Co-incineration effect of sewage sludge and municipal solid waste on the behavior of heavy metals by phosphorus

Treatment of chromium-containing sludge using sintering and ironmaking combined technology: A risk-reducing strategy for environmental impact

The efficient, safe and eco-friendly disposal of the chromium-containing sludge (CCS) has attracted an increasing concern. In this study, Co-processing of CCS was developed via employing sintering and ironmaking combined technology for its harmless disposal and resource utilization. Crystalline phase and valence state transformation of chromium (Cr), technical feasibility assessment, leaching risk, characteristics of sintered products, and pollutant release during CCS co-processing were investigated through a series of laboratory-scale sintering pot experiments and large scale industrial trials. The results showed that the content of Cr(VI) in sintered products first increased then decreased with increasing temperature ranges of 300 °C-800 °C, and reached a maximum of 2189.64 mg/kg at 500 °C. 99.99% of Cr(VI) can be reduced to Cr(III) at above 1000 °C, which was attributed to the transformation of the Cr(VI)-containing crystalline phases (such as, MgCrO4 and CaCrO4) to the (Mg, Fe2+)(Cr, Al, Fe3+)2O4. The industrial trial results showed that adding 0.5 wt‰ CCS to sintering feed did not have adverse effects on the properties of the sintered ore and the plant's operating stability. The tumbler index of sinter was above 78% and the leaching concentrations of TCr (0.069 mg/L) was significantly lower than the Chinese National Standard of 1.0 mg/L (GB5085.3-2007). The TCr contents of sintering dust and blast furnace gas (BFG) scrubbing water were less than 0.19 wt‰ and 0.11 mg/L, respectively, which was far below the regulatory limit (1.5 mg/L, GB13456-2012). The mass balance evaluation results indicated that at least 89.9% of the Cr in the CCS migrated into the molten iron in the blast furnace (BF), which became a useful supplement to the molten iron. This study provided a new perspective strategy for the safe disposal and resource utilization of CCS in iron and steel industry.

Effect of NaCl on the migration of heavy metals during the non-isothermal and isothermal combustion of sludge: Static and dynamic analyses

Chlorine has been proven to promote the volatilization of heavy metals during sludge combustion. This work compared the migration of heavy metals with NaCl addition under different combustion modes at 900 ℃. The combustion modes have less effect on the mineral phase of residues, but the volatilization and toxicity reduction of heavy metals were more pronounced under isothermal combustion. The mineral evolution, release of Cl, and migration of metals were dynamically tracked by the continuous sampling at different combustion time under isothermal combustion. It was found that the volatile matter and fixed carbon burned almost simultaneously, and the addition of NaCl promoted them. As combustion proceeded, the minerals gradually crystallized and the heavy metals were volatilized due to the direct and indirect chlorination. Meanwhile, the chlorination and volatilization of Zn was less than that of Pb due to its effective solidification by minerals. The combination of the adsorption by exposed char and solidification by sludge minerals influenced the dynamic leaching behavior of metals. These results will help understand the interactions between heavy metals, inorganic Cl, and Fe-Si-Al minerals during combustion, which will further help optimize the combustion strategy for both stabilization or enrichment of heavy metals when inorganic chlorine exists.

Bioavailability transition path of phosphorus species during the sewage sludge incineration process

Sewage sludge incineration ash (SSIA) is rich in phosphorus (P), thus being considered as a reliable source of phosphorus recovery. Different P species behaved significant bioavailability. Based on this, a comprehensive investigation into the bioavailability transition path of P species during sewage sludge (SS) incineration was conducted. P predominantly existed in the form of inorganic phosphorus (IP) in SS with a higher concentration of non-apatite inorganic phosphorus (NAIP) and less concentration of apatite inorganic phosphorus (AP). During the SS incineration process, OP existed in the flocs and cell structures of SS underwent destruction, the released P then combined with metal elements such as Ca, Mg, Fe, and Al to form AP species (Ca/Mg-P) and NAIP species (Fe/Al/Mn-P), and the NAIP decomposition to release into gas phase. This was the initial step for enhancing the bioavailability of P species. As temperature increased and the incineration process progressed, the low-temperature-resistant NAIP dissociated, and the metal-binding sites of Al, Fe and Mn in NAIP species were gradually replaced by the Ca and Mg thus forming thermal stability AP species (Ca/Mg-P, such as CaHPO4, Ca2PO4Cl, and Mg3(PO4)2 et al.). This step was crucial for the bioavailability improvement of P species during the incineration process. Therefore, the IP proportions in TP were extremely high (＞98%), and this value gradually increased as incineration temperature raised. The higher incineration temperature, the lower NAIP concentration and higher AP concentration. Besides, additives such as coal/rice husk/eggshell played a significant affect. Additives wither higher Ca content were inclined to react with P to form Ca/Mg-P (AP), while the presence of SO2 would react with Ca metals to form CaSO4 thus inhibiting the formation of AP species (such as CaHPO4 and CaPO4Cl). This results could provide theoretical support for the efficient and directional migration of P during sewage sludge incineration.

