Anaerobic mesophilic co-digestion of sewage sludge with glycerol: Enhanced biohydrogen production

Anaerobic mesophilic co-digestion of mixed sewage sludge from wastewater treatment plants, WWTP, with crude glycerol, the major byproduct of the biodiesel industry, has been examined using a two-phase digestion process in a semi-continuous CSTR at laboratory scale. The objective was to determine the operational conditions that enhanced biohydrogen and methane production and to evaluate the effect of the organic loading rate (OLR) applied to the process. It was concluded that the Hydraulic Retention Time HRT of the methanogenic stage did not have an important influence on the operational process of co-digestion of sewage sludge and glycerol in terms of efficiency of organic removal and biogas yield. Hence, the results obtained were 73–77% organic matter removal (as CODr) with 0.032 LH₂/gCODr and 0.16 LCH₄/gCODr when the system operated at OLRs in the range of 15.33–17.90 gCODs/L d with HRTs of 3 days in the acidogenic digester and 6, 8, and 10 days in the methanogenic digester. In terms of volatile solids, the results obtained were 92–88% organic matter removal (as VSr) with 0.20 LH₂/gVSr and 1.27 LCH₄/gVSr when the system operated at OLRs in the range of 1.94–2.79 gVS/L d.     

Enhanced bio-methane and bio-hydrogen production using banana plant waste and sewage sludge through anaerobic co-digestion

The yield of bio-methane and bio-hydrogen was enhanced by co-digesting banana plant waste (BPW) and sewage sludge (SS). In stage 1, acclimatisation of the inoculum SS is performed in a continuous stirred tank reactor (CSTR), and its various parameters are analyzed daily. In stage 2, six different BPW to SS ratios is optimized and incubated in manual methane potential test setup (MMPTS) for 40 days. The highest bio-methane and bio-hydrogen yields were observed from R4 (60% BPW and 40% SS). In stage 3, the best-achieved ratio of BPW and SS are then treated with different doses of NaOH as an alkaline pre-treatment. The highest bio-methane was achieved from dose 2 (0.75 mol NaOH) as 620.8 NmL/day. The study shows that the co-digestion of the BWP with SS has promising potential for enhanced bio-methane and bio-hydrogen production.     

Biochemical assays of potential methane to test biogas production from dark fermentation of sewage sludge and agricultural residues

This study aims to study the methane generation potential (BMP tests) of different samples from the dark acid fermentation of sewage sludge:wine vinasse, and sewage sludge:wine vinasse:poultry manure. Specifically, mixtures of sewage sludge (S) and wine vinasse (V) were used in a 50:50 ratio and mixtures of sewage sludge and wine vinasse with 10 g/L of poultry manure (PM) (50:50 + 10 g/L) (S:V + PM). The goal was to determine the effect of the high ammonia concentrations in poultry manure when was used as co-substrate in the anaerobic methanogenic degradation of sewage sludge and vinasse. Results obtained show that the addition of 10 g/L of poultry manure to the SV mixture improves the production of methane generation, reaching values of 166 mL of accumulated methane. The SVPM mixture shows the highest purification percentages, with 63.90% TCOD removal, 79.51% SCOD removal and a yield of 52.05 mLCH₄/gSV_added. The SVPM test showed a higher concentration of microorganisms during the BMP test, although the population of microorganisms for the SV test was doubled and presented greater activity with values of 2.27 versus 1.73E-11 LCH₄/Cells.     

Hydrogen production by anaerobic co-digestion of rice straw and sewage sludge

In this study, the rich carbon content of rice straw and the high nitrogen content of sewage sludge make the straw a good potential substrate and the sludge a viable inoculum for biohydrogen production. Two treatment conditions for the sewage sludge (raw and heat-treated) were used in the present experiments. Batch test using a mixture of rice straw and sewage sludge were carried out to investigate the optimum carbon to nitrogen (C/N) ratio for effective biohydrogen production. The experimental results indicate that untreated sludge could be used as the inoculum for efficient hydrogen production when mixed with the appropriate proportion of rice straw. According to our results, biogas and hydrogen production in all raw sludge cases ramped up more quickly and also exhibited longer and higher hydrogen production in comparison with heat-treated cases. At the C/N ratio of 25 in untreated sludge, hydrogen production was 33% higher than heat-treated one. Additionally, under the same conditions, high and stable hydrogen content (58%) and the maximal hydrogen yield (0.74 mmol H₂/g-VS added straw) were obtained.     

