Psychrophilic methane fermentation of excess sludge by enrichment culture

This study was carried out to determine the possibility of the psychrophilic methane fermentation of the excess sludge produced from fish-processing wastewater treatment. Three different kinds of seed sludge, named M seed sludge, K seed sludge and KL seed sludge were used for batch methane fermentation in Erlenmeyer flasks. M seed sludge was mesophilic, while the other two were prepared in our laboratory by soil collection to obtain a psychrophilic biological community. The effects of temperature on the total volume of biogas generated and on the rate of biogas generation were examined. The activity of K seen sludge with respect to gas yield at 15°C was found to be 1.6-fold higher than that of M seed sludge. Enrichment techniques were then used to accumulate psychrophilic bacteria in KL seed sludge, and the activity at 15°C increased by about two times due to these techniques. Comparison of the three kinds of seed sludge indicates that methane fermentation at low temperature may be feasible, and therefore further study to isolate the psychrophilic bacteria in anaerobic digestion is needed.     

Petroleum coke supplementation for enhanced biogas production and phosphate removal under mesophilic conditions

The use of carbon-based conductive materials has been shown to lead to an increase in biogas and methane yields during anaerobic digestion (AD). The effect of these additives on AD using synthetic substrates has been extensively studied, yet their significance for wastewater sludge digestion has not been adequately investigated. Therefore, the aim of this research was to optimize the concentration of petroleum coke (PC) that is a waste by-product of oil refineries, for the anaerobic digestion of wastewater sludge and investigation of phosphate removal in the AD process in the mesophilic temperature range. According to the results of the experiments, supplementing reactors with PC could significantly improve biogas and methane production. Supplementation of reactors with 1.5 g/L PC led to 23.40 ± 0.26% and 42.55 ± 3.97% increase in biogas production and methane generation, respectively. Moreover, the average volatile solids (VS), phosphate, and chemical oxygen demand (COD) removals were 43.43 ± 0.73, 46.74 ± 0.77%, and 60.40 ± 0.38%, respectively.     

Stirring and biomass starter influences the anaerobic digestion of different substrates for biogas production

Here, we present the results of lab‐scale experiments conducted in a batch mode to determine the biogas yield of lipid‐rich waste and corn silage under the effect of stirring. Further semi‐continuous experiments were carried out for the lipid‐rich waste with/without stirring. Additionally, it was analyzed how the starter used for the batch experiment influences the digestion process. The results showed a significant stirring effect on the anaerobic digestion only when seed sludge from a biogas plant was used as a starter. In this case, the experiments without stirring yielded only about 50% of the expected biogas for the investigated substrates. The addition of manure slurry to the batch reactor as part of the starter improved the biogas production. The more diluted media in the reactor allowed a better contact between the bacteria and the substrates making stirring not necessary.     

Peracetic acid oxidation as an alternative pre-treatment for the anaerobic digestion of waste activated sludge

Anaerobic digestion is generally considered to be an economic and environmentally friendly technology for treating waste activated sludge, but has some limitations, such as the time it takes for the sludge to be digested and also the ineffectiveness of degrading the solids. Various pre-treatment technologies have been suggested to overcome these limitations and to improve the biogas production rate by enhancing the hydrolysis of organic matter. This paper studies the use of peracetic acid for disintegrating sludge as a pre-treatment of anaerobic digestion. It has been proved that this treatment effectively leads to a solubilisation of organic material. A maximum increase in biogas production by 21% is achieved. High dosages of PAA lead to a decrease in biogas production. This is due to the inhibition of the anaerobic micro-organisms by the high VFA-concentrations. The evolution of the various VFAs during digestion is studied and the observed trends support this hypothesis.     

