Technical Yearbook 2024

MAY

Biosand reactors and anaerobic digestion – a zero-waste model for winery wastewater (Part 2) By Gareth Holtman, Walusungu Kaira, Ashton Mpofu, Elyse Kimpiab, Rainer Haldenwang & Pamela Welz

quite frequently (3 to 10 times per year) or very frequently (>10 times per year). Disposal takes place more frequently in spring-summer, which was expected as most winery wastewater is generated during the summer/autumn crush season. A definite trend between disposal frequency and winery size was noted, with larger wineries tending to have higher disposal rates. Of the wineries that took part in the survey, 33% compost their sludge, 17% digest their sludge via anaerobic digestion (AD) while 25% indicated that they use commercial companies for off-site disposal, which is costly. None of the smaller wineries that took part in the survey use commercial companies for sludge removal. The propensity for the medium to larger wineries to contract commercial companies for sludge removal and disposal, together with higher disposal frequencies at larger wineries, is notable from the perspective of economies of scale. It is a justification for the beneficiation of this organic-rich waste. Experiments known as biomethane potential experiments (BMPs) were then conducted to determine whether PWWS is amenable to AD. Specific microorganisms (methanogens) are needed to generate methane during AD. These methanogens are sensitive to oxygen and grow slowly. An inoculum, therefore, needs to be prepared in the absence of oxygen to enrich the growth of methanogens and other functional microorganisms in a particular substrate (in this case, PWWS), or AD will be ineffective. The preparation of a suitable inoculum typically takes around three months. It is then important to apply the correct amount of inoculum to the substrate amount to maximise AD’s efficiency. This optimal inoculum-to-substrate ratio (ISR) can differ from substrate to substrate. In full-scale AD systems, a pre-determined amount of the digested material (which contains the functional microorganisms) is typically retained after the first inoculation of the reactors. This serves as the inoculum for the fresh substrate for the next batch of AD at the correct ISR. In the PWWS studies, samples were taken during the crush and non-crush periods and tested under different conditions (ISR, temperature and addition/non-addition of nutrients). The specific yields of methane from AD of the PWWS are shown in Figure 1. In general, the yields from the crushing season PWSS were lower, which was due to the presence of residual inorganic matter in the settling delta that was not generated during the crushing

Abstract This article proposes a zero-waste model for

Anaerobic digestion of primary winery wastewater sludge A number of studies have been conducted focussed on optimising the operation of biosand reactors (BSRs) for remediating winery effluent (for more information, please refer to the previous article and related literature references). During the pilot operation of these BSRs at local wineries, it was noted that copious amounts of primary winery wastewater sludge (PWWS) required periodic removal from the settling deltas. This PWWS contains the solids directly settled from the wastewater and is different from the secondary wastewater sludge (SWWS) formed in biological wastewater treatment reactors in larger wineries. In response, a Survey Monkey ® questionnaire was conducted to determine how much PWWS and SWWS are generated by Western Cape wineries and how they dispose of their sludges. This formed part of a Water Research Commission (WRC) scoping study focussed on valorisation (beneficial use) of winery wastewater sludge. Twenty-four wineries responded to the questionnaire, and 67% and 46%, respectively, indicated that they generate PWWS and/or SWWS. Almost 60% of participants indicated that they dispose of sludge only 1 to 2 times per year, while 21% indicated that they dispose of sludge remediation and beneficial reuse of winery wastewater and primary winery wastewater sludge. The strategy is based on the use of biosand reactors containing locally available dune sand (Phillipi sand) with particles <0.425 mm removed for remediation of winery wastewater and anaerobic digestion of winery wastewater sludge. It is proposed that (1) the residual sand containing the smaller particles can be used in self-compacting high-performance concrete mixes, (2) the treated effluent is suitable for beneficial irrigation, (3) the primary winery wastewater sludge can be anaerobically digested, and (4) the remaining solid fraction (digestate) can be used as an organic agricultural fertiliser.

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TECHNICAL YEARBOOK 2024