Pretreatment of pulp mill secondary sludge for high-rate anaerobic conversion to biogas

Three pretreatment methods were compared based on their ability to increase the extent and rate of anaerobic bioconversion of pulp mill secondary sludge to biogas. The pretreatment technologies used in these experiments were: (i) thermal pretreatment performed at 170 °C; (ii) thermochemical (caustic) pretreatment performed at pH 12 and 140 °C; and (iii) sonication performed at 20 kHz and 1 W mL⁻¹. Sludge samples were obtained from a sulfite and a kraft pulp mill, and biochemical methane potential (BMP) assays were performed using microbial granules obtained from a high-rate anaerobic digester operating at a pulp mill. Biogas production from untreated sludge was 0.05 mL mg⁻¹ of measured chemical oxygen demand (COD) and 0.20 mL mg⁻¹ COD for kraft and sulfite sludge, respectively. Thermal pretreatment had the highest impact on sludge biodegradability. In this case, biogas yield and production rate from sulfite sludge increased by 50% and 10 times, respectively, while biogas yield and production rate from kraft sludge increased by 280% and 300 times, respectively. Biogas yield correlated to soluble carbohydrate content better than soluble COD.     

Electricity generation from bio-treatment of sewage sludge with microbial fuel cell

A two-chambered microbial fuel cell (MFC) with potassium ferricyanide as its electron acceptor was utilized to degrade excess sewage sludge and to generate electricity. Stable electrical power was produced continuously during operation for 250 h. Total chemical oxygen demand (TCOD) of sludge was reduced by 46.4% when an initial TCOD was 10,850 mg/l. The MFC power output did not significantly depend on process parameters such as substrate concentration, cathode catholyte concentration, and anodic pH. However, the MFC produced power was in close correlation with the soluble chemical oxygen demand (SCOD) of sludge. Furthermore, ultrasonic pretreatment of sludge accelerated organic matter dissolution and, hence, TCOD removal rate in the MFC was increased, but power output was insignificantly enhanced. This study demonstrates that this MFC can generate electricity from sewage sludge over a wide range of process parameters.     

Improvement of sludge anaerobic degradability by combined electro-flotation and electro-oxidation treatment

Recently, bioenergy recovery from sludge biomass has attracted increasing attention due to the high demand for renewable energy resources. In order to enhance methane production from sludge biomass, electrochemical treatment can be used as a novel and efficient pretreatment for the hydrolysis of sludge biomass. In this study, a combined electro-flotation and electro-oxidation pretreatment was employed to improve the anaerobic degradability of sludge biomass. Electro-flotation was efficient in separating flocs in the mixed liquor and led to a sludge volume reduction greater than 60% after 10 min of operation at a current density of 4.72 mA cm⁻². Electro-oxidation using IrO₂/Ti anode was performed to improve the anaerobic degradability of sludge and resulted in a 30% increase in COD solubilization after 30 min of operation at current density of 9.45 mA cm⁻². The factors affecting electro-oxidation, i.e. the gap width between anode and cathode, current density and applied voltage, were investigated to optimize the operating conditions. A biochemical methane potential assay demonstrated that the anaerobic biodegradability of sludge was enhanced by combined electro-flotation and electro-oxidation pretreatment.     

Biological hydrogen production by anaerobic digestion of food waste and sewage sludge treated using various pretreatment technologies

The purpose of this study was to enhance the efficiency of anaerobic co-digestion with sewage sludge using pretreatment technologies and food waste. We studied the effects of various pretreatment methods (thermal, chemical, ultrasonic, and their combination) on hydrogen production and the characteristics of volatile fatty acids (VFAs) using sewage sludge alone and a mixture of sewage sludge and food waste. The pretreatment combination of alkalization and ultrasonication performed best, effecting a high solubilization rate and high hydrogen production (13.8 mL H₂/g VSS_consumed). At a food waste:pretreated sewage sludge ratio of 2:1 in the mixture, the peak hydrogen production value was 5.0 L H₂/L/d. As the production of hydrogen increased, propionate levels fell but butyrate concentrations rose gradually.     

