Sludge Carbon Research Articles

The properties and combustion behaviors of hydrochars derived from co-hydrothermal carbonization of sewage sludge and food waste

The experiments of co-hydrothermal carbonization (co-HTC) of sewage sludge and food waste in different mixing ratio (30%, 50% and 70%) and process temperature (180 °C, 230 °C and 280 °C) were conducted in this paper. And the hydrochars properties and thermal behaviors were investigated to determine the effects of the conditions. The results showed that the hydrochars derived from co-HTC possessed higher C content, higher HHV compared with the hydrochar of sewage sludge. Meanwhile, it maintained low N, S and O content relatively. It ascribed to the carbonization, dehydration and decarboxylation reactions according to the ultimate analysis and proximate analysis. And the TGA indicated that the combustion behaviors got better compared with the hydrochar of sewage sludge. Therefore, the co-HTC with food waste is an effective way to transform sewage sludge into clean solid fuel in the field of energy utilization.

Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application

Hydrothermal carbonization has been considered effective to reduce sewage sludge volume and provide an opportunity to generate valuable byproducts for a wide range of potential applications. The fate and distribution of nutrients during hydrothermal carbonization of sewage sludge, along with that of heavy metals, is very important for realizing nutrient recovery and reuse by both hydrochar and liquid phase directly as soil amendments and/or organic fertilizer. In this study, we systematically investigated the migration and transformation of nutrients and heavy metals using chemical extractions and reaction severity (Ruyter model) methods at a temperature range of 120–300 °C for 30–180 min. With increasing reaction severity, carbon and nitrogen efficiency of hydrochar showed a liner and exponential decrease respectively, while phosphorus accumulated positively in hydrochar. Nitrogen species in hydrochar are mostly nitrogen-containing aromatic heterocycles whereas in liquid phase are predominantly ammonia-N and organic-N. The organic phosphorus in sewage sludge was transformed to inorganic species but non-apatite phosphorus showed an exponential reduction in hydrochar with increasing reaction severity, together with the decrease of orthophosphate in liquid phase. As for heavy metals, hydrothermal carbonization promoted decrease in direct bioavailable fractions (except arsenic) and increase in stable fraction (except cadmium and arsenic) in hydrochar as reaction severity elevated. Since hydrothermal effects led to redistribution of nutrients speciation and immobilization of heavy metals, non-farm land application of both hydrochar and liquid phase seems to be possible and unrestricted. This study provides fundamental knowledges for the construction of sludge management strategies to better nutrients recycling and reclamation.

Oxidation of high iron content electroplating sludge in supercritical water: stabilization of zinc and chromium.

The stabilization of heavy metals (zinc and chromium) and the degradation of organic pollutants during supercritical water (SCW) and supercritical water oxidation (SCWO) treatment of electroplating sludge (EPS) with a high iron content were studied. Experiments were performed in a batch reactor at temperatures in the range from 623.15 to 823.15K with an oxygen coefficient (OE) from 0 to 2.0, a reaction time of 7min and pressure of 25MPa to examine the effect of the operation conditions. Chemical oxygen demand (COD) and total organic carbon (TOC) in raw sludge and liquid products under different reaction conditions were detected. The results indicated that more organic pollutant degradation occurred under supercritical conditions than in subcritical water. Additionally, as the temperature and amount of oxidant increased, the organic pollutant removal rate increased. In addition, the Zn and Cr removal efficiency from sludge was more than 98% under all conditions. Temperatures under 773.15K had a positive effect, whereas the oxygen ratio was more significant than the other factors above 773.15K. Furthermore, leaching toxicity tests of the heavy metals in solid products were conducted based on the toxicity characteristic leaching procedure (TCLP). All heavy metals showed greatly reduced leaching toxicity due to their stabilization. The Zn in the EPS is more easily converted into a solid product after SCWO treatment; however, Cr is more difficult to leach from the solid product. Oxides of iron, zinc, and chromium were detected by X-ray diffraction and an electron probe microanalyzer, and the yield of the oxides increased with increasing temperature and oxidant amount. Using the obtained data and analysis results, the effect of Fe on the stabilization of Zn and Cr was studied.

