Enhancing hydrolysis and syntropy simultaneously in solid state anaerobic digestion: A GRA – Taguchi based study and techno-economic assessment for sustainable bioeconomy

26 Background: Solid-state anaerobic digestion of agricultural stubble is attractive technology 27 for energy and bioeconomy as well as it may lead to transitioning towards greenhouse gas 28 neutrality; yet hydrolysis and syntropy affects the process and makes it economically 29 nonviable. In this regard, present study investigates the effect of alkali and biochar addition for 30 simultaneous increment of hydrolysis and syntropy for higher methane yield from pearl millet 31 straw. Further, taguchi based design of experiment was coupled with grey relation analysis for 32 multiple output evaluation and detailed techno-economic assessment was performed. 33 Results: Study showed that 0.5 g/100g pearl millet straw of alkali and 10 g/L of biochar was 34 the optimised dosing along with 20% total solid concentration and 4 as feedstock/inoculum 35 ratio. Statistically, contribution of biochar and alkali was 48 and 21% respectively on the 36 multiple output. The confirmation test revealed that hydrolysis rate constant, k for reactor 37 having optimised conditions was 0.0521 d -1 while for control, it was 0.0595 d -1 . Cumulative 38 methane yield was also increased by 1.8-fold for optimised condition. Techno-economic 39 assessment showed that capital cost and electrical efficiency of combined heat and power unit 40 have dominant effect on the investment. Solid state anaerobic digestion of pearl millet straw 41 with alkali and biochar showed US$ 25652 of net present value and showed to have payback 42 time of 8.2 years with 11% of internal rate of return. 43 Conclusion: The simultaneous increment of hydrolysis rate and syntrophic activity in 44 optimized condition helped to achieve higher methane yield. Techno-economic assessment 45 showed that shorter payback time and higher internal rate of return, making large scale project 46 profitable and viable which may endorse sustainable bioeconomy with lower greenhouse gases.

L9 orthogonal array (3 4 ) employed for experimental setup and output responses were calculated 158 for 60 days of SSAD under batch system. Methane yield, VS reduction, pH of the trials and 159 TVFA/alkalinity ratios were measured after the end of L9 orthogonal setup of experiment. 160 Cumulative methane yield for all the trials is shown in Figure 1. Experimental investigation 161 showed that maximum methane yield was observed at 20% TS which was 226 L/kg VS in trial 162 R2. It was also observed that trial R2 showed 91 and 38% higher methane yield to that of trial 163 R1 and R3 having same TS content (20%). This may be ascribed to the fact that VS reduction 164 in the case of trial R1 and R3 was 39 and 44% which was 22 and 8% less as compared to R2 165 respectively ( Figure 2). Further, increasing TS content from 20% to 22.5 and 25% did not help 166 to improve the methane yield significantly. The maximum methane yield observed in trial R6 167 and R8 which was 184 and 174 L/kg VS for 22.5% and 25% TS respectively. This observed 168 methane yield was 52 and 43% lesser as compared to trial R2 respectively. The subsequent VS 169 reduction in R6 and R8 was examined as 43 and 41% which was 1.11 and 1.16-fold less to the 170 best performing trial (R2) (Figure 2). Similar observations were made by Abbassi-Guendouz 171 et al. [18] for SSAD of cardboard with TS ranging between 10-35%. It was reported that rate 172 of methane production was inversely proportional to the TS content in the SSAD reactor 173 increased. It was also contemplated that beyond 30% TS, methane production rate was highly inhibited. Suksong et al. [19] also reported that increasing TS content from 16 to 25 and 30% 175 did more harm than good in SSAD of palm oil mill effluent. 176 The role of pH is also crucial as pH may help to understand the pattern of methane yield as 177 methanogens are sensitive to change in pH (Figure 2). The suggested pH is 7.4 for AD reactor 178 to function properly [8]. Results obtained were supporting the statement as the lowest pH was 179 observed in the case of R1 which was 6.9 at the end of the experiment. Interestingly, the 180 cumulative methane yield was also lowest in the trial R1 which was 118 L/kg VS (Figure 2). 181 The pH of trial R2 was 7.5 which was near the suggested value and methane yield was also 182 maximum (226 L/kg VS). The second and third highest methane yield have noted the pH 7.8 183 and 8.8 respectively after the end of experiment which was 0.4 and 1.3 unit more to that of R2. 184 Measurement of TVFA/alkalinity ratio is also required to monitor the stability of SSAD reactor 185 as pH is not sole indicator of reactor's wellness. Simultaneous KOH or NaOH addition may 186 enhance the lignin disruption and subsequently pH drop may be observed due to TVFA 187 accumulation at acidogenesis stage [11,15]. However, PHWBC may enable a balanced 188 electron transfer network trough DIET within the anaerobic reactor. Furthermore, presence of 189 PHWBC in excess amount may increase the alkalinity of the reactor due to alkaline nature of 190 PHWBC. In this regard, the TVFA/alkalinity ratio may be observed as stress indicator of SSAD 191 reactor [20]. Though this ratio will be unique for each reactor, the ratio of 0.4 and 0.6 is 192 considered as optimal and as excess organic loading in liquid AD [21]. The maximum 193 TVFA/alkalinity ratio was observed as 0.86 at the loading of 0.5% (w/w) KOH and 10 g/L of 194 PHWBC in the case of trial R1 followed by trial R9 as 0.73 at 1% (w/w) KOH and 10 g/L 195 PHWBC respectively (Figure 2). The minimum TVFA/alkalinity ratio was observed in trial R5