Thermophilic and mesophilic temperature phase anaerobic co-digestion (TPAcD) compared with single-stage co-digestion of sewage sludge and sugar beet pulp lixiviation

The performance of temperature phase anaerobic co-digestion (TPAcD) for sewage sludge and sugar beet pulp lixiviation (using the process of exchanging the digesting substrate between spatially separated thermophilic and mesophilic digesters) was tested and compared to both single-stage mesophilic and thermophilic anaerobic co-digestion. Two Hydraulic Retention Times (HRT) were studied in the thermophilic stage of anaerobic digestion in two temperature phases, maintaining the optimum time of the mesophilic stage at 10 days, obtained as such in single-stage anaerobic co-digestion. In this way, we obtained the advantages of both temperature regimes.Volatile solids removal efficiency from the TPAcD system depended on the sludge exchange rate, but fell within the 72.6–64.6% range. This was higher than the value of 46.8% obtained with single-stage thermophilic digestion and that of 40.5% obtained with mesophilic digestion. The specific methane yield was 424–468 ml CH₄ per gram of volatile solids removed, similar to that of single-stage mesophilic anaerobic digestion. The increase in microbial activity inside the reactor was directly proportional to the organic loading rate (OLR) (or inversely proportional to the HRT) and inversely proportional to the size of the microbial population in single-stage anaerobic co-digestion systems.     

Feasibility of biohydrogen production by anaerobic co-digestion of food waste and sewage sludge

Anaerobic co-digestion of food waste and sewage sludge for hydrogen production was performed in serum bottles under various volatile solids (VS) concentrations (0.5–5.0%) and mixing ratios of two substrates (0:100–100:0, VS basis). Through response surface methodology, empirical equations for hydrogen evolution were obtained. The specific hydrogen production potential of food waste was higher than that of sewage sludge. However, hydrogen production potential increased as sewage sludge composition increased up to 13–19% at all the VS concentrations. The maximum specific hydrogen production potential of 122.9 ml/g carbohydrate-COD was found at the waste composition of 87:13 (food waste:sewage sludge) and the VS concentration of 3.0%. The relationship between carbohydrate concentration, protein concentration, and hydrogen production potential indicated that enriched protein by adding sewage sludge might enhance hydrogen production potential. The maximum specific hydrogen production rate was 111.2 ml H₂/g VSS/h. Food waste and sewage sludge were, therefore, considered as a suitable main substrate and a useful auxiliary substrate, respectively, for hydrogen production. The metabolic results indicated that the fermentation of organic matters was successfully achieved and the characteristics of the heat-treated seed sludge were similar to those of anaerobic spore-forming bacteria, Clostridium sp.     

Hydrogen production through anaerobic co-digestion of food waste and crude glycerol at mesophilic conditions

Hydrogen production from mesophilic anaerobic co-digestion of food waste (FW) and crude glycerol (GLC) was investigated in this study. Batch experiments were carried out at 35 °C for 36 h to assess the effect of supplementation of different glycerol concentrations (1%, 3% and 5% (v/v)) on dark fermentation of FW. The maximum hydrogen yield (180 mLH₂/gVS) was obtained at 5% GLC while the maximum specific production rate (around 13 mLH₂/(gVS.h)) was similar for all experiments with glycerol addition. Besides contributing to increase H₂ productivity, the presence of glycerol reduced the microorganisms acclimation time (Lag phase) in comparison to the control tests conducted without this co-substrate. In addition, the increment of glycerol concentration also enhanced volatile fatty acids generation and favoured the production of 1,3-propanediol (1,3-PDO). In the experimental conditions studied (i.e., 1–5% (v/v) of GLC), the results revealed that co-digestion of FW and GLC is promising and can be potentially used to maximize energy production while contributing to the management and treatment of these wastes.     

Hydrogen and methane production from untreated rice straw and raw sewage sludge under thermophilic anaerobic conditions

Bioenergy produced from co-digestion of sewage sludge (SS) and rice straw (RS) as raw materials, without pretreatment and additional nutrients, was compared for the one-stage system for producing methane (CH₄) and the two-stage system for combined production of hydrogen (H₂) and CH₄ in batch experiments under thermophilic conditions. In the first stage H₂ fermentation process using untreated RS with raw SS, we obtained a high H₂ yield (21 ml/g-VS) and stable H₂ content (60.9%). Direct utilization of post-H₂ fermentation residues readily produced biogas, and significantly enhanced the CH₄ yield (266 ml/g-VS) with stable CH₄ content (75–80%) during the second stage CH₄ fermentation process. Overall, volatile solids removal (60.4%) and total bioenergy yield (8804 J/g-VS) for the two-stage system were 37.9% and 59.6% higher, respectively, than the one-stage system. The efficient production of bioenergy is believed to be due to a synergistically improved second stage process exploiting the well-digested post-H₂ generation residues over the one-stage system.     

