Excess Sewage Sludge Hydrolization And Its Hydrogen Production By Anaerobic Fermentation Process

Large amounts of waste sludge are produced from the biological processes of sewage plants, and the disposal of waste sludge is becoming an important and complex question to many sewage plants. The sludge contains many organic matters, for example, proteins and carbohydrates. Anaerobic digestion has traditionally been employed to reduce the volume and weight of sludge and to recover the biomass energy from sludge by producing methane. Most of the organic matters in sludge are microbial cytoplasm, the microbial cell walls must be disrupted or hydrolyzed before those organics can be used and this process is very slow and is the key of sludge utilization. On the other hand, hydrogen is an intermediate product of anaerobic sludge digestion, which is rapidly taken up and conversed into CH4 by methanogens or H2S by sulfate-reducing bacteria. It is difficult to harvest hydrogen during the anaerobic digestion of sludge. However, hydrogen is a clean, environmentally'friendly'fuel that produces water instead of greenhouse gases when combusted. Furthermore, hydrogen has a high energy yield (142.35 kJ/g) that is about 2.75 times that of hydrocarbon fuels, hydrogen has more economic value than methane, and it is more advantaged to harvest hydrogen than to harvest methane. Basing on the latest researches in both domestic and abroad, the waste sludge solubilization and hydrogen production were investigated in this paper systematically.Three kind methods of thermal treatment, ultrasonication and microwave treatment were used to pretreat the waste sludge. The influence on sludge solubilization was inspected through detecting different pretreatments at different treating strength and time. The sludge solubilization was enhanced with treating strength. When ultrasonication at energy density (ED)=2 W/L conditions, the sludge solubilization effect was excellent, the maximal value of soluble protein and total sugar were 1013.7 mg/L and 512.6 mg/L at continuous radiation 10 min; the maximal value of soluble chemical oxygen demand (SCOD) concentration was 4184 mg/L at pulse radiation 10 min. SCOD/TCOD value increased 41.73% compared to the raw sludge. Considering from the economics, low strength and long time waste sludge treatment could reach relatively good solubilization effect, namely thermal treatment (t=75 min, T=45℃), ultrasonic treatment (t=10 min, ED=0.5 W/L), microwave treatment (t=300 s, P=70 W). In addition, under the same energy consumption, the solubilization effect of sludge is: ultrasonic treatment > microwave treatment > thermal treatment.The effect of hydrolization by thermophilic enzymes (S-TE) pretreatment on waste sludge solubilization was investigated. A thermophilic bacterium Bacillus sp. AT07-1 (registration number: FJ231108) was isolated from soil and shown that it could produce extracellular enzyme to accelerate solubilization of excess sludge. At 65℃and microaeration, the strain of AT07-1 was inoculated in the waste sludge with different concentration (TSS were 14 g/L and 21 g/L respectively). The result showed that two reaction mechanisms of enzyme-catalyzed and thermal-hydrolyzed were involved in the S-TE process, which began with depolymerization of sludge flocs by extracellular enzyme (including proteases and amylases), followed by cell-walls and organic matters disintegration and hydrolyzation, the inoculation was advantage for solubilization compared with non-inoculation. It was revealed that under microaeration and inoculation operation, the maximal accumulation of SCOD in the waste sludge were 8222.5 mg/L and 11265.8 mg/L respectively, and the maximal accumulation of volatile fatty acid (VFA) were 4285.8μg/L and 5578.4μg/L respectively, which was beneficial for the followed anaerobic digestion and biogas generation. Moreover, the protein and total sugar released from the sludge was hydrolyzed and consumed by thermophilic bacterium rapidly, which resulted in an increase first and then a decrease at the later. The effect of S-TE method to the solid material in waste sludge was also investigated. The strain of AT07-1 was inoculated in the waste sludge of different total suspended solid (TSS) concentrations (7.5, 14.5, 20.5 g/L) at 65℃and the dynamic analysis of VSS was also made. The result showed that the solubilization process with Bacillus sp. AT07-1 inoculation was enhanced comparing to those with non-inoculation. The volatile suspended solid (VSS) solubilization rate reached to 61.4%, 53.8% and 41.9% at 60 h after the inoculation at different concentration, 27.6%, 25.2% and 16.3% higher than the non-inoculation respectively. The VSS dissolving process (at the first 3 days) accorded with first-order reaction dynamic model, and low concentration and inoculation, the dissolving rate constant Kd (t=3 d) increased evidently.In