The harmless treatment and energy conversion of the sewage sludge can be implemented by pyrolysis,and the understanding of speciation distribution of the heavy metals in pyrolysis residue is particularly important.The pyrolysis experiments were conducted at different temperatures (500,600,700,800 and 900 ℃),and the contents of heavy metals in raw sludge(RS)and dewatered sludge cake(FS) were studied.The concentrations of Cu,Zn,Cr,Pb and Cd in the pyrolysis residue were determined and the speciation of heavy metals was analyzed following Tessier sequential extraction procedure.An enrichment effect of heavy metals was found during the pyrolysis process.An enrichment of Cu and Zn in the pyrolysis residue was found for the raw sludge and an enrichment of Cr,Pb and Cd was found for the FS.From the chemical state analysis of the pyrolysis residue,most of the heavy metals (Cu,Zn,Cr,Pb,Cd) were presented in a more chemical stable form than those from raw sludge.

This study investigated the synergistic effect of water content and a composite conditioner of Fenton's reagent combined with red mud (Fenton-RM) on the pyrolytic products (fuel gas, tar, and solid char) of deep-dewatered sludge. The catalytic effect of metal oxides in Fenton-RM could be promoted by the presence of water during sludge pyrolysis, showing higher gas yield with increased water content. Maximum gas outputs of the deep-dewatered sludge conditioned with Fenton-RM (S-Fenton-RM) and the conventional dewatered sludge conditioned with polyacrylamide (S-PAM), both appeared at 900 °C with a water content of 65 wt%, and were 0.257 and 0.189 L/g dry solid (DS), respectively. At the same temperature and with the same water content, the hydrogen (H2) yields of the S-Fenton-RM samples were always higher than those of the S-PAM samples. At 900 °C, the maximum H2 yield of the S-Fenton-RM samples was 0.102 L/g DS, which was 85.5% higher than that of the S-PAM samples. The results indicated that water in the wet sludge provided the steam atmosphere for pyrolysis, and the water vapor then involved in secondary cracking reformation of tar and char gasification reactions, which would be catalyzed by the presence of metal oxides in the Fenton-RM conditioner, thus increasing the yield of fuel gas, especially hydrogen. The H2 production cost from the S-Fenton-RM system is less than that from the S-PAM system. The results suggest that pyrolysis of the wet deep-dewatered sludge conditioned with Fenton-RM is an economical and promising alternative for sewage sludge dewatering and disposal/reuse.

该文在前期研究的市政污泥经Fenton试剂与赤泥复合调理,机械压滤脱水,得到深度脱水污泥泥饼的基础上,重点探讨脱水泥饼热解气化实验,首次研究了复合调理剂中不同种类赤泥对热解产气效果的影响.结果表明,经过高铁含量的澳洲矿赤泥复合调理的深度脱水泥饼热解产气量在700、800、900℃时与Fenton试剂单独调理泥饼相比均有明显提高.经过低铁含量的河南矿赤泥和印尼矿赤泥复合调理的深度脱水泥饼热解产气量仅在900℃时明显提高.经过赤泥复合调理的深度脱水泥饼在700、800、900℃时的热解产氢量都比Fenton试剂单独调理泥饼高,其中经过澳洲矿赤泥复合调理的深度脱水泥饼产氢量最高.这表明:赤泥中F2O3能够有效促进泥饼中有机物的裂解产H2、产CH4等气体,并具有裂解催化效果.

This study investigated the pyrolytic performances of deep-dewatered sludge cakes which were previously conditioned with Fenton's reagent and red mud. The pyrolytic products, including fuel gas, tar and solid char, were characterized by GC, GC-MS and FTIR. The results showed that pyrolysis of sludge cakes conditioned with Fenton's reagent and red mud produced more gas, especially higher H2 yield, compared with pyrolysis of raw sludge. In addition, red mud promoted PAHs formation and tar re-formation through side-chain-cleaving, dehydrogenation, polycondensation and aromatization of hydrocarbon during the pyrolysis, resulting in enhancement of the H2 yield. The sharp decrease in the absorbance of CHaromatic group of chars from the conditioned cakes implied that red mud could intensify the cracking and re-formation of aromatics. The higher the iron oxide content in red mud was, the better catalytic efficiency in the decomposition of organics and tar re-formation was, which led to more H2 production.

Raw and conditioned dewatered sewage sludges of sewage treatment plant were analysed by the TGA thermal analysis technique,and TG and DTG curves were obtained at different heating rates.Experimental results showed that the pyrolysis characteristics of different conditioned dewatered sludge were complex and pyrolysis kinetics with different conversion ratios corresponded to different kinetic reations.M-KAS model and Discrete DAEM model were also applied to analyze the kinetic parameters of different conditioned dewatered sewage sludge.The results showed that the nonisothermal decomposition process of dewatered sewage sludge contained three weight loss stages and the heating rate influenced the characteristic temperature zones of pyrolysis.With increasing heating rate,the weight loss curve slipped to the high temperature zone.The variations of kinetic parameters calculated by the above models showed that composite conditioning of Fenton and red mud had a great influence on the pyrolysis characteristics of dewatered sewage sludge.Fenton reagent can break the extracellular polymeric substances of sludge,thus affecting the nucleation process in the thermal decomposition reaction,thereby affecting the pyrolysis dynamics trend of sewage sludge.With red mud conditioning,red mud was easily combined with small sludge particles,increasing the number of pyrolysis reactions,thus improving the whole pyrolysis process.It can be concluded from the results that the pyrolysis reactions of dewatered sewage sludge were composed of several kinetic reactions with variable kinetic parameters.

Pilot-scale sewage sludge dewatering experiments were conducted using two composite conditioners: FeCl3 + lime (Fe-lime) and Fenton's reagents + red mud (Fenton-RM).

Red mud was evaluated as an alternative skeleton builder combined with Fenton's reagent in sewage sludge conditioning. The results show that red mud combined with Fenton's reagent showed good conditioning capability with the pH of the filtrate close to neutrality, indicating that red mud acted as a neutralizer as well as a skeleton builder when jointly used with Fenton's reagent. Through response surface methodology (RSM), the optimal dosages of Fe2+, H2O2 and red mud were proposed as 31.9, 33.7 and 275.1 mg/g DS (dry solids), respectively. The mechanism of the composite conditioner could be illuminated as follows: (1) extracellular polymeric substances (EPS), including loosely bound EPS and tightly bound EPS, were degraded into dissolved organics, e.g., proteins and polysaccharides; (2) bound water was released and converted into free water due to the degradation of EPS; and (3) morphology of the conditioned sludge exhibited a porous structure in contrast with the compact structure of raw sludge, and the addition of red mud formed new mineral phases and a rigid lattice structure in sludge, allowing the outflow of free water. Thus, sludge dewatering performance was effectively improved. The economic assessment for a wastewater treatment plant of 370,000 equivalent inhabitants confirms that using red mud conditioning, combined with Fenton's reagent, leads to a saving of approximately 411,000 USD/y or 50.8 USD/t DS comparing with using lime and ordinary Portland cement combined with Fenton's reagent, and approximately 612,000 USD/y or 75.5 USD/t DS comparing with the traditional treatment.