| Anaerobic digestion is a widely-used municipal sludge stabilization technology with good economic and environmental benefits,but traditional anaerobic digestion has the disadvantages of low organic matter reduction,long digestion cycle and high construction cost.Combined pretreatment can effectively improve the sludge anaerobic digestion performance and can also reduce energy consumption and usage of chemicals,thus becoming a new direction of research today.In this study,the properties of sludge and semi-continuous thermophilic anaerobic digestion performance after several combined pretreatments were investigated to determine the optimal pretreatment.In addition,the optimal combined pretreatment and its single pretreatments were compared in the respect of pretreatment performance.This study also analyzed the changes in colony structure and estimated cost of anaerobic digestion of the thermophilic anaerobic digestion system under untreated condition and optimal pretreatment.The main conclusions are as follows:（1）Pretreatments can significantly promote the solubilization of organic matter in sludge.The solubilization effect of organic matter under NaOH-low temperature hydrothermal combined pretreatment is better than that under HCl-low temperature hydrothermal combined pretreatment.However,HAc pretreatment cannot further promote the solubilization of organic matter.The combined pretreatment has a better solubilization effect on sludge organic matter than single pretreatments.The NaOH-low temperature hydrothermal combined pretreatment and NaOH-low temperature hydrothermal-HAc combined pretreatment had the best solubilization effect,and their soluble chemical oxygen demand（SCOD）/total chemical oxygen demand（TCOD）ratio could reach 24.5%and 24.7%,respectively.Pretreatments can also promote the release of ammonia nitrogen,the increase of total alkalinity and the decrease of viscosity in sludge.（2）NaOH-low temperature hydrothermal combined pretreatment and HCl-low temperature hydrothermal combined pretreatment can improve gas production performance of thermophilic anaerobic digestion.The addition of HAc is not conducive to the biogas production of sludge thermophilic anaerobic digestion.Combined pretreatment produces better gas performance of anaerobic digestion than a single pretreatment.NaOH-low temperature hydrothermal combined pretreatment possesses the optimal gas production performance.Its biogas yield and methane yield are 182.9 m L/g VSadded and 101.3 m L/g VSadded,respectively,which can be increased by 6.4%and 31.0%compared with untreated raw sludge,respectively.（3）The thermophilic anaerobic digestion of sludge under NaOH-low temperature hydrothermal combined pretreatment has the best removal ability of organic matter,and the volatile solids（VS）reduction rate and TCOD reduction rate could reach 37.4%and 36.8%,respectively.The anaerobic digestion system under different pretreatments does not suffer any inhibition with stable state.（4）NaOH-low temperature hydrothermal combined pretreatment can significantly influence the colony structure of sludge thermophilic anaerobic digestion system.It can also significantly improve species uniformity and species diversity.The number of dominant bacterial phylum in the system increase,including the six phyla of Firmicutes,Chloroflexi,Synergistetes,Proteobacteria,Actinobacteria and Thermotogae.The relative abundance of dominant phyla are more uniform.In addition,the number and uniformity of dominant bacterial genuses in the system have also increased.（5）The operating costs under untreated,low-temperature hydrothermal pretreatment and NaOH-low temperature hydrothermal combined pretreatment were 54.96 yuan/ton sludge（moisture content of 80%）,70.13 yuan/ton sludge（moisture content of 80%）and 95.30 yuan/ton sludge（moisture content of 80%）,respectively.Although the treatment cost of NaOH（p H=11,24h）-low temperature hydrothermal（90°C,30min）combined pretreatment has been increased,it still possesses economic benefits when it receives government subsidies. |
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