Browsing by Author "Netshivhumbe, Rudzani"

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    Anaerobic co-digestion of fish sludge originating from a recirculating aquaculture system
    (Stellenbosch : Stellenbosch University, 2022-12) Netshivhumbe, Rudzani; Goosen, Neill Jurgens; Faloye, F.; Gorgens, Johann F.; Mamphweli, N.S.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Recirculation aquaculture systems (RAS) are considered as sustainable and environmentally friendly aquaculture systems capable of meeting the growing demand of seafood for human consumption. However, RAS produce large quantities of waste sludge from uneaten feed and fish faecal matter, which need to be removed from the recirculating water and treated to prevent adverse environmental impacts. Anaerobic digestion (AD) has been considered as an alternative method to stabilize the amount of organic waste in the environment before its disposal, with the simultaneous production of bio-methane that can serve as a source of energy within RAS. However, there are some drawbacks in the mono-digestion process of fish sludge (FS) such as process inhibition, unbalanced nutrient contents, and low methane yields. The biomethane production from FS, food waste (FW), and fruit & vegetable waste (FVW) was optimized during anaerobic co-digestion using a mixture design. The synergistic and antagonistic interaction effects of the three substrates on specific methane yield, volatile solids reduction, and process stability were evaluated in both batch and semi-continuous mode. A mixture design was used to determine the best mixture compositions of FS, FW, and FVW for specific methane yield and volatile solids removal during the anaerobic co-digestion process based on biomethane potential (BMP) measurements. The results showed that the optimum mixture proportions of FS, FW, and FVW were 63 %, 18 %, and 19 %, respectively. The results showed the maximum methane production and VS removal of 401 mL CH4/gVS and 64%, respectively under the optimum mixture. Anaerobic co-digestion of FS with FW and FVW enhanced the methane yields by 8 folds compared with mono-digestion of FS. The optimum mixture proportions obtained from batch BMP tests were further evaluated in 50 L batch and 30 L semi-continuous pilot-scale digesters to evaluate the effect of organic loading rate (OLR) on biogas production and process performance stability. The methane yield obtained from the batch pilot-scale digester was 272 NmLCH4 /gVS. This was 71 % of the methane yield obtained from the BMP test under the same optimum mixture condition. The batch digester showed no substantial inhibition of the system due to its strong buffering capacity. In semi-continuous mode, the digester was conducted under different OLRs of 1, 2, and 3 𝑔𝑉𝑆𝐿−1𝑑−1 to investigate the impacts of OLR on biogas and methane production, and process performance stability of the anaerobic co-digestion of FS, FV, and fruit and FVW. The highest total biogas and methane production of 388 L/gVS and 67 L/gVS, with a methane content of 66.8% obtained at an OLR of 2 𝑔𝑉𝑆𝐿−1𝑑−1 compared to OLRs of 1 and 3 𝑔𝑉𝑆𝐿−1𝑑−1. The digester showed instabilities or failure at an OLR of 3 𝑔𝑉𝑆𝐿−1𝑑−1 due to acid crash and accumulation of VFA of 11g/L. An OLR of 1- 2 𝑔𝑉𝑆𝐿−1𝑑−1 is recommended for anaerobic co-digestion of FS, FW, and FVW in semi-continuous digesters because of less inhibitor indicators observed.