Key Ecological Factors Strengthen The Study Of The Anaerobic Digestion Of Municipal Solid Waste

The integrative anaerobic digestion is a new bioremediation technique with advantages in environment protection, energy and resource recovery treating municipal solid waste (MSW), among that Dranco, Valorga, Kompogas and Biocel processes have been industrialized. However, there are several disadvantages such as low gas production rate and gas production, long digestion time and inefficient organic compounds-decomposing in anaerobic process because of lack of effective microorganism and anaerobic reactor, complex-variability of organic fraction of municipal solid waste (OFMSW) and instability of anaerobic process. On basis of domestic and abroad references, there have been some systematic researches of key factors enhancement of anaerobic digestion for treating MSW including comminution and separation of MSW, variability and biodegradability of MSW, filtration and enrichment culture of inoculum, aerobic microorganism pretreatment of OFMSW enhancing anaerobic process, codigestion of OFMSW with waste sludge, influence of organic loading rate (OLR) and temperature on anaerobic process in this dissertation.MSW play an important role in anaerobic process, which is the key abiological factor having influence on anaerobic performance. On basis of the variability of MSW, the comminuting and sorting experiments were carried out for MSW in Kunming University of Science and Technology.The analyses were taken about the physical compositions and organic chemical components of MSW respectively by machine-sorted, hand-sorted, and source-sorted method which called MS-OFMSW, HS-OFMSW and SS-OFMSW. Based on the data from the experiments it was about 47.8%/49.1%,66.7%/68.0%,80.5%/81.5% and 86.1%/86.9% at 35℃/55℃respectively for biodegradability percentage while their biodegradable fractions (BF) based lignin content was 51.4%,63.7%,72.1% and 74.0% respectively. The biochemical methane potential (BMP) of MS-OFMSW, HS-OFMSW and SS-OFMSW were 215.9mLCH₄/g VS, 302.5mLCH₄/gVS, 368.8mLCH₄/gVS and 391.9mLCH₄/gVS in the mesophilic condition,221.8mLCH₄/gVS,308.4mLCH₄/gVS,373.7mLCH₄/gVSand 395.4mLCH₄/g VS in the thermophilic condition respectively.The results of biological factors enhancement under the mesophilic anaerobic digestion of OFMSW showed that each inoculum possessed particular characteristics because of their different source, among that the initial specific methanogenic activity (SMA) were the most important. The mesophilic anaerobic sludge (MAS) presented the highest SMA value. UASB sludge (UAS) and septic tank bottom sludge (STBS) were much of SMA value which were 196mLCH₄/gVSS·d and 184.8mLCH₄ /gVSS·d respectively. These inoculums after incubation and enrichment shorted the lag time of anaerobic process and improved the biodegradability of organic waste. The relationship between OFMSW and anaerobic degradability was positive, namely the more the SMA was, the more the anaerobic degradability was.The inoculum-substrate ratio (ISR) had influence on the mesophilic anaerobic performance of OFMSW evidently. When ISR being one was the best condition under the same mass (based volatile solid) which kept anaerobic process high stability; meanwhile the biodegradability, gas production rate (GPR) and accumulative gas production were higher than that of others. The biodegradability of organic waste and utilization of reactor capacity improved reasonably.Aerobic microorganism was regarded as biological factor to pre-treat OFMSW, which improved the biodegradability of OFMSW.Therefore the anaerobic process stability, gas production and digestion efficiency improvement presented in the anaerobic process, especially in the OFMSW pretreated for 12 hours by aerobic microorganism, which gas production increased 25.8% being the most gas production (29927.98mL) of all others. The reaction rate constants of anaerobic digestion of OFMSW pretreated for Oh, 6h, 12h, 24h, 48h and 72h by aerobic microorganism were 0.1517d^-1, 0.1453d^-1, 0.1428d^-1, 0.1396d^-1, 0.1748d^-1 and 0.1819d^-1 respectively.The experiments of OFMSW and waste sludge codigestion were carried out in order to improve the nutrition ingredients and C/N of OFMSW and their anaerobic performance. The results of batch codigestion of OFMSW and waste sludge showed that theoretical methane production (TMP) decreased with OFMSW content increase from 560CH₄mL/gVS to 521CH₄mL/gVS; while BMP increased with OFMSW content increase from 223CH₄mL/gVS to 414CH₄mL/gVS. The results of semicontinuous codigestion of OFMSW and waste sludge showed that gas production rate(GPR) increased with OFMSW content increase. The appropriate proportion of OFMSW and waste sludge was 90/10(based VS content),which GPR was the highest under the conditions that HRT and OLR was 15d and 4.8gVS/L·d respectively. The GPR was 2456mL/L·d.OLR fluctuation or sudden variation had influence on stability of anaerobic process distinctly by the fluctuation of some important parameters. The influence degree of OLR fluctuation on some important parameters as follows:VFA concentration > GPR > methane content > pH. Therefore, the anaerobic process stability was estimated by inspecting these parameters above-mentioned, among that pH assay was the most convenient and effective.During the biological and abiological factors enhancing anaerobic digestion dynamics model of semi-continuous and batch anaerobic process have been studied including gas production, degradability and fecal coliform destruction. The parameters concerned obtained according to these data and dynamics model. The reaction rate constant of semicontinuous anaerobic process was 0.119d^-1; while the rate constant of fecal coliform destruction according to the class A criterion of sludge (USA) was 0.69d^-1 under the mesophilic (35℃) condition. However, the reaction rate constants of batch anaerobic process were 0.013d^-1,0.047d^-1 and 0.1254d^-1;meanwhile the rate constant of fecal coliform inactivation of the criterion in GB8172-87 were 0.001d^-1,0.0256d^-1 and 16.11d^-1 under the psychrophilic (25℃), mesophilic (35℃), and thermophilic (55℃) conditions respectively. The activation energy of anaerobic reaction of OFMSW was 58.93kJ/mol.Laboratory-scale experiments were carried out to assess the influence of temperature enhancement on anaerobic treatment of OFMSW. The results showed that it was 55℃for the optimum temperature. The parameters were obtained such as 180.80mL/gTS for average biogas production, 70.0% for average methane concentration in biogas, about 20d for digestion time, 477mLCH4/gVSS·d for the residue SMA, 10^-3MPN/g for fecal coliform and 100% for destruction rate of roundworm eggs under thermophilic condition, which exceeded the psychrophilic and mesophilic conditions. The inactivation of pathogens in the residue under thermophilic condition was in accordance with the citerion in GB8172-87.