digestion of chicken manure and food waste in a two-stage mesophilic fermentation process. Results showed that no hydrogen was produced in the first stage of sole chicken manure fermentation, while methane yield was 247.3 mL·g-1-VS. By comparison, the co-digestion process with food waste proportions of 50-85% obtained hydrogen yields of 15.5-57.5 mL·g-1-VS, and the methane yields and maximum specific methane production rates were also improved by 7.0-16.7% and 80%, respectively. Moreover, the highest hydrogen and methane yields were achieved during sole food waste fermentation process. The acetate was the main volatile fatty acid (VFA) produced during sole chicken manure fermentation process in the first stage. Statistical analysis revealed that hydrogen production from co-digestion process and sole food waste fermentation process followed the n-butyrate-type pathway. Meanwhile, it should be noticed that the co-fermentation of chicken manure and food waste had antagonistic effects on the hydrogen fermentation, implying that there might be some inhibition factors existing in chicken manure or produced during the co-fermentation process. At the beginning of methane fermentation, the VFA profiles were similar to those at the end of hydrogen fermentation, and the main VFA compositions changed to acetate and propionate in the latter period of methane production. The volatile solid removal efficiencies were also promoted in co-digestion process compared with sole chicken manure digestion, which were increased by 9.7-14.4% with food waste proportions of 50-80%.

Improvement of the anaerobic digestion of sewage sludge by co-digestion with wine vinasse and poultry manure: Effect of different hydraulic retention times

This study arose in response to the management of different organic wastes generated locally, which cause environmental damage. These wastes could be used to obtain biogas and class A biosolids through anaerobic digestion. For this purpose, the mesophilic anaerobic tri-digestion of sewage sludge (S), wine vinasse (V) and poultry manure (PM) (49.5:49.5:1) was studied to obtain biogas in an improved way compared to bi-digestion of SV (50:50) and mono-digestion of S. Tests were carried out in anaerobic digesters, at laboratory scale, to compare the benefits at different hydraulic retention time (HRT) (20, 15, 13, 10, 8 and 6 days). From the results obtained, anaerobic tri-digestion of the waste gave the best results, with a total chemical oxygen demand (TCOD) removal efficiency of 51%, volatile solids (VS) removal efficiency of 57%, and methane yield values of 261 mLCH4/gVSadded at a HRT of 13 days. In relation to the pathogens, both co-digestion studies addressed managed to inactivate enough pathogens in the effluent to be classified as class A biosolids. This would allow considering the anaerobic tri-digestion as a promising technology, which allows valorizing organic waste, obtaining biogas and class A biosolids, moving towards a circular bioeconomy.

Anaerobic Co-Digestion of Food Waste with Sewage Sludge: Simulation and Optimization for Maximum Biogas Production

Anaerobic co-digestion (ACD), where two or more substrates are digested simultaneously, is able to prevent the problems associated with mono-digestion. The aim of this study is to develop a simulation model of ACD of food waste (FW) with sewage sludge (SS) for biogas production coupled with pre-treatment, sludge handling and biogas upgrading using SuperPro Designer v9.0. The Design Expert v13 is employed to perform optimization and evaluate the effect of hydraulic retention time (HRT), sludge recycle ratio, water to feed ratio (kg/kg) and SS to FW ratio (kg/kg) on the methane flow, chemical oxygen demand (COD) and volatile solids (VS). The results show that the methane yield of 0.29 L CH4/g COD removed, COD removal efficiency of 81.5% and VS removal efficiency of 69.2% are obtained with a HRT of 38.8 days, water to feed ratio (kg/kg) of 0.048, sludge recycle ratio of 0.438 and SS to FW ratio (kg/kg) of 0.044. Economic analysis has shown this study is feasible with a payback time of 6.2 years, net present value (NPV) of $5,283,000 and internal return rate (IRR) of 10.2%. This indicates that the ACD of FW and SS is economically feasible in a larger scale.

Study of internal and external temperatures and their influence on covered lagoon digester performance

There is an evident gap in terms of understanding the interactions among variables that directly affect anaerobic digestion in full-scale biodigesters. The objective of this study was to assess the behavior of temperatures inside and outside the biodigesters and their relationship with organic matter removal and CH4 yield. The experiment was carried out in a pig farm located in the municipality of Teixeiras, Minas Gerais, Brazil, which has two parallel covered lagoon digesters (CLDs). According to the results, air and biogas temperatures largely varied throughout the day (24-h period), in contrast to the soil and substrate temperatures. The CLD system presented a low removal efficiency in terms of chemical oxygen demand (COD) and volatile solids (VS). The mean CH4 content in the biogas was 67%, and the mean biogas yield was 740.2 m³.d−1. When assessing the correlation matrix between internal and external temperatures, and their correlation with treatment efficiency, in terms of COD and VS removal, biogas yield presented a linear correlation with VS removal efficiency (r = 0.60). All temperatures inside and outside the CLD were intercorrelated.

