Thermal characterization and syngas production from the pyrolysis of biophysical dried and traditional thermal dried sewage sludge

Syngas-driven sewage sludge conversion to microbial protein through H2S- and CO-tolerant hydrogen-oxidizing bacteria

Treating excess municipal sewage sludge (MSS) by means of thermochemical processes could enable its conversion into high-value microbial protein (MP) through syngas. Nevertheless, the variable composition and content of inhibitory compounds of the latter hinders the application potential of such a biorefinery scheme. Through a series of short- (48 to 96 h) and long-term (30 days) batch aerobic bioconversion tests, the present study aimed at investigating the potential of a mixed culture of hydrogen-oxidizing bacteria (HOB) to produce MP from a simulated syngas mixture characterized by variable H2 and CO2 concentrations, and different levels of CO and H2S as potential inhibitors of the HOB-driven process. Syngas was converted into MP with a protein content as high as 74 %, reaching biomass yields of 0.25 g VSS/g H2-COD, close to the maximum reported HOB yield of 0.28 g VSS/g H2-COD,  and volumetric productivities of 16 mg VSS/L/h. The potential of the process to provide between 50 and 100 % of the total nitrogen requirement of HOB solely by means of the gaseous ammonia nitrogen recovered through syngas was also preliminarily calculated. The presence of H2S and CO concentrations up to 0.4 % and up to 40 %, respectively, and a wide range of H2/CO2 ratios (2 - 10) had no negative influence on the main process performances. The role played by H2S- and CO-tolerant HOB species was fundamental to guarantee a high tolerance to microbial inhibitors, and demonstrated the high potential of mixed cultures for resource recovery and valorisation.

Study on the Thermogravimetric Kinetics of Dehydrated Sewage Sludge Regulated by Cationic Polyacrylamide and Sawdust

With the continuous increase in sewage-sludge production worldwide, the pyrolytic disposal of sludge has received great attention. To build knowledge on the kinetics of pyrolysis, first, sludge was regulated using appropriate amounts of cationic polyacrylamide (CPAM) and sawdust to study their enhancing effect on dehydration. Due to the effects of the charge neutralization and skeleton hydrophobicity, a certain dose of CPAM and sawdust reduced the sludge's moisture content from 80.3% to 65.7%. Next, the pyrolysis characteristics of the dehydrated sludge regulated by CPAM and sawdust were investigated at a heating rate of 10~40 °C/min by using TGA method. The addition of sawdust enhanced the release of volatile substances and reduced the apparent activation energy of the sample. The maximum weight-loss rate decreased with the heating rate, and the DTG curves moved in the direction of high temperature. A model-free method, namely the Starink method, was adopted to calculate the apparent activation energies, which ranged from 135.3 kJ/mol to 174.8 kJ/mol. Combined with the master-plots method, the most appropriate mechanism function ultimately obtained was the nucleation-and-growth model.

Sludge low-temperature drying with mainly non-phase change in mere seconds based on particle high-speed self-rotation in cyclone

Improper sludge treatment will cause serious environmental problems, and sludge drying is the key to effective treatment. Almost all the existing sludge drying technologies use heating to overcome the great latent heat of moisture vaporization, which leads to high drying energy consumption. In this study, based on the particle high-speed self-rotation in the cyclone and micro-interface oscillations, the cyclone self-rotation drying (CSRD) technology was developed. It can realize drying of the dewatered landfill sludge (DLS) and the urban sewage dewatered sludge (UDSS) with mainly non-phase change. The obtained results reveal that at low carrier gas temperatures (< 100 °C) and very short residence time (< 15 s), the moisture content of the DLS decreased from 53% to 6.85%, and that of the UDSS decreased from 67% to 18.92%. Through calculation, the proportions of moisture non-phase change removal during the CSRD process touched 68.94% and 63.39%, respectively. Based on the experimental studies, we proposed an enlarged industrial application program (50 t/d) for the UDSS drying by employing the CSRD technology. The operating cost was 159.69 CNY/t H₂O, showing prominent advantages. This study can provide guidelines for the practical application of CSRD technology and fill the scientific gap in the field of moisture non-phase change separation for sludge drying.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus

Phosphorus is a nutrient that is essential to nature and human life and has attracted attention because of its very limited reserves. Dwindling phosphorus reserves and soaring prices have made the recovery of phosphorus from waste biosolids even more urgent. Waste activated sludge, as the final destination of most of the phosphorus in human domestic and industrial water, has been considered as a reliable source of phosphorus recovery. The thermal treatment method of sewage sludge is currently a relatively environmentally friendly disposal method, which mainly includes incineration, pyrolysis and hydrothermal carbonization. This paper reviews the methods for the recovery of different forms of phosphorus (wet chemical, thermochemical and electrodialysis) from solid products obtained from different sludge thermal treatment methods (incinerated sewage sludge ash, pyrolysis of sewage sludge char and hydrochar) and the bioavailability of the recovered phosphorus products. Incineration of sewage sludge is currently the most established and effective method for recovering phosphorus from the thermal treatment products of sewage sludge. One of the wet chemical methods has been applied on a commercial scale and is expected to be further developed for future industrial applications. Pyrolysis and hydrothermal carbonation still have many research gaps in this field. Based on their principles and laboratory performance, both of them have the potential to recover phosphorus and should be further explored.

