Efficiencies Of Mesophilic Two-phase Anaerobic Digestion Of Pretreated Surplus Sludge

With the acceleration of urbanization and the improvement of environmental quality standards in China, the disposal rate and degree of sewage is raising year by year while the by product—surplus sludge generated in the course of sewage treatment is increasing rapidly. As surplus sludge contains a great deal of organic matter, it needs a stabilization treatment before the ultimate disposal. The sludge anaerobic digestion treatment technology should be the most widespread sludge stabilization technology at present, but all the existing sludge anaerobic digestion techniques have the deficiencies like long digestion time, low yield of methane.Dealing with surplus sludge by applying mesophilic two-phase anaerobic digestion technique, this study first compares the effect separate thermal hydrolysis, separate high pressure and the combined application of thermal hydrolysis and high pressure have on surplus sludge anaerobic digestion performance; then conducts mesophilic two-phase anaerobic digestion for the pre-treated sludge and keeps the acidogenic phase in the stage of ethanol-type fermentation. On the basis of the stable operation of the system, this study discussed the operating characteristic of mesophilic two-phase anaerobic digestion, researched the change of sludge VS, settling property and dewatering performance as well as species distribution of the heavy metals in sludge after anaerobic digestion, built a dynamic model of surplus sludge anaerobic digestion and studied the property of surplus sludge after ethanol-type fermentation with supernatant as denitrifying carbon source.The combined treatment of high pressure and thermal hydrolysis is obviously better for the improvement of anaerobic digestion performance of sludge than the separate treatment of high pressure or thermal hydrolysis, and can reduce the treatment time of thermal hydrolysis. The anaerobic digestion performance of surplus sludge after 30 minutes of combined treatment at 5MPa,150℃is slightly higher than that of the sludge after 45 minutes of the separate treatment of thermal hydrolysis at 170℃.With surplus sludge after being treated for 30 minutes at 5MPa,150℃as matrix, the operating efficiency of mesophilic two-phase anaerobic digestion system was examined. The result showed that the removal rate of sludge VS decreased as organic load increased and the removal rate of the system VS was 59.4%,54.7%,50.21% and 42.2% respectively when the organic load was 0.98,1.57,2.35 and 3.13kgVS/m³Â·d. Inoculating sludge of ethanol-type fermentation after acclimation can rapidly maintain acidogenic phase in the stage of ethanol-type fermentation. The composition study of acidogenic phase VFA found that under different organic loads, the proportion of ethanol or acetic acid in the total VFA and specific hydrogen production rate were different. The specific hydrogen production rate was 0.0183,0.0238,0.0221 and 0.0194 L·H₂/g·VS respectively when the organic load of acidogenic phase was 0.98, 1.57, 2.35 and 3.13kgVS/m³Â·d.Both the settling property and dewatering performance of the surplus sludge were significantly improved after combined pre-treatment of thermal hydrolysis and high pressure. After mesophilic two-phase anaerobic digestion, both the settling property and dewatering performance of surplus sludge were deterioration, but better than the original surplus sludge; the stability of heavy metals Zn, Cu in the sludge improves remarkably.Based on the Lawrence– McCarty model and according to the rule of microbial growth and degradation of organic substrates, we obtained four basic parameters of reactor anaerobic digestion dynamics with surplus sludgeafter the combined pre-treatment of high temperature and pressure as matrix: microbial theoretical yield per unit of matrix Y , microbial attenuation constant K_d, matrix maximal ratio removal velocity v_max and saturation constant K_s are 0.2471,0.3697,2.1767 and 10.4158 respectively.When the organic load of acidogenic phase was 10.56kgVS/m³Â·d and in the condition that C/N and MLVSS are about the same, the denitrification rates in the denitrification processes with supernatant as denitrifying carbon source and with supernatant and methane as denitrifying carbon source show little difference, being 2.26, 2.23mgNO₃^--N/gVSS·h respectively, 2.86 times as much as that of sanitary sewage.