Treatment of 3,3′-dimethoxybenzidine in sludge by advance oxidation process: Degradation products and toxicity evaluation

Studies on the oxidation products of organic pollutants and their toxicity in textile dyeing sludge after the sludge was treated by the advance oxidation processes were limited, since textile dyeing sludge was a complicated mixture. For the first time, simulated sludge was used to study the degradation mechanism of 3,3′-dimethoxybenzidine (DMB) during the combined ultrasound-Mn(VII) treatment. The toxicity of DMB and its products was also evaluated. The results indicated that the compositions and microstructures of polyaluminium chloride (PAC)- and polyferric sulphate (PFS)-based simulated sludge were similar to those of real textile dyeing sludge. The optimum conditions of ultrasound-Mn(VII) treatment were: a KMnO4 dosage of 40 μM, an ultrasound power density of 0.36 W cm−3, and a reaction time of 20 min. 98.24% of DMB and 63.04% of total organic carbon (TOC) in the simulated sludge were removed. Six products, that is, 2-nitroanisole, 3-methoxy-4-nitrophenol, vanillylmandelic acid, vanillyl alcohol, m-anisic acid, and benzoic acid, were identified by GC-MS and LC-MS-MS. It was noted that all of these identified products were also detected in the real textile dyeing sludge after the ultrasound-Mn(VII) treatment. All of them were less toxic than DMB. Moreover, 53.30% and 54.80% of toxicity toward the alga Desmodesmus subspicatus and the bacterium Vibrio fischeri were removed in simulated sludge, respectively. Therefore, simulated sludge was helpful for studying a pollutant's degradation mechanism in the complex sludge mixtures. The results would also provide some useful suggestions for the sludge disposal after the sludge was treated by the advance oxidation processes.

Pyrolysis kinetics and thermodynamic parameters of the hydrochars derived from co-hydrothermal carbonization of sawdust and sewage sludge using thermogravimetric analysis

The thermal behavior of the hydrochars from co-hydrothermal carbonization (co-HTC) of sawdust (SD) and sewage sludge (SS) was investigated using thermogravimetric analysis. The comprehensive devolatilization index indicated that the devolatilization performance of SS was decreased by HTC, while it was significantly improved 7.38–23.69 times by co-HTC. The kinetic analysis showed that HTC of SS decreased the average activation energy from 308.96 and 314.78 kJ mol−1 to 220.86 and 221.27 kJ mol−1 by Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), respectively, while it was increased from 182.37 to 207.06 kJ mol−1 and from 181.06 to 207.05 kJ mol−1 with the increasing proportion of SD from 25% to 75% during co-HTC, respectively. The thermodynamic parameters revealed that pyrolysis reactivity of the hydrochar derived from SD was improved by co-HTC of SD and SS. Kinetic and thermodynamic findings were useful for the design of pyrolysis process using hydrochar as solid fuel.

Phosphorous Retention and Release by Sludge-Derived Hydrochar for Potential Use as a Soil Amendment.

To solve both the problems of P deficiency in arable soil and excessive waste sludge disposal, we evaluated hydrothermal carbonization of sludge with the aim of recycling sludge P resources for soil amendment. In contrast with pyrochars obtained through pyrolysis, hydrochars generated from hydrothermal carbonization often feature variable properties and therefore require detailed characterization. In this study, sludge-derived hydrochars were evaluated to determine their P content and fractionation, release and availability of P, and P adsorption and desorption behavior. We also assessed changes in P availability after soil was amended with the hydrochars. Our results showed that the chars were rich in total P (up to 25,175 mg kg), but most of the free fractions were transformed to bound fractions, thereby reducing the available P concentrations. However, available P content was >417 mg kg, which was far higher than soil demand. The hydrochars shifted from releasing to adsorbing P adsorbent when the environmental P concentration increased above 20 mg L. The hydrochars showed a high phosphate adsorption capacity (up to 23,815 mg kg) and the adsorbed P could be readily released. The addition of 1% P-laden hydrochar significantly enhanced the soil available P content by 8.93 mg kg. These findings have important implications for further development of hydrochar-based P carriers as a slow-releasing fertilizer.