Anaerobic digestion of glycerol derived from biodiesel manufacturing

The anaerobic digestion of glycerol derived from biodiesel manufacturing, in which COD was found to be 1010 g/kg, was studied in batch laboratory-scale reactors at mesophilic temperature using granular and non-granular sludge. Due to the high KOH concentration of this by-product, H₃PO₄ was added to recover this alkaline catalyst as agricultural fertilizer (potassium phosphates). Although it would not be economically viable, a volume of glycerol was distilled and utilised as reference substrate. The anaerobic revalorisation of glycerol using granular sludge achieved a biodegradability of around 100%, while the methane yield coefficient was 0.306 m³ CH₄/kg acidified glycerol. Anaerobic digestion could be a good option for revalorising this available, impure and low priced by-product derived from the surplus of biodiesel companies. The organic loading rate studied was 0.21–0.38 g COD/g VSS d, although an inhibition phenomenon was observed at the highest load.     

Degradation and characteristic changes of organic matter in sewage sludge using microbial fuel cell with ultrasound pretreatment

Microbial fuel cell (MFC) could be an efficient sludge treatment unit in regard of rates and extents of total chemical oxygen demand (TCOD) removal, particularly when ultrasound was applied to pretreat the sludge. This study characterized the organic matter in sludge before and after MFC treatment, with or without ultrasound as a pretreatment stage. The 5-d MFC tests with electric load significantly enhanced TCOD removal rate from 11.3% to 19.2% for raw sludge and from 25% to 57% for sludge pretreated with >0.6 W ml^?1 ultrasound, using conventional anaerobic digestion test (without electric load) as control. The aromatic proteins, soluble microbial byproduct-like fluorescent compounds and carboxylic components, aliphatic components (C–H related), hydrocarbon and carbohydrate materials were identified to be principally released by ultrasound pretreatment and the fuels in the present MFC study.     

Anaerobic storage as a pretreatment for enhanced biodegradability of dewatered sewage sludge

Dewatered sewage sludge is often stored still before further processing and final disposal. This study showed that anaerobic storage of dewatered sewage sludge could hydrolyze organic matter from the sludge matrix, and increase soluble organic acid content from 90 to 2400 mg/L and soluble organic carbon content from 220 to 1650 mg/L. Correspondingly, the contents of proteins, celluloses and hemicelluloses were reduced by 2-9%. Applying anaerobic storage markedly enhanced the efficiency of the subsequent bio-drying process on stored sludge. Correspondingly, biogas and odor gas were produced immediately after commencing the sludge storage. Anaerobic storage with odor control can be applied as a pretreatment process for dewatered sewage sludge in wastewater treatment plants.     

Effect of seed sludge microbial community and activity on the performance of anaerobic reactors during the start-up period

In this study, two laboratory-scale anaerobic batch reactors started up with different inoculum sludges and fed with the same synthetic wastewater were monitored in terms of performance and microbial community shift by denaturant gradient gel electrophoresis fingerprinting and subsequent cloning, sequencing analysis in order to reveal importance of initial quality of inoculum sludge for operation of anaerobic reactors. For this purpose, two different seed sludge were evaluated. In Reactor1 seeded with a sludge having less diverse microbial community (19 operational taxonomic unit (OTU’s) for Bacterial and 8 OTU’s for Archaeal community, respectively) and a methanogenic activity of 150 ml CH₄ g TVS⁻¹ day⁻¹, a chemical oxygen demand (COD) removal efficiency of 78.8 ± 4.17% was obtained at a substrate to microorganism (S/X) ratio of 0.38. On the other hand, Reactor2, seeded with a sludge having a much more diverse microbial community (24 OTU’s for Bacterial and 9 OTU’s for Archaeal communities, respectively) and a methanogenic activity, 450 ml CH₄ g TVS⁻¹ day⁻¹, operated in the same conditions showed a better start-up performance; a COD removal efficiency of over 98% at a S/X ratio of 0.53. Sequence analysis of Seed2 revealed the presence of diverse fermentative and syntrophic bacteria, whereas excised bands of Seed1 related to fermentative and sulfate/metal-reducing bacteria. This study revealed that a higher degree of bacterial diversity, especially the presence of syntrophic bacteria besides the abundance of key species such as methanogenic Archaea may play an important role in the performance of anaerobic reactors during the start-up period.     

