Characteristics And Energy Efficiency Evaluation Of High-solid Sludge Anaerobic Digestion

Comparing to conventional anaerobic digestion(CAD)of sewage sludge,high-solids anaerobic digestion(HSAD)has higher volumetric biogas yield and less energy consumption on heating.On the other hand,high solids sludge could lead to poor rheological property of sludge,and correspondingly,increase the energy consumption on sludge stirring markably.Moreover,the possible accumulation of VFAs and ammonia caused by the blocked mass transfer may inhibit anaerobic digestion processes.Up to date,the characteristics and energy efficiency of HSAD,especially when fed with low-organic-content sludge,are not explicit.Hence,sewage sludge with total solids(TS)content 3%~15% and volatile solids(VS)content 40%~65% was used as substrate to investigate the charateristics of HSAD.The energy-efficiency level of HSAD was also evaluated based on its performance.The main conclusions are as following:(1)The increasing TS resulted in an exponential increase in sludge viscosity but an exponential decrease in diffusive coefficient.The blocked mass transfer led to the accumulation of free ammonia,and then inhibited the biochemical reactions of HSAD.(2)Mass transfer was the key factor influencing the performance of HSAD.The diffusion coefficient in sludge decreased sharply when sludge TS increased from 6% to12%,because of the decrease of free water.When sludge TS increased further to 15%,the diffusion coefficient was determined by capillary diffusion,and the value decreased gradually to a lower level.The diffusion coefficients in the digested sludge were an order of magnitude lower than in the diluted dewatered sludge at the same water content,because the digested sludge had more fine particles and less free water.(3)HSAD exhibited similar performance to CAD with the assistance of enhanced agitation.HSAD was more tolerant to the shock load.Besides sludge solid concentration and digestion time,organic matter in feed sludge was also a key factor influencing the performances of HSAD.(4)The increasing VS content resulted in an linear increase in the biogas yield and organic degradation rate.The oligotrophic environment with less bioavailable substrate resulted in the decrease of microbial activity and the switch of metabolic pathway.Inthe experiments using low-organic-content substrate,propionic fermentation competed over butyric fermentation.The first-order model described biogas production from low-organic-content sludge very well,and the degradation coefficient k exhibited an exponential relation with organic content in substrate.For the low-organic-content sludge(VS/TS<60%),the digestion time should be no less than 25 days.(5)HSAD has the advantage of energy self-sustaining,and consequently it is more suitable for the treatment of low-organic-content sludge.When TS content of feed sludge increased from 5% to 15%,the energy required for temperature maintenance can be reduced 67%.When organic content in feed sludge increased from 40% to 50%,the corresponding biogas production increased by 2.2 times.And the energy-efficiency rate of HSAD was 1.9~2.3 times the value of CAD.For the integrated system of HSAD and thermal drying,the biogas generated from anaerobic digestion of low-organic-content sludge could only meet 19%~46% of the energy required by sludge drying.Therefore,it cannot improve the energy-efficiency of the integrated system using HSAD instead of CAD.