Dual role of O2 concentration on the reducing gases produced and NO reduction during sewage sludge combustion in pilot scale cement precalciner

The reducing gases produced and NO reduction by sewage sludge combustion were investigated in a self-made cement precalciner. The dual role of O₂ concentration (0-5 vol%) in the production characteristics of reducing gases and the reduction efficiency of NO were evaluated experimentally. TG-FTIR analysis demonstrated that the key reducing gaseous species produced by sewage sludge combustion were HCN, NH₃, CO, and CH₄. And experiments demonstrated that O₂ concentration had pronounced effects on NH₃ distribution, the maximum production rate was obtained at an O₂ concentration of 3 vol%. Meanwhile, the reducing gases NH₃ and CO were the key species for NO reduction in the cement precalciner, and the reduction efficiency of NO, when reduced by NH₃, increased with an increase in O₂ concentration, while the reduction performance of NO by CO was limited by O₂ concentration. Therefore, O₂ concentration greatly influences NO reduction efficiency by sewage sludge combustion; the maximum NO reduction efficiency was 61.67% at an O₂ concentration of 3 vol%. The difference in NO reduction by sewage sludge combustion under different O₂ concentrations was primarily attributed to NH₃ production rate and NO reduction by NH₃ and CO, which is greatly affected by O₂ concentration. Sewage sludge combustion can result in NO reduction in the cement kiln flue gas and resource utilisation of sewage sludge.

Pretreatment, modification and applications of sewage sludge-derived biochar for resource recovery- A review

With the quick increase in industrialization and urbanization, a mass of sludge has been produced on the account of increased wastewater treatment facilities. Sewage sludge (SS) management has become one of the most crucial environmental problems because of the existence of various pollutants. However, SS is a carbon-rich material, which has favored novel technologies for biochar production, which can be utilized for dissimilar applications. This review systematically analyzes and summarizes the pretreatment, modification, and especially application of sewage sludge-derived biochar (SSBC), based on published literature. The comparative assessment of pretreatment technology such as pyrolysis, hydrothermal carbonization, combustion, deashing, and co-feeding is presented to appraise their appropriateness for SS resource availability and the production of SSBC. In addition, the authors summarize and analyze the current modification methods and divide them into two categories: physical properties and surface chemical modifications. The applications of SSBC as absorbent, catalyst and catalyst support, electrode materials, gas storage, soil amendment, and sold biofuel are reviewed in detail. Furthermore, the discussion about the existing problems and the direction of future efforts are presented at the end of each section to envisage SS as a promising opportunity for resources rather than a nuisance.

The evaluation of immobilization behavior and potential ecological risk of heavy metals in bio-char with different alkaline activation

The bio-char was prepared by co-pyrolysis of municipal sewage sludge and biomass with chemical activation. The alkaline activating agents of KOH and K₂CO₃ were used to develop multilevel pore structure without heavy metal. The proximate analysis, ultimate analysis, SEM, and surface area and porosity analyzer were applied to present the physico-chemical properties and multilevel pore structure of bio-char. After impregnation pretreatment, the KOH provided more functional ingredients and reacted with C to expand pore structure for bio-chars. It was confirmed the specific surface area reached 2122.43 m²/g, and micropore area was 1674.85 m²/g after co-pyrolysis at 800 °C. Through the pretreatment of alkaline activation, the novel evaluation of heavy metal immobilization behavior in bio-char matrix were investigated by BCR sequential extraction and leaching tests. The KOH activation showed prominent immobilization behavior relatively, and the K₂CO₃ activation had more noticeable effects on leaching behavior. For Cu, Ni, Cr, Cd, Pb, and Zn, after co-pyrolysis at 900 °C, the proportion of unstable fraction decreased significantly, and the residual fractions of heavy metals were above 89.44% according to BCR sequential extraction procedure. Under optimal pyrolysis temperature, the E_r value of bio-char reduced to 41.93, and the potential ecological risks decreased from considerable risk to low risk to ensure the further eco-friendly application.