Transformation behavior of heavy metal during Co-thermal treatment of hazardous waste incineration fly ash and slag/electroplating sludge

In this study, the behavior of heavy metal transformation during the co-thermal treatment of hazardous waste incineration fly ash (HWIFA) and Fe-containing hazardous waste (including hazardous waste incineration bottom slag (HWIBS) and electroplating sludge (ES)) was investigated. The findings demonstrated that such a treatment effectively reduced the static leaching toxicity of Cr and Pb. Moreover, when the treatment temperature exceeded 1000 °C, the co-thermal treated sample exhibited low concentrations of dynamically leached Cr, Pb, and Zn, indicating that these heavy metals were successful detoxified. Thermodynamic analyses and phase transformation results suggested that the formation of spinel and the gradual disappearance of chromium dioxide in the presence of Fe-containing hazardous wastes contributed to the solidification of chromium. Additionally, the efficient detoxification of Pb and Zn was attributed to their volatilization and entry into the liquid phase during the co-thermal treatment process. Therefore, this study sets an excellent example of the co-thermal treatment of hazardous wastes and the control of heavy metal pollution during the treatment process.

Bidirectional Catalysis Disintegration and Mineral Polymerization via Endogenous Iron(III) from Iron-Rich Sludge in Synergy with Waste Incineration Fly Ash

To enhance the utilization of solid waste in cement kiln co-processing, this study analyzed the multifaceted synergy of pyrolysis and mineralization processes of iron-rich sludge (SS) and waste incineration fly ash (FA) at optimal blending ratios. Based on the physicochemical properties of SS and co-pyrolysis experiments, it was found that Fe acted as a positive catalyst in pyrolysis between 700 and 1000 °C, while the endogenous polymerization effect of Fe(III) mineral groups dominated above 800 °C. Additionally, the study investigated the solidification and migration of heavy metals and the transformation of harmful elements (S, Cl, and P). Results indicated that the best mixture ratios for SS and FA were 6:4 and 9:1, respectively, and synergistic pyrolysis and mineral co-curing effects were observed in the pyrolysis temperature range of 50-1000 °C. The synergy between SS and FA allowed for the decomposition and solidification of harmful organic components and heavy metals, reducing environmental risks. Furthermore, in actual production, by mixing 100 tons of SS and FA with Portland cement with a daily output of 2500 tons, the compressive strength during early hydration stages can reach 34.52 MPa on the third day, exceeding the highest performance of Portland cement (62.5R) strength index specified in the standard.

Study on properties of sewage sludge cemented paste backfill and leaching mechanism of heavy metals

In order to achieve sustainable development and control environmental pollution, this paper proposes sewage sludge (SS) as an auxiliary cementitious material, which is mixed with ordinary Portland cement (OPC), fly ash (FA), and gangue to produce sewage sludge cemented paste backfill (SS-CPB) material. The fluidity and mechanical properties of backfill materials with different SS contents and the heavy metal leaching mechanism of SS-CPB are investigated. The results reveal that (1) with the increase of SS content from 10 to 30%, the slump of fresh SS-CPB mortar decreased from 21.7 to 18.2 cm, the initial setting time decreased from 2.83 to 0.58 h, and the final setting time decreased from 4.92 to 0.83 h. (2) Compared with the control group, the 3-day unconfined compressive strength (UCS) of the SS-CPB mixed with 10% SS increased by 49.5%, and the UCS decreased slightly in the later stage, but it also met the actual needs of coal mines. (3) Scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy were used to study the SS-CPB samples. It was found that the free Al³⁺ in SS promoted the formation of ettringite (AFt), provided part of the early UCS, and accelerated the setting time. (4) The leaching rule of heavy metal ions was analyzed in combination with leaching kinetics, and the change of heavy metal ion mass concentration in the rising stage was in line with the contraction core model controlled by diffusion.