Start-up and stabilization of an Anammox process from a non-acclimatized sludge in CSTR

Development of an Anammox (anaerobic ammonium oxidation) process using non-acclimatized sludge requires a long start-up period owing to the very slow growth rate of Anammox bacteria. This article addresses the issue of achieving a shorter start-up period for Anammox activity in a well-mixed continuously stirred tank reactor (CSTR) using non-acclimatized anaerobic sludge. Proper selection of enrichment conditions and low stirring speed of 30 ± 5 rpm resulted in a shorter start-up period (82 days). Activity tests revealed the microbial community structure of Anammox micro-granules. Ammonia-oxidizing bacteria (AOB) were found on the surface and on the outer most layers of granules while nitrite-oxidizing bacteria (NOB) and Anammox bacteria were present inside. Fine-tuning of influent NO₂ ⁻/NH₄ ⁺ ratio allowed Anammox activity to be maintained when mixed microbial populations were present. The maximum nitrogen removal rate achieved in the system was 0.216 kg N/(m³ day) with a maximum specific nitrogen removal rate of 0.434 g N/(g VSS day). During the study period, Anammox activity was not inhibited by pH changes and free ammonia toxicity.     

Enhancement of biochemical methane potential from excess sludge with low organic content by mild thermal pretreatment

Excess sludge with low organic content always led to the failure of anaerobic digestion for methane production. Recently, the mild thermal pretreatment, which is usually operated at temperatures below 120 °C, has drawn much attention due to less energy consumption and no chemical addition. In this study the effect of mild thermal pretreatment (50–120 °C) on the solubilization and methane potential of excess sludge with a low concentration of organic matters was investigated. Experimental results showed that the concentration of soluble organic matters increased gradually with temperature during the mild thermal pretreatment of excess sludge. Biochemical methane potential experiments demonstrated that the potential of methane production from excess sludge was greatly enhanced by mild thermal pretreatment, and under the conditions of pretreatment temperature 100 °C and digestion time 20 d the methane yield was as high as 142.6 ± 2.5 mL/g of volatile solids. Mechanism investigation on the enhancement of methane production from excess sludge exhibited that the consumptions of sludge protein and carbohydrate, the adenosine 5′-triphosphate content of anaerobic microorganisms, the activities of key enzymes related to anaerobic digestion, and the amount of methanogens were all improved by mild thermal pretreatment, in correspondence with the production of methane.     

Bioelectricity production from acidic food waste leachate using microbial fuel cells: Effect of microbial inocula

The effects of three different inocula (domestic wastewater, activated sludge, and anaerobic sludge) on the treatment of acidic food waste leachate in microbial fuel cells (MFCs) were evaluated. A food waste leachate (pH 4.76; 1000 mg chemical oxygen demand (COD)/L) was used as the substrate. The results indicate that the leachate itself can enable electricity production in an MFC, but the co-addition of different inocula significantly reduces the start-up time (approximately 7 days). High COD and volatile fatty acids removal (>87%) were obtained in all MFCs but with only low coulombic efficiencies (CEs) (14–20%). The highest power (432 mW/m³) and CE (20%) were obtained with anaerobic sludge as the co-inoculum. Microbial community analysis (PCR-DGGE) of the established biofilms suggested that the superior performance of the anaerobic sludge-MFC was associated with the enrichment of both fermentative (Clostridium sp. and Bacteroides sp.) and electrogenic bacteria (Magnetospirillum sp. and Geobacter sp.) at the anode.     

Long-term effects of multi-walled carbon nanotubes on the performance and microbial community structures of an anaerobic granular sludge system

Multi-walled carbon nanotubes (MWCNTs) released into the sewage may cause negative and/or positive effects on the treatment system. The objective of this study was to explore over 110 days’ effect of MWCNTs on the performance of anaerobic granular sludge and microbial community structures in an upflow anaerobic sludge blanket (UASB) reactor. The results showed that MWCNTs had no significant effect on the removal of chemical oxidation demand (COD) and ammonia in UASB reactor, but the total phosphorus (TP) removal efficiency increased by 29.34%. The biogas production of the reactor did not change. The anaerobic granular sludge tended to excrete more EPS to resist the effects of MWCNTs during the long-term impact. Illumina MiSeq sequencing of 16S rRNA gene revealed that MWCNTs did not affect the microbial diversity, but altered the composition and structure of microbial community in the reactor. In this process, Saccharibacteria replaced Proteobacteria as the highest abundant bacterial phylum. MWCNTs promoted the differentiation of methanogen structure, resulting in increase of Methanomassiliicoccus, Methanoculleus, and the uncultured WCHA1–57. These results indicated that MWCNTs impacted the performance of UASB reactor and the structures of the microbial community in anaerobic granular sludge.

