Microbial Enrichment Techniques on Syngas and CO2 Targeting Production of Higher Acids and Alcohols.

(1) Background: Microbial conversion of gaseous molecules, such as CO2, CO and H2, to valuable compounds, has come to the forefront since the beginning of the 21st century due to increasing environmental concerns and the necessity to develop alternative technologies that contribute to a fast transition to a more sustainable era. Research efforts so far have focused on C1-C2 molecules, i.e., ethanol and methane, while interest in molecules with higher carbon atoms has also started to emerge. Research efforts have already started to pay off, and industrial installments on ethanol production from steel-mill off-gases as well as methane production from the CO2 generated in biogas plants are a reality. (2) Methodology: The present study addresses C4-C6 acids and butanol as target molecules and responds to how the inherent metabolic potential of mixed microbial consortia could be revealed and exploited based on the application of different enrichment methods (3) Results and Conclusions: In most of the enrichment series, the yield of C4-C6 acids was enhanced with supplementation of acetic acid and ethanol together with the gas substrates, resulting in a maximum of 43 and 68% (e-mol basis) for butyric and caproic acid, respectively. Butanol formation was also enhanced, to a lesser degree though and up to 9% (e-mol basis). Furthermore, the microbial community exhibited significant shifts depending on the enrichment conditions applied, implying that a more profound microbial analysis on the species level taxonomy combined with the development of minimal co-cultures could set the basis for discovering new microbial co-cultures and/or co-culturing schemes.

Prediction of methane yield and pretreatment efficiency of lignocellulosic biomass based on composition.

Lignocellulosic biomass is considered a key resource for the future expansion of biogas production through anaerobic digestion (AD), and research on the development of pretreatment technologies for improving biomass conversion is an intensive and fast-growing field. Consequently, there is a need for creating tools able to predict the efficiency of a certain pretreatment on different biomass types, fast and accurately, and to assist in selecting a pretreatment technology for a specific biomass. In this study, seven different types of raw lignocellulosic biomass of industrial relevance were systematically analyzed regarding their composition (carbohydrates, lignin, lipids, ash, extractives, etc.) and subjected to a common pretreatment. The aim of the study was to identify the most important characteristics that make a biomass good receptor of the specific pretreatment prior to AD. A simple ammonia pretreatment was chosen as a case study and partial least squares regression (PLS-R) was used for modeling initially the ultimate methane yield of raw and pretreated biomass. In the sequel, PLS-R was used for modeling the efficiency of the pretreatment on increasing the ultimate methane yield and hydrolysis rate as a function of the biomass composition. The fit of the models was satisfactory, ranging from R2 = 0.89 to R2 = 0.97. The results showed that the most decisive characteristics for predicting the efficiency of the pretreatment were the lipid (r = -0.88), ash (r = +0.79), protein (r = -0.61), and hemicellulose/lignin (r = -0.53) content of raw biomass. Finally, the approach followed in this study facilitated an improved understanding of the mechanism of the pretreatment and presented a methodology to be followed for developing tools for the prediction of pretreatment efficiency in the field of lignocellulosic biomass valorization.

Effects of different treatments of manure on mitigating methane emissions during storage and preserving the methane potential for anaerobic digestion.

Current agricultural practices in regards to storage of manure come with a significant GHG contribution, due, to a big extent, to CH4 emissions. For example, in Denmark, the agricultural sector is responsible for about 11.1 metric tons of CO2 equivalents; only about 0.2 metric tons come directly from CO2, while 6.0 tons come from CH4. The present study aims at evaluating and comparing two methods based on their effect on suppressing CH4 emissions during storage as well as on preserving and enhancing CH4 yield in a subsequent anaerobic digestion step: the commonly applied acidification with H2SO4 as acidifying agent and thermal treatment at the mild temperatures of 70 and 90 °C (pasteurization). Although both treatments effectively suppressed CH4 emissions during storage, they exhibited a significant difference in preserving and/or enhancing the CH4 potential of manure. Specifically, thermal treatment resulted in 16-35% enhancement of CH4 potential, while acidification resulted in decreasing the CH4 yield by 6-23% compared to non-treated manure. Further investigation showed that storage itself positively affected the CH4 potential of treated manure in a subsequent anaerobic digestion step; this was attributed to microbial activity other than biomethanation during storage. In overall and based on the results obtained regarding suppression of CH4 emissions during storage as well as CH4 potential enhancement, pasteurization at the temperatures tested is a promising alternative to the broadly applied acidification of manure.

