Biosand reactors and anaerobic digestion – a zero-waste model for winery wastewater (Part 2)

Abstract

This article proposes a zero-waste model for 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.

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.¹ This formed part of a Water Research Commission (WRC) scoping study focussed on valorisation (beneficial use) of winery wastewater sludge.¹

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 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.² 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 season and skewed the results. Nonetheless, the results were promising, especially because high yields were also obtained at ambient temperatures. This suggests that PWWS could be easily digested without heating, particularly during the warmer grape harvesting and crushing months. The composition of the residual matter (digestate) after AD indicated that it may be suitable as an agricultural fertiliser, with high concentrations of N (21.5 to 27.7 g/kg dry weight) and C (229 to 277 g/kg dry weight), as well as the presence of all essential micronutrients. The composition was compared favourably with commercial agricultural organic fertilisers based on supplemented chicken manure.²

FIGURE 1. Results of biochemical methane potential tests using primary winery wastewater sludges from the crush and post-crush periods.² The results from the first study at ambient temperatures are shown on the left of the vertical black line, while those from a second study comparing results from ambient and mesophilic temperatures (37°C) are shown on the right of the line. ISR = inoculum to substrate ratio; N = nutrients (Co, Zn, Cu).

The volumes of wastewater and PWWS generated from wineries crushing from 1 000 to 2000 tons of grapes per year were estimated.³ The AD experimental data was applied to the latter to extrapolate the potential volumes of methane and the amount of methane-derived energy that could be generated from AD of PWWS (Table 1).

Overall, the results indicated that it might be feasible for larger wineries that generate significant volumes of PWWS/SWWS to utilise AD as a means of (1) generating utilisable biogas for energy, (2) producing fertiliser, and (3) limiting the economic and environmental burden associated with disposal of PWWS/SWWS to landfill. The next steps are: (1) to pilot AD of PWWS at a medium-large winery, and (2) to conduct pot growth experiments to compare the performance of PWWS digestate with organic commercial fertilisers on plant growth and soil health.

TABLE 1. Estimated amount of energy from primary winery wastewater sludge for different-sized wineries.³

ML = megalitres; CH₄ = methane.

Zero-waste model

Building on the results obtained from this work, a zero-waste model for remediation of winery wastewater and PWWS/SWWS is proposed as outlined in the schematic (Figure 2). The intent is to contribute to minimising waste formation and maximising resource recovery in wineries.

The model consists of the following elements:

• Fractionation of Phillipi sand.
• Application of the fraction of sand consisting of larger particles (≥0.425 mm) in BSRs.
• Remediation of winery wastewater in BSRs and reuse of the treated effluent for beneficial irrigation.
• Application of the smaller sand particles (<0