Bacterial dynamics and functions driven by bulking agents to enhance organic degradation in food waste in-situ rapid biological reduction (IRBR).

This study investigated the effects of different bulking agents (i.e., sawdust, wheat straw, rice straw, and corncob) on bacterial structure and functions for organic degradation during food waste in-situ rapid biological reduction (IRBR) inoculated with microbial agent. Results showed that the highest organic degradation (409.5g/kg total solid) and volatile solids removal efficiency (41.0%) were achieved when wheat straw was used, largely because the degradation of readily degradable substrates and cellulose was promoted by this bulking agent. Compared with other three bulking agents, the utilization of wheat straw was conducive to construct a more suitable environmental condition (moisture content of 18.0-28.2%, pH of 4.91-5.87) for organic degradation during IRBR process, by virtue of its excellent structural and physiochemical properties. Microbial community analysis suggested that the high-moisture environment in rice straw treatment promoted the growth of Staphylococcus and inhibited the activity of the inoculum. By contrast, lowest bacterial richness was observed in corncob treatment due to the faster water loss. Compared with these two bulking agents, sawdust and wheat straw treatment led to a more stable bacterial community structure, and the inoculated Bacillus gradually became the dominant genus (36.6-57.8%) in wheat straw treatment. Predicted metagenomics analysis showed that wheat straw treatment exhibited the highest carbohydrate metabolism activity which improved the pyruvate, amino sugar and nucleotide sugar metabolism, and thereby promoted the organic degradation and humic substrate production. These results indicated that wheat straw was a more desirable bulking agent, and revealed the potential microbial organics degradation mechanism in IRBR process.

Solid and Soluble Organic Compounds Changes During Thermophilic Aerobic Digestion of Sewage Sludge

Over the past decade, thermophilic aerobic digestion (TAD) has been considered a practical approach to treating highstrength wastewater because of its several advantages. In this study, batch-mode TAD of sewage sludge was conducted using a lab-scale bioreactor to evaluate the changes in solid and soluble organic compounds. The TAD process achieved rapid degradation of the solid compound at 55oC. The removal efficiency of volatile suspended solids (VSS) increased rapidly at the early operation stage and achieved 25% at 48 h. Then, the VSS removal efficiency increased gradually up to approximately 40% at the end of the digestion (168 h). As a result of VSS degradation, the soluble chemical oxygen demand (SCOD) increased significantly up to 12 g/L at 36 h and then showed a slow decline pattern. Furthermore, the pattern of protease activity was similar to that of the SCOD. This result indicates that the rapid degradation of VSS is highly linked to the increase of protease activity and thermophilic conditions. This study used acetic acid and propionic acid. The variation patterns were similar under both acetic acid and propionic acid. The concentrations of acetic acid and propionic acid rapidly increased up to 3.05 g COD/L at 48 h and 2.51 g COD/L at 36 h, respectively. Then, major volatile fatty acids concentrations declined sharply and became undetectable after 144 h. Similar to the SCOD variation pattern, the soluble total nitrogen and NH4+-N also decreased sharply after 72 h. This might be because ammonia stripping is enhanced by pH rising and continuous aeration during the TAD.

Simulation and Optimization of Anaerobic Co-Digestion of Food Waste with Palm Oil Mill Effluent for Biogas Production

Food waste (FW) utilized as substrate for anaerobic digestion (AD) to produce biogas is promising. Simultaneously, waste is handled and value-added products such as biogas and fertilizer are produced. Palm oil mill effluent (POME) is used as the co-substrate. This study aims to simulate the complete process flow of anaerobic co-digestion (AcoD), consisting of pre-treatment of feedstock, biogas upgrading, wastewater treatment and sludge dying using SuperPro Designer. Parameters, namely hydraulic retention time (HRT), recycle ratio of sludge, water to FW ratio (kg/kg) and co-substrate to FW ratio (kg/kg), would affect the performance of digester. The optimization of these parameters is performed using Design-Expert software, involving response surface methodology (RSM). The effects on responses such as methane flow, chemical oxygen demand (COD) and volatile solid (VS) removal efficiencies are analyzed. In treating 25,000 kg/h of feed, the optimized values for HRT, recycle ratio, water to feedstock ratio, POME to FW ratio are 37.2 days, 0.381, 0.027 and 0.004, respectively. The methane yield is 0.30 L CH4/g of COD removed, with COD and VS removal efficiencies of 81.5% and 68.9%, respectively. The project is profitable, with a payback period of 6.14 years and net present value (NPV) of $5,680,000. A comprehensive understanding of AD matures it for commercialization purposes.

