Research On Pretreatment Technologies Of Enhancing The Biogas Production In The Anaerobic Digestion Process

Conventional anaerobic digestion is a common process for wasteactivated sludge stabilization. However, the hydrolysis process of sludgesolid is very difficult in the anaerobic digestion, which results in the largevolume and low rate of gas production. Pretreatment can destroy the cellstructure in order to increase nutrient release and promote the rate ofhydrolysis of sludge, thereby improving the biogas production ofanaerobic digestion. Thermal pretreatment, thermal-alkaline pretreatmentand alkaline pretreatment, are more effective methods of applicationprospects among many sludge pretreatment techonics. Based on ourprevious study, electrochemical pretreatment as a novel technology used inthe sludge treatment, also has obvious effect on the enhancingperformance of anaerobic digestion. In this thesis, the effects of thermalpretreatment, thermal-alkaline pretreatment, alkaline pretreatment as wellas electrochemical pretreatment on the sludge anaerobic digestion arecompared. The biogas production of subsequent mesophilic anaerobicdigestion and the mechanism of different pretreatments are studied. Themain conclusions are as follows:(1) Different pretreatment techonics can improve the disruption efficiency of sludge, and promote the hydrolysis process. Theexperimental results showed that, electrochemical pretreatment was morebenefited to the effect of disruption and the release rate of organisms. Thesolubilization growth rate of SCOD, protein, and carbohydrates was thehighest after electrochemical pretreatment. Meanwhile, the changes of pHand ORP value by different pretreatments were different. The pH valueafter electrochemical pretreatment rose to7.94±0.10. The ORP value ofsludge after thermal pretreatment, thermal-alkaline pretreatment, andalkaline pretreatment dropped to about-300mV, while the ORP value afterelectrochemical pretreatment rose to28±6mV. It could be implied thatelectrochemical pretreatment process produced strong oxidants. Inaddition, the compositions of VFAs in supernatant were also different. Theproportion of individual VFA after electrochemical pretreatment wascloser to that in conventional anaerobic digestion system.(2) The anaerobic fermentation cumulative biogas productionincreased by63.38%when treated by electrochemical pretreatment, whilethermal-alkaline pretreatment, alkaline pretreatment and thermalpretreatment increased by52.78%,41.41%and32.07%, respectively. Thelinear fitting curves of cumulative biogas production showed that,(ⅰ)average daily biogas yield of the alkaline pretreatment was maximum,reaching Kalkaline=90.3L/(kgVS d) when anaerobic digestion of sludgeoperated in0-5days;(ⅱ) in6-10days, the biogas production rate wasKthermal-alkaline=32.8L/(kgVS d) after thermal-alkaline pretreatment, andKthermal=31.3L/(kgVS d), Kelectrochemical=26.6L/(kgVS d) was obtained whenthermal pretreatment and electrochemical pretreatment processed,respectively;(ⅲ) in the anaerobic fermentation period of11-20days, theaverage daily biogas production of electrochemical pretreatment couldreach Kelectrochemical=25.7L/(kgVS d) larger than those of other pretreatments;(ⅳ) in the later anaerobic fermentation period (21-45days),the average daily biogas production in the systems with differentpretreatments were less than3L/(kgVS d).(3) The methane content of the anaerobic digestion with theelectrochemical pretreatment was higher than other pretreatment groups,up to64.9%. The CH4/CO2ratio in electrochemical pretreatment groupand alkaline pretreatment group were both high, between2.0and2.4;while pretreated by alkaline, the CH4/CO2ratio maintained the low level of1.5to1.6.(4) In different pretreatment groups, the final VS removal rates were43.7%in thermal pretreatment group,46.2%in thermal-alkalinepretreatment group,43.5%in alkaline pretreatment group,45.5%in theelectrochemical pretreatment group, respectively. All of those were higherthan38.9%of the control group. Taking the relationship betweencumulative biogas production and VS removal into account, it was knownthat the efficiency of biogas production and cell proliferation afterelectrochemical pretreatment was the highest.(5) The pH, ORP, C/N ratio and VFAs content in the anaerobicfermentation process of electrochemical pretreatment group were moreconducive to the growth of methane bacteria than other pretreatmentgroups. There were oxidizing substances remaining in the sludge afterelectrochemical pretreatment which played a very important role inpromoting the dissolution and hydrolysis of the sludge in the fermentationprocess, and the improment of overall efficiency of biogas production andorganics removal was obtained.