order to enhance the yield of hydrogen production and hydrogen concentration, hungate method was used to separate hydrogen bacteria. An effective hydrogen production bacteria of Pseudomonas sp. GZ1 (registration number: EF551040) was separated from anaerobic granular sludge, the other one of Enterococcus sp. LG1 (registration number: FJ231108) was separated from river sediment, and the 16s rRNA gene sequence was also analysis. The two kinds of bacteria were used for hydrogen production from waste sludge.Batch tests of anaerobic fermentative hydrogen production by Pseudomonas sp. GZ1 were investigated using thermal treatment(sterilization), microwave and ultrasonication pretreated sludge as substrate.The changes of SCOD, protein, total sugar and pH value during the fermentation process were monitored, and the impact of different pretreatments on hydrogen production using waste sludge was also analysed. The results showed that only H2 and CO2 were produced and CH4 was not observed during the process. A maximal hydrogen yield (15.0 mlH2/gCOD) and hydrogen content (81.5%) were obtained from the sterilization pretreated sludge. The shortest lag time for hydrogen production was obtained for using ultrasonication pretreated sludge (3 h), while the longest one was obtained from thermal (sterilization)pretreated sludge (15 h). It was found that the changes of sludge substrates were various with diferent pretreated sludge during the fermentation process, especially in the thermal (sterilization) sludge run, which implied that the pretreatment method could affect substrate utilization by Pseudomonas sp. GZ1. The heavy metal and VFA released from waste sludge were the key factor influencing the hydrogen production from waste sludge. The sludge after thermal (sterilization) pretreatment and their filtrate were used as the substrates for hydrogen production. After inoculated Pseudomonas sp. GZ1, the hydrogen yield and the changes of SCOD, protein, total sugar and pH value during the fermentation processes were analyzed and measured. The results showed that using filtrate could produce more hydrogen gas than using the bio-solid waste sludge. It could produce a hydrogen yield of 4.5 mgH2/gCOD, approximately 3.3 times higher than the pretreated sludge. It was demonstrated that the solids matter in waste sludge could release more nutrients during fermentation, and not all the substrate could be used for producing hydrogen. A new style of heavy metals sorbent-carbonate hydroxylapatite (CHAP) was utilized to remove the dissolved heavy metals from microwave pretreated waste sludge. Then the pretreated sludge in which different dosages of CHAP added was used as the substrates for anaerobic fermentive hydrogen production by Enterococcus sp. LG1. The results showed when the dosage of CHAP in the sludge was 5 g/L (sludge), the hydrogen yield reached to 10.6 mlH2/gCOD and improved nearly 30% camparing pretreated sludge without adding CHAP. However, when the dosage of CHAP in the sludge was more than 10 g/L (sludge), the anaerobic fermentation was obviously restrained. In this study, the hydrogen yield and the changes of heavy metals, SCOD, protein, total sugar and pH value during the fermentation processes were measured, and it was demonstrated that the hydrogen yield could be effectively improved though removing the appropriate heavy metals from waste sludge.Four batch tests were carried out to analyse the effects of anaerobic fermentative hydrogen production using waste sludge pretreated by S-TE method with and without inoculateion of thermophilic bacteria Bacillus sp. AT07-1 (65℃thermal treatement) , and with and without inoculation of hydrogen producing bacteria (Enterococcus sp. LG1). The changes of substrates and pH value during the fermentation process were also monitored. The results showed that sludge pretreated by S-TE without inoculation could make good bio-hydrogen production. The maximal hydrogen yield was 16.3 mlH2/gVS, 26.4% higher than the sludge pretreated by 65℃thermal treatement inoculated Enterococcus sp. LG1 and 15.6% than 65℃thermal treatement without inoculation. H2 and CO2 were only produced and CH4 was not observed during the process. The lag time for hydrogen production was only 3~4 hours, and after reaching its maximum, the hydrogen production was able to maintain stable for about 10 h with little consuming. The hydrogen yield of the S-TE pretreated sludge inoculated Enterococcus sp. LG1 was very low, only 10.7 mlH2/gVS. It implied that using S-TE pretreated sludge directly could produce hydrogen well, and the solubilization effect and the adaptability of hydrogen production bacterial in the waste sludge were the main reasons of hydrogen production diversity. Using 65℃thermal treatement and S-TE pretreated sludge to produce hydrogen, it was benefical for hydrolization of sludge which inoculated hydrogen production bacteria.