Calcium hypochlorite pretreatment improves thermophilic digestion of waste activated sludge in an upflow anaerobic sludge blanket reactor

Anaerobic wasted activated sludge (WAS) digestion has been widely applied to reduce sludge volume and generate bioenergy in the form of methane. However, anaerobic WAS digestion performance is often challenged with poor hydrolysis of biomass cellular structures. In the present study, the feasibility of using calcium hypochlorite (Ca(ClO)2) to improve the thermophilic digestion of WAS was studied. Two thermophilic upflow anaerobic sludge bed (UASB) reactors (one with and one without Ca(ClO)2 pretreatment) were operated for 120 days under low and high organic loading rate (OLR) conditions, corresponding hydraulic retention time (HRT) of 10 days and 6 days, respectively. Both reactors achieved satisfied performance during the studied period. Under the low OLR condition, Ca(ClO)2 pretreatment significantly improved WAS total volatile solids (VS) removal efficiency (from 48.06 ± 2.63% to 57.34 ± 3.54%) and methane yield (from 289.2 ± 27.6 to 362.2 ± 36.7 N mL/g VS). However, no significant improvement was observed under the high OLR condition. g_S1 and g_Fervidobacterium were predominant bacteria in the thermophilic UASB reactor fed with Ca(ClO)2 pretreated WAS. Methanosarcina was dominant archaea in both reactors. The treatment mechanism and application potential of using Ca(ClO)2 to enhance the WAS digestibility were further discussed.

Strengthen “the sustainable farm” concept via efficacious conversion of farm wastes into methane

With escalating global demand for renewable energy, exploitation of farm wastes (i.e., agriculture straw wastes (ASWs), livestock wastewater (LW) and sewage sludge (SS)) has been considered to attain maximum methane yield (MY) via anaerobic digestion (AD). Results pointed that mixture of SS and LW as anaerobes’ source with 20 g of ASWs/300 mL of working volume achieved maximum MY and volatile solid (VS) removal efficiency of 0.44 (±0.05) L/gVS and 51.4 (±4.1)%, respectively. This was mainly because of emerging heavy duty bacterial species (i.e., Syntrophorhabdaceae and Synergistaceae) and archaeal community (i.e, Methanosarcina and Methanoculleus) after 70 days of anaerobic incubation. This was acquired along with boosting enzymatic activity, especially xylanase, cellulase and protease up to 71.5(±7.9), 179.3(±14.3) and 207.2(±16.2) U/100 mL, respectively. Furthermore, the digestate contained high concentrations of NH4+ (960.1±(76.8) mg/L), phosphorus (126.3±(10.1) mg/L) and trace metals, making it a good candidate as organic fertilizer.

Recycling anaerobic digestate enhances the co-digestion potential of agro-industrial residues: Influence of different digestates as sources of microbial inoculum

Anaerobic codigestion (AcD) of agroindustrial residues was investigated. Granular sludge from bench-scale bioreactors digesting different manure were acclimated and recycled as microbial seed sludge to demonstrate inoculum-type influence on digestion performance. The biomethane potential (BMP) assay was operated for 30 days at 40 ± 2 °C in batch-type laboratory-scale reactors (100 mL). In inoculum amended reactors, codigestion showed significant, yet distinctive, biomethanation than monodigestion with a 5-fold increase (p < 0.05) in average biogas (248.3 ± 5.30 mL gVS-1) and CH4 yield (207.5 ± 4.15 mL gVS-1). The pH, soluble chemical oxygen demand (sCOD) and volatile fatty acids (VFAs) concentrations were within limits for stable AcD process with elevated total solids (TS) and volatile solids (VS) removal efficiencies. This study reinforces advancements in the recycling of digestate in biodigesters and suggests the appropriate selection of inoculum, preferably cow manure, to essentially boost methane production from these wastes.