Enzyme research and applications in biotechnological intensification of biogas production

Biogas technology provides an alternative source of energy to fossil fuels in many parts of the world. Using local resources such as agricultural crop remains, municipal solid wastes, market wastes and animal waste, energy (biogas), and manure are derived by anaerobic digestion. The hydrolysis process, where the complex insoluble organic materials are hydrolysed by extracellular enzymes, is a rate-limiting step for anaerobic digestion of high-solid organic solid wastes. Biomass pretreatment and hydrolysis are areas in need of drastic improvement for economic production of biogas from complex organic matter such as lignocellulosic material and sewage sludge. Despite development of pretreatment techniques, sugar release from complex biomass still remains an expensive and slow step, perhaps the most critical in the overall process. This paper gives an updated review of the biotechnological advances to improve biogas production by microbial enzymatic hydrolysis of different complex organic matter for converting them into fermentable structures. A number of authors have reported significant improvement in biogas production when crude and commercial enzymes are used in the pretreatment of complex organic matter. There have been studies on the improvement of biogas production from lignocellulolytic materials, one of the largest and renewable sources of energy on earth, after pretreatment with cellulases and cellulase-producing microorganisms. Lipids (characterised as oil, grease, fat, and free long chain fatty acids, LCFA) are a major organic compound in wastewater generated from the food processing industries and have been considered very difficult to convert into biogas. Improved methane yield has been reported in the literature when these lipid-rich wastewaters are pretreated with lipases and lipase-producing microorganisms. The enzymatic treatment of mixed sludge by added enzymes prior to anaerobic digestion has been shown to result in improved degradation of the sludge and an increase in methane production. Strategies for enzyme dosing to enhance anaerobic digestion of the different complex organic rich materials have been investigated. This review also highlights the various challenges and opportunities that exist to improve enzymatic hydrolysis of complex organic matter for biogas production. The arguments in favor of enzymes to pretreat complex biomass are compelling. The high cost of commercial enzyme production, however, still limits application of enzymatic hydrolysis in full-scale biogas production plants, although production of low-cost enzymes and genetic engineering are addressing this issue.     

Enzyme research and applications in biotechnological intensification of biogas production

Biogas technology provides an alternative source of energy to fossil fuels in many parts of the world. Using local resources such as agricultural crop remains, municipal solid wastes, market wastes and animal waste, energy (biogas), and manure are derived by anaerobic digestion. The hydrolysis process, where the complex insoluble organic materials are hydrolysed by extracellular enzymes, is a rate-limiting step for anaerobic digestion of high-solid organic solid wastes. Biomass pretreatment and hydrolysis are areas in need of drastic improvement for economic production of biogas from complex organic matter such as lignocellulosic material and sewage sludge. Despite development of pretreatment techniques, sugar release from complex biomass still remains an expensive and slow step, perhaps the most critical in the overall process. This paper gives an updated review of the biotechnological advances to improve biogas production by microbial enzymatic hydrolysis of different complex organic matter for converting them into fermentable structures. A number of authors have reported significant improvement in biogas production when crude and commercial enzymes are used in the pretreatment of complex organic matter. There have been studies on the improvement of biogas production from lignocellulolytic materials, one of the largest and renewable sources of energy on earth, after pretreatment with cellulases and cellulase-producing microorganisms. Lipids (characterised as oil, grease, fat, and free long chain fatty acids, LCFA) are a major organic compound in wastewater generated from the food processing industries and have been considered very difficult to convert into biogas. Improved methane yield has been reported in the literature when these lipid-rich wastewaters are pretreated with lipases and lipase-producing microorganisms. The enzymatic treatment of mixed sludge by added enzymes prior to anaerobic digestion has been shown to result in improved degradation of the sludge and an increase in methane production. Strategies for enzyme dosing to enhance anaerobic digestion of the different complex organic rich materials have been investigated. This review also highlights the various challenges and opportunities that exist to improve enzymatic hydrolysis of complex organic matter for biogas production. The arguments in favor of enzymes to pretreat complex biomass are compelling. The high cost of commercial enzyme production, however, still limits application of enzymatic hydrolysis in full-scale biogas production plants, although production of low-cost enzymes and genetic engineering are addressing this issue.     

