Abstract:
The ever-increasing food waste generated globally attracts grave concerns due to associated environmental and socio-economic implications. One way to mitigate these adverse consequences is to use the food waste as feedstock for valuable energy and fertilizer generating anaerobic digestion (AD) process. While AD is a widely accepted method for managing and valorising food waste (FW), in South Africa, its use is in the infancy stage. Lack of awareness and adequate knowledge of the potential of FW for biogas and manure production could be the reason for the observed slow uptake of AD in South Africa. This study evaluated the energy and manure production potential of solid FW generated by a typical South African university (the University of Venda) canteen using the AD co-digestion of solid FW and cow dung (CD). In the first part of the study, a 2 x 5 factorial experimental design was used to assess the effect of substrate mixing ratios and temperature on biogas yield, methane content, and biochemical methane potential (BMP) of pure FW and CD and co-digestion of FW and CD for predetermined mixing ratios. FW and CD mixed on a wet mass basis at ratios 0:100, 25:75, 50:50, 75:25, and 100:0 (w/w) were batch incubated at 35±1℃ and 55±1℃. Experimental results were subjected to analysis of variance (ANOVA) at a 5% level of significance. Where a significant ANOVA result was obtained, the mean separation was done using Fisher’s least significant difference (LSD) test at a 5% level of significance. The obtained results showed that the temperature and substrate mixing ratios significantly (p < 0.05) affected biogas yield, biogas methane content, and BMP. The highest biogas yield and BMP were 4903.33 ± 38.84 mL and 301.879 ± 2.07 mLCH4/g VS at 35 °C, and 7151.67 ± 11.55 mL and 401.88 ± 1.98 mLCH4/g VS at 55 °C. The lowest biogas and BMP yields were 5301.67 ± 62.51 and 328.278 ± 4.265 mLCH4/g VS at 55 °C, and 3291.67 ± 81.45 mL and 328.28 ± 4.26 mLCH4/g VS, respectively, for mono-digestion of FW at 55 ° and 35 °C. Overall, co-digestion of FW and CD produced higher biogas yield and BMP than mono-digestion of the substrates at both test temperatures. These findings can be used to size heated and unheated biogas plants using co-digested FW and CD substrates. In the last part of the study, a 2 x 2 factorial experimental design was used to assess the effect of substrate type and temperature on the quality of bio-slurry. The bio-slurry samples were collected from mono-digestion of FW and CD which were incubated at 35±1℃ and 55±1℃. The obtained results indicated that bio-slurry quality was significantly affected (p < 0.05) by substrate type, temperature, and the interaction between substrate type and temperature. Bio-slurries had lower total solids (TS %) and (volatile
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solids/total solids) VS/TS% compared to undigested FW and CD. Bio-slurry from the AD of FW and CD at 55 ℃ had significantly higher (p < 0.05) NPK concentrations than bio-slurry obtained from AD at 35 ℃. The highest nitrogen (0.25%), phosphorous (1.61%) and potassium (1.25%) concentrations were obtained from bio-slurry obtained the AD of CD at 55 ℃. Characterization data for bio-slurry from mono-digested FW and CD can be used to advise biogas plant owners, agricultural extension officers and the public on the application rates of co-digested FW and CD slurry as manure depending on crop nutrient requirements.