Variables and Mechanisms Affecting Electro-Membrane Extraction of Bio-Succinic Acid from Fermentation Broth.

The production of succinic acid from fermentation is a promising approach for obtaining building-block chemicals from renewable sources. However, the limited bio-succinic yield from fermentation and the complexity of purification has been making the bio-succinic acid production not competitive with petroleum-based succinic acid. Membrane electrolysis has been identified to be a promising technology in both production and separation stages of fermentation processes. This work focuses on identifying the key operational parameters affecting the performance of the electrolytic cell for separating succinic acid from fermentation broth through an anionic exchange membrane. Indeed, while efforts are mainly focused on studying the performance of an integrated fermenter-electrolytic cell system, a lack of understanding remains in how to tune the electrolytic cell and which main parameters are involved. The results show that a single electrolytic cell of operating volume 250 mL was able to extract up to 3 g L-1 h-1 of succinic acid. The production of OH- ions by water electrolysis can act as a buffer for the fermenter and it could be tuned as a function of the extraction rate. Furthermore, as the complexity of the solution in terms of the quantity and composition of the ions increased, the energy required for the separation process decreased.

Biofouling Mitigation Approaches during Water Recovery from Fermented Broth via Forward Osmosis.

Forward Osmosis (FO) is a promising technology that can offer sustainable solutions in the biorefinery wastewater and desalination fields, via low energy water recovery. However, microbial biomass and organic matter accumulation on membrane surfaces can hinder the water recovery and potentially lead to total membrane blockage. Biofouling development is a rather complex process and can be affected by several factors such as nutrient availability, chemical composition of the solutions, and hydrodynamic conditions. Therefore, operational parameters like cross-flow velocity and pH of the filtration solution have been proposed as effective biofouling mitigation strategies. Nevertheless, most of the studies have been conducted with the use of rather simple solutions. As a result, biofouling mitigation practices based on such studies might not be as effective when applying complex industrial mixtures. In the present study, the effect of cross-flow velocity, pH, and cell concentration of the feed solution was investigated, with the use of complex solutions during FO separation. Specifically, fermentation effluent and crude glycerol were used as a feed and draw solution, respectively, with the purpose of recirculating water by using FO alone. The effect of the abovementioned parameters on (i) ATP accumulation, (ii) organic foulant deposition, (iii) total water recovery, (iv) reverse glycerol flux, and (v) process butanol rejection has been studied. The main findings of the present study suggest that significant reduction of biofouling can be achieved as a combined effect of high-cross flow velocity and low feed solution pH. Furthermore, cell removal from the feed solution prior filtration may further assist the reduction of membrane blockage. These results may shed light on the challenging, but promising field of FO process dealing with complex industrial solutions.

Cryopreservation and fast recovery of enriched syngas-converting microbial communities.

Over the last decades, the use of mixed microbial communities has attracted increasing scientific attention due to their potential biotechnological applications in several emerging technological platforms such as the carboxylate, bioplastic, syngas and bio-electrochemical synthesis platforms. However, this increasing interest has not been accompanied by a parallel development of suitable cryopreservation techniques for microbial communities. While cryopreservation methods for the long-term storage of axenic cultures are well established, their effectiveness in preserving the microbial diversity and functionality of microbial communities has rarely been studied. In this study, the effect of the addition of different cryopreservation agents on the long-term storage of microbial communities at -80 °C was studied using a stable enrichment culture converting syngas into acetate and ethanol. The cryopreservation agents considered in the study were glycerol, dimethylsulfoxide, polyvinylpyrrolidone, Tween 80 and yeast extract, as well as with no addition of cryopreservation agent. Their effectiveness was evaluated based on the microbial activity recovery and the maintenance of the microbial diversity and community structure upon revival of the microbial community. The results showed that the commonly used glycerol and no addition of cryopreservation agent were the least recommendable methods for the long-term frozen storage of microbial communities, while Tween 80 and polyvinylpyrrolidone were overall the most effective. Among the cryoprotectants studied, polyvinylpyrrolidone and especially Tween 80 were the only ones assuring reproducible results in terms of microbial activity recovery and microbial community structure preservation.

On the Effect of Aqueous Ammonia Soaking Pre-Treatment on Continuous Anaerobic Digestion of Digested Swine Manure Fibers.