Simulation of Seed Digestion by Birds: How Does It Reflect the Real Passage Through a Pigeon’s Gut?

Simulation of seed passage through a bird’s gut is an important tool for comparing the effect of bird digestion and thus the potential for plant dispersal by endozoochory. However, sufficient methodology is missing. Thus, we subjected seeds of 20 plant species to seven different simulations of gut passage and to the real passage through a pigeon’s gut to determine which simulation type best reflects the effects of real bird digestion. We also measured various seed traits to identify the traits responsible for differences between species. Results show that four out of seven simulations were significant predictors of seed survival after gut passage. The fit between direct digestion by the pigeon and the different simulation treatments was, however, species-specific and depends not only on the commonly tested traits such as seed mass and water permeability, but also on other unmeasured traits. Seed mass was the best predictor of differences between real digestion and simulation. Selecting one type of simulation to be a good predictor of seed survival after gut passage is difficult. The strongest simulation (24-h scarification and 240-min acid immersion) is the best predictor and may be used to compare the ability of seeds to be dispersed by bird endozoochory. Such knowledge can be included in databases of species traits, as is currently done for many other species traits.     

Developments in pre-treatment methods to improve anaerobic digestion of sewage sludge

During wastewater treatment, most organic matter is transferred to a solid phase commonly known as sludge or biosolids. The high cost of sludge management and the growing interest in alternative energy sources have prompted proposals for different strategies to optimize biogas production during anaerobic sludge treatment. Because of the high solid content and complex structure of sludge-derived organic matter, methane production during digestion is limited at the hydrolysis step. Therefore, large digester volume and long retention times of over 20 days are necessary to achieve adequate stabilization. Pre-treatments can be used to hydrolyze sludge and consequently improve biogas production, solids removal and sludge quality after digestion. This paper reviews the main pre-treatment processes, with emphasis on the most recent developments. An overview of the different technologies is presented, discussing their effects on sludge properties and anaerobic digestion. Future challenges and concerns related to pre-treatment assessment and implementation are also addressed.     

Optimisation of biogas production from manure through serial digestion: lab-scale and pilot-scale studies

In the present study, the possibility of optimizing biogas production from manure by serial digestion was investigated. In the lab-scale experiments, process performance and biogas production of serial digestion, two methanogenic continuously stirred tank reactors (CSTR) connected in series, was compared to a conventional one-step CSTR process. The one-step process was operated at 55 degrees C with 15d HRT and 5l working volume (control). For serial digestion, the total working volume of 5l was distributed as 70/30%, 50/50%, 30/70% or 13/87% between the two methanogenic reactors, respectively. Results showed that serial digestion improved biogas production from manure compared to one-step process. Among the tested reactor configurations, best results were obtained when serial reactors were operated with 70/30% and 50/50% volume distribution. Serial digestion at 70/30% and 50/50% volume distribution produced 13-17.8% more biogas and methane and, contained low VFA and residual methane potential loss in the effluent compared to the one-step CSTR process. At 30/70% volume distribution, an increase in biogas production was also noticed but the process was very unstable with low methane production. At 13/87% volume distribution, no difference in biogas production was noticed and methane production was much lower than the one-step CSTR process. Pilot-scale experiments also showed that serial digestion with 77/23% volume distribution could improve biogas yields by 1.9-6.1% compared to one-step process. The study thus suggests that the biogas production from manure can be optimized through serial digestion with an optimal volume distribution of 70/30% or 50/50% as the operational fluctuations are typically high during full scale application. However, process temperature between the two methanogenic reactors should be as close as possible in order to derive the benefits of serial coupling.     

