Moisture variation associated with water input and evaporation during sewage sludge bio-drying

Inoculation with thermophiles enhanced the food waste bio-drying and complicated interdomain ecological networks between bacterial and fungal communities

Bio-drying is a practical approach for treating food waste (FW). However, microbial ecological processes during treatment are essential for improving the dry efficiency, and have not been stressed enough. This study analyzed the microbial community succession and two critical periods of interdomain ecological networks (IDENs) during FW bio-drying inoculated with thermophiles (TB), to determine how TB affects FW bio-drying efficiency. The results showed that TB could rapidly colonize in the FW bio-drying, with the highest relative abundance of 5.13%. Inoculating TB increased the maximum temperature, temperature integrated index and moisture removal rate of FW bio-drying (55.7 °C, 219.5 °C, and 86.11% vs. 52.1 °C, 159.1 °C, and 56.02%), thereby accelerating the FW bio-drying efficiency by altering the succession of microbial communities. The structural equation model and IDEN analysis demonstrated that TB inoculation complicated the IDENs between bacterial and fungal communities by significantly and positively affecting bacterial communities (b = 0.39, p < 0.001) and fungal communities (b = 0.32, p < 0.01), thereby enhancing interdomain interactions between bacteria and fungi. Additionally, inoculation TB significantly increased the relative abundance of keystone taxa, including Clostridium sensu stricto, Ochrobactrum, Phenylobacterium, Microvirga and Candida. In conclusion, the inoculation of TB could effectively improve FW bio-drying, which is a promising technology for rapidly reducing FW with high moisture content and recovering resources from it.

Effects of 3D-printed bulking agent on microbial community succession and carbohydrate-active enzymes genes during swine manure composting

The bulking agent plays an important role in aerobic composting, but their shape, porosity, and homogeneity need to be optimized. In the present work, a bulking agent with a uniform shape was prepared by 3D printing to explore its influence on physicochemical parameters, microbial community succession, and gene abundance of carbohydrate-active enzymes (CAZymes) in swine manure aerobic composting. The results showed that adding 3D-printed bulking agents can increase maximum temperature, prolong the thermophilic period, and improve the degradation rate of volatile solids, which was attributed to ameliorative air permeability by the porous 3D-printed bulking agent. The abundances of some pathogenic bacteria decreased and CAZymes genes increased respectively in response to the addition of the 3D-printed bulking agent, implying it has a certain positive effect on improving the safety of compost products and promoting the degradation of organic matter. In summary, the 3D-printed bulking agent has good application potential in laboratory-scale aerobic composting.

Recovery of phosphate from aqueous solution by dewatered dry sludge biochar and its feasibility in fertilizer use

Discharging of dewatered dry sludge into the environment can trigger irreversible environmental damage. The treatment of dewatered dry sludge into a valuable biochar product is essential for the sustainability of resources and environmental safety. In this study, we examined the removal of phosphate from water bodies by adsorption of biochar that had been prepared from dewatered dry sludge with different pyrolysis temperatures. We showed that being a composite material rich in carbon, CaO and MgO were produced in the biochar preparation when the pyrolysis temperature was increased to 700 °C. The phosphate adsorption of biochar has strengthened with the increase of pH. The phosphate adsorption of composite was fitted with the pseudo-second-order kinetic model, while the Langmuir adsorption isotherm model yielded a maximum phosphate adsorption of 51.79 mg/g. The phosphate adsorption by sludge biochar was mainly affected by the deposition of phosphate crystals and electrostatic attraction. Overall, biochar prepared from dewatered dry sludge demonstrated acceptable phosphate removal performance and phosphate-loaded biochar had a slow release of phosphorus, therefore, can be used as a phosphate fertilizer to promote plant growth.

Water-heat balance characteristics of the sewage sludge bio-drying process in a full-scale bio-drying plant with circulated air

Sewage sludge bio-drying technology has attracted considerable attention in recent years. In this study, we explored the water-heat balance under two ventilation strategies for the first time in bio-drying plants with circulated air, and examined the influence of air circulation on water removal and heat recovery. We want to obtain the relationships of pile temperature, ventilation, and water removal. Then, it provides support for optimizing the bio-drying process conditions and improving the efficiency through analysis of the water-heat relationship. In the low-ventilation and high-ventilation trials, water removed was mainly on Days 9-12 and 1-4, respectively. Ventilation and pile temperature jointly determine the water removed during the bio-drying process. Water balance indicated that more than 30% of the water was removed under the nonventilated process. More organic matter was degraded to maintain a higher pile temperature under low-ventilation than under high-ventilation, which also led to more radiation heat being lost. High-ventilation trial input less energy (3.36 MJ/kg water removed) but obtained a higher bio-drying index I (7.04) and heat utilization efficiency Qeffic (94.1%). Heat balance showed that lower energy consumption by dry air (Qdryair) was obtained due to circulation air with high temperature. Circulation air also has a higher carried capacity of water vapor but carries more water into the pile due to higher humidity.

