Hydrothermal Treatment Coupled With Mechanical Expression For Excess Sludge Deep Dewatering

Dewatering is very important for excess sludge treatment and disposal. The only way to realize the resource utilization of excess sludge is deep dewatering. However, the deep dewatering process of excess sludge with high energy consumption has always been the the challenge of technologies development for excess sludge dewatering. To solve this problem, a novel dewatering technology, hydrothermal treatment coupled with mechanical expression, was proposed, in which a conventional pressure dewatering is combined with hydrothermal effect to realize an improved liquid/solids separation with low energy consumption. In this study, the process was performed by way of that the excess sludge was hydrothermally treated first and then the mechanical expression was employed immediately at increased temperature in two separate cells respectively.Based on the verification of different performance of hydrothermal sludge dewatering at increased and room temperatures, the influences of the processing parameters (hydrothermal temperature, residence time and mechanical pressure) on the dewatering process were investigated, and the dewatering characteristics of hydrothermal sludge during mechanical expression stage were also evaluated based on the Terzaghi-Voigt rheological model. The results demonstrated that the mechanical expression employed at increased temperature showed a significant advantage than that at room temperature, given a further reduction of 19 47% of the moisture content. The dewatering process at room temperature was mostly depended on the effect of mechanical expression. Hydrothermal process, more importantly than mechanical effect at increased temperatures, seemed to govern the extent to which the dewatering process occurred. Harsher hydrothermal treatment (temperature and residence time) led to greater water removal and mechanical pressure became less significant as it increased. The dewatering began to show a positive effect when the temperature was exceeded the threshold temperature (between 120 and 150℃). The residence time of 30 min promoted a substantial conversion in the sludge surface properties. After dewatering at temperature of 180-210℃, the moisture content decreased from 52 to 20% and the corresponding total water removal as filtrate was between 81 and 93%. The whole expression stage was completely described by the modified Terzaghi-Voigt rheological model. The role of tertiary consolidation stage in the water removal was reduced with hydrothermal treatment being stronger, meaning the bound water content of hydrothermal sludge was lower.After that the compositions and properties of the obtained products (hydrothermal sludge, hydrochar and filtrate) were identified in order to seek to provide comprehensive information about the dewatering mechanisms. Results indicate that the hydrothermal treatment is mainly a devolatilization process. The observed changes in H/C and O/C for hydrothermal sludge suggested dehydration was the major reaction mechanism and decarboxylation only occurred significantly at higher temperature. It was observed that the moisture content of filter cake correlated with surface charge (Rp=-0.93, p< 0.05) and relative hydrophobicity (Rp=-0.99, p<0.05). The solubilization and decomposition of proteins, polysaccharides and DNA were determined to be temperature and residence time dependent. The improvement of dewaterability was closely correlated to the variation of these biopolymers. The higher heating value correlated well with carbon content of sludge, which was increased by 4.8% for hydrothermal sludge at 210℃ for 60 min and significantly decreased by 15.4% for hydrochar after 6.0 MPa for 20 min. The filtrates collected above 150℃ were found to be acidic. The increase of humic substances and the melanoidins formed by Maillard reaction were largely responsible for the filtrate colour.Meanwhile, the dewatering energy consumption associated with the energy input and potential energy recovery was estimated. The computational results indicate that the heating energy consumption is the major part of energy input. The calculated energy balance based on the assumptions of potential recoverable energy suggested that no additional external energy input is needed to support the dewatering process for excess sludge and no enhancement of energy comsumption is found with the severity of hydrothermal conditions. The dewatering process needs an obviously lower energy input compared to thermal drying and electro-dewatering to produce a higher solid content cake.Finaly, the resources utilization of the process products (hydrochar and filtrate) were estimated. The produced hydrochar with large amount of nutrient elements may be unsuitable for land use due to heavy metal contents far exceeding the standard guidelines for pollutants in sludges from agricultural use of GB4284-1984 and GB18918-2002 in China. However, the activation energy of hydrochar in combustion is reduced, leading to the lower requirement for combusion temperature and be easy to combustion. The process filtrate at hydrothermal temperature of 180℃ showed a best methanogenesis, with a total methane production of 234 ml·g-1 COD and the corresponding COD removal of 64%. It is noteworthy that the anaerobic digsetion is helpless to attenuate the coulor of filtrate.In summary, the dewatering process of hydrothermal treatment coupled with mechanical expression at increased temperature completely changed the picture of the dilemma of established technologies for excess sludge dewatering, either still with a higher moisture content after the dewatering run or with a high energy consumption, highlighting an energy-saving method for strengthening excess sludge dewatering.