Effect of enzymes on anaerobic digestion of primary sludge and septic tank performance

Enzyme additives are believed to improve septic tank performance by increasing the hydrolysis and digestion rates and maintaining a healthy microbial population. Previous studies reported mixed results on the effectiveness of enzymes on mesophilic and thermophilic digestion, and it is not clear whether enzymes would be effective under septic tank conditions where there is no heating or mixing, quantities of enzymes added are small, and they can be washed out quickly. In this study, batch reactors and continuous-flow reactors designed and operated as septic tanks were used to evaluate whether enzymatic treatment would increase the hydrolysis and digestion rates in primary sludge. Total solids, volatile solids, total suspended solids, total and soluble chemical oxygen demand, concentrations of protein, carbohydrate, ammonia and volatile acids in sludge and effluent samples were measured to determine the differences in digestion rates in the presence and absence of enzymes. Overall, no significant improvement was observed in enzyme-treated reactors compared with the control reactors.     

城市污泥对作物种子发芽及幼苗生长影响的初步研究

不同类型污泥及其堆肥对通菜、菜心种子发芽和幼苗生长影响的研究表明 ,深圳生污泥、佛山生污泥和广州生污泥及其堆肥对通菜种子的发芽率有明显的影响 ,前者仅为73% ,其余的在 85%左右 ,均低于对照处理 ( 93% ) .而它们对菜心种子的发芽率 ,除了深圳生污泥有抑制作用外 ( 75% ) ,其余均表现为促进作用 ( 90 % ) ,均高于对照处理 ( 85% ) .它们对于两种作物幼苗的生长则均表现为不同程度的抑制作用甚至是损害作用 .污泥及其堆肥中抑制作物种子发芽和幼苗生长的物质主要是有机酸和醛类等有机物 .生污泥经过消化作用或适当的堆沤作用可降解这些有机物 ,从而减弱或消除这种抑制作用 .因此 ,生污泥经过消化作用或适当的堆沤作用后用于农业生产更为安全 .但对刚施用污泥或污泥堆肥的耕地最好不要直接播种 .     

Monofermentation of grass silage under mesophilic conditions: measurements and mathematical modeling with ADM 1

In this paper experimental data from grass fermentation and simulation results with the Anaerobic Digestion Model (ADM) No. 1 are described. Two laboratory reactors were operated under mesophilic conditions with volumetric loading rates in between 0.3 and 2.5 kg(VS)/(m(3) x d). Two different kinds of grass silage were used as substrates, resulting in an average specific biogas production of 600 L/kg(VS). The ADM 1 was calibrated both manually and with the help of a Genetic Algorithm in Matlab/Simulink. Results from calibration indicate that the NH3 inhibition constant used to model the inhibition of acetate uptake is three to five times higher compared with digested activated sludge. The hydrogen inhibition constants applied for propionate and valerate/butyrate uptake are around two orders of magnitude lower than for sludge digestion.     

Biogas production and microbial community change during the Co-digestion of food waste with chinese silver grass in a single-stage anaerobic reactor

Chinese silver grass (CSG), a potential subtropical energy crop, was investigated as a co-substrate to enhance the anaerobic digestion of food waste for municipal solid waste treatment. Results showed that 88.1% of food wastes were degraded using CSG as a co-substrate with 45 days of digestion, where the food waste, CSG, and sludge on VS/TS/working volume was 93.14 g/111.55 g/1 L, in which the average biogas production was at 429.3 L/kg solids, and the average methane content was around 60%. During the digestion, the concentrations of ammonium and free ammonia gradually increased to 1448.2 and 265.2 mg/L respectively, without any significant inhibitory effects on biogas production, which is probably due to the buffering effects of CSG. Microbial community analysis showed that microorganisms from the class of Firmicutes and Bacteroidetes were dominant during digestion, and that the microbial community diversity increased with active methanogenesis, suggesting that the addition of substrates contribute to the increase of microbial diversity, and could be beneficial for biogas production. Therefore, using CSG as a co-substrate in the single-stage food waste anaerobic digestion system is a potential simple method to convert CSG into renewable energy and to simultaneously improve food waste treatment.     

