Anaerobic co-digestion of fruit juice industry wastes with lignocellulosic biomass
Thesis (MEng)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: The fruit juice industry in South Africa forms an important part of the South African economy, however it generates large quantities of liquid and solid organic wastes. Landfilling is typically used to dispose of these wastes, resulting in uncontrolled greenhouse gas emissions (GHG). Anaerobic digestion (AD) offers an alternative waste disposal method and produces two valuable by-products: biogas (a renewable energy source) and a liquid fertiliser. The high sugar content of fruit waste alone often results in AD failure due to acidification, resulting in poor quality biogas. Consequently, there is relatively little information available on the AD of apple fruit juice process wastes (FJPW). Identification of substrate combinations that improve the energy value of the resultant biogas may mitigate GHG emissions and generate valuable by-products which provide additional revenue streams to industry. This study thus aimed to identify optimal substrate combinations to aid in waste disposal of FJPW and energy value of biogas from fruit juice industry waste based on seasonal availability of waste streams. Five waste streams: manure, food waste, retentate, pomace and waste apples were incorporated into a five-factor mixture design to assess food waste and manure as co-substrates of FJPW. This design was carried out in a series of biomethane potential (BMP) tests performed in 100 mL serum bottles. A second mixture design was performed using BMP tests in 100 mL bottles to evaluate lignocellulosic biomass (LCB) as a potential co-substrate of FJPW. A biogas and methane optimisation substrate mixture (50% manure, 30% LCB, 20% Retentate) and a manure minimisation mixture (30% manure, 30% LCB, 30% retentate, 10%waste apples) were selected and scaled up in 50 L CSTR reactors in batch process for 32 days with intermittent mixing. Two substrate combinations based on biogas optimisation and manure minimisation were scaled-up in 50 L reactors in semi-continuous process and fed increasing organic loading rates (OLRs) from 1-4 gVS/L/day over the course of 32 days to identify the maximum OLR that can be stably operated for each point. The results indicated food waste was highly variable and behaved similarly to FJPW when digested, thus food waste was deemed unsuitable as a co-substrate for FJPW. An ANOVA was performed on the results of the LCB mixture design revealing both biogas and methane production to be significant (p< 0.05). The standardised effect estimates of all five feedstocks revealed manure, LCB and retentate to have a significant (p<0.05) effect on biogas and methane production. LCB addition was found to significantly improve biogas production and prevent acid crash, however it mainly did so when compensating for the fruit waste fraction rather than the manure fraction except for two mixtures: 20% manure, 30% LCB, 30% pomace and 20% retentate and 20% manure; 30% LCB, 30% waste apples and 20% retentate. The highest yields obtained from the LCB supplementation experiment were 410.01 mL.gVS-1 fed biogas and 167.10 mL.gVS-1 fed methane for the fruit-juice producing season and 325.69 mL.gVS-1 fed and 131.95 mL.gVS-1 fed for the non- juice producing season. The improved biogas and methane yields in the batch experiment compared to lab-scale were as a result of slow intermittent mixing at 125 rpm for 5-10 minutes twice daily. The biogas optimisation point gave the highest yields at an OLR of 4 gVS/L/day. The manure minimisation point demonstrated the highest biogas and methane production at an OLR of 3.5 gVS/L/day, with the system showing signs of organic overloading at a higher OLR. To conclude, this study found a 30% LCB addition to improve digestibility of fruit process waste mixture for certain combinations of pomace and retentate, and waste apples and retentate with 20% manure. As this study only investigated 0%, 20% and 30% LCB supplementation, future research should focus on a broader array of supplementation levels in order to further maximise fruit waste disposal via AD.