(1) Background: The continuously increasing demand for renewable energy sources renders anaerobic digestion as one of the most promising technologies for renewable energy production. Due to the animal production intensification, manure is being used as the primary feedstock for most biogas plants. Their economical profitable operation, however, relies on increasing the methane yield from the solid fraction of manure, which is not so easily degradable. The solid fraction after anaerobic digestion, the so-called digested fibers, consists mainly of hardly biodegradable material and comes at a lower mass per unit volume of manure compared to the solid fraction before anaerobic digestion. Therefore, investigation on how to increase the biodegradability of digested fibers is very relevant. So far, Aqueous Ammonia Soaking (AAS), has been successfully applied on digested fibers separated from the effluent of a manure-fed, full-scale anaerobic digester to enhance their methane productivity in batch experiments. (2) Methods: In the present study, continuous experiments at a mesophilic (38 °C) CSTR-type anaerobic digester fed with swine manure first and a mixture of manure with AAS-treated digested fibers in the sequel, were performed. Anaerobic Digestion Model 1 (ADM1) previously fitted on manure fed digester was used in order to assess the effect of the addition of AAS-pre-treated digested manure fibers on the kinetics of anaerobic digestion process. (3) Results and Conclusions: The methane yield of AAS-treated digested fibers under continuous operation was 49-68% higher than that calculated in batch experiments in the past. It was found that AAS treatment had a profound effect mainly on the disintegration/hydrolysis rate of particulate carbohydrates. Comparison of the data obtained in the present study with the data obtained with AAS-pre-treated raw manure fibers in the past revealed that hydrolysis kinetics after AAS pre-treatment were similar for both types of biomasses.

Enrichment of syngas-converting mixed microbial consortia for ethanol production and thermodynamics-based design of enrichment strategies.

BACKGROUND: The production of ethanol through the biochemical conversion of syngas, a mixture of H2, CO and CO2, has been typically studied using pure cultures. However, mixed microbial consortia may offer a series of benefits such as higher resilience and adaptive capacity, and non-sterile operation, all of which contribute to reducing the utility consumption when compared to pure culture-based processes. This work focuses on the study of strategies for the enrichment of mixed microbial consortia with high ethanologenic potential, investigating the effect of the operational conditions (pH and yeast extract addition) on both the ethanol yield and evolution of the microbial community along the enrichment process. The pH was selected as the main driver of the enrichment as it was expected to be a crucial parameter for the selection of carboxydotrophic bacteria with high ethanologenic potential. Additionally, a thermodynamic analysis of the network of biochemical reactions carried out by syngas-converting microbial consortia was performed and the potential of using thermodynamics as a basis for the selection of operational parameters favoring a specific microbial activity was evaluated. RESULTS: All enriched consortia were dominated by the genus Clostridium with variable microbial diversity and species composition as a function of the enrichment conditions. The ethanologenic potential of the enriched consortia was observed to increase as the initial pH was lowered, achieving an ethanol yield of 59.2 ± 0.2% of the theoretical maximum in the enrichment at pH 5. On the other hand, yeast extract addition did not affect the ethanol yield, but triggered the production of medium-chain fatty acids and alcohols. The thermodynamic analysis of the occurring biochemical reactions allowed a qualitative prediction of the activity of microbial consortia, thus enabling a more rational design of the enrichment strategies targeting specific activities. Using this approach, an improvement of 22.5% over the maximum ethanol yield previously obtained was achieved, reaching an ethanol yield of 72.4 ± 2.1% of the theoretical maximum by increasing the initial acetate concentration in the fermentation broth. CONCLUSIONS: This study demonstrated high product selectivity towards ethanol using mixed microbial consortia. The thermodynamic analysis carried out proved to be a valuable tool for interpreting the metabolic network of microbial consortia-driven processes and designing microbial-enrichment strategies targeting specific biotransformations.

Combined polyhydroxyalkanoates (PHA) and 1,3-propanediol production from crude glycerol: Selective conversion of volatile fatty acids into PHA by mixed microbial consortia.

Crude glycerol is an important by-product of the biodiesel industry, which can be converted into volatile fatty acids (VFA) and/or 1,3-propanediol (1,3-PDO) by fermentation. In this study, a selective conversion of VFA to polyhydroxyalkanoates (PHA) was attained while leaving 1,3-PDO in the supernatant by means of mixed microbial consortia selection strategies. The process showed highly reproducible results in terms of PHA yield, 0.99 ±â€¯0.07 Cmol PHA/Cmol S (0.84 g COD PHA/g COD S), PHA content (76 ±â€¯3.1 g PHA/100 g TSS) and 1,3-PDO recovery (99 ±â€¯2.1%). The combined process had an ultimate yield from crude glycerol of 0.19 g COD PHA and 0.42 g COD 1,3-PDO per g of input COD. The novel enrichment strategy applied for selectively transforming fermentation by-products into a high value product (PHA) demonstrates the significance of the enrichment process for targeting specific bio-transformations and could potentially prove valuable for other biotechnological applications as well.