Biomass fuel production from cellulosic sludge through biodrying: Aeration strategies, quality of end-products, gaseous emissions and techno-economic assessment

This study assesses the technological, environmental and economic feasibility of biodrying to valorise cellulosic sludge as a renewable energy source. Specifically, three different aeration strategies were compared in terms of biodrying performance, energetic consumption, gaseous emissions, quality of end-products and techno-economic analysis. These strategies were based on different combinations of convective drying with biogenic heat produced. Two innovative biodrying performance indicators (Energetic Biodrying Index and Biodrying Performance Index) were proposed to better assess the initial and operational conditions that favour the maximum energy process efficiency and the highest end-product quality. The end-products obtained consistently presented moisture contents below 40% and lower heating values above 9.4 MJ·kg-1. However, the best values achieved were 32.6% and 10.4 MJ·kg-1 for moisture content and lower heating value, respectively. Low N₂O and CH₄ emissions confirmed the effective aeration of all three strategies carried out, while NH₄ and tVOCs were related either to temperature or biological phenomena. A techno-economic analysis proved the economic viability and attractiveness of the biodrying technology for cellulosic sludge in all the strategies applied.

Application of composite degradable modified starch-based flocculant on dewatering and recycling properties

Sludge dewatering is an important step for wastewater treatment. Composite degradable flocculant (CDF) was prepared by cationic polyacrylamide (PAM) grafting onto modified starch with a novel initiator, and characterized by Fourier transform infrared spectroscopy. The microstructure of flocculated sludge was characterized by scanning electron microscopy. The study investigated the properties of CDF compared to PAM, which showed that the prepared CDF exhibited a highly effective flocculation on sludge dewatering, a higher transmittance and chemical oxygen demand removal rate, and a lower value of effluent ammonia nitrogen and total phosphorus. The fermentation process was also analyzed by testing the performance of dewatered sludge (temperature, pH, ammonia nitrogen, E4/E6 (humic acid absorbance at 465 nm (E4) and 665 nm (E6))). The dehydrated sludge with CDF could be easily compressed into cakes by belt-filter for easy transportation and storage. With the continuous addition of CDF and PAM, the corresponding index of capillary suction time (CST) increased. Moreover, the total value of CST with CDF was low, showing a good dewaterability. In addition, the sludge index of pumping time and moisture content with CDF were low in contrast with PAM. Fermentation experiments demonstrated that sludge with CDF had a comparatively high temperature and low value of E4/E6. Such novel CDF shows enormous potential in wastewater treatment and sludge fermentation.

Thermal drying of industrial sludge using forced aeration

This study investigated an industrial biosludge drying system using hot gases from a coal furnace, seeking to increase the solids content of the biosludge above 50% (w.b.), considered suitable for combustion in biomass boilers. Biosludge was collected from a paper mill activated sludge plant. Biosludge mixtures with eucalyptus chips and eucalyptus bark in two different proportions (15% and 25%) were placed into a drying chamber. Hot gases generated by the furnace, with a flowrate of 0.64 ± 0.02 m³ s^-1 at 100 ± 20°C, were applied to the piles through a blowing system. The results demonstrated that the 75% biosludge/25% eucalyptus bark mixture achieved the best drying ratio, increasing the total solids content from 31% to 72%, over a 5-h drying period. Nevertheless, all other treatments involving the addition of a bulking agent achieved solids content above 50%, confirming the positive effect of adding dried material to the sludge. These results indicate a potential use of hot gases that are currently available and released into the atmosphere by paper mills.

Insight into the potentiality of big biochar particle as an amendment in aerobic composting of sewage sludge

To assess the potential of big biochar as composting amendment, the study was performed on aerobic composting of sewage sludge amended with litchi wood biochar (10% of fresh mixture weight, 20-40 mm) in a 400 L bioreactor system. Physicochemical properties and microbial activities were monitored during the first fermentation process of aerobic compost. Application of big biochar reduced peak temperature and shortened thermophilic phase, but increased the activities of aryl-sulfatase, β-glucosidase, and dehydrogenase, which led to higher removal efficiency of total carbon, nitrogen, and sulphur. Big biochar inhibited degradation of organic matters in composting. Therefore, a comprehensive assessment including physicochemical and microbial properties can help to better understand the effect of big biochar on sewage sludge composting.