Biogas production potential from cotton wastes

The anaerobic treatability and methane generation potential of three different cotton wastes namely, cotton stalks, cotton seed hull and cotton oil cake were determined in batch reactors. In addition, the effects of nutrient and trace metal supplementation were also investigated. To this purpose biochemical methane potential (BMP) experiments were performed for two different waste concentrations, namely 30 and 60 g/l. The results revealed that cotton wastes can be treated anaerobically and are a good source of biogas. Approximately 65, 86 and 78 ml CH₄ were produced in 23 days from 1 g of cotton stalks, cotton seed hull and cotton oil cake in the presence of basal medium (BM), respectively. BM supplementation had an important positive affect on the production of biogas.     

Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm³) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH₄ and H₂S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH₄ potential than dairy manure on a volatile solids basis. While no synergistic effect on CH₄ production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH₄ production. Initial H₂S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H₂S concentrations (0.10–0.14%) and higher CH₄ content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH₄ yield (372 ± 12 L kg⁻¹ VS). The co-digestion mixture containing 40% radish had a lower specific CH₄ yield (345 ± 2 L kg⁻¹ VS) but also showed significantly less H₂S production at start-up and high quality biogas (58% CH₄). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH₄ production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH₄ production of manure digesters in the fall/winter.     

Hydrogen and syngas production from sewage sludge via steam gasification

High temperature steam gasification is an attractive alternative technology which can allow one to obtain high percentage of hydrogen in the syngas from low-grade fuels. Gasification is considered a clean technology for energy conversion without environmental impact using biomass and solid wastes as feedstock. Sewage sludge is considered a renewable fuel because it is sustainable and has good potential for energy recovery. In this investigation, sewage sludge samples were gasified at various temperatures to determine the evolutionary behavior of syngas characteristics and other properties of the syngas produced. The syngas characteristics were evaluated in terms of syngas yield, hydrogen production, syngas chemical analysis, and efficiency of energy conversion. In addition to gasification experiments, pyrolysis experiments were conducted for evaluating the performance of gasification over pyrolysis. The increase in reactor temperature resulted in increased generation of hydrogen. Hydrogen yield at 1000 °C was found to be 0.076 g_gas g_sample⁻¹. Steam as the gasifying agent increased the hydrogen yield three times as compared to air gasification. Sewage sludge gasification results were compared with other samples, such as, paper, food wastes and plastics. The time duration for sewage sludge gasification was longer as compared to other samples. On the other hand sewage sludge yielded more hydrogen than that from paper and food wastes.     

Feasibility of microalgal and macroalgal biomass co-digestion on biomethane production

Algal biomass is a promising candidate for biofuels/chemicals production in recent decades due to their huge availability and ease of cultivation methods when compared to terrestrial crops. Anaerobic digestion (AD) of algal biomass is viable option for green and sustainable biorefinery to produce energy and waste minimization. In this study, the feasibility of microalgal and macroalgal biomass on biomethane production and evaluated for mono-digestion and co-digestion process. The experiments resulted showed that mono-digestion gave relatively lower methane yield (MY) of 102–180 mL/g VS than co-digestion experiments. Co-digestion of microalgae and macroalgae biomass in the ratio of 2:8 provided the peak MY of 256 mL/g VS with an increment in MY over 40–70% than the individual algal biomass. The kinetic analysis showed that synergic effect of co-digestion with proper nutrient balance promoted the methane conversion yield from algal biomass with reduction in lag-phase time and overall improved process performances. Co-digestion of mixed algal strain is a feasible strategy to boost-up the performance of AD with relative easiness in real-field applications.     

