Study Of Thermophilic Anaerobic Fermentation Process With Pig Manure As Raw Material

The microbiology of the anaerobic digestion process is a complex ecosystemthat involves several groups of microorganisms. Degradation of organic matter andproducing a combustible gas (CH4) are the two basic functions of biogas technology.Therefore, it has a very distinct advantage in treating sewage,agricultural and forestryresidues,industrial waste,livestock manure and other organic waste.There are verymany factors that can affect the performance of anaerobic fermentation,in which thetemperature,hydraulic retention time (HRT）,hydraulic retention time),organic loadingrate(OLR,organic loading rate) are major factors.And these factors areinterrelated,such as with different temperatures, HRT and OLR are will change aswell.In China,the application of anaerobic fermentation engineering is mesophilicanaerobic fermentation technology basically.Thermophilic fermentation is mainly juststay at laboratory research.In this paper,the experimental study conducted in two phases. The first stageconsists of two anaerobic fermentation experiment:under the temperature conditionsof35℃and the total solids concentration of6%, with pig manure as thefermentation material, effluent of a mesophilic fermentation tank as inoculum forstarting experiment,10days later,investigating the fermentation conditions of the caseof HRT=20d,OLR=3.125gTS·L-1d-1, the another group fermentation experiment isunder fermentation conditions of the initial total solids concentration of10%,10dayslater,investigating the fermentation conditions of the case of HRT=20d, OLR=5.208gTS·L-1d-1.The second stage is based on the first stage experiment.Taking thebetter appropriate fermentation conditions, domesticating culture by the way ofslowly heating to50℃. In order to select the optimal fermentation conditions,wecarry out the thermophilic manure fermentation experiment by gradually changing theHRT and OLR.Through experiments we get these conclusions:(1)At the case of HRT=20d, even shorter HRT,the mesophilic fermentation isnot suitable for high-density fermentation of manure,and the initial fermentation oftotal solids concentration can not higher than10%.When OLR from3.125gTS·L-1d-1 increased to5.208gTS·L-1d-1:(a)the volatility of daily gas production is significantlyincreased.Although gas production from31.14L/d rise to34.49L/d, the content ofmethane is reduced from67.53%to39.13%; methane gas production rate from280.20mLCH4·g-1TS-1fall to106.63mLCH4·g-1TS-1;volume of methane gasproduction rate from0.700LCH4/L·d to0.450LCH4/L·d.(b)The final concentrationof ammonia is increased from454.051mg/L to848.527mg/L.According to relevantreports,within this concentration range,it does not occur ammonia inhibition.(c) Thefinal concentration of volatile fatty acids increase from3579.833mg/L to6525.635mg/L.This VFA concentration has been able to cause inhibition to themethanogenic bacteria,which is the reason of decline of methane content.(d) Judgingfrom the pH value,when the initial TS concentration is10%, there is a clearacidification. The pH value continues8d at about6.7,then appears upward trend.(e)Since the degradation rate of the mesophilic fermentation is not high,the TS and VSremoval rate of two fermentation experiments is not high, only from32.12%to41.06%.(2) In the process of high-temperature culture domesticating, the temperaturerange of37～43℃, changes of CH4content, VFA concentration and daily gasproduction are very intense relative to those between43～50℃. Thus, in thetemperature range of37～43℃,fermentation bacteria is more sensitive totemperature changes, so fermentation of pig manure is not recommended between thistemperature range.(3) High-temperature fermentation has a very distinct advantage at gasproduction,hydraulic retention time(HRT) and the degradation rate. Compared to themesophilic fermentation at the case of OLR=3.125and5.208gTS·L-1d-1, the gasproduction of high-temperature fermentation higher than the mesophilic fer-mentation16.67%and78.66%,respectively, he methane content was5.73%and30.54%,respectively; the removal rate of TS,VS is12.22%,15.30%and27.41%,31.81%,respectively. However,the concentration of ammonia is higher than the mesophilicfermentation38.30%and50.50%, respectively. Therefore it is prone to occurinhibition of ammonia at high temperature fermentation. Throughout hightemperature fermentation experiments,when HRT=5d,OLR=10.650gTS· L-1d-1,the optimal volume of methane gas production rate can reach2.552LCH4·L-1d-1,at thispoint,TS methane gas production was299.33mLCH4· g-1TS-1;When HRT=20d,OLR=3.125gTS·L-1d-1,TS methane gas production rate reached the highest level of354.87mLCH4·g-1TS-1,the volume of methane gas production is only0.888LCH4·L-1d-1.(4) By comparing the energy surplus, thermophilic anaerobic fermentationdominant than mesophilic anaerobic fermentation.When the OLR=3.125gTS·L-1d-1and5.208gTS· L-1d-1, energy surplus of thermophilic fermentation higher thanmesophilic anaerobic fermentation198881.6kJ and1052973.4kJ,respectively.And itcan generate higher economic efficiency. Considering about the utilization of rawmaterials and energy surplus, HRT=5d,OLR=10.567gTS·L-1d-1,fermentation systemcan get the optimal energy surplus of2716753kJ.