Biodegradation of volatile solids and water mass balance of bio-drying sewage sludge after electro-dewatering pretreatment

Using pressurized electro-osmotic dewatering (PEOD) as the pretreatment process for sewage sludge (SS) bio-drying can improve the dewatering performance, but the kinetics of volatile solids biodegradation and the water mass balance are still unknown. These processes were first investigated in this study. Experiments were conducted with three different initial materials, which were composed of SS, bio-dried product and SS dewatered by PEOD (EDSS) as different mass ratios. Six kinetic models and a nonlinear regression method were used to estimate the kinetic parameters, and the models were analyzed using four statistical indicators. Satisfactory fitting of the proposed kinetic model to the experimental data was achieved. Through the water mass balance, the results showed that EDSS had the best dewatering performance for bio-drying. EDSS provided the most appropriate conditions for the bio-drying process; the highest correlation coefficient was 0.9291 and the total water removal rate was 51.13% in the bio-drying of all EDSS.

Humic substances developed during organic waste composting: Formation mechanisms, structural properties, and agronomic functions

Aerobic composting is a typical biochemical process of stabilization and harmlessness of organic wastes during which organic matter degrades, and then aggregates, to produce humic substances (HSs). HSs are a core product of-and a crucial indicator of-the maturation of compost that can be used in soil amendments. The formation of HSs is affected by the characteristics of the raw materials involved, the presence of compost additives, microbial activity, temperature, pH, the C/N ratio, moisture content, oxygen content and particle size, all of which can interact with each other. The formation of HSs is therefore complex. Moreover, it is difficult to identify definitive structures of humic acids (HAs) and fulvic acids (FAs), which are the two major components of HSs. However, HSs represent the same functional groups and structural arrangements, which helps to predict their structures. Functional groups represented by phenol and carboxylic acid groups of HAs and FAs can provide various agronomic functions, such as plant growth enhancement, water and nutrient retention, and disease suppression capacity. Overall, HSs can act as a soil amendment, fertilizer, and plant growth regulator. These functions of HSs enhance the reuse potential of organic waste compost products; however, this requires scientific control of various composting parameters and appropriate application of final products.

Energetic enhancement of thermal assistance in the cooling stage of biodrying by stimulating microbial degradation

In this study, thermal assistance was employed in the cooling stage of conventional biodrying. The results indicated that thermal assistance greatly enhanced water removal with improved vapor-carrying capacity of air-flow, and rapidly decreased moisture contents (MCs) from 45.15% to 49.42% to 15.20-25.85% in 6 days, which were much lower than those of conventional biodrying (CB, 34.90-40.85%). More importantly, a synergistic enhancement of physical and biological effects was observed in thermally assisted biodrying (TB) in terms of stimulated enzymes activity and microbial metabolism (higher oxygen uptake rate and degradation coefficient k). Among the degraded organics, lignocellulose was noted to be important for bio-heat generation in cooling stages, especially for straw as bulking agent. Heat balance results suggested that small fractions of thermal heat (19.76-24.73%) were required to upgrade CB processes for water evaporation with higher energy efficiency. Based on economic viability analysis and with consideration of the further drying for CB products, thermally assisted biodrying presented more economic benefits with less investment and shorter payback period. This research provided an efficient engineering approach to upgrade the cooling stage of conventional biodrying with low external heat cost.

Exploiting community structure, interactions and functional characteristics of fungi involved in the biodrying of storage sludge and beer lees

In this study, the dynamic changes in fungal biodiversity, community structure, fungal associations and functional characteristics were investigated in the biodrying of storage sludge and beer lees by using high throughput sequencing, network and correlation matrix analyses, and FUNGuild database. Additionally, a hypothetical model was provided to better understand the biodrying system. The results showed that fungal diversity decreased after biodrying, while community richness increased in the mesophilic stage and decreased as biodrying progressed. Fungal communities differed in different stages of the biodrying process. Ascomycota and Basidiomycota were the dominant phyla throughout the biodrying process, while Pichia was the dominant genus in the thermophilic stage. Network and correlation matrix analyses provided useful tools for insight into the fungal interactions, allowing us to propose a conceptual model of how succession in fungal associations regulates the dynamics of biodrying systems. Biodrying treatment had a significant effect on fungal trophic modes, with most pathogenic fungi fading away over the process, illustrating that biodrying is an effective bio-treatment method to eliminate pathogenic fungi. These findings provide information that elucidates the fungal interactions and functional characteristics during the biodrying process.