Nymphoides peltatum as a novel feedstock for biomethane production via anaerobic co-digestion with waste sludge

Nymphoides peltatum (NP) is exploited as a novel feedstock for biomethane production via anaerobic co-digestion with waste sludge (WS). Batch experiments are conducted under mesophilic condition at NP/WS of 1/3, 2/2, 3/1, 0/4 and 4/0 based on volatile solids (VS). Prior to anaerobic digestion (AD), NP undergoes only natural drying and grinding. The maximum net cumulative methane yield (265.16 mL CH₄·g VS_added^?1) and the highest gross VS removal rate (56.12%) are obtained at NP/WS of 1/3. The kinetic analysis by the modified Gompertz model fit hinted that 28 days is adequate for methane recovery and co-digestion significantly accelerates the digestion rate. Synergetic effect is corroborated to exist in co-digestion due to amiable conditions in term of total ammonia nitrogen, free ammonia, pH, volatile fatty acids and total alkalinity. High-throughput 16S rRNA pyrosequencing reveals that Bacteroidetes, Firmicutes, Methanosarcina and Methanosaeta are conducive to AD of NP.     

Anaerobic co-digestion of sewage sludge with switchgrass: Experimental and kinetic evaluation

Sewage sludge removal via anaerobic digestion provides energy production in addition to waste minimization. Several strategies, such as anaerobic co-digestion, were developed to increase energy production from sewage sludge by improving C/N balance. In this study, anaerobic co-digestion of sewage sludge with an energy crop, namely switchgrass, was evaluated. As a result of studies implemented at different mixing ratios, maximum methane production was measured as 272.06 mLCH₄/gVS at the mixing ratio of 0.4:0.6 (sewage sludge:switchgrass). According to modified kinetic models used for interpretation of synergetic and/or antagonistic effects, anaerobic co-digestion has a synergetic effect on biogas production from both biomass.     

Recent developments of hydrogen production from sewage sludge by biological and thermochemical process

Hydrogen is a kind of clean effective resource. Sewage sludge is regarded as a promising material for hydrogen production because it owns a wide range of sources and the methods are consistent with the goal of sustainable development. This work reviews existing hydrogen production technologies from sewage sludge, including photo-fermentation, dark-fermentation, sequential dark- and photo-fermentation, pyrolysis, gasification, and supercritical water gasification (SCWG). Overall comparison for the involving approaches is conducted based on their inherent features and current development status along with the technical and environmental aspect. Results show that sequential dark- and photo-fermentation and pyrolysis have improved hydrogen yields, but the emissions of carbon dioxide are also remarkable. Biological processes have an advantage in cost, but the reaction rates are inferior to those of thermochemical method. Enhancing methods and improvements are proposed to guide future research on hydrogen production from sewage sludge and promote the effectiveness both technically and economically.     

Enhanced hydrogen production in co-gasification of sewage sludge and industrial wastewater sludge by a pilot-scale fluidized bed gasifier

This research provides a perspective on sludge-to-energy using sewage sludge (SS) and industrial wastewater sludge (IS) co-gasification in a pilot-scale fluidized bed gasifier with temperature controlled at (600–800 °C) using IS addition ratio (0%–60%), and steam-to-biomass ratio (S/B) (0–1.0). The experimental results show that the increase in thermal reaction activity occurred in concordance with the increase in the IS addition. The explanation for such phenomena is that relatively high catalytic Fe/Mn content in industrial wastewater sludge could lower the activation energy. Hydrogen production was increased from 9.1% to 11.94% with an increase in industrial wastewater sludge ratios from 0% to 60%. The produced gas heating value ranged from 4.84 MJ/Nm³ to 5.11 MJ/Nm³, which was coupled with the cold gas efficiency (CGE) ranging from 33.91% to 36.15%. Enhanced hydrogen production in sewage sludge and industrial wastewater sludge co-gasification is investigated in this study.     

Phosphorus recovery from sewage sludge char ash

Phosphorus was recovered from the ash obtained after combustion at different temperatures (600 °C, 750 °C and 900 °C) and after gasification (at 820 °C using a mixture of air and steam as fluidising agent) of char from sewage sludge fast pyrolysis carried out at 530 °C. Depending on the leaching conditions (extraction time, acid load and acid concentration, and type of acid) 90% mass of the original P was recovered. Regarding char combustion ash, higher phosphorus yields are obtained from ash obtained at 900 °C than at 600 °C and 750 °C when using sulphuric acid. Combustion temperature does not affect phosphorus leaching with oxalic acid. A contact time of 2 h and an oxalic acid load of 10 kg kg⁻¹ of P seem sufficient for phosphorus extraction. Almost all phosphorus present in gasification ash is leached after 2 h with both sulphuric and oxalic acid using an acid load of 14 kg kg⁻¹ of P. Char ash is a possible renewable source of phosphorus and it can be an alternative to rock phosphate in fertilizer production. The combination of sewage sludge pyrolysis, combustion or gasification of the char and phosphorus extraction from the final solid residue contributes to the integral exploitation of sewage sludge.     

