| The sustainable low-cost self-supporting system utilizing waste to bioenergy opportunities are increasing option given the global energy crises and climatic changes due to intense use of fossil fuel. In this study, we developed and tested a small-scale sustainable system that produces high-yield outputs of biomethane, biofertilizer and lipid for biodiesel (via alga).The microalga was cultivated using digestate refuse blends of poultry manure (PM), paper pulp and algae waste sludge as the initial input. We blended the poultry manure, alga sludge, and paper waste in varying ratios and anaerobically co-digested in batch mode. Varied blending ratios were studied for biomethane yield. The digestates (the residues left in the digester after biogas is removed) from these co-digestions were analyzed for their value as biofertilizer and as supplemental feed to enhance microalga Chlorella vulgaris biomass for biodiesel production. The system that we investigated is a three-step process;1. The first step was the biogas production in various blends (poultry manure, paper pulp and algae sludge) producing biomethane gas plus the remaining digestates (semi-solid and aqueous). The carbon/nitrogen (C/N) ratios were 26,30,31,34, and 37. The C/N 26 achieved highest (1045 ml/L/day,74% biomethane content) biogas in comparison to other blends. Also the C/N 26 resulted in nutrient rich-semi solid digestate (comparing the nitrogen mineralization), a potential crop fertilizer. The nutrient variations linearly increased with biomethane production. Therefore, C/N26 produced more methane and nutrients-rich digestate. Generally, comparative result suggested that using PM blends with C/N ratio 26 and 30 as initial input resulted is an excellent tri-part yield of biomethane, fertilizer, and algae biomass. The improved biomethane yield occurred when PM blends were diluted to a 1:1.5 ratio and pre-treated. Considerable alga yield occurred with C/N 26 and 30 digestates, and produced satisfactory biofertilizer. The second step was assessing the biofertilizer value of each blending in terms in nitrogen mineralization.2. The third step was alga biomass enhancement using the aqueous digestates as supplemental feed. In this step, we developed two-stage digestate feeding where exponential and stationary growth phases were enhanced by digestate feeding that improved algal biomass. Feeding 20,30,40 ml/L/day digestate improved cell biomass with noticeable differences. Dry cell weight of 9 g/L and lipid of 3 g/L were significantly achieved at 165 h compared to others that achieved that much in 180 h. By supplementing with glucose at 2 g/L at stationary phase, lipid reached to 4.9 g/L. Supplementing a little glucose at stationary phase depleted the cells (starve) of nitrogen as available nitrogen was used in metabolizing glucose, hence, lipid soared.The study revealed that a feasible sustainable system of using common waste can commercialize algae biomass including those generating wastes in small-scale industries. From this study, it was recommended that algae biofuel can reach significant commercialization if the following can be adequately addressed:Disseminate adequate knowledge on utilizing small scale-industrial and agro-biowastes plus emphasis the'know how'knowledge of algae cultivation to the mass of population. Feasible alga cultivation and digestate feeding strategies investigated had already given the feasibility signal increasing commercial status of alga that may possibly elevate.
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