Products distribution and sulfur fixation during the pyrolysis of CaO conditioned textile dyeing sludge: Effects of pyrolysis temperature and heating rate

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

Co-hydrothermal carbonization of sludge and food waste for hydrochar valorization: Effect of mutual interaction on sulfur transformation

Co-hydrothermal carbonization of sludge and food waste is a promising method for hydrochar valorization. The sulfur content and form of hydrochar are the key parameters that determine its further utilization. However, the effect of the chemical composition of food waste on sulfur redistribution remains unknown. Herein, the sulfur transformation behavior during the co-hydrothermal carbonization of sludge and model compounds (cellulose, starch, xylan, and palmitic acid) of food waste was investigated, with focus on the detailed reaction pathways from inorganic-S/organic-S media in aqueous to hydrochar. The added model compounds, particularly the starch and xylan, increased the sulfur retention ratio from 41.0 to 44.7- 49.2 % in hydrochar. Among them, starch and xylan can react with aliphatic-S in aqueous via cyclization and oxidization to form the thiophene-S/aromatic-S and sulfone-S and can react with SO42--S to form sulfone-S via sulfonate reaction. These formed organic-S can polymerize with hydrolyzed intermediates (i.e., 5 hydroxymethyl-furfural, glucose, and xylose) from model compounds to transform into hydrochar. Cellulose enhanced the formation of sulfone-S in hydrochar via the reactions between the water-insoluble partial hydrolysate and SO42- in the aqueous. Additionally, palmitic acid hydrolysate provided an acidic environment that facilitated the polymerization of thiophene-S/aromatic-S from aqueous to hydrochar. Generally, the chemical composition of food waste largely affects the redistribution behavior of sulfur during co-hydrothermal carbonization, and this occurs primarily due to the differences in the hydrolysate and degree of hydrolysis for various model compounds. The results can provide guidance for preparing sludge-based hydrochar possessing different sulfur content and species, that can be used as clean fuel or carbon material.

Acid gas emission and ash fusion characteristics of multi-component leather solid waste incineration in bubbling fluidized bed

The tanning sludge (TS) and other tanning solid wastes are produced in significant quantities by the leather industry. To evaluate the combustion properties, acid gaseous pollutant conversion, and ash management, co-firing of TS with various wastes was investigated in a bubbling fluidized bed. TG-FTIR test indicated that tanning solid wastes had superior combustion properties and include more gaseous pollutants than TS. The leather mixed solid waste (LMSW) formed by mixing had better fuel characteristics than TS. The conversion rates of SO2 and HCl of LMSW incineration were 67% and 40%, respectively. The co-combustion of TS and solid wastes reduces the conversion rate of acid gas. Increasing the proportion of high-inorganic chlorine raw material could further reduce the conversion rate and increase the ash fusion temperature appropriately. Because ash and slag were primarily composed of Ca and Fe elements, the addition of calcium carbonate (CaCO3) can increase ash melting point while reducing acid gas emissions. When CaCO3 was added at a calcium to sulfur (Ca/S) ratio of 2, the acid gas emission was reduced by more than 80% and the softening temperature was raised by 90 °C. When Ca/S is greater than 2, the economics of adding CaCO3 decreased.

Effect of electromagnetic induction drying on the drying-incineration process of dyeing sludge: focus on migration and conversion of sulfur

Secondary sulfur pollution in dyeing sludge (DS) during drying and incineration is a major environmental problem necessitating in-situ control. To robustly immobilise sulfur during drying-incineration, the authors introduce an electromagnetic induction (EMI) drying method and reveal the corresponding migration and conversion of sulfur in DS. The EMI-drying efficiency reached 10.69%/min, five times that of thermal drying. EMI drying increases the relative sulfoxide ratio from that of thermal drying. In a sludge-sulfur model, the proposed treatment promoted the oxidation and decomposition of organic sulfur without noticeably affecting the inorganic sulfur. The selective oxidation process during EMI drying promotes sulfur stabilisation in dried DS, decreasing the performance and stability of DS combustion. The sulfur-containing pollutants released during the incineration of DS mainly contain H2S, followed by CH3SH and SO2. EMI drying increases the outputs of SO2 and CH3SH but decreases the outputs H2S and total sulfur compared with the outputs of thermal drying. Under the sulfur-model conditions, EMI promoted the conversion of inorganic sulfur to sulfur-containing gases (especially H2S) during incineration. In contrast, the sulfur stabilised by partial oxidation of organic sulfur in the EMI-dried DS was not easily converted to gaseous sulfur during subsequent combustion. Overall, EMI inhibits the release of sulfur during the combined drying-incineration process of DS.

Study on the effect of biomass on sulfur release behavior from dyeing sludge incineration: Focusing on in-situ sulfur fixation mechanism based on model compounds

Although incineration is a recommended disposal strategy for dyeing sludge (DS), sulfurous gases problem is severe. Wood sawdust (WS) and rice husk (RH) are eco-friendly and CO₂-neutral additives to relieve sulfur emission from DS incineration. However, the interaction between organic sulfur and biomass is uninterpreted. This study explores the effect of WS and RH on the combustion behavior and sulfur evolution from organic sulfur model compound combustion via thermogravimetry (TG) with mass spectrometry (MS). Results indicated that the sulfone and mercaptan combustion activities in DS were more drastic than in other forms. WS and RH additives generally deteriorated the combustibility and burnout performance of model compounds. The combustion of mercaptan and sulfone in DS contributed to most gaseous sulfur pollutants, where CH₃SH and SO₂ were the predominant forms. WS and RH minimized the sulfur release from mercaptan and sulfone incineration, whose in-situ retention ratios reached 20.14 % and 40.57 %. The retention mechanism to sulfur could be divided into: (1) Diffusion stage: the closed structure of biomass residue restrained sulfurous gases from escaping. (2) Chemical reaction stage: multiple sulfation occurred and inhibited sulfur release. Ca/K sulfate and compound sulfates were predisposed and thermostable sulfur-fixing products for the mercaptan-WS and sulfone-RH co-combustion systems.