Insights into the succession of the bacterial microbiota during biodrying of storage sludge mixed with beer lees: Studies on its biodiversity, structure, associations, and functionality

Biodrying was first used for post-treatment of storage sludge mixed with beer lees. In this study, dynamic changes in dissolved organic matter (DOM), bacterial community structure, bacterial associations as well as metabolic functions were investigated using Excitation-Emission Matrix (EEM) spectra, high-throughput sequencing, network and correlation matrix analyses, and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt). Furthermore, a hypothetical model was proposed to better understand the biodrying process. The results showed that desired performance was obtained and DOM variations revealed that biodrying can increase biostability of the matrix. The bacterial communities differed among different stages of the biodrying. At the phylum level, the dominant phyla were Proteobacteria and Bacteroidetes in the mesophilic and cooling phases, whereas Firmicutes became the most dominant phylum in the thermophilic phase. At the genus level, the dominant bacteria in the mesophilic and cooling phases were not obvious, while Ureibacillus and Bacillus were the dominant genera in the thermophilic phase. Network and correlation matrix analyses were useful tools for insights into the bacterial interactions. PICRUSt metagenome inference indicated that metabolism, genetic information processing, and environmental information processing were the primary metabolic pathways. These results allowed us to advance a hypothetical model explaining how succession in bacterial associations regulates the dynamics of a biodrying system.

Sewage sludge drying method combining pressurized electro-osmotic dewatering with subsequent bio-drying

In this study, pressurized electro-osmotic dewatering (PEOD) as a pretreatment process, instead of the conventional practice of adding bulking agents, for sewage sludge bio-drying was proposed. Initially, various parameters were optimized for obtaining dewatered sewage sludge (DSS), treated by an efficient, quick, and energy-saving PEOD process. The results show that the moisture content (MC) of sewage sludge could decrease from 83.41% to 60.0% within 7.5 min in the optimum conditions of the PEOD process. Subsequently, two DSS bio-drying tests were carried out to investigate the effects of inoculation. The highest temperature (68.1 °C) was obtained for T2 (inoculation), which was 3.6 °C higher than that for T1 (non- inoculation). The MC accumulative removal rate for T1 (41.49%) was slightly less than that for T2 (44.60%). Lastly, the volatile solid degradation dynamics model parameters were measured. The degradation rate constants (k) for T1 and T2 were 0.00501 and 0.00498, respectively.

Co-biodrying of sewage sludge and organic fraction of municipal solid waste: Role of mixing proportions

This study investigated the performance of co-biodrying sewage sludge and organic fraction of municipal solid waste (OFMSW) at different proportions. Cornstalk was added at 15% (of total wet weight) as the bulking agent. Results show that increasing OFMSW percentage promoted the biodegradation of organic matter, thus enhancing the temperature integration value and water removal to above 75% during sludge and OFMSW co-biodrying. In particular, adding more OFMSW accelerated the biodegradation of soluble carbohydrates, lignins, lipids, and amylums, resulting in more organic loss and thus lower biodrying index (3.3-3.7 for 55-85% OFMSW). Water balance calculation indicated that evaporation was the main mechanism for water removal. Heat used for water evaporation was 37.7-48.6% of total heat consumption during co-biodrying. Our results suggest that sludge and OFMSW should be mixed equally for their efficient co-biodrying.

New insights into biodrying mechanism associated with tryptophan and tyrosine degradations during sewage sludge biodrying

Sewage sludge biodrying is a treatment that uses bio-heat generated from organic degradation to remove water from sewage sludge. Dewatering is still limited during biodrying, due to the presence of extracellular polymeric substances (EPS) in sludge. To study the biodrying mechanism associated with EPS compositions tryptophan and tyrosine degradations, this study investigated the microbial function in sludge biodrying material. This study conducted a taxonomic analysis of biodrying material; determined the most abundant genetic functions; analyzed the functional microorganisms involved in the degradations of tryptophan and tyrosine; and summarized the metabolic pathways. The results indicated efficient degradations of tryptophan and tyrosine were observed during the initial thermophilic phase; functional microorganisms were mainly from the phyla Firmicutes, Actinobacteria, and Proteobacteria, enriched with genes involved in amino acid transport and metabolism. These findings highlight the potentially important microorganisms and typical pathways that may help improve dewaterability during biodegradation.

Effects of aeration on matrix temperature by infrared thermal imager and computational fluid dynamics during sludge bio-drying

The effect of aeration on the pile matrix temperature was investigated using thermocouples and Infrared Thermal Imager (IRI) for temperature sensing, and Computational Fluid Dynamics (CFD) for modelling of temperature variation during aeration in a full-scale sludge biodrying plant. With aeration saving of 20%, the improved strategy speeded up biodrying from 21 days to 14 days, while achieving similar drying effect. A persistent thermocouple recorded the one-dimensional (1D) total temperature variation of all aeration strategies. The IRI captured the rapid two-dimensional (2D) pile temperature dropped from 72.5 °C to 30.3 °C during 6 min of aeration, which mechanism suggested as the latent heat of moisture evaporation and sensible heat of air exchange. The CFD three-dimensional (3D) CFD results highlight the importance of latent heat rather than sensible heat. Therefore, the pile temperature drop inferred is ΔT = 5.38 °C theoretically and ΔT = 5.17 ± 4.56 °C practically, per unit of MC removed. These findings also emphasize the possibility of a pile temperature valley, due to excessive aeration under unsaturated vapour conditions. Surface temperature monitored by IRI coupled with 3D temperature simulated by CFD rapidly gives a clear matrix temperature evolution, empowering biodrying by more accurate temperature and aeration.

