The Internal Circulation Authermal Thermophilic Aerobic Digestion For Sewage Sludge Treatment

One-stage autothermal thermophilic micro-aerobic digestion(ATMAD)is a promising aerobic process,which is suitable for small-and medium-sized WWTPs due to its limited land scale occupied,short SRT and efficient pathogen inactivation.However,the reaction system maintains anoxic operation condition because of the specific aerating system.Therefore,the sludge stabilization can be accelerated by chancing the structure of reactor.In order to improve the efficiency of sludge digestion,an internal circulation autothermal thermophilic aerobic digestion with an inner cylinderwas investigated.The sludge was cycled between inner and outer cylinder.Bench-scale experiments have revealed the efficacy of internal circulation ATMAD system and the optimal circulating velocity.In an attempt to verify the efficacy of the internal circulation ATMAD technology further and provide more technological data for its practical operation,a pilot-scale digester with an effective volume of 80 m~3 was designed and operated continuously over one year.The thermal equilibrium and economic cost of the reactor have been analyzed.The main conclusions were as follows.In internal circulation ATMAD system,the removal of volatile solids(VS)at 16d was 41.52%.At 20d,the removal of VS was increased 11.39%than no internal circulation.The sludge could be mixed more homogeneous with the inner cylinder.When the circulation rate was 10 times per hour and 20 times per hour,the removal of VS had over 40%at 16d.The removal of VS was also reached stability requirements at 20d when the circulation rate was 5 times per hour.Considering the removal of VS and other indexes,the optimal circulating velocity is between 8~11 times per hour.In the pilot-scale system,the average removals of VS at SRTs of 12,8and 6 days were 44.73,47.88 and 48.63%,respectively.For the HRT less than 6 days,the system was unable to maintain a steady state due to a large amount of sludge replacement.Hence,in the actual operation of small and medium sized sewage treatment plant,6-12 days is the suitable HRT level to reached stability requirements.Through analysis of heat balance,we found that in internal circulation ATMAD system,the heat transformed from mechanical energy made up a unnegligible proportion of the total heat generated.The main reasons for heat loss were the steam and discharging sludge.At SRT of 6 days,the cost of treating sludge was 36.3 yuan per ton.