Gasification performance of the hydrochar derived from co-hydrothermal carbonization of sewage sludge and sawdust

Co-hydrothermal carbonization (co-HTC) is recognized as a promising pretreatment for upgrading fuel quality of high moisture biomass prior to further thermal conversion. The properties of the hydrochar derived from co-HTC of sewage sludge (SS) and sawdust (SD), and CO2 gasification characteristics of the hydrochar were investigated. The results showed that the hydrochar had enhanced aromatic degree and increased metals content compared to raw sludge. The hydrochar also exhibited an enhanced gasification reactivity, resulting in high carbon monoxide content in the syngas than that from SS under identical conditions. The temperature and SD/SS mass ratio had a significant influence on the syngas composition mainly by the Boudouard reaction and water gas reaction. The gasification reactivity was strongly associated with the inorganic elements, and the KAlSi3O8 inhibited while the K2CO3 promoted the gasification reaction of hydrochars. Under optimal conditions, a high gasification efficiency of 77.73% was obtained, and the lower heating value of the syngas reached 8.15 MJ/Nm3. This study indicated that co-HTC of SS and SD combined with subsequent gasification had promising potential towards syngas production with high quality.

Urea-modified grass ash activated sludge carbon: structure and adsorption properties towards H2S and CH3SH

The deodorization structure of H2S and CH3SH adsorbed by sludge carbon was studied, and the deodorized product was analyzed.

Comparative study of the methods for the determination of organic carbon and organic matter in soils, compost and sludge

The aim of this study was to compare the most commonly used methods for the determination of organic carbon (OC) and organic matter (SOM), and evaluate the capability of LOI (loss on ignition method) to estimate OC, using reference soils, compost and sludge as standards. The use of a titrimetric endpoint (adding phosphoric acid prior to titration) is more suitable for assessing the level of organic carbon in soils with low, medium, high and very high content. The use of a photometric endpoint is more appropriate for samples of medium and high carbon content. LOI 400 and LOI 450 may also be used for the determination of samples with a high organic content whereas LOI 450 - in the determination of samples with an average organic content. LOI 450 can be used to evaluate SOC in a wide range of organic matter concentrations (3-55%). Indirect assessment of the organic carbon content or organic matter using a correction coefficient is not very accurate for samples with average organic carbon content. Keywords: methods, organic carbon, organic matter, certified samples

Fecal sludge as a fuel: characterization, cofire limits, and evaluation of quality improvement measures.

In many low-income cities, a high proportion of fecal sludge, the excreta and blackwater collected from onsite sanitation systems such as pit latrines, is not safely managed. This constitutes a major danger to environmental and human health. The water, sanitation, and hygiene sector has recognized that valorization of treated fecal sludge could offset the upfront cost of treatment by using it as a fuel source. The few quantitative studies on fecal sludge fuel published to date have focused on heating value, moisture, ash fraction, and heavy metals. However, other factors impacting fuel utility, specifically ash speciation, have not been adequately quantified for fecal sludge. This study contributes to closing that gap and shows the value of more detailed quantification. It first characterizes fecal sludge samples from Colorado and Uganda, confirms that the fuel is better if cofired with other biomass, and outlines a framework for determining safe cofire ratios. Second, the study evaluates two methods for improving fecal sludge as a fuel: carbonization and ash leaching. Carbonization of fecal sludge did not improve fuel quality, but leaching showed promise in ash reduction.

Enhancing denitrification efficiency for nitrogen removal using waste sludge alkaline fermentation liquid as external carbon source.

