Effect Of Rice Alcohol Wastewater On Anaerobic Co-Digestion Of Molasses And Impact Of Nickel On Molasses Mono-Digestion

Many by-products are produced after agro-industrial processes containing highly biodegradable organic content and are considered an attractive feedstock for bio-refineries.However,these by-products can be transformed into high valued chemicals.Sugarcane derived products are among the agro-industrial products that can be used for the bio-methane,bio-hydrogen,and organic acid production through the anaerobic digestion（AD）process.Molasses is a highly dense material produced after sugarcane crystallization,composed approximately of 45%carbohydrates such as sucrose,glucose and fructose as an organic content.This high organic fraction in molasses laid the by-product as an attractive source for biogas production.However,the anaerobic digestion of molasses can be inhibited by its high COD and some toxic chemicals present,such as melanoidins and high salt concentration like potassium.Anaerobic digestion technology is a highly sustainable and cost-effective technique.It is considered the preferred method for treating agro-industrial by-products and other organic wastes to produce biogas.The anaerobic co-digestion technique is suitable for anaerobic technology in which two or more substrate share their nutrients for the stable digestion process.At present,there is less research on the anaerobic co-digestion of molasses with other substrates,and most importantly,on the constant increase of the OLR.Anaerobic co-digestion of molasses with rice alcohol wastewater was carried out in this research to compare their digestion stability with the mono-digestion of molasses and increase organic loading rates（OLR）.This study aimed to optimize molasses’digestion process and improve their digestion efficiency by combining it with rice alcohol wastewater.In this study,anaerobic co-digestion of molasses and rice alcohol wastewater at different ratios and increasing OLR was studied to optimize the process with the highest OLR.The increased loading rates and substrates ratios were optimized based on the study of anaerobic co-digestion of molasses with rice alcohol wastewater.Later on,with the activated sludge already digesting bagasse spray wastewater,anaerobic mono-digestion of molasses was compared with the co-digestion of molasses and rice alcohol wastewater in the second set of experiments.Finally,at lab scale,using the sludge from the second set of experiment,the effects of different nickel concentrations were studied on the stability of mono-digestion of molasses at constant OLR.Furthermore,various stability parameters like COD removal efficiency,methane contents,alkalinity,p H,and biogas were detected in response to nickel effects.The response of microbial communities was also studied in co-digestion and mono-digestion processes.The key results of the above experiments are as follows.In the anaerobic co-digestion of molasses and rice alcohol wastewater at increasing OLR and various ratios,the organic load rate reached its maximum concentration of 13 g COD/L.d at the ratio of 6:1 of molasses and RAW where stable COD removal rate was observed.The co-digestion was then followed by mono-digestion of molasses at OLR 17.5 g COD/L.d and 18.5 g COD/L.d.However,at OLR 18.5 g COD/L.d,the digestion process failed quickly within a week.During the co-digestion stage,an increase in the process efficiency was observed,and the highest COD removal rate at feeding OLR from 7 to 13 g COD/L.d was within the range of 90.20±0.44%to 92.65±0.38%.The highest methane content observed was69.14±1.4%at the influent OLR 3.75,when the substrate’s ratio was 2:1 on day 36.Stability in p H was observed during the entire co-digestion stage and was in the range suitable for the methanogenesis.Changes in the microbial community at various periods were identified by the high throughput sequencing technique（Hi Seq）.The results reveal that Methanosaetaceae dominated the methanogenic community over Methanobacteriaceae and other methanogens that clearly shows the dominance of acetoclastic methanogens in the whole process.After the severe failure of the digestion process in the first experiment,the anaerobic co-digestion of molasses and RAW was compared parallel to its mono-digestion at the increasing OLR as the second experiment.Both processes’efficiency was evaluated by observing the COD removal rate,methane content in biogas,Volatile fatty acids（VFAs）,and the corresponding dynamic changes in microbial communities at different stages of both processes.Stability in the co-digestion was seen with an increasing OLR to 16 g COD L^-1d^-1.In comparison,instability in molasses mono-digestion was observed two times at the maximum OLR of 9 and 10g COD L^-1d^-1.Mono-digestion instability was attributed to the increase in VFAs concentrations,which were 2059.66 mg/L and 1896.9 mg/L.The corresponding COD removal rate and methane content observed in the co-digestion process was detected as 90.72±0.63%and 64.47%±0.59%.While in the mono-digestion process,the high COD removal rate and methane contents observed were 89.29±0.094%and 61.37±1.06%,respectively.The microbial community analysis showed that the bacteria and methanogens diversity changed at unlike stages in both processes.However,the bacterial community was dominated by Propionibacteriaceae,and Methanosaetaceae dominated the methanogenic community.Based on the stability parameters observed in the anaerobic co-digestion and mono-digestion,it was found that co-digestion is the most suitable technique to digest molasses at high OLR compared to its mono-digestion.Besides,RAW used for the first time in this study could be a good co-substrate for molasses digestion,integrating molasses into progressive biogas production with high OLR.The effects of nickel(Ni²⁺)on the stability of anaerobic digestion of molasses were established by studying the degradation of organic matter（COD removal rate）,methane content,p H,and alkalinity.The results showed that there were no significant effects on the stability of p H and alkalinity.Increased COD removal rate was observed by 2-3%in the digesters receiving 2mg/L of Ni²⁺in the first phase of the experiment.While in the third phase of the experiment,when Ni²⁺was supplemented to digesters,no significant effects on COD removal rate were seen.Increased methane content was observed in the reactors supplemented with Ni²⁺concentrations of 2mg/L and 4mg/L.However,in the third phase,the methane content was higher by 2 to 3%in the Ni²⁺added reactors than in control.Overall,it is suggested that the addition of Ni²⁺has some effects on the enhancement of methane contents but has no obvious effects on the long-lasting stability of the molasses digestion.Results:A quick startup and stabilization of anaerobic digestion can be attained feeding with a small OLR range and increased gradually at the beginning,and RAW could be a good co-substrate for molasses digestion.