Anaerobic Co-Digestion of Water Hyacinth (Eichhornia crassipes) with Ruminal Slaughterhouse Waste under Mesophilic Conditions

Abstract

Water hyacinth (Eichhornia crassipes), an invasive aquatic weed with large biomass poses serious socio-economic and environmental challenges in fresh water bodies such as Lake Victoria in East Africa. Efforts towards its control and removal can be complemented by biogas production for use as energy source. However, knowledge of chemical and nutritional composition of its largely complex lignocellulosic biomass is important in determining its conversion into biogas. The complex structure of lignocellulosic biomass can affect its biodegradability and limit biogas production. On the other hand, co-digestion with complimentary substrates can potentially make it more amenable to biodegradation and improve biogas production. Biomethanation involves a combination of physicochemical conditions in anaerobic reactors and the action of microbial community. Understanding the dynamics of the microbial community can provide insights on how co-digestion influences biogas generation. This study evaluated synergy in co-digestion of WH from Lake Victoria with ruminal slaughterhouse waste (RSW) from Dagoretti slaughterhouses in Nairobi. The study characterized WH and RSW as co-substrates in biogas production by conducting proximate, crude fiber, elemental and biochemical analysis. It evaluated influence of processes parameters that included pH, temperatures, reaction times and substrate mix proportions on biogas production for WH and RSW substrates digested separately and in co-digestion. The study also investigated the dynamics of microbial communities in digestion of single substrates and in co-digestion at critical stages of biogas production by isolation, identification, DNA extraction, 16S rRNA gene amplification and sequencing using the Basic Local Alignment Search Tool (BLAST) technique. xvii Water Hyacinth had significant concentrations of cellulose, hemicellulose and carbohydrates of 331,200, 231,800 and 447,800 mg/L, respectively and lesser concentration of lignin of 99,400 g/L, which is desirable in biomass for biogas production. The concentration for C, N, P and K in WH were 15480, 1654, 51 and 137 mg/L, respectively compared to 26,220, 1,390, 34 and 7,475 mg/L for RSW. The concentration of potassium for WH was below the optimum range for biogas production of 200 - 400 mg/l while that for RSW was in the inhibitory range. Both biomass exhibited phosphorous deficiency at C/P of 310 and 656 for WH and RSW, respectively against optimum ratio of 100 and 150 for hydrolysis and acidogenesis stages, and methanogenesis stage, respectively. The C/N ratio for WH was 9.4 that tended towards ammonia toxicity at the lower limit of 8 to 20 C/N ratio for optimal biogas production while that for RSW was 18.8, which bordered on nitrogen deficiency. The results indicated potential for complementing of nutrients in co-digestion of WH with RSW substrates, for enhanced biogas production. Profiles of biogas production exhibited distinct acclimatization, lag and active biomethanation phases. Water hyacinth and RSW digested separately experienced lag phase of 7 and 20 days, respectively in which pH dropped from 7.5 to 7.0 for WH and to acidic pH of 6.0 for RSW. The pH drop was attributed to rapid generation of volatile fatty acids by hydrolysis and acidogenesis process without corresponding consumption by acetogenesis and methanogenesis processes. Depressed pH inhibited methanogenic organisms resulting in lag in biogas yield. However, co-digestion of WH with RSW led to a consistent and improved biogas production that was attributed to collation of processes parameters restraining of pH drop alkaline to levels that were conducive for biomethanation. A WH: RSW co-digestion ratio of 70:30 exhibited the most consistent and largest biogas yield over a residence time of 60 days. Co-digestion of WH with xviii 30% RSW proportion at 24°C improved WH biogas yield by 75% from 8.05 to 14.1 L/Kg biomass and the proportion of methane component by 9% from 59 to 68% suggesting synergisms in the co-digestion. The morphologies of microbial colonies isolated from reactor sludge were dominated by short and long rods bacilli, but also cocci, and streptococcus mainly in WH samples. About 77% of the isolates were Gram positive that indicated dominance of the Firmicutes phyla that includes the bacillus genus, while 23% were Gram negative. Molecular analysis identified a shift in microbial community during the acidic lag phase from Bacillus genus to Lysinibacillus and Solibacillus genera. Recovery of alkaline conditions resulted in re-emergence of diverse species of Bacillus genus including Bacillus aerophilus, Bacillus pumilus, Bacillus glycinifermentans, Bacillus cereus, Bacillus thuringiensis, Bacillus licheniformis and Bacillus aquimaris, which were associated with active biomethanation. Consequently, consistent and improved biogas production in co-digestion of WH with RSW was attributed to collation of processes parameters that restrained pH to alkaline levels allowing growth of Bacillus sp. associated with active biomethanation.