Anaerobic co-digestion of Palm Oil Mill Effluent (POME) with Decanter cake (DC): Effect of mixing ratio and kinetic study

The characteristics of Palm Oil Mill effluent (POME) have captivated attention in harnessing its high organic contents to generate biogas via anaerobic digestion. However, biogas production is unstable during low crop season. Therefore, this research focuses on improving the biogas generation by adopting the co-digestion method which treats POME and Decanter cake (DC) simultaneously. Results show that DC:POME ratio of 0.8:1 (in term of volatile solids (VS)) with organic loading of 11 g VS/L and HRT of 28 days generated the highest methane yield of 0.515 L CH4/g VS, which is a 186% improvement as compared to mono-digestion of POME. The co-digestion of substrates has shown good biodegradability of organic contents where up to 99.42% chemical oxygen demand (COD) and VS removal efficiencies were attained. Logistic Function Model (LFM) is the most suitable model to predict the performance of co-digestion with the highest R2 of 0.9810.

Impact of nickel nanoparticles on biogas production from cattle manure

The presence of nano-nutrients can be stimulatory for the anaerobic digestion (AD) of hardly degradable wastes and thus enhance process performance. One of the essential trace elements, nickel (Ni), is involved in multiple important enzymes necessary for efficient AD. This study investigated the effects of Ni nanoparticles (NPs) with different concentrations on cattle manure AD performance and effluent chemical composition. The results showed that the highest specific biogas production was attained with 4 mg/L Ni NPs (p<0.05) and was 792.00 mL/g VS compared with cattle manure-only which yielded only 589.20 mL/g VS. Moreover, the addition of 2 mg/L Ni NPs significantly increases the methane yield (p<0.05) by 70.46% compared with cattle manure-only. Besides, H2S production decreased by 16.50%, 90.47%, and 51.89% compared with cattle manure-only when the cattle manure was treated by 1, 2, and 4 mg/L Ni NPs, respectively. The supplementation of bio-digester with 2 mg/L Ni NPs improved the total and volatile solids removal efficiencies by 19.2% and 12.1% and maintained pH at a suitable level (6.8–7.3). FTIR spectrum of effluent presented the addition of Ni NPs changed the intensity and shift of peaks compared with cattle manure-only peak. Moreover, all of the effluent samples from the AD systems using Ni NPs showed stronger fertilizer values, as well as the effluent with 2 mg/L Ni NPs, had the highest sulfide content (31.2%, increase), leading to higher H2S removal efficiency by 47.5%. A first-order kinetic model showed an increase up to 1.88-fold in the hydrolysis rate constant (K), in 4mg/L Ni NPs compared to cattle manure-only, leading to higher biogas yield.

Effect of organic loading rate on the anaerobic digestion of swine waste with biochar addition.

The aim of this study was to investigate the impact of biochar addition on the mesophilic semi-continuous anaerobic digestion of swine waste with a focus on the effects of the organic loading rate (OLR) on biogas production, methane yield, total volatile fatty acids (TVFA), alkalinity, ammonium, volatile solids (VS) removal efficiency and process stability. Four reactors, two with amended biochar (R1 and R2) and two without biochar addition as controls (R3 and R4), were operated at OLRs in the range of 2-7 g VS/(L d), which corresponded to hydraulic retention times (HRTs) in the range of 7-2 days, respectively. The addition of biochar initially caused an increase in the generation of biogas and methane when compared to the control reactors when the process operated at OLRs of 2 and 3 g VS/(L d). This behaviour could be attributed to the presence of several trace elements (such as Fe, Co, Ni and Mn) in the biochar, which are involved in the action of acetyl-CoA synthase and methyl coenzyme M reductase to catalyse key metabolic steps, especially the methanogenic stage. The pH, alkalinity, TVFA and TVFA/Alkalinity ratio values for the effluents remained within the optimal ranges for the anaerobic digestion process. It was also found that the increase in OLR in the range of 2-5 g VS/(L d) determined a proportional increase in the VS removal rate. However, when the OLR increased up to 7 g VS/(L d), a drastic decrease in the VS removal rate was found for the control reactors. Biochar amendment contributed to a more balanced state of the anaerobic process, preventing biomass washout.