     

Effect of Iron Hydroxide on Phosphate Removal during Anaerobic Digestion of Activated Sludge

The addition of iron (III) hydroxide during methanogenic digestion of activated sludge by anaerobic sludge displaying an iron-reducing activity resulted in a microbial reduction of iron (III) with the formation of iron (II), capable of precipitating phosphates. The feasibility of eliminating 66.6 to 99.6% of the dissolved phosphate at initial concentrations of 1000 to 3500 mg PO^3- ₄/l by adding 6420 mg/l iron (III) hydroxide into a reactor for anaerobic fermentation of activated sludge was analyzed. The optimal ratio of iron (III) added to dissolved phosphate removed (mg) providing a 95% removal amounted to 2 : 1. These results may be used in new technology for anaerobic wastewater treatment with phosphate removal.     

Influence of bulking agent on sewage sludge composting process

Four types of compost, consisting of mixtures of Acacia dealbata (A) with sewage sludge (SS) were studied in a laboratory reactor. Composting time was 80 days and parameters monitored over this period included temperature, organic matter, pH, CO₂, O₂, C/N ratio, Kjeldahl-N, as well as maturity indexes. All the studied parameters were influenced by the bulking amount used. The highest profile temperature measured was for the A/SS 1/2 (w/w) mixture that reached a maxima temperature of 67 °C and lower maximum temperatures of 52, 48 and 46 °C were observed for A/SS 1/3, 1/1 and 1/0 composts, respectively. The kinetic model used showed that a descent of sewage sludge in the composting mixtures favored the enzyme–substrate affinity. However, an increase in depending on the parameters of the process factors was observed when the sewage sludge ratio was increased in mixtures. The optimal amounts of sewage sludge for co-composting with Acacia indicate that moderate amounts of sludge (1/1) would be the best compromise.     

Anaerobic granule development for removal of pentachlorophenol in an upflow anaerobic sludge blanket (UASB) reactor

The granulation process using synthetic wastewater containing pentachlorophenol (PCP) in four 1.1 l laboratory scale upflow anaerobic sludge blanket (UASB) reactors was studied, and the anaerobic biotransformation of PCP during the granulation process investigated. After 110 days granular sludge was developed and up to 160 and 180 mg/l of PCP was added into the reactors R1 and R2, respectively, when they were inoculated with acclimated anaerobic sludge from an anaerobic digester of a citric acid plant. The inoculum was predominately composed of bacilli and filamentous bacteria. Granulation did not occur in reactors R3 and R4 which were inoculated with acclimated anaerobic sludge from aerobic sludge of the municipal sewage treatment plant which consisted mainly of cocci. Despite similar bacilli in the granule, the filamentous bacteria from reactor R1 were thicker than those of reactor R2. The granular sludge had a maximum diameter of 2.5 and 2.2 mm, and SMA of 1.44 and 1.32 gCOD/gTVS per day for reactors R1 and R2, respectively. Over 98% chemical oxygen demand (COD) removal rate and 99% of PCP removal rate were achieved when reactors R1 and R2 were operated at PCP and COD loading rates of 150 and 7.5 g/l per day, respectively. H₂-producing acetogens were the dominant anaerobes in the granular sludge.     

Evaluation of pretreatment methods on mixed inoculum for both batch and continuous thermophilic biohydrogen production from cassava stillage

Anaerobic sludges, pretreated by chloroform, base, acid, heat and loading-shock, as well as untreated sludge were evaluated for their thermophilic fermentative hydrogen-producing characters from cassava stillage in both batch and continuous experiments. Results showed that the highest hydrogen production was obtained by untreated sludge and there were significant differences (p < 0.05) in hydrogen yields (varied from 32.9 to 65.3 mlH₂/gVS) among the tested pretreatment methods in batch experiments. However, the differences in hydrogen yields disappeared in continuous experiments, which indicated the pretreatment methods had only short-term effects on the hydrogen production. Further study showed that alkalinity was a crucial parameter influencing the fermentation process. When the influent was adjusted to pH 6 by NaHCO₃ instead of NaOH, the hydrogen yield increased from about 40 to 52 mlH₂/gVS in all the experiments. Therefore, pretreatment of anaerobic sludge is unnecessary for practical thermophilic fermentative hydrogen production from cassava stillage.     