196
(1.5% KOH, and 10g/L PHWBC) which was 0.17. The optimal ratio is reported as 0.4 and the 197 TVFA/alkalinity ratio observed for trial R2, R6 and R8 was 0.37, 0.36 and 0.44 which was 198 nearby optimal value. Interestingly, trial R2, R6 and R8 also showed maximum methane yield 9 in the set of 20, 25.5 and 25% TS respectively. Alike results were observed by Liew et al. [11] 200 in which simultaneous NaOH treatment of corn stover in SSAD not only increased the 201 digestibility but also helped to maintain the buffering capacity of the reactor against excess 202 acid accumulation.

203
Analysis of Taguchi based GRA 204 Table 1, 2 and 3 shows the values of normalizing sequence, deviation sequence and grey 205 relation coefficient for GRA respectively. All the output response such as cumulative methane 206 yield, VS reduction, pH and TVFA/alkalinity was normalized first. The normalization of 207 cumulative methane yield and VS reduction is performed as "larger-the-better" using Eq. 5.

208
For pH and TVFA/Alkalinity ratio, "nominal-the-better" was applied using Eq. 7 as pH and 209 TVFA/alkalinity ratio should be in a specific range. The target value in Eq. 7 was given as 7.4 210 for pH and 0.5 for TVFA/alkalinity ratio [8,21]. As per the normalized sequence, grey relation coefficient was determined for every individual 224 output using Eq. 8 in which deviation sequence was calculated using Eq. 9. The grey relation 225 grade (GRG) was then determined with the help of grey relation coefficient using Eq. 12 (Table   226 4). The obtained weighted GRG for all the trials were used for obtaining the optimum condition                 while for untreated condition, it was 11.9 years. This is because the revenue generated by shows that if methane yield is increased per unit mass, the NPV will also increase in the case 360 of electricity price as methane yield governs the electricity generation. Also, compared to 361 untreated scenario, the change in NPV due to ±15% change in methane yield was more than 362 59% in scenario of simultaneous treatment. This clearly shows that simultaneous increment of 363 hydrolysis and syntropy will help to achieve monetary benefits despite the fact that OPEX was 364 US$ 3650/year more in the case of simultaneous treatment ( ). In this study, hydrolysis and syntrophic activity enhanced simultaneously in SSAD process.

373
The increment in the hydrolysis rate and syntrophic activity helped to achieve 1. number of experiments were 9 (L9) for given number of input parameters and levels (Eq. 1).

421
The inner orthogonal array formulated for selected parameters were shown in table 11.     The batch SSAD was carried out in a glass bottle of volume 600 mL. A total of 9 set were 444 prepared. The PHWBC and KOH were pre-dissolved into inoculum and mixed with straw qualitatively and quantitatively [35]. The GRA technique categorizes dynamic characteristics 453 and relative influence of input factor. GRA requires data pre-processing based on the selected 454 criteria. In pre-processing, data are normalized based on "larger-the-better", "smaller-the-455 better" or "nominal-the-better" conditions using Eq. 5, 6 and 7.