Single-Stage and Two-Stage Anaerobic Digestion of Food Waste: Effect of the Organic Loading Rate on the Methane Production and Volatile Fatty Acids

The high organic content of food waste (FW), which represents the largest proportion of municipal solid waste (MSW) (in Latinamerican countries 50–75%), makes its treatment increasingly common trough technologies such as anaerobic digestion (AD), to obtain value-added by-products, such as renewable energy in the form of methane, digestates and other by-products of biotechnological applications such as long-chain fatty acids. In this study, the influence of semi-continuous reactors in single-stage (R1) and two-stage (R2: acidogenic and R3: methanogenic reactors) configurations on the AD-FW was evaluated (including parameters related to process monitoring, organic matter conversion and process reactions) with the following organic loading rate (OLR: kgVS m-3 d-1) values: i. R1: 0.7, 1.5, 3.0 and 6.0; ii. R2: 3.0, 4.0, 9.0 and 15.0; and R3: 1.0, 2.0, 4.0 and 7.0. The two-stage configuration showed a better performance in terms of: i. the OLRs: 35% higher than that in the single-stage configuration, with chemical oxygen demand (CODtotal) and volatile solid (VS) removal efficiencies > 80%; ii. the best performance in terms of methane production, with statistically significant differences (p<0.05) in the quantity and quality of biogas and iii. obtaining other by-products with high added value, such as behenic and caproic acid, which are useful in biotechnological applications. Additionally, it was found that total reducing sugars (TRS) are an important parameter in the monitoring and conversion of matter organic, mainly in two-stage configuration.

Anaerobic co-digestion of rice straw and de-oiled rice bran for biomethane production

The main aim of anaerobic co-digestion of rice straw is to enhance volumetric methane yields and process stability to achieve synergetic effects. However, high carbon to nitrogen ratio of rice straw causes poor process stability for mono-digestion. This study investigated the co-digestion of rice straw and de-oiled rice bran, a nitrogen-rich by-product of the rice milling industry. The spent bio-slurry (having total solids concentration of 2.19%) from running de-oiled rice bran which inherently contains various types of microbes was used as an inoculum for process start-up. A series of experiments were conducted to optimise the methane yield in semi-continuous anaerobic digester operating at 30 days of hydraulic retention time at mesophilic temperature conditions. Co-digestion studies of rice straw and de-oiled rice bran with three different mixing ratios were conducted by maintaining a specific total solids concentration of 5.0, 7.5, and 10.0% total solids concentration for the designated digester. The biogas production, methane contents, volatile matter degradation efficiency were determined to evaluate the stability and performance of the system. The digester maintained at 5% and 7.5% TS showed nearly similar results in terms of methane yield and volatile matter conversion efficiency. During the entire experimentation the percentage of methane content in the produced biogas was in the range of 24.8–55.4%. The average methane yield of 0.169 and 0.173 L/g-VS was found for digester maintained at 5.0% and 7.5% TS respectively. However, the volatile solid mass removal efficiency of 40.13% and 37.94% was recorded for the digester maintained at 5% and 7.5% TS respectively. Since no chemical or thermal pre-treatment was adopted during the entire process it is highly economical and adaptable for concerned industries. Ultimately, the results of the present study were compared to the previous studies on anaerobic co-digestion of RS, which shows the promising result with no extra added input cost.

Anaerobic co-digestion of food waste and cardboard in different mixing ratios: Impact of ultrasound pre-treatment on soluble organic matter and biogas generation potential at varying food to inoculum ratios

The aim of the present work is to investigate the impact of acetic acid assisted ultrasound pre-treatment of food waste (FW) and cardboard (CB) to overcome the barriers of substrate characteristics. Anaerobic co-digestion (ACoD) experiments were conducted with FW and CB in the mixing ratios of 100:0, 80:20, 60:40 and 50:50 at a food to inoculum (F/I) ratio of 1:1 and 1:2 to investigate its impact on biogas yield, volatile solids (VS) removal, solubilization of organic matter and the specific energy (SE) required. Results revealed that the biogas yield obtained from ultrasound pre-treated samples regardless of mixing ratio was higher at a F/I ratio of 1:2 compared to 1:1. The formation of bubbles, cavities and floc matrix significantly increased and the particle size decreased from 100 μm to 10 μm for FW:CB of 60:40 and 50:50 after pre-treatment. Ultrasound pre-treatment for 45 min was adequate for FW:CB of 100:0 and 80:20 while it was 60 min for 60:40 and 50:50 for increased sCOD between 37 % - 53 %. The VS removal efficiencies increased between 3–15 % from pre-treated samples compared to untreated. The CB mixing ratio is inversely related to SE while directly related to sCOD. Overall, during ACoD of FW and CB, the FW:CB of 80:20 gave a higher biogas yield of 471 mL/g VSadded followed by 60:40 and 50:50.