Bio-Methane potential of exotic food waste and water hyacinth

Abstract

Region specific foods in the Niger Delta like yam and cassava are consumed on a daily basis by at least 70% of the population. In addition to other commonly consumed foods, high volumes of unavoidable food wastes are generated. With 78% of the households in the region disposing their waste by burying, burning or in unauthorised heaps, environmental degradation is sustained. The region also suffers an infestation of Water Hyacinth (WH). Anaerobic Digestion (AD) presents a viable way of managing these wastes in addition to providing a clean source of energy. Limited research has been conducted on the characterisation and biogas potential of these exotic food wastes due to their localised availability, leading to a knowledge gap. My original contribution to knowledge is the Specific Waste Index (SWI), nutrient characterisation and biogas potential of the local food wastes and WH. Also novel is the design and optimisation of the AD process for mono and codigestion including the quarter hourly analysis of CH4 and CO2 content of the biogas composition over the complete duration of an AD test. The research approach was experimental and involved using conventional research methods in new fields of investigation. SWI was determined by replicating local food processing practices while nutrient composition was obtained using standard analytical methods. The Bio-Methane Potential (BMP) tests were carried out on the four most common food wastes, Yam Peel (YP), Cassava Peel (CP), Cocoyam Peel (CoP), Plantain Peel (PP), following VDI 4630 guidelines and using a newly designed cost-effective bioreactor. Laboratory scale batch reactors ran over 20 days at 37⁰C (310 K). The food wastes were anaerobically co-digested with WH in the ratio 2:1 g Volatile Solid (VS) with a total substrate mass of 8.4 g VS. The S:I ratio was 1:2 g VS and tests were carried out in duplicates to give an indication of repeatability. The results showed a wide range of SWI from 0.2-1.5. The Total Solid (TS) content varied from 7% for WH to 82% for Egusi Shell. Crude Protein and Crude Fibre were highest for Ugwu Stalk at 37% VS and Egusi Shell at 82% VS respectively. Cassava Peel had the highest oil content at 25% VS. NFE which was the major nutrient for 80% of the samples was highest for Yam Peel at 82% VS. YP+WH, CP+WH, CoP+WH and PP+WH had specific biogas yields of 0.42, 0.29, 0.39, and 0.38 m3/kg VS respectively. The yields represented 76%, 48%, 70% and 69% of their respective theoretical values. The samples had their highest methane content during the Technical Digestion Time (T80) period, which lasted up to the 8th day of digestion. The pH values ranged from 7.3 to 7.9 indicating that there was no inhibitory accumulation of organic acids. The results of the mono-digestion tests showed that co-digestion with WH reduced the biogas yields for YP, CP, CoP and PP by 16%, 22%, 7% and 7%. This drop in gas production was due to presence of complex molecules in the WH co-substrate, which cannot be digested by the anaerobic microbes. Further tests showed that fresh waste produced more biogas than dry samples, while a lower S:I produced more biogas due to increased microbial population. It was concluded that waste and nutrient content varied widely between different types of Niger Delta foods. In addition mesophilic digestion of food wastes have good biogas potentials which reduce when co-digested with water hyacinth. The methane content is shown to vary widely throughout an AD test. The findings of this research would provide valuable information to AD databases and its implementation would support clean energy production, environmental remediation and allow researchers in poor regions to perform BMP test on novel feedstock using cost-effective reactors. Key Words: Anaerobic Digestion, Co-Digestion, Water Hyacinth, Niger Delta, Yam, Cassava, Cocoyam, Food Waste, Specific Waste Index, Biogas.