Sludge reduction using aquatic worms under different aeration regimes

Adding aquatic worms to a wastewater treatment system can reduce sludge production through predation. The aeration level is crucial for success. To evaluate aeration impacts on sludge reduction and determine an optimal aeration regime, this study investigated the processes of in-situ sludge reduction, using aquatic worms exposed to different aeration levels. The experiment also compared treatment results between a conventional reactor and an aquatic worm reactor (WR). Results indicated that the recommended concentration of dissolved oxygen (DO) was 2.5 mg L^-1. The removal rate of chemical oxygen demand remained steady at 80% when the DO concentration was higher than 2.5 mg L^-1, while the removal rate of ammonia nitrogen continued to moderately increase. Increasing the DO concentration to 5 mg L^-1 did not improve sludge reduction, and consumed more power. With a DO concentration of 2.5 mg L^-1 and a power of 0.19 kWh t^-1 water, the absolute sludge reduction and relative sludge reduction rates in the WR were 60.0% and 45.7%, respectively, and the daily aquatic worm growth rate was 0.150 d^-1 during the 17-d test. Therefore, at the recommended aeration regime, aquatic worms reduced the sludge without increasing the power consumption or deteriorating the effluent.

Energy-efficient co-biodrying of dewatered sludge and food waste: Synergistic enhancement and variables investigation

In this study, dewatered sludge (DS) and food waste (FW) were co-biodried by balancing substrate's property and microbial aspect. A series of experiments were conducted to explore the effects of mixing ratio, particle size of bulking agent, air-flow rate and initial moisture content (MC). A synergistic enhancement of co-biodrying of FW and DS was observed in terms of a stable temperature profile and long high-temperature duration. The biodrying index (water removal/VS consumption) indicated that the co-biodrying had a high efficiency for water removal with less organics consumption, especially for DS/FW=2/2. The small size (<3mm) of bulking agent and initial MC of 62.68% was preferable for the biodrying process by providing adequate free air space and extra carbon source. A moderate air-flow rate of 0.04m(3)h(-1)kg(-1) showed the best water carrying capacity. This finding suggests that the co-biodrying strategy could be a promising approach to treating different organic wastes with synergistic enhancement.

Structure modification and extracellular polymeric substances conversion during sewage sludge biodrying process

Biodrying, an economical and energy-saving biomass waste treatment, removes water from waste using the biological heat generated by organic matter degradation. Technical limitations associated with dewatering complicate the biodrying of sewage sludge. This study investigated the sludge alteration associated with its water removal, focusing on sludge form, extracellular polymeric substances, and free water release. An auto-feedback control technology was used for the biodrying; a scanning electron microscope was used to record the morphological change; three-dimensional excitation-emission matrix fluorescence spectroscopy was used to analyze extracellular polymeric substances (EPS) variation, and time domain reflectometry was used to assess the free water release. Over the 20-day biodrying, there was a 62% water removal rate during the first thermophilic phase. Biodrying created a hollow and stratified sludge structure. Aromatic proteins and soluble microbial byproducts in the EPS were significantly degraded. The thermophilic phase was the phase resulting in the greatest free water release.

Biodrying process: A sustainable technology for treatment of municipal solid waste with high moisture content