External carbon source was usually added to enhance denitrification efficiency for nitrogen removal in wastewater treatment. In this study, waster sludge alkaline fermentation liquid was successfully employed as an alternative carbon source for biological denitrification. The denitrification performance was studied at different C/Ns (carbon-to-nitrogen ratios) and HRTs (hydraulic retention times). A C/N of 7 and an HRT of 8h were the optimal conditions for denitrification. The nitrate removal efficiency of 96.4% and no obvious nitrite accumulation in the effluent were achieved under the optimal conditions with a low soluble chemical oxygen demand (SCOD) level. The sludge carbon source utilization was analyzed and showed that the volatile fatty acids (VFAs) were prior utilized than proteins and carbohydrates. The excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) was adopted to analyze the compositional and variations of dissolved organic matters (DOM). Moreover, a high denitrification rate (VDN) and potential (PDN) with low heterotroph anoxic yield (YH) was exhibited at the optimal C/N and HRT condition, indicating the better denitrification ability and organic matter utilization efficiencies.

Preparation of biological activated carbon (BAC) using aluminum salts conditioned sludge cake for the bio-refractory organic contaminants removal from anaerobically digested liquor

Waste sludge management continues to pose a great challenge with the increasing development of municipal wastewater treatment. Preparation of biological activated carbon (BAC) with waste activated sludge (WAS) is a promising method which could realize the synchronous harmlessness and recycling of WAS. The chemical conditioning is an indispensable step during the high-pressure dewatering process, which may greatly affect the physicochemical properties of BAC and adsorptive performance for various contaminants. In this work, sludge cake conditioned with typical inorganic polymeric flocculants – polymeric aluminum chloride (PAC) was used to prepare BAC for anaerobically digested liquor (ADL) treatment, and the effects of hydroxyl aluminum on physicochemical properties and adsorptive performance were systematically investigated. The results indicated that hydroxyl aluminum speciation had important effects on pore structure and functional groups on the surface of BAC. In comparison with sludge activated carbon, BAC conditioned with PAC exhibited better thermal stability and contained more functional groups. Furthermore, BAC conditioned with PAC of alkalisation degrees of 0.5 and 1.5 (PAC0.5-BAC and PAC1.5-BAC) had remarkable mesoporous structure and notable adsorption capacities for organic contaminants, and the organic contaminants removal could be fitted Freundlich model better. In addition, PAC1.5-BAC performed best in removing chroma and biopolymers (humic and protein-like substances) in filtrate. In brief, this paper proposed a novel sludge carbonization process by coupling enhanced dewatering with catalytic pyrolysis to prepare efficient BAC for wastewater treatment.

Optimization of polyhydroxyalkanoates (PHA) synthesis with heat pretreated waste sludge

To reduce the cost of polyhydroxyalkanoates (PHA) production and disposal amount of waste sludge simultaneously, the feasibility of using different heat pretreated sludge (60 °C, 80 °C, 100 °C, 120 °C) as external carbon source to synthesize PHA was examined in this study. The maximal PHA accumulation (24.1% of the dry cell weight) was achieved with 60 °C pretreated waste sludge, with the utilization efficiency of COD, proteins, carbohydrate and VFAs were 74.3%, 82.3%,47.2%,81.4%, respectively. Both of VFAs and non VFAs organics could be used as carbon source for PHA synthesis. The results of kinetic parameter analysis showed that the highest PHA production rate (0.23 mg COD/mg X·h) and the PHA conversion rate (0.46 mg COD/mg COD) all occurred when using 60 °C pretreated waste sludge. In order to further investigate the utilization of sludge carbon source for PHA synthesis, the three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis were introduced.

Phytoremediation potential of sewage sludge using native plants: Gossypium hirsutum L. and Solanum lycopersicum L.