Enhancement of hydrogen production in steam gasification of sewage sludge by reusing the calcium in lime-conditioned sludge

Gasification is a promising alternative process for sewage sludge energy utilization. CaO has been identified as an effective additive which can increase H₂ content of syngas produced by coal, biomass, and sludge gasification. Considering that lime (CaO) is a widely applied conditioner for sewage sludge dewatering in filter press, this study investigated the enhanced efficiency of syngas, especially regarding H₂ yield, in the catalytic steam gasification of dry dewatered sludge with physically mixed CaO and dry sludge dewatered with CaO as conditioner. The experiments were conducted in an electrically heated reactor at 873 K, 973 K and 1073 K, respectively. According to the results, conditioner CaO improved the H₂ and syngas production more remarkably than additive CaO. It was identified by XRD and SEM-EDX that conditioner CaO was completely converted into Ca(OH)₂ while additive CaO was still presented mainly as CaO. Furthermore, the Ca species of conditioner CaO was evenly distributed over the sludge matrix while Ca species of additive CaO maintained the original state with uneven distribution, both of which could increase the formation of H₂ through interacting with produced gas and catalyzing thermal cracking of tar to some extent. In addition, the pore structure tests and XPS analyses revealed that, comparing to additive CaO, conditioner CaO was more favorable for the formation of pores, and it had a greater potential to encourage partial cleavages of C–C bonds and C–H bonds, resulting in the decomposition of organic macromolecules into relative small molecules, which might be more easily converted to the gaseous products. These indicate that it is valuable to reuse the Ca in lime-conditioned sludge during gasification process.     

Biohydrogen production by anaerobic co-digestion of municipal food waste and sewage sludges

Food waste (FW), primary sludge (PS) and waste activated sludge (WAS) were characterized and found to be complementary in the concentrations of carbohydrates, total Kjeldahl nitrogen (TKN), PO₄–P and some metal for biological hydrogen production. Moreover, FW was found to have low pH buffering capacity while the values for PS and WAS were relatively higher. An anaerobic toxicity analysis (ATA) derived from a methanogenic ATA protocol showed that these waste materials had no toxicity to hydrogen production. Adding phosphate buffer to the FW significantly improved hydrogen production while initial pH was 7.0. Co-digestion of FW and sewage sludge was studied using a batch respirometric cultivation system. All combinations of the feedstocks (FW+PS, FW+WAS and FW+PS+WAS) showed enhanced hydrogen production potential as compared with the individual wastes. A mixing ratio of 1:1 was found to be the best among the ratios tested for all three co-digestion groups. A hydrogen yield of 112 mL/g volatile solid (VS) added was obtained from a combination of FW, PS and WAS. This yield was equivalent to 250 mL/g VS added if only FW contributed to hydrogen production. The reason for the enhancement of hydrogen production was postulated to be multifold in which the increase in buffer capacity in the co-digestion mixture was verified.     

Air gasification of high-ash sewage sludge for hydrogen production: Experimental,sensitivity and predictive analysis

In this work, air gasification of sewage sludge was conducted in a lab-scale bubbling fluidized bed gasifier. Further, the gasification process was modeled using artificial neural networks for the product gas composition with varying temperatures and equivalence ratios. Neural network-based prediction will help to predict the hydrogen production from product gas composition at various temperatures and equivalence ratios. The gasification efficiency and lower heating values were also established as a function of temperatures and equivalence ratios. The maximum H₂ and CO was recorded as 16.26 vol% and 33.55 vol%. Intraileally at ER 0.2 gas composition H₂, CO, and CH₄ show high concentrations of 20.56 vol%, 45.91 vol%, and 13.32 vol%, respectively. At the same time, CO₂ was lower as 20.20 vol% at ER 0.2. Therefore, optimum values are suggested for maximum H₂ and CO yield and lower concentration of CO₂ at ER 0.25 and temperature of 850 °C. A predictive model based on an Artificial Neural network is also developed to predict the hydrogen production from product gas composition at various temperatures and equivalence ratios. The network has been trained with different topologies to find the optimal structure for temperature and equivalence ratio. The obtained results showed that the regression coefficients for training, validation, and testing are 0.99999, 0.99998, and 0.99992, respectively, which clearly identifies the training efficiency of the trained model.