Polyhydroxyalkanoates (PHA) production from fermented crude glycerol: Study on the conversion of 1,3-propanediol to PHA in mixed microbial consortia.

Crude glycerol, a by-product from the biodiesel industry, can be converted by mixed microbial consortia into 1,3-propanediol (1,3-PDO) and volatile fatty acids. In this study, further conversion of these main products into polyhydroxyalkanoates (PHA) was investigated with the focus on 1,3-PDO. Two different approaches for the enrichment of PHA accumulating microbial consortia using an aerobic dynamic feeding strategy were applied. With the first approach, where nitrogen was present during the whole cycle, no net production of PHA from 1,3-PDO was observed in the fermented effluent, not even in a nitrogen-limited PHA accumulation assay. Nevertheless, experiments in synthetic substrates revealed that the conversion of 1,3-PDO to PHA was possible under nitrogen limiting conditions. Thus, a different enrichment strategy was formulated where nitrogen was limited during the feast phase to stimulate the storage response. Nitrogen was still supplied during the famine phase. With the latter strategy, a net production of PHA from 1,3-PDO was observed at a yield of 0.24 Cmol PHA/Cmol 1,3-PDO. The overall yield from the fermented effluent was 0.42 Cmol PHA/Cmol substrate. Overall, the PHA yield from 1,3-PDO seemed to be limited, similarly to when using glycerol as a substrate, by a decarboxylation step and accumulation of other storage polymers such as glycogen, and possibly, lipid inclusions.

Recent Advances and Challenges towards Sustainable Polyhydroxyalkanoate (PHA) Production.

Sustainable biofuels, biomaterials, and fine chemicals production is a critical matter that research teams around the globe are focusing on nowadays. Polyhydroxyalkanoates represent one of the biomaterials of the future due to their physicochemical properties, biodegradability, and biocompatibility. Designing efficient and economic bioprocesses, combined with the respective social and environmental benefits, has brought together scientists from different backgrounds highlighting the multidisciplinary character of such a venture. In the current review, challenges and opportunities regarding polyhydroxyalkanoate production are presented and discussed, covering key steps of their overall production process by applying pure and mixed culture biotechnology, from raw bioprocess development to downstream processing.

Optimization of Aqueous Ammonia Soaking of manure fibers by Response Surface Methodology for unlocking the methane potential of swine manure.

Swine manure mono-digestion often results to economically non-feasible processes, due to the high dilution and ammonia concentration together with the low degradation rates it presents. The effects of different parameters of Aqueous Ammonia Soaking (AAS) as a pretreatment for improving the digestion of manure fibers when coupled to an ammonia removal step were investigated in this study. Response Surface Methodology was followed and the influence and interactions of the following AAS parameters were studied: NH3 concentration, duration and solid-to-liquid ratio. The mild conditions found to be optimal (7%w/w NH3, 96h, and 0.16kg/L) in combination to a significant increase of the short term CH4 yield (244% in 17days), make this pretreatment a promising solution for improving swine manure mono-digestion. Furthermore, compositional analysis of the manure fibers revealed significant solubilization of hemicellulose, while no lignin removal or loss of cellulose occurred under optimal conditions.

Effect of in situ acids removal on mixed glucose and xylose fermentation by Clostridium tyrobutyricum.

In the present study, the effect of potassium ions and increasing concentrations of glucose and xylose on the growth of a strain of Clostridium tyrobutyricum, adapted to wheat straw hydrolysate, was investigated. Application of continuous fermentation of a mixture of glucose and xylose and in situ acid removal by reverse electro enhanced dialysis (REED) was investigated as a method to alleviate potassium and end-product inhibition and consequently enhance the sugar consumption rates and butyric acid productivity. It was found that glucose and xylose were not inhibitory up to a concentration of 50 and 37 g L(-1) respectively, and that they were consumed at comparable rates when fermented alone. However, continuous fermentation of a mixture of glucose and xylose resulted in a significantly decreased xylose consumption rate compared to that of glucose alone, supporting the conclusion that C. tyrobutyricum has a lower affinity for xylose than for glucose. Potassium ions negatively affected the effective maximum growth rate of C. tyrobutyricum at concentrations higher than 5 g L(-1) exhibiting a non-competitive type of inhibition. Continuous fermentation of a glucose and xylose mixture with simultaneous acid removal by REED resulted in a two to threefold increase of the glucose consumption rate, while the xylose consumption rate was enhanced sixfold compared to continuous fermentation without in situ acid removal. Similarly, butyric acid productivity was enhanced by a factor of 2-3, while the yield remained unaffected.

Enhancement of methane yield from wheat straw, miscanthus and willow using aqueous ammonia soaking.

The increasing demand for methane production cannot be satisfied by the use of anaerobic digestion only from waste/wastewater treatment. Perennial energy crops, such as miscanthus and willow, as well as agricultural residues can be considered as options for increasing the methane production through biomass digestion, due to their high organic content and biomass yield. These materials present a great potential, which is only limited by the rigid lignocellulosic structure. In this case, it is possible to apply a pretreatment step in order to achieve increased biogas production. In the present study, aqueous ammonia soaking (AAS) has been investigated as a method to disrupt the lignocellulosic structure and increase the methane yield of wheat straw, miscanthus and willow. Among the three biomasses tested, wheat straw and miscanthus were the most promising in terms of methane production, yielding around 200 and 230 ml of methane per gram of total solids. In all three cases, AAS resulted to an increase in methane yield of 37-41%, 25-27% and 94-162% for wheat straw, miscanthus and willow, respectively. A comparison of the methane yields after 20 and 50 days of anaerobic digestion revealed that AAS affected positively the methane production rate as well. AAS also resulted to a low solubilization of sugars, with a 15.4% and 8.9% increase in soluble xylose concentration in miscanthus and willow, respectively, and a 5% solubilization of glucose in AAS-pretreated miscanthus.

ADM1-based modeling of methane production from acidified sweet sorghum extract in a two stage process.

The present study focused on the application of the Anaerobic Digestion Model 1 on the methane production from acidified sorghum extract generated from a hydrogen producing bioreactor in a two-stage anaerobic process. The kinetic parameters for hydrogen and volatile fatty acids consumption were estimated through fitting of the model equations to the data obtained from batch experiments. The simulation of the continuous reactor performance at all HRTs tested (20, 15, and 10d) was very satisfactory. Specifically, the largest deviation of the theoretical predictions against the experimental data was 12% for the methane production rate at the HRT of 20d while the deviation values for the 15 and 10d HRT were 1.9% and 1.1%, respectively. The model predictions regarding pH, methane percentage in the gas phase and COD removal were in very good agreement with the experimental data with a deviation less than 5% for all steady states. Therefore, the ADM1 is a valuable tool for process design in the case of a two-stage anaerobic process as well.

Hydrogen and methane production through two-stage mesophilic anaerobic digestion of olive pulp.

The present study focused on the anaerobic biohydrogen production from olive pulp (two phase olive mill wastes, TPOMW) and the subsequent anaerobic treatment of the effluent for methane production under mesophilic conditions in a two-stage process. Biohydrogen production from water-diluted (1:4) olive pulp was investigated at hydraulic retention times (HRT) of 30 h, 14.5 h and 7.5 h while methane production from the effluent of hydrogenogenic reactor was studied at 20 d, 15 d, 10d and 5d HRT. In comparison with previous studies, it has been shown that the thermophilic hydrogen production process was more efficient than the mesophilic one in both hydrogen production rate and yield. The methanogenic reactor was successfully operated at 20, 15 and 10 days HRT while it failed when an HRT of 5 days was applied. Methane productivity reached the maximum value of 1.13+/-0.08 L/L/d at 10 days HRT whereas the methane yield increased with the HRT. The Anaerobic Digestion Model no. 1 (ADM1) was applied to the obtained experimental data from the methanogenic reactor to simulate the digester response at all HRT tested. The ability of the model to predict the experimental results was evident even in the case of the process failure, thus implying that the ADM1 could be a valuable tool for process design even in the case of a complex feedstock. In general, the two-stage anaerobic digestion proved to be a stable, reliable and effective process for energy recovery and stabilization treatment of olive pulp.

Improving anaerobic sewage sludge digestion by implementation of a hyper-thermophilic prehydrolysis step.

The present study focuses on a two-step process for treatment and stabilisation of primary sludge. The process consists of a hyper-thermophilic hydrolysis step operated at 70 degrees C and a hydraulic retention time (HRT) of 2 days followed by a thermophilic (55 degrees C) anaerobic digestion step at a HRT of 13 days. A one-step anaerobic digester operated at 55 degrees C and 15 days HRT was used as a reference process. The two-step process was characterized by a 12% higher organic suspended solids removal efficiency and better pathogen reduction effect than the conventional one-step digestion. The microbial community of the digester fed with pre-treated sludge was characterised by a higher activity compared to that of the digester treating raw sludge. Moreover, the pre-treatment of the primary sludge resulted up to 48% increase of the methane potential (20.09 and 13.56 mmol CH(4)g(-)VS(-1) with and without pre-treatment, respectively) and up to 115% increase of the methane production rate. Finally it was shown that the extra energy requirements for the operation of a pre-treatment step would be covered by the energy produced from the extra methane production and in addition there would be a significant energy surplus of 2.17 kJ d(-1) for the system tested.

Biofuels generation from sweet sorghum: fermentative hydrogen production and anaerobic digestion of the remaining biomass.

The present study focuses on the exploitation of sweet sorghum biomass as a source for hydrogen and methane. Fermentative hydrogen production from the sugars of sweet sorghum extract was investigated at different hydraulic retention times (HRT). The subsequent methane production from the effluent of the hydrogenogenic process and the methane potential of the remaining solids after the extraction process were assessed as well. The highest hydrogen production rate (2550 ml H(2)/d) was obtained at the HRT of 6h while the highest yield of hydrogen produced per kg of sorghum biomass was achieved at the HRT of 12h (10.4l H(2)/kg sweet sorghum). It has been proved that the effluent from the hydrogenogenic reactor is an ideal substrate for methane production with approximately 29l CH(4)/kg of sweet sorghum. Anaerobic digestion of the solid residues after the extraction process yielded 78l CH(4)/kg of sweet sorghum. This work demonstrated that biohydrogen production can be very efficiently coupled with a subsequent step of methane production and that sweet sorghum could be an ideal substrate for a combined gaseous biofuels production.

Thermal and enzymatic pretreatment of sludge containing phthalate esters prior to mesophilic anaerobic digestion.

The present study aimed at investigating the effect of thermal pretreatment of sludge at 70 degrees C on the anaerobic degradation of three commonly found phthalic acid esters (PAE): di-ethyl phthalate (DEP), di-butyl phthalate (DBP), and di-ethylhexyl phthalate (DEHP). Also, the enzymatic treatment at 28 degrees C with a commercial lipase was studied as a way to enhance PAE removal. Pretreatment at 70 degrees C of the sludge containing PAE negatively influenced the anaerobic biodegradability of phthalate esters at 37 degrees C. The observed reduction of PAE biodegradation rates after the thermal pretreatment was found to be proportional to the PAE solubility in water: the higher the solubility, the higher the percentage of the reduction (DEP > DBP > DEHP). PAE were slowly degraded during the pretreatment at 70 degrees C, yet this was probably due to physicochemical reactions than to microbial/biological activity. Therefore, thermal pretreatment of sludge containing PAE should be either avoided or combined with a treatment step focusing on PAE reduction. On the other hand, enzymatic treatment was very efficient in the removal of PAE. The enzymatic degradation of DBP, DEP, and DEHP could be one to two orders of magnitude faster than under normal mesophilic anaerobic conditions. Moreover, the enzymatic treatment resulted in the shortest half-life of DEHP in sludge reported so far. Our study further showed that enzymatic treatment with lipases can be applied to raw sludge and its efficiency does not depend on the solids concentration.

Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature.

Anaerobic digestion is an appropriate technique for the treatment of sludge before final disposal and it is employed worldwide as the oldest and most important process for sludge stabilization. In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization, enhancement of dewatering of the sludge, reduction of the numbers of pathogens and could be realized at relatively low cost especially at low temperatures. The present study investigates (a) the differences between mesophilic and thermophilic anaerobic digestion of sludge and (b) the effect of the pre-treatment at 70 degrees C on mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. The pre-treatment step showed very positive effect on the methane potential and production rate upon subsequent thermophilic digestion of primary sludge. The methane production rate was mostly influenced by the pre-treatment of secondary sludge followed by mesophilic and thermophilic digestion whereas the methane potential only was positively influenced when mesophilic digestion followed. Our results suggest that the selection of the pre-treatment duration as well as the temperature of the subsequent anaerobic step for sludge stabilization should depend on the ratio of primary to secondary sludge.