Ultrasonically-assisted KOH Pretreatment And Anaerobic Digestion Process For Effective Biogasification Of Wheat Straw

Wheat straw is one of an important agricultural waste resource.It can be converted to a renewable green-energy biogas,through an anaerobic digestion process.However,wheat straw contains higher lignocellulosic content with poor biodegradability,which results in lower performance during anaerobic digestion processes with longer digestion time.In order to address this problem,the biodegradability of wheat straw can be increased by applying effective pretreatment method,to increase the biogas and methane productions.Moreover,the conversion efficiency of the substance can be further improved by strengthening the anaerobic digestion process.The current pretreatments are mostly single pretreatments,such as physical,chemical or biological pretreatment,with limited pretreatment effects.Therefore,the present study proposed a method of ultrasonically assisted KOH pretreatment,to physically and chemically disrupt the structural behavior of wheat straw owing to improve the anaerobic digestion performance.Similarly,the application of ultrasonic medium during anaerobic digestion was also investigated,to further increase the anaerobic conversion efficiency of wheat straw with minimal ultrasonic application.For this purpose,the studies were carried out in three stages and the results were found feasible to support positive effects of ultrasonic application for enhanced anaerobic digestion performances,increased biogas and methane production.(1)The ultrasonic assisted KOH pretreatment method was applied and its performances during anaerobic digestion process were analyzed by batch digestion.The KOH concentration of 2%,4%and 6%,respectively were used.A constant ultrasonic frequency of 20 kHz,with 20? temperature,and 36 hours of the pretreatment time were used.The untreated and pretreated wheat straw was then subjected to anaerobic digestion in a batch reactor.The digestion temperature was 35?.and the organic load rate was 50 g TSL-1.The results showed that ultrasonic assisted KOH pretreatment(KOHUpt)can significantly disrupt more organic material in reduced time(50%time reduction),resulting in increased biogas and methane production.The maximum reduction in LHC content(22.38 and 27.51%)was successfully achieved with 4 and 6%KOHUpt of wheat straw,with highest TVFA release at 3011.71±17.99,and 3188.51±10.42 mgL-1,respectively.Similarly,the SEM images confirmed the significant destruction in the lignocellulosic structure of all the KOHUpt wheat straw samples,promoting increased accessibility of anaerobic microbes,for increased biogas and methane production.The 4%KOH upt showed the highest biogas and methane production with 555.49 ± 6.96 and 282± 6.90(mLg-1 VSloaded),respectively.This increment in biogas production and methane production was 34.92 and 46.88%,respectively,compared with untreated wheat straw digester bottles.Finally,the highest biodegradability of 68.5%from 4%KOHUpt AD was successfully achieved.The predictions from modified Gompertz model were also in agreement with all the experimental results.(2)The comparative application of ultrasonic medium during anaerobic digestion process for untreated,single KOH and KOHUpt wheat straw was also studied.The anaerobic digestion was performed in a batch reactor,with and without ultrasonic medium and,the results were compared to determine whether ultrasound addition can improve AD performance and the biodegradation rate of the wheat straw,during AD.The digestion temperature of 35?,with OLR 50 g TSL-1,and 50 d of digestion time was selected.For ultrasonic assisted AD reactors,the ultrasonic frequency was 20 kHz,and the ultrasonic treatment was 10 min/d.The results show that ultrasonic application during anaerobic digestion can effectively increase biogas and methane production.The biogas and methane produced by ultrasonic-assisted anaerobic digestion of 4%KOH wheat straw were 569.89±0.79 and 305.58±0.85 mL g-1 VSloaded respectively,which were 32.86 and 63%higher than the untreated wheat straw AD,without ultrasonic application during AD.The Bd for untreated wheat straw digester without ultrasonic addition reached only 46.72%compared to 4%KOH-ADUas,which successfully attained 76.15%of Bd,after 50 days of anaerobic digestion.Similarly,the TS and VS reduction rate also increased to 64.10 and 76.08%compared with the untreated TS and VS reductions.The time for ultrasonic application during AD was also found reduced to 8.33 hours(76.9%reduction),compared to its application during pretreatment stage(36hours),which reflected lesser energy input with higher biodegradation rate of wheat straw during AD.The model prediction for the methane production values were also consistent with the experimental data and confirmed the successful simulation.(3)Based on the outcomes from phase(1)and(2),the experimental study of temperature phase anaerobic digestion(TPAD),with different temperature combinations were carried out.The untreated and 4%KOH pretreated wheat straw were used as raw materials.The anaerobic digestion performance of wheat straw with and without ultrasonic application was studied using different temperature combinations(55?+35? and 35?+55?).A total of 10 combinations were designed.The results showed that the behavior of anaerobic digestion performance for wheat straw was different,with operative parameters.Generally,both groups digestion showed better AD performance when ultrasonic applications were applied continuously during both temperature ranges.However,the discontinuation in either stage,showed lower biogas and biomethane productions,indicating unbalanced composition of degrading components.Among them,the experimental group AR1 with TPAD55-35 obtained the highest biogas and methane productions with 568.04±0.72 and 282.61±0.30 mLg-1VSloaded,respectively.This increment was 31.59 and 50.91%higher,than the respective untreated AD bottles of AR5.The other operative parameters such as pH,alkalinity and total ammonia were also found more stable during TP AD with thermophilic to mesophilic temperature shift digesters(TPAD55-35).In addition,the highest biodegradation with AR1 digester bottles reached to 74.63%,which confirmed the positive effect of continuous ultrasonic application during the TPAD55-35.(4)The above research results showed that both ultrasonic-assisted KOH pretreatment and anaerobic digestion can effectively increase the anaerobic digestion performance by increasing the conversion rate of wheat straw,and enhancing the biogas and methane production capacity,using wheat straw.