Municipal solid waste with high moisture content is the major hindrance in the field of waste to energy conversion technologies and here comes the importance of biodrying process. Biodrying is a convective evaporation process, which utilizes the biological heat developed from the aerobic reactions of organic components. The numerous end use possibilities of the output are making the biodrying process versatile, which is possible by achieving the required moisture reduction, volume reduction and bulk density enhancement through the effective utilization of biological heat. In the present case study the detailed research and development of an innovative biodrying reactor has been carried out for the treatment of mixed municipal solid waste with high moisture content. A pilot scale biodrying reactor of capacity 565 cm(3) was designed and set up in the laboratory. The reactor dimensions consisted of an acrylic chamber of 60 cm diameter and 200 cm height, and it was enveloped by an insulation chamber. The insulation chamber was provided to minimise the heat losses through the side walls of the reactor. It simulates the actual condition in scaling up of the reactor, since in bigger scale reactors the heat losses through side walls will be negligible while comparing the volume to surface area ratio. The mixed municipal solid waste with initial moisture content of 61.25% was synthetically prepared in the laboratory and the reactor was fed with 109 kg of this substrate. Aerobic conditions were ensured inside the reactor chamber by providing the air at a constant rate of 40 litre per minute, and the direction of air flow was from the specially designed bottom air chamber to the reactor matrix top. The self heating inside reactor matrix was assumed in the range of 50-60°C during the design stage. Innovative biodrying reactor was found to be efficiently working with the temperature inside the reactor matrix rising to a peak value of 59°C by the fourth day of experiment (the peak observed at a height of 60 cm from the air supply). The process analyses results were promising with a reduction of 56.5% of volume, and an increase of 52% of bulk density of the substrate at the end of 33 days of biodrying. Also the weight of mixed MSW substrate has been reduced by 33.94% in 20 days of reaction and the average moisture reduction of the matrix was 20.81% (reduced from the initial value of 61.25% to final value of 48.5%). The moisture reduction would have been higher, if the condensation of evaporated water at the reactor matrix has been avoided. The non-homogeneous moisture reduction along the height of the reactor is evident and this needs further innovation. The leachate production has been completely eliminated in the innovative biodrying reactor and that is a major achievement in the field of municipal solid waste management technology.

Bacterial communities and their association with the bio-drying of sewage sludge

Bio-drying is a technology that aims to remove water from a material using the microbial heat originating from organic matter degradation. However, the evolution of bacterial communities that are associated with the drying process has not been researched systematically. This study was performed to investigate the variations of bacterial communities and the relationships among bacterial communities, water evaporation, water generation, and organic matter degradation during the bio-drying of sewage sludge. High-throughput pyrosequencing was used to analyze the bacterial communities, while water evaporation and water generation were determined based on an in situ water vapor monitoring device. The values of water evaporation, water generation, and volatile solids degradation were 412.9 g kg(-1) sewage sludge bio-drying material (SSBM), 65.0 g kg(-1) SSBM, and 70.2 g kg(-1) SSBM, respectively. Rarefaction curves and diversity indices showed that bacterial diversity plummeted after the temperature of the bio-drying pile dramatically increased on d 2, which coincided with a remarkable increase of water evaporation on d 2. Bacterial diversity increased when the pile cooled. During the thermophilic phase, in which Acinetobacter and Bacillus were the dominant genera, the rates of water evaporation, water generation, and VS degradation peaked. These results implied that the elevated temperature reshaped the bacterial communities, which played a key role in water evaporation, and the high temperature also contributed to the effective elimination of pathogens.

Energy recovery from thermal treatment of dewatered sludge in wastewater treatment plants

Sewage sludge is a by-product generated from municipal wastewater treatment (WWT) processes. This study examines the conversion of sludge via energy recovery from gasification/combustion for thermal treatment of dewatered sludge. The present analysis is based on a chemical equilibrium model of thermal conversion of previously dewatered sludge with moisture content of 60-80%. Prior to combustion/gasification, sludge is dried to a moisture content of 25-55% by two processes: (1) heat recovered from syngas/flue gas cooling and (2) heat recovered from syngas combustion. The electricity recovered from the combined heat and power process can be reused in syngas cleaning and in the WWT plant. Gas temperature, total heat and electricity recoverable are evaluated using the model. Results show that generation of electricity from dewatered sludge with low moisture content (≤ 70%) is feasible within a self-sufficient sludge treatment process. Optimal conditions for gasification correspond to an equivalence ratio of 2.3 and dried sludge moisture content of 25%. Net electricity generated from syngas combustion can account for 0.071 kWh/m(3) of wastewater treated, which is up to 25.4-28.4% of the WWT plant's total energy consumption.

Effect of back mixing on thin-layer drying characteristics of sewage sludge by the appropriate foaming pretreatment

Herein, we report the creative combination of foaming and back mixing.

Integrating particle physical geometry into composting degradation kinetics

The study was carried out to integrate physical geometry of compost particle with degradation kinetics to model biological reactions, which revealing additional dynamic approaches. A sphere and its circumscribing cube were used to represent compost particles. An inner sphere, representing anaerobic zone, was introduced to describe variations of substrate volume without sufficient oxygen supply. Degradation of soluble substrates and hydrolysis of insoluble substrates were associated with the particle geometry. Transportation of soluble substrates produced from hydrolysis was expressed using Fick's law. Through the integration of degradation kinetics with geometry models, degradation models could describe varying volume of composting materials involving aerobic or anaerobic digestion and transportation of soluble substrates in a unit compost particle.

