Enhancement Of Autothermal Thermophilic Micro-aerobic Digestion For Sewage Sludge By Ferric Nitrate And Its Mechanism

Sludge stabilization is a key step in the harmless treatment of sludge,and the discharge of sludge without stabilization will cause great threat to ecological environment and human health.Generally,the method of sludge stabilization is divided into physical method,chemical method and biological method,and biological method is mainly anaerobic digestion and aerobic digestion.The autothermal thermophilic micro aerobic digestion?ATMAD?comes from aerobic digestion,and the process has the advantages of high efficiency and good sterilization effect of sludge digestion.But the lack of oxygen at the initial stage of ATMAD sludge digestion process caused volatile organic acid?VFA?accumulation,which would lead to the inhibition of dominant microbial activity within the system.Thus,the digestibility of the process can not be further improved.For the shortage of ATMAD process,the effect of ferric nitrate and ferric chloride etc,were investigated on the ATMAD process of sludge.On this basis,the effect of parameters such as dosage and dosing time of ferric nitrate on the ATMAD process of sludge was discussed.Subsequently,the effect of the factors such as digestion temperature and aeration rate on the improvement of ATMAD efficiency of sludge was studied.Finally,through the analysis of the interaction between the organic metabolism and changes of microbial community structure in the ATMAD system of sludge enhanced by ferric nitrate,the mechanism of how ferric nitrate enhanced ATMAD process of sludge was elucidated.In addition,the kinetic of organic matter degradation in the ATMAD system of sludge improved by ferric nitrate is established.The effect of ferric nitrate on the digestion process was the most obvious,followed by ferric chloride,and the effects of potassium nitrate and potassium chloride were not obvious.The concentration of TVFA in group with ferric nitrate decreased from 5499 mg/L to 716 mg/L and the concentration of acetic acid decreased from 2790 mg/L to 541 mg/L at the end of digestion,compared with the blank group,which effectively improved the microbial activity.Hence,it greatly shortened the sludge stabilization time by 9 days and improved the stability level by increase of 8%VS removal comparing to the control,and the concentrations of carbon,nitrogen and phosphorus were also lower in digestive fluid.The results showed that the sludge treatment of ATMAD technology enhanced by the ferric nitrate had obtained a remarkable effect.The optimum dosage of ferric nitrate was equivalent amount corresponding to the removal of 1000 mg/L acetic acid,the best dosing time was the sixth day,and the best pH when ferric nitrate dosed was 6.5.With ferric nitrate,sludge digestion effect by ATMAD process increased with digestion temperature,the optimum temperature was 55?.Aeration rate increases which in favor of accelerating the stabilization rate of sludge,and aeration rate of 1.0 m³/?h�m³ sludge?was appropriate.The feed sludge concentration of TS increased in the digestive had an adverse effect,the best concentration of TS was 5.5%.Feed sludge with higher percentage of secondary sedimentation sludge was more conducive to digestion process,best with 100%secondary sedimentation sludge.The effects of the sludge stabilization in dosing groups were all higher than that of the control group,but it could not improve the biodegradability of sludge.The dewatering performance of digested sludge was not improved by the addition of ferric salt.The increases of the aeration rate and the proportion of the secondary sedimentation sludge in the feed sludge deteriorated the sludge dewatering performance.The addition of ferric nitrate not only decreased the concentration of VFA,but also changed the distribution of VFA species,which made it more conducive to microbial degradation.The microbial abundance remained basically unchanged in the ATMAD system after addition of ferric nitrate,but the diversity of microbes was greatly reduced,showed that the microbial community structure had been simplified selectively and strengthened.The Gram positive bacteria in the ATMAD process,especially the relative abundance of Firmicutes had been increased significantly.The dominant bacteria in the control as traditional Tepidiphilussp.belongingtoProteobacteriatransformedinto Symbiobacterium sp.belonging to Firmicutes.The optimization of microbial community structure led to biodegradable composition exponentially of dissolved organic matter?DOM?in the supernatant of the ATMAD grew multiply,so as to promote the digestion process,and greatly improve the removal rate of VS,and shorten the time of sludge stabilization.At the end of digestion,the SCOD and TCOD of the ferric nitrate dosing group were lower than that of the control group,which indicated that the level of sludge stabilization in the dosing group was higher than that in the control.In the whole process of digestion,the kinetic equation of organic matter degradation was piecewise linear,which was first fast and then slow.The effect of temperature with the chemical was negligible,and the enhancing coefficient of ferric nitrate(K_T+A/K_T)was 1.8700.The effect of enhancement by organic matter ratio in the feed sludge was amplified after addition of ferric nitrate.Organic matter degradation rate increased with the increase of feed sludge concentration before ferric nitrate adding,however,the result was contrary after addition of ferric nitrate.Furthermore,the effect of sludge concentration on organic matter degradation rate enhanced with ferric nitrate addition.