Anaerobic co-digestion of sewage sludge and grease trap: Assessment of enzyme addition

Anaerobic co-digestion of grease trap and sewage sludge from a wastewater treatment plant is evaluated. Enzyme-lipase application, both addition and dosage, are evaluated by fitting the methane production of biochemical potential tests with the first order model. The enzyme addition effect, at 2, 5 and 10% of grease trap (%GT VS_FED^?1) and the enzymes doses, between 0.25 and 1.67% (v/v), without and with grease trap presence were studied. Grease trap addition showed a negative effect on the waste biodegradability, which was completely overcome by the addition of lipase. Enzyme addition improved notably the methane production for all grease trap fractions studied. In regards to the dosage, the best result was achieved between 0.33 and 0.83% (v/v) of enzyme. The co-digestion of sewage sludge and grease trap may be a feasible process by using lipases due to the saving in operational costs and the increase in the biogas production     

Biodegradation of nitrobenzene by a sequential anaerobic-aerobic process

Nitrobenzene was completely degraded by mixed cultures using a sequential anaerobic-aerobic treatment process. Under anaerobic conditions in a fixed-bed column aniline was formed from nitrobenzene through gratuitous reduction by cells of sewage sludge. This reaction was accelerated by the addition of glucose. Complete mineralization of aniline was accomplished by subsequent aerobic treatment using activated sludge as inoculum. The maximum degradation rate of nitrobenzene (4.5 mM) in the two-stage system was 552 mg l^–1d^–1, referring to 154 mg of nitrobenzene per gram of glucose. In a second experimental phase glucose as cosubstrate and H-donor was replaced by synthetic waste containing ethanol, methanol, isopropanol and acetone. Again, nitrobenzene (1.9 mM) was completely degraded (maximum degradation rate of 237 mg l^–d^–1, referring to 251 mg per gram of solvents). The major advantage of the described two-stage process is that the reduction of nitrobenzene by anaerobic pretreatment drastically reduces emission by stripping during aerobic treatment.Abbreviations HRT hydraulic retention time - OD₅₄₆ optical density at 546 nm     

Biodegradation of petroleum industry oily-sludge using Jordanian oil refinery contaminated soil

The main objective of this study was to evaluate the effect of oily sludge concentration on its biodegradability in soil. Oily sludge was collected and applied to microcosms at full-, half-, or quarter-strength concentrations equivalent to 44.2, 22.2, and 11.1 g kg^?1 soil, respectively, of total petroleum hydrocarbons (TPH) contained in oily sludge. The biodegradability of oily sludge was evaluated by measuring CO₂ evolution and by measuring removal of TPH as well as its main composing fractions; namely; alkanes, aromatics, NSO-compounds, and asphaltenes. The collected soil contained 3.63 × 10⁶ cfu g^?1 soil of hydrocarbon-degrading bacteria, which is satisfactory to drive successful biodegradation of hydrocarbons in soil. These numbers increased significantly with oily sludge addition at a rate proportional to the added TPH reaching 3.35 × 10⁷ cfu g^?1 soil in the half-strength treatment. TPH mineralization rate followed the same pattern. However, TPH-mineralization efficiency was the greatest in quarter-strength treatment at 18.3%. TPH-removal efficiency was also highest in quarter-strength treatment at 30.9%. Nutrients addition caused mineralization inhibition. Since nutrients were added as a ratio of the added carbon, inhibition was the greatest with the highest TPH treatment. While alkanes were degraded, aromatics and asphaltenes were not, and NSO-compounds were enriched. Although SDS was completely biodegradable in soil, its addition promoted mineralization and removal of TPH from soil.     

Enhanced saccharification of wheat straw with the application of ultrasonic-assisted quaternary ammonium hydroxide pretreatment

Lignocelluloses featuring complicated structure and poor degradability usually require pretreatment before its utilization. In this study, an ultrasonic-assisted pretreatment by using quaternary ammonium hydroxide was introduced to enhance biodegradability of lignocellulosic biomass. The synergistic chemical and mechanical pretreatment were supposed to be responsible for both external surface destruction and internal structure disruption of lignocelluloses. High-efficient lignin removal accompanied with obvious structural (crystallinity) transformation was achieved in the pretreated straws. Process analysis indicated that factors of time, temperature, concentration of solvent, and ultrasound power intensity turned out to be significant for pretreatment, and a 4-fold increased saccharification yield of around 92.4% as compared to untreated straw was obtained from the wheat straw pretreated by 15% solvent at 50 °C for 0.5 h in power intensity 344 W/cm². All results suggest that the combined chemical and mechanical treatment can significantly improve the bio-accessibility of lignocelluloses, leading to the enhanced utilization efficiency.