Prediction of moisture variation during composting process: A comparison of mathematical models

This study was carried out to develop and compare three models for simulating the moisture content during composting. Model 1 described changes in water content using mass balance, while Model 2 introduced a liquid-gas transferred water term. Model 3 predicted changes in moisture content without complex degradation kinetics. Average deviations for Model 1-3 were 8.909, 7.422 and 5.374 kg m(-3) while standard deviations were 10.299, 8.374 and 6.095, respectively. The results showed that Model 1 is complex and involves more state variables, but can be used to reveal the effect of humidity on moisture content. Model 2 tested the hypothesis of liquid-gas transfer and was shown to be capable of predicting moisture content during composting. Model 3 could predict water content well without considering degradation kinetics.

Simultaneous effect of initial moisture content and airflow rate on biodrying of sewage sludge

The simultaneous effect of initial moisture content (initial Mc) and air-flow rate (AFR) on biodrying performance was evaluated. For the study, a 3(2) factorial design, whose factors were AFR (1, 2 and 3 L/min kg(TS)) and initial Mc (59, 68 and 78% w.b.), was used. Using energy and water mass balance the main routes of water removal, energy use and efficiencies were determined. The results show that initial Mc has a stronger effect on the biodrying than the AFR, affecting the air outlet temperature and improving the water removal, with higher maximum temperatures obtained around 68% and the lowest maximum matrix temperature obtained at initial Mc = 78%.Through the water mass balance it was found that the main mechanism for water removal was the aeration, with higher water removal at intermediate initial Mc (68%) and high AFR (3 L/min kg(TS)). The energy balance indicated that bioreaction is the main energy source for water evaporation, with higher energy produced at intermediate initial Mc (68%). Finally, it was found that low values of initial Mc (59%) improve biodrying efficiency.

Two-dimensional FTIR correlation spectroscopy reveals chemical changes in dissolved organic matter during the biodrying process of raw sludge and anaerobically digested sludge

2D FTIR COS analysis is a feasible technique to explore the degradation characteristics of sludge organic matter, and supplies the first evidence for the complementarities of anaerobic and aerobic process in sludge organic compound degradation.

Simulation of water removal process and optimization of aeration strategy in sewage sludge composting

Reducing moisture in sewage sludge is one of the main goals of sewage sludge composting and biodrying. A mathematical model was used to simulate the performance of water removal under different aeration strategies. Additionally, the correlations between temperature, moisture content (MC), volatile solids (VS), oxygen content (OC), and ambient air temperature and aeration strategies were predicted. The mathematical model was verified based on coefficients of correlation between the measured and predicted results of over 0.80 for OC, MC, and VS, and 0.72 for temperature. The results of the simulation showed that water reduction was enhanced when the average aeration rate (AR) increased to 15.37 m(3) min(-1) (6/34 min/min, AR: 102.46 m(3) min(-1)), above which no further increase was observed. Furthermore, more water was removed under a higher on/off time of 7/33 (min/min, AR: 87.34 m(3) min(-1)), and when ambient air temperature was higher.

Biodrying of pulp and paper secondary sludge: kinetics of volatile solids biodegradation

This study focuses on the kinetics of volatile solids (VS) biodegradation of the biodrying process using pulp and paper secondary sludge. The experiments were carried out with air-flow rates of 0.51, 1.61, 3.25 and 5.26L/minkgVS) and initial moisture content of 64-66% w.b. Using five kinetic models and a nonlinear regression method, kinetic parameters were estimated and the models were analyzed with two statistical indicators. Higher air-flow rates cause greater moisture content reduction, lower temperature in the matrix, and lower VS reduction. At an air-flow rate as high as 5.26L/minkgVS there is no biodrying but only convective drying. The kinetic models used successfully simulate the VS biodegradation under biodrying conditions, with a root mean square error (RMSE) between 0.007929 and 0.02744. In conclusion, we show for the first time that VS biodegradation in the biodrying process can be successfully modeled with a kinetic model.

The biodrying concept: an innovative technology creating energy from sewage sludge

A full-scale biodrying installation was treating 150 kton (wet weight) of dewatered waste activated sludge per year. The waste was treated at thermophilic conditions (65-75 °C) in a 2-step forced aeration process reducing the total wet sludge weight by 73%. The final product had a high caloric value (7700-10,400 (kJ/kg)), allowing a combustion for energy generation in external facilities. The resulting product met the European microbial and heavy metal quality standards needed for an application as organic fertilizer. The facility used <0.5 MW of electricity and recovered 9.3 MW from biologically produced heat, which was internally used for the heating of office buildings. Produced ammonia, originating from the microbial conversion of organic matter, was recovered from the ventilated air in an acid gas scrubber as an ammonium sulphate solution 40% (w/w) (7.3 kton/year) and was sold as substitute for artificial fertilizers. The sustainability of this process is discussed relative to other sludge handling processes.

