Study On Anaerobic Digestion Of Product From Hydrothermal Carbonized Sewage Sludge

With the increasing of sludge production and the deterioration of environment,the traditional methods have long been unable to meet the requirements of sludge treatment.Recently,hydrothermal carbonization technology as a thermochemical treatment has received more and more attention because of its short processing cycle and high conversion efficiency.Although the hydrothermal carbonization treatment can obtain a good carbon material,the disposal of its liquid product should be payed more attention.Anaerobic digestion is the preferable way to solve this problem.However,the influence of different temperatures on the anaerobic performance of hydrothermal carbonization products is serious,so the research on the anaerobic digestion properties of sludge hydrothermal products is necessary.The preliminary results indicated that an inhibitory effect occurred during anaerobic digestion of hydrothermal carbonization products.It is an urgent task to find a way to increase performance of anaerobic digestion.In this paper,the properties of the products under different hydrothermal temperatures were analyzed.And then,the anaerobic digestion performances of sludge hydrothermal carbonization products at different temperatures were studied in detail.In addition,hydrothermal pretreatment,dilution effect or solid-liquid separated on anaerobic digestion were investigated.Finally,a series of experiments to improve the anaerobic performance were carried out by using activated carbon.The experimental results were as follows:Firstly,the properties of sludge changed significantly after the hydrothermal treatment,and the hydrothermal product was an appropriate substrate for anaerobic digestion.With the increasing of hydrothermal temperature,the volatile content in product significantly decreased and the content of the fixed carbon increased.For example,the proportion of volatiles and fixed carbon were 49.41% and 6.35% for raw sludge,respectively,and 35.74%,7.95% for the products treated at 240 °C.At the same time,the dewaterability of hydrothermal product was significantly improved.The analytical results of liquid part showed that SCOD increased first and then decreased with increasing treatment temperature and the maximum SCOD of 63.00 g/L was obtained at 160 °C.The hydrothermal treatment also resulted in production of SCFAs in liquid,which increased with increasing treatment temperature.For instance,the SCFAs concentration increased from 2629.72 mg/L at 121 °C to 4106.88 mg/L at 240 °C.Secondly,medium and low temperature hydrothermal treatment improved the biodegradability of sludge.However,when the hydrothermal temperature exceeds 180 °C,anaerobic digestion is significantly inhibited.For example,methane productions of sludge treated at 121°C,160 °C and 180 °C were increased by 148 m L,138 m L and 130 m L,respectively.The peak value of SCFAs increased quickly and decreased faster.However,when the hydrothermal temperature is 220 °C and 240 °C,the methane production of the hydrothermal sludge was only 45.26% and 47.37% of the raw sludge,respectively,and the accumulation of SCFAs was as high as 4500 mg/L.GC-MS analysis of the products treated at 180 °C and 240 °C showed that the inhibitory organic components effect on anaerobic digestion involved in hydrothermal products might be: 2,5-dihydro-2,5-Dimethyl-furan,pyridine,2-methyl-pyridine,2,5-dione-piperidine,3-methyl-pyridine and 2,6-dimethoxy-phenol.Thirdly,the inhibitor was both present in the hydrothermal liquid phase and the solid phase product,and the inhibitory effect of the hydrothermal product on anaerobic digestion could be effectively reduced by diluting the hydrothermal product,thereby the production of anaerobic methane was improved.In the anaerobic experiments of hydrothermal solid phase and liquid phase products,the anaerobic performances of the solid or liquid products were all better than those of the mixed products.The methane production decreased by 45.32% and 84.59%,respectively,when the organic loads of liquid product and solid product were increased by 4.29 and 4.78 times.At the same time,when it was treated at 200 °C and 240 °C,the methane production of the 4-times diluted hydrothermal sludge was 1.28 and 1.77 times that of the 2-times diluted hydrothermal sludge,respectively.Finally,activated carbon reduced the impact of inhibitors on microorganisms by adsorbing the inhibitors,thereby improving the anaerobic performance.After the addition of activated carbon to the anaerobic system,the lag phase was significantly shortened.For instance,by adding activated carbon,the lag phase of the group which containing inhibitors from 10 g COD /L 240 °C-hydrothermal carbonization products was reduced from 12 days to 8 days;and the group which containing inhibitors from 15 g COD /L 240 °C-hydrothermal carbonization products was reduced from 26 days to 16 days.The anaerobic performance of the hydrothermal liquid products pretreated with activated carbon was basically the same as those adding activated carbon directly,and the methane production was 1.19-time and 1.84-time higher than that of the untreated group,respectively.