Comparative analysis for pyrolysis of sewage sludge in tube reactor heated by electromagnetic induction and electrical resistance furnace

A comparative study was conducted on the pyrolysis of sewage sludge in tube reactors heated by electromagnetic induction (EMI) and conventional electrical resistance furnace (ERF). A minimal effect of pyrolysis temperature and initial moisture content on the distribution of pyrolytic products was obtained. Compared with the counterpart from ERF pyrolysis, the bio-char from EMI pyrolysis exhibited less ash content (46.38 wt%) and higher organic matter content (53.62 wt%). SEM and FTIR test showed similar microstructure characterizations in the two bio-chars. The specific area of bio-char from EMI pyrolysis was 8.6 m²/g. EMI pyrolysis increased the total content of aliphatic/aromatics in the bio-oil from 10.8 wt% to 15.6 wt% and the hydrogen/carbon monoxide in the bio-gas from 33.8 vol% to 41.1 vol% because of possible cracking and reforming reactions. Increased sulfur content in the bio-oil and decreased hazard gas content (such as hydrogen sulfide and sulfur dioxide) in the bio-gas were obtained during EMI pyrolysis. The actual energy consumption for EMI and ERF pyrolysis were 4.62 MJ/kg and 6.65 MJ/kg. Increasing the feedstock content would reduce the energy consumption unit energy consumption. Less system energy loss during EMI pyrolysis might explain the higher energy recovery from EMI pyrolysis than that from ERF. Despite some disadvantages, EMI pyrolysis shows potential in real-plant applications.

Preparation of Porous Biochars by the Co-Pyrolysis of Municipal Sewage Sludge and Hazelnut Shells and the Mechanism of the Nano-Zinc Oxide Composite and Cu(II) Adsorption Kinetics

Municipal sewage sludge (MSS) and hazelnut shells were selected for co-pyrolysis and chemically activated with K2CO3 in a N2 atmosphere. The biochar was then modified by photocatalysis. Hazelnut shells, as a solid waste, were selected as a carbon source additive because of its high cellulose content and similar structure to natural wood. Using hazelnut shells could increase the specific surface area, enhance the porosity, and improve the adsorption capacity of the biochar. Hazelnut shells could also reduce the content of heavy metals in the raw biochar materials, along with increasing the ecological security of biochar and contributing to its further development and utilization. FTIR was used to study the changes in the functional groups on the biochar surface. The layered porous structure of the biochar was observed by SEM. The Cu(II) adsorption capacity of the biochar was 42.28 mg/g after 24 h. The Langmuir and pseudo-second-order models effectively described Cu(II) adsorption.

Spent coffee ground as a new bulking agent for accelerated biodrying of dewatered sludge

The feasibility of using spent coffee ground (SCG) as a new bulking agent for biodrying of dewatered sludge (DS) was investigated in comparison with two other frequently-used bulking agents, air-dried sludge (AS) and sawdust (SD). Results showed that the moisture contents (MC) of 16-day DS biodrying with AS (Trial A), SCG (Trial B) and SD (Trial C) decreased from 70.14 wt%, 68.25 wt% and 71.63 wt% to 59.12 wt%, 41.35 wt% and 57.69 wt%, respectively. In case of Trial B, the MC rapidly decreased to 46.16 wt% with the highest water removal (70.87%) within 8 days because of the longest high-temperature period (5.8 days). Further studies indicated that the abundant biodegradable volatile solids (BVS) and high dissolved organic matter (DOM) contents in SCG were the main driving forces for water removal. According to pyrosequencing data, Firmicutes, most of which were recognized as thermophiles, was rapidly enriched on Day 8 and became the dominant phylum in Trial B. Four thermophilic genera, Bacillus, Ureibacillus, Geobacillus and Thermobifida, which can produce thermostable hydrolytic extracellular enzymes, were the most abundant in Trial B, indicating that these thermophilic bacteria evolved during the long high-temperature period enhanced the biodegradation of BVS in SCG. The 8-day biodried product of Trial B was demonstrated to be an excellent solid fuel with low heating value (LHV) of 9284 kJ kg^-1, which was 2.1 and 1.8 times those of biodried products with AS and SD, respectively. Thus SCG was found to be an excellent bulking agent accelerating DS biodrying and producing a solid fuel with a high calorific value.

Kinetics evaluation and thermal decomposition characteristics of co-pyrolysis of municipal sewage sludge and hazelnut shell

Hazelnut shell, as novel biomass, has lower ash content and abundant hydrocarbon, which can be utilized resourcefully with municipal sewage sludge (MSS) by co-pyrolyisis to decrease total content of pollution. The co-pyrolysis of MSS and hazelnut shell blend was analyzed by a method of multi-heating rates and different blend ratios with TG-DTG-MS under N₂ atmosphere. The apparent activation energy of co-pyrolysis was calculated by three iso-conversional methods. Satava-Sestak method was used to determine mechanism function G(α) of co-pyrolysis, and Lorentzian function was used to simulate multi-peaks curves. The results showed there were four thermal decomposition stages, and the biomass were cracked and evolved at different temperature ranges. The apparent activation energy increased from 123.99 to 608.15kJ/mol. The reaction mechanism of co-pyrolysis is random nucleation and nuclei growth. The apparent activation energy and mechanism function afford a theoretical groundwork for co-pyrolysis technology.

Bioconversion of carbon dioxide to methane using hydrogen and hydrogenotrophic methanogens

Biogas produced from organic wastes contains energetically usable methane and unavoidable amount of carbon dioxide. The exploitation of whole biogas energy is locally limited and utilization of the natural gas transport system requires CO₂ removal or its conversion to methane. The biological conversion of CO₂ and hydrogen to methane is well known reaction without the demand of high pressure and temperature and is carried out by hydrogenotrophic methanogens. Reducing equivalents to the biotransformation of carbon dioxide from biogas or other resources to biomethane can be supplied by external hydrogen. Discontinuous electricity production from wind and solar energy combined with fluctuating utilization cause serious storage problems that can be solved by power-to-gas strategy representing the production of storable hydrogen via the electrolysis of water. The possibility of subsequent repowering of the energy of hydrogen to the easily utilizable and transportable form is a biological conversion with CO₂ to biomethane. Biomethanization of CO₂ can take place directly in anaerobic digesters fed with organic substrates or in separate bioreactors. The major bottleneck in the process is gas-liquid mass transfer of H₂ and the method of the effective input of hydrogen into the system. There are many studies with different bioreactors arrangements and a way of enrichment of hydrogenotrophic methanogens, but the system still has to be optimized for a higher efficiency. The aim of the paper is to gather and critically assess the state of a research and experience from laboratory, pilot and operational applications of carbon dioxide bioconversion and highlight further perspective fields of research.

Investigation on the fast co-pyrolysis of sewage sludge with biomass and the combustion reactivity of residual char

Gaining the valuable fuels from sewage sludge is a promising method. In this work, the fast pyrolysis characteristics of sewage sludge (SS), wheat straw (WS) and their mixtures in different proportions were carried out in a drop-tube reactor. The combustion reactivity of the residual char obtained was investigated in a thermogravimetric analyzer (TGA). Results indicate that SS and WS at different pyrolysis temperatures yielded different characteristic gas compositions and product distributions. The co-pyrolysis of SS with WS showed that there existed a synergistic effect in terms of higher gas and bio-oil yields and lower char yield, especially at the WS adding percentage of 80wt%. The addition of WS to SS increased the carbon content in the SS char and improved char porous structures, resulting in an improvement in the combustion reactivity of the SS char. The research results can be used to promote co-utilization of sewage sludge and biomass.

Co-pyrolysis characteristics of municipal sewage sludge and hazelnut shell by TG-DTG-MS and residue analysis

Co-pyrolysis characteristics of municipal sewage sludge and hazelnut shell blend have been studied in this work. The behavior of co-pyrolysis was researched by a method of multi-heating rates and different blend ratios to analyze thermal decomposition stages. The experimental data of the blended samples in TG-DTG plots were compared with calculated data to investigate the interactions during co-pyrolysis. The bio-chars investigated by SEM and FTIR spectra were used to examine the physical and chemical changes. The results showed there are four thermal decomposition stages during co-pyrolysis, with hydrocarbon transforming to gas evolution in the second and the third stages. The inhibitive interaction occurred between 260 and 400°C and the accelerative interaction occurred between 450 and 900°C during co-pyrolysis. The activation energy of the blended sample was 51.97-178.84kJ/mol in the second stage and 207.04-630.73kJ/mol in the third stage calculated by DAEM.

Hydrogen-rich gas production via fast pyrolysis of biophysical dried sludge: Effect of particle size and moisture content on product yields and syngas composition

After biophysical drying, a novel biophysical dried sludge particle was obtained. This work aims to investigate the function and effects of particle sizes and moisture contents on the fast pyrolysis of biophysical dried sludge particles. The results showed that large particles (>4 mm) favoured the oil generation with a maximum value of 19.0%, and small particles (<0.27 mm) favoured the char yield with a maximum value of 60.6%. Medium particle fractions (between 0.27 mm and 4 mm) benefited syngas production and induced higher H2 and CO emission, owing to the well-developed microstructure, enrichment of cellulose, and enhanced catalytic effects during the charring process. The introduction of proper moisture content (53.9% to 62.6%) to biophysical dried sludge was found to dramatically enhance syngas yield, hydrogen production, and carbon conversion efficiency. H2 molar concentration reached a maximum of 46.02% at a moisture content of 53.9%, which was attributed to the steam reforming and steam gasification accompanying the initial biophysical dried sludge pyrolysis.