Application of a recyclable plastic bulking agent for sewage sludge composting

A recyclable plastic bulking agent (RPBA) that can be screened and reused was developed to improve sludge composting and to reduce costs. Two RPBAs were developed: RPBA35 (35 mm in diameter) and RPBA50 (50mm in diameter). The objective was to study the influence of size and quantity of RPBA on temperature, oxygen content, water removal during sludge composting, and phytotoxicity of the compost. RPBAs of both sizes improved the temperature, oxygen supply, and water removal compared with the treatment with no RPBA, and obtained phytotoxic-free compost. RPBA50 more effectively removed water than RPBA35. Oxygen diffusion rate in the composting pile containing RPBA50 was higher than in the treatment with no RPBA. When the RPBA50: sludge mixture ratio was above 1:1.5, the period over which the temperature exceeded 55 °C was insufficient to meet the harmless treatment requirement. The water evaporation rate was highest at a ratio of 1:2.

Influence of forced air volume on water evaporation during sewage sludge bio-drying

Mechanical aeration is critical to sewage sludge bio-drying, and the actual water loss caused by aeration can be better understood from investigations of the relationship between aeration and water evaporation from the sewage sludge bio-drying pile based on in situ measurements. This study was conducted to investigate the effects of forced air volume on the evaporation of water from a sewage sludge bio-drying pile. Dewatered sewage sludge was bio-dried using control technology for bio-drying, during which time the temperature, superficial air velocity and water evaporation were measured and calculated. The results indicated that the peak air velocity and water evaporation occurred in the thermophilic phase and second temperature-increasing phase, with the highest values of 0.063 ± 0.027 m s(-1) and 28.9 kg ton(-1) matrix d(-1), respectively, being observed on day 4. Air velocity above the pile during aeration was 43-100% higher than when there was no aeration, and there was a significantly positive correlation between air volume and water evaporation from day 1 to 15. The order of daily means of water evaporation was thermophilic phase > second temperature-increasing phase > temperature-increasing phase > cooling phase. Forced aeration controlled the pile temperature and improved evaporation, making it the key factor influencing water loss during the process of sewage sludge bio-drying.

Resource recovery of organic sludge as refuse derived fuel by fry-drying process

The organic sludge and waste oil were collected from the industries of thin film transistor liquid crystal display and the recycled cooking oil. The mixing ratio of waste cooking oil and organic sludge, fry-drying temperatures, fry-drying time, and the characteristics of the organic sludge pellet grain were investigated. After the fry-drying process, the moisture content of the organic sludge pellet grain was lower than 5% within 25 min and waste cooking oil was absorbed on the dry solid. The fry-drying organic sludge pellet grain was easy to handle and odor free. Additionally, it had a higher calorific value than the derived fuel standards and could be processed into organic sludge derived fuels. Thus, the granulation and fry-drying processes of organic sludge with waste cooking oil not only improves the calorific value of organic sludge and becomes more valuable for energy recovery, but also achieves waste material disposal and cost reduction.

Settling and dewatering characteristics of granulated methane-oxidizing bacteria

We evaluated the settling ability and dewaterability of granulated methane-oxidizing bacteria (GMOB) after granulation using a continuous-flow reactor. A comparative analysis on settling and dewatering characteristics due to changes in sludge retention time (SRT, 10, 15 and 20 days) during cultivation of GMOB was conducted. In assessing dewaterability, the specific resistance to filtration (SRF) of activated sludge and GMOB was found to be 8.21 x 10(13)-2.38 x 10(14) and 4.88 x 10(12) - 1.98 x 10(13) m/kg, respectively. It was confirmed that as SRT decreased, SRF of GMOB increased. In the case of bound extracellular polymeric substance (EPS), activated sludge registered 147.5 mg/g-VSS while GMOB exhibited 171-177.2 mg/g-VSS. In the case of extracellular polymeric substance soluble EPS in effluent, activated sludge measured 62 mg/L and GMOB had 17.4-21.4 mg/L. The particle size analysis showed that mean particle diameters of GMOB were 402, 369, and 350 microm, respectively, at SRTs of 20, 15 and 10 days. In addition, it was found that GMOB had a larger mean particle diameter and exhibited much better settleability and dewaterability than activated sludge did.

Online monitoring of volatile organic compound production and emission during sewage sludge composting

The production and emission of volatile organic compounds (VOCs) were studied using an online monitoring method in a well-operated sludge composting plant. Results indicated that VOC production within the pile was different from emission at the pile surface. The total mass of VOC production was 1.09gCkgDM(-1), which was 2.3 times as high as the total mass of emission. The maximum production and emission masses occurred in the mesophilic phase of composting and were 444 and 202mgkgDM(-1)d(-1), respectively. VOC production and emission rates also varied rapidly at different times. The relationship of VOC production rates and time in an on/off aeration cycle at different periods could be expressed as a quadratic equation, while the emission rate could be expressed as a linear equation.