The aim of the study was to evaluate the potential of Gossypium hirsutum L. and Solanum lycopersicum L. plants for sewage sludge stabilization. The plants were cultured for 120 days in two plastic beds of 30 cm × 60 cm × 50 cm (W. L. H) in dimension. The highest removal efficiency of organic carbon, total Kjeldahl nitrogen, total phosphorous, lead and cadmium was obtained 36, 33.6, 23, 74.29 and 91.2%, respectively. The removal rate of above parameters was considerably higher than control (P < 0.05), and in Solanum lycopersicum L., it was higher than Gossypium hirsutum L. (P < 0.05). The pH, electrical conductivity and sodium absorption ratio variation in control with plants were significant (P < 0.05). By increasing electrical conductivity and sodium absorption ratio, the removal efficiency of heavy metals increased. The highest translocation factor (2.44) and bio-concentration factor (1.63) were observed in the Solanum lycopersicum L. for lead. Maximum and minimum cadmium concentrations in the shoot parts of Solanum lycopersicum L. and Gossypium hirsutum L. plants were 1.21 and 0.03 mg/kg, respectively. TF mean for lead and cadmium in Gossypium hirsutum L. was and 0.65 and 1.77; in Solanum lycopersicum L., it was 0.44 and 1.5, respectively. The results showed that the mean of bio-concentration factor in Solanum lycopersicum L. bed for Cd and Pb was 0.39 and 0.63, and for Gossypium hirsutum L., it was 0.3 and 0.58, respectively. The changes in organic carbon, nutrients and heavy metal in sludge were showed that Solanum lycopersicum L and Gossypium hirsutum L. can be used properly to stabilize sewage sludge.

A Evaluation on Drying Performances of Hydrothermal Carbonization of Sewage Sludge by using Low-grade Heat Source

A Evaluation on Drying Performances of Hydrothermal Carbonization of Sewage Sludge by using Low-grade Heat Source

Accelerated phosphorus recovery from aqueous solution onto decorated sewage sludge carbon

In search of efficient phosphorus resource recovery and pollution remediation should be highly concerned due to the view of phosphorus nonrenewable and eutrophication. This work presented a new insight into conversion of sewage sludge into favorable carbonaceous adsorbent for accelerated removing and recovering phosphorus from aqueous solution, what addressed the issues of phosphorus recovery and pollution remediation as well as sludge disposal. Ca and water hyacinth were evolved to decorate sludge derived carbon. Effect of mass ratio of sludge, water hyacinth and calcium carbonate on the morphologies and adsorption kinetics was investigated. The adsorbents (SW-Ca-112) resulted from sludge in the presence of water hyacinth and CaCO3 in a mass ratio of 1:1:2 had the highest adsorption capacity of 49.50 mg/g P and adsorption rate. Decoration of Ca favored adsorption ability and the presence of water hyacinth accelerated the adsorption rate due to the enhanced porosity. Formation of acicular Ca5(PO4)3OH nanoparticles contributed to the favorable adsorption process. Thus, the contribution of decorated Ca and water hyacinth to the adsorption ability and rate to phosphorus was understand, providing important information on resource utilization of sewage sludge as efficient adsorbent for immobilizing phosphorus from aqueous solution.

Potential enhancement of direct interspecies electron transfer for anaerobic degradation of coal gasification wastewater using up-flow anaerobic sludge blanket (UASB) with nitrogen doped sewage sludge carbon assisted

Waste sewage sludge was converted into the novel conductive material of nitrogen doped sewage sludge carbon (N-SC) to enhance anaerobic degradation of coal gasification wastewater (CGW). The results indicated that N-SC played a significant role in enhanced efficiencies, with chemical oxygen demand (COD) removal efficiency increased by 25.4%, methane production rate improved by 68.1% and total volatile fatty acids (VFA) decreased by 37.5% than that of controlled reactor. The conductivity, activity of electron transport, and extracellular polymeric substances (EPS) of anaerobic sludge were remarkably enhanced with N-SC, which promoted sludge granulation and supplied better conductive environment for microorganisms. The microbial community analysis revealed that potential enhancement of direct interspecies electron transfer (DIET) was achieved by electrical connection between enriched Geobacter, Pseudomonas and Methanosaeta with N-SC assisted, which enhanced the anaerobic degradation of CGW. Moreover, anaerobic degradation with N-SC had higher capacity to resist acidic shocks, facilitating the process stability.

