Effect of Variation in Co-Digestion Ratios of Matooke, Cassava and Sweet Potato Peels on Hydraulic Retention Time, Methane Yield and Its Kinetics

Abstract

This paper presents the results of batch anaerobic co-digestion of matooke, cassava, and sweet potato peels and vines. These agricultural wastes and others form the biggest portion of household wastes in developing countries. However, they have remained an unexploited resource amidst the ever increasing needs of clean energy and waste disposal challenges. Efforts to use them individually as biogas substrates have been associated with process acidification failure resulting from their fast hydrolysis. The aim of this work was to exploit agricultural wastes is co-digestion among themselves and assess their effect on methane yield and its kinetics, pH and hydraulic retention time (HRT). Sixteen ratios of Matooke peels (MP), cassava peels (CP) and sweet potato peels (SP) were assessed in duplicate. Methane yield and its kinetics, pH and HRT demonstrated dependence on the proportion of substrates in the mixture. Depending on the ratio mixture, HRT increased to 15 days compared to less than 5 days for single substrates, hydrolysis rate constant (k) reduced to a range of 0.1 -0.3 d −1 compared to single substrates whose k-values were above 0.5 d −1 , pH was maintained in the range of 6.38 -6.43 and CH 4 yield increased by 15% -200%. Ratios 2:1:0, 2:0:1, 0:1:2, 1:1:1 and 1:1:4 were consistent all through in terms of model fitting, having a positive synergetic effect on HRT, hydrolysis rate constant, lag phase and methane yield. However, more research is needed in maintaining the pH near the neutral for process stability assurance if household wastes are to be used as standalone substrates for biogas production without being co-substrates with livestock manure.

Effect of temperature fluctuation, substrate concentration, and composition of starchy substrates in mixture and use of plant oils as antifoams on biogas production

“…In some digesters or incubators, temperature continuously fluctuates in such a way that may hinder digester performance if certain limits are exceeded. This scenario of temperature fluctuation was observed in previous investigations on hydraulic retention time and methane production kinetics with incubator temperature set at mesophilic temperature of 37 C. However, the actual temperature inside the incubator fluctuated between 28 C and 46 C and at one point, the lower temperature limit abruptly went as low as 26 C, causing an abrupt sharp decrease in biogas production [2]. Nevertheless, biogas production recovered after this temperature shock as reported by other researchers [3][4][5] investigating temperature fluctuation effects on biogas production.…”

“…Only results of the mono‐substrates (1:0:0, 0:1:0, and 0:0:1) have been reproduced in this work (Table ) from Tumutegyereize et al . as reference substrates to show the effect of co‐digestion and plant oil addition on biogas production. This confirms that addition of plant oils as antifoams has a synergetic effect on biogas production which agrees with findings of Kougias et al .…”

“…Procedures already described by Tumutegyereize et al . were followed for gas measurements and data processing.…”

“…The ratios that performed were therefore selected for concentration and temperature fluctuation investigation. Inoculum was collected from a household biogas digester fed with household organic waste and analyzed as described in Tumutegyereize et al [2]. The characteristics of the selected substrate ratios and inoculum are presented in Table 1 in addition to their parent substrate.…”

“…Control serum bottles with inoculum and plant oils were also setup to determine the methane potential of the oils during antifoaming. Procedures already described by Tumutegyereize et al [2] were followed for gas measurements and data processing. Table 1 presents the characteristics of the substrate ratios used.…”