Effects of temperature, substrate-to-inoculum ratio, nutrient augmentation, and inoculum acclimation on the anaerobic digestion of primary winery wastewater sludge

Kaira, Walusungu Maclean

Effects of temperature, substrate-to-inoculum ratio, nutrient augmentation, and inoculum acclimation on the anaerobic digestion of primary winery wastewater sludge

Author(s)

Kaira, Walusungu Maclean

Date Issued

2026

Type

master thesis

Publisher

Cape Peninsula University of Technology

Abstract

The wine industry generates substantial volumes of organic-rich primary winery wastewater sludge (PWWS), posing both economic and environmental challenges if not managed effectively. Currently, only 17% of wineries surveyed in the Western Cape province of South Africa employ anaerobic digestion (AD) for treatment of their PWWS, with others opting for composting and off-site disposal. With rising energy costs and increasingly stringent environmental regulations, circular waste management practices are essential for achieving long-term economic and environmental sustainability. This study addresses the need for a circular bioeconomy approach by evaluating the feasibility of AD as a waste management and energy solution for wineries, aligned with South Africa’s National Development Plan 2030 and UN Sustainable Development Goals (SDGs 7, 12, and 13). This thesis investigated the variability of PWWS from six wineries (designated AF) and the optimization of anaerobic digestion (AD) of the PWWS for methane (CH₄) production. The study evaluated the influence of key parameters such as inoculum acclimation, substrate composition, operational temperature, inoculumto-substrate ratio (ISR), and micronutrient supplementation on CH₄ yields and process stability. The variability of PWWS from winery A during the crush and post-crush seasons was examined. Results revealed significant intra-site variability (p<0.05), likely due to differences in winery operations during those periods. The total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) content of post-crush PWWS were found to be 74%, 171%, and 132% higher, respectively, than those of crush-season PWWS. Inter-site variability was also assessed across PWWS from wineries B through E, as well as lees from winery F. Substantial variation was observed, with COD, total organic carbon (TOC), TS, and VS concentrations ranging between 33.0–181.7 g/L, 6.8–20.2 g/L, 32.8–153.1 g/L, and 26.0–123.5 g/L, respectively. Theoretical biomethane yields at standard temperature and pressure (STP) were Theoretical biomethane yields at standard temperature and pressure (STP) were determined based on the COD:VS ratio of each sample. The estimated CH₄ yields were 567 mLCH₄/gVSadded and 490 mLCH₄/gVSadded for winery A crush and post- crush season PWWS, respectively, 654 mLCH₄/gVSadded for winery B PWWS, 251 mLCH₄/gVSadded for winery C PWWS, 470 mLCH₄/gVSadded for winery D PWWS, 544 mLCH₄/gVSadded added for winery E PWWS, and 1 434 mLCH₄/gVSadded for lees from winery F. The actual CH4 yields achieved at 37oC without added nutrients were 204 mLCH₄/gVSadded, 65 mLCH₄/gVSadded, 43 mLCH₄/gVSadded, 53 mLCH₄/gVSadded, and 765 mLCH₄/gVSadded for PWWS from wineries A-E and lees from winery F, respectively. With the addition of nutrients at 37°C, the CH4 yields achieved were 154 mLCH₄/gVSadded, 28 mLCH₄/gVSadded, 114 mLCH₄/gVSadded, 62 mLCH₄/gVSadded, and 584 mLCH₄/gVSadded for PWWS from wineries A-E and lees from winery F, respectively. This study found that in comparison to ambient temperatures, when the temperature was elevated to 37°C, CH₄ production was enhanced and time required to reach maximum yield was reduced. However, heating is only justified when the increase in CH₄ production exceeds 30 mL CH₄/g VS compared to unheated conditions. For small-scale wineries, lower-cost strategies such as insulating digesters or painting them black to enhance solar heat absorption may be more appropriate and cost-effective than active heating. The addition of a micronutrient blend containing 14 trace elements produced mixed results, likely due to substrate-specific nutrient imbalances and potential synergistic effects with temperature. These findings suggest that nutrient supplementation strategies must be tailored to the specific characteristics of the PWWS substrate and the operating conditions of the digester. The study also highlighted the critical role of inoculum acclimation and substrate type in determining methanogenic pathways and overall process performance. When the inoculum was switched from a municipal waste-activated sludgeacclimated inoculum (MWASi) to a PWWS-acclimated inoculum (PWWSi), biomethane yields increased from 0 to 49 mL CH₄/g VS added. Moreover, analysis revealed that the dominant methanogenic populations in PWWSi were composed of both acetoclastic and hydrogenotrophic communities, indicating active methanogenesis through multiple metabolic routes. Kinetic modelling showed that the Chen and Hashimoto and Cone models provided the best fit for predicting AD performance of PWWS from wineries B through E at 37°C. At ambient temperature, the Cone and Logistic models were more accurate. For the digestion of lees from winery F, the Logistic model consistently provided the best prediction regardless of temperature or nutrient supplementation. These findings reveal significant variability in PWWS characteristics across wineries and seasons, underscoring the importance of a tailored approach to AD. Effective AD of PWWS requires optimization of process parameters based on specific substrate properties and the use of a well-acclimated inoculum. The study emphasizes the need for customized, site-specific strategies to enhance biogas production and improve the overall feasibility of AD in the wine industry. en_

Additional information

Thesis (MEng (Chemical Engineering))--Cape Peninsula University of Technology, 2026

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