A two-step process for sewage sludge treatment: Hydrothermal treatment of sludge and catalytic hydrothermal gasification of its derived liquid

The worldwide increase in amount of sewage sludge has recently attracted considerable interest in developing an efficient and economical sludge disposal method. A sewage sludge treatment process combining the hydrothermal treatment (HT) of sludge and the catalytic hydrothermal gasification (CHTG) of its derived liquid (HT&CHTG-Process) is proposed in this paper. Liquid hourly space velocity (LHSV) value of 25–100 h−1 and liquid flow rate of 10–40 mL/min were employed with a self-designed CHTG experimental setup. During the first stage of the HT-Process, approximately 56% of the carbon in the sludge was transferred into the derived liquid at 300 °C. During the second stage of the CHTG-Process, the derived liquid was gasified into CH4, H2, and CO2 using a Ni/carbon catalyst, and the carbon conversion reached 88% at 350 °C and 20 MPa with a LHSV of 25 h−1. Using the integrated HT&CHGT-Process, approximately 0.78 Nm3 of CH4, 0.62 Nm3 of H2 and 0.31 Nm3 of CO2 were generated from 1 kg of carbon in the sewage sludge. The combined process was expected to be an efficient method for sewage sludge treatment due to its mild conditions (≤350 °C and ≤20 MPa) and high yield of fuel gases.

Hydrothermal carbonization of anaerobic granular sludge: Effect of process temperature on nutrients availability and energy gain from produced hydrochar

Anaerobic granular sludge (AGS) has been applied for most highly efficacy anaerobic digestion systems like upflow anaerobic sludge blanket and expanded granular sludge bed reactors. As a by-product from these systems, AGS is prospected as a promising resource for energy and nutrients recovery from wastewater. In this study, hydrothermal carbonization (HTC) of AGS was investigated at different temperatures (160–240 °C) regarding the distributions of C, N and P in the hydrothermal products to maximize the utilization efficiency of AGS. Elemental composition and fuel characteristics of the hydrochar were evaluated. Results indicated that the percentages of C in hydrochar increased from 43.79% to 49.81% with the increase in HTC temperature, while N showed an opposite trend, decreasing from 9.58% to 5.49%. The higher heating value of hydrochar increased up to a maximum of 24 MJ/kg at 240 °C from 20 MJ/kg at 160 °C. However, the hydrochar yield decreased remarkably from 62% to 32%. As a result, the highest net energy output was about 6.86 MJ/kg achieved at 160 °C. Results from the van Krevelen diagram suggested that dehydration and decarboxylation reactions occurred during the HTC of AGS. In addition, the thermogravimetric analysis implied that the combustion of the produced hydrochar could be completed in two phases rather than the one phase as the raw AGS. With regard to other resources utilization, HTC was proved to be effective for AGS to immobilize and recycle phosphorus. The increase in HTC temperature exerted a limited effect on P distribution, resulting in less than 5% being released into the liquid at all tested HTC temperatures. Most of P were immobilized into the produced hydrochar where the bioavailable P fractions > 80%.

Zn-Fe-rich granular sludge carbon (GSC) for enhanced electrocatalytic removal of bisphenol A (BPA) and Rhodamine B (RhB) in a continuous-flow three-dimensional electrode reactor (3DER)

Recycling sewage sludge from wastewater treatment plants is one of the most challenging environmental problems. In this work, we develop a one-step facile synthesis method of converting iron-rich sewage sludge into granular sludge carbon (GSC), further applying as particle electrodes in three-dimensional electrochemical reactor (3DER) for wastewater treatment. Iron in sludge and zinc in activator are in-situ loaded on particle electrodes as catalytic ingredients. The prepared GSC under different pyrolysis temperature are characterized to investigate the physical, chemical and electrochemical properties. The treatment performances of GSC as particle electrodes are evaluated by degrading bisphenol A (BPA) and Rhodamine B (RhB) in a continuous-flow 3DER. The stability of the GSC are evaluated and possible reaction mechanism are explored. Results indicate that GSC fabricated are typical macroporous material, exhibiting good electrocatalytic activity. The 500 °C/GSC packed 3DER shows the best performances in contaminants removal, that was attributed to larger specific surface and superior electrochemical properties with cooperation of iron oxide and zinc oxide components. The 500 °C/GSC prepared are proved to be suitable for utilizing as particle electrodes in 3DER. Our work offers a new strategy for sewage sludge reuse and provides a promising future for 3DER applications.