Increase of methane formation by ethanol addition during continuous fermentation of biogas sludge

Very recently, it was shown that the addition of acetate or ethanol led to enhanced biogas formation rates during an observation period of 24 h. To determine if increased methane production rates due to ethanol addition can be maintained over longer time periods, continuous reactors filled with biogas sludge were developed which were fed with the same substrates as the full-scale reactor from which the sludge was derived. These reactors are well reflected conditions of a full-scale biogas plant during a period of 14 days. When the fermenters were pulsed with 50–100 mM ethanol, biomethanation increased by 50–150 %, depending on the composition of the biogas sludge. It was also possible to increase methane formation significantly when 10–20 mM pure ethanol or ethanolic solutions (e.g. beer) were added daily. In summary, the experiments revealed that “normal” methane production continued to take place, but ethanol led to production of additional methane.     

Herb endozoochory by cockatoos: Is ‘foliage the fruit’?

Establishing whether herb seed endozoochory is accidental or has evolved independently or in combination with other dispersal mechanisms may be valuable in the study of plant–animal interactions, but it remains unexplored for birds. We tested whether an Australian cockatoo, the galah (Eolophus roseicapilla), swallows entire seeds when feeding on other tissues without subsequent seed digestion, thus enhancing seed dispersal (the ‘foliage is the fruit’ hypothesis). Our preliminary sampling provides strong evidence supporting that this seed predator also acts as a legitimate endozoochorous disperser. A large proportion of droppings contained numerous seeds of six herb species of three plant families, surviving gut passage to be dispersed as viable propagules. The wide range in the number of seeds found in combinations with up to five species in particular droppings suggests both simultaneous and sequential passive ingestion without seed digestion and/or focused seed predation and digestion. As expected for inadvertent ingestion and inefficient digestion, our findings suggest that seed number and richness of dispersed plants are associated traits in this particular mutualistic interaction. This relationship can have important implications in community‐wide processes, favouring herbs whose seeds are disseminated in a viable state over those predated or negatively affected by gut transit.     

Comparison of the effects of microwave irradiation with different intensities on the biodegradability of sludge from the dairy- and meat-industry

Microwave (MW) irradiation is a relatively new possibility of conditioning and pretreating for wastewater sludge. Following its application in the telecommunications and food-industries, the environmental use of this technique has come into the limelight in recent years, and has become increasingly popular. Various publications have dealt with the examination of the effects of MW irradiation in municipal sludge-handling processes. We focused on the effects of MW irradiation at different power levels on solubilization (sCOD/tCOD), biodegradation and anaerobic digestion of sludge from the food-industry. For evaluating the efficiency of MW pre-treatment, the changes in the soluble fraction of the organic matter, the VS/TS ratio, the biogas yield, the methane content in the biogas, and the rate of batch mesophilic digestion were used as control parameters. Additionally, the energetic efficiency of MW pre-treatment was also examined. The results were compared with those of conventional heat (CH) treatments of the same sludge. The MW treatment proved to increase both the sCOD/tCOD and the VS/TS ratio. Furthermore, the biogas and methane yields increased during the digestion of the MW-pretreated food-industry sludge. A higher MW power level generally enhanced the biogas and methane production. Energetically, the most economic pre-treatment of sludge from dairy and meat processing was at a power level of 1.5 Wg⁻¹ and 2.5 Wg⁻¹ MW respectively; the surplus energy content of the enhanced biogas product could not compensate the extra energy demand of the stronger MW pre-treatments.     

Enhancing the start-up of a UASB reactor treating domestic wastewater by adding a water extract of Moringa oleifera seeds

Water extract of Moringa oleifera seeds (WEMOS) was used to enhance the start-up of a self-inoculated upflow anaerobic sludge blanket (UASB) reactor treating raw domestic wastewater. Two reactors labelled control (RC) and WEMOS addition (RM) were started without special inoculum. Both reactors were fed continuously for 22 weeks with domestic wastewater containing an average total chemical oxygen demand (COD) of 320 mg O2/l and suspended solid (SS) of 165 mg/l. The reactors operated during the entire experimental period at 29 degrees C and at a hydraulic retention time (HRT) of 4 h. The RM reactor received 2 ml WEMOS per litre of influent. WEMOS solution was prepared on the basis of 2.5% (w/v) ground M. oleifera seeds in water. The results of 22 weeks' operation showed an improvement in the performance of the RM compared to that of the RC. The dosage of WEMOS in the feed (1) shortened the biological start-up period by 20%, (2) increased acidogenic and methanogenic activity by a factor of 2.4 and 2.2 respectively, (3) increased the specific biogas production by a factor of 1.6, (4) favoured fast growth of the sludge bed, and (5) allowed the aggregation of coccoid bacteria and growth of microbial nuclei, which are precursors of anaerobic granulation.     

Biogas plasticization coupled anaerobic digestion: batch test results

Biogas has unique properties for improving the biodegradability of biomass solids during anaerobic digestion (AD). This report presents batch test results of the first investigation into utilizing biogas plasticization to "condition" organic polymers during active digestion of waste activated sludge (WAS). Preliminary design calculations based on polymer diffusion rate limitation are presented. Analysis of the 20 degrees C batch test data determined the first order (k(1)) COD conversion coefficient to be 0.167 day(-1) with a maximum COD utilization rate of 11.25 g L(-1) day(-1). Comparison of these batch test results to typical conventional AD performance parameters showed orders of magnitude improvement. These results show that biogas plasticization during active AD could greatly improve renewable energy yields from biomass waste materials such as MSW RDF, STP sludges, food wastes, animal manure, green wastes, and agricultural crop residuals.     

Laboratory-scale ultrasound pre-treated digestion of sludge: Heat and energy balance

The objective of this work was to maximize the digestibility of biological sludge to elucidate the feasibility of a new sludge management strategy to recover good quality sludge for agricultural use. The combined effects of organic loading rates (from 0.7 to 2.8 g VS L⁻¹ d⁻¹) and the degree of disintegration by anaerobic digestion of sonicated activated sludge were discussed, and the thermal and energetic balances were evaluated. Despite low sonication inputs, sludge digestion performance improved in terms of solids degradation and biogas production depending on the soluble organic load. The biogas production by sonicated sludge was higher (up to 30%) with respect to the control. Filterability improved during digestion of sonicated sludge at medium OLR due to a significant abatement of the fines. Thermal balances indicated that sonication may be a proper system to guarantee self-sustaining WAS mesophilic digestion. Nevertheless, thickening is a pre-requisite to achieve a positive energy balance.     

Potential of biogas recirculation to enhance biomass accumulation on supporting media

Two lab-scale anaerobic hybrid reactors (AHR) were operated to investigate the effect of recirculated biogas on the development of biomass on supporting media during the start-up. The reactor comprised of two distinct zones; sludge bed on the bottom and packed bed using nylon fiber as the media on the upper half of the reactor. Both reactors were continuously fed with cassava starch wastewater. The organic loading rate (OLR) was increased from 0.3 to 5.5 g COD/L/day by gradually decreasing the hydraulic retention time (HRT) from 37 to 3.5 days in two months. The biogas at 2.6 L/L/day was recirculated merely in the first month of the operation in order to allow the attached biomass to grow according to the organic matters present in the reactor at the final stage of the start up. Chemical oxygen demand (COD) removal efficiency of over 80% was achieved throughout the study. The result demonstrated a better COD removal efficiency for the reactor with biogas recirculation, especially at low HRTs. The amounts of biomass accumulated on the media in both reactors were slightly different with 11.9 gVSS found on the one with biogas recirculation compared to 9.8 gVSS on the other. In addition, 16.3% increase of the sludge bed was achieved with biogas recirculation as opposed to 9% in the control one. The attached biomass activity test indicated a greater amount and more favorable ratio of the methanogenic bacterial group on the media with the recirculation correlating well to a relatively higher methane content in biogas. As a result, the recirculation of biogas has a potential of improving the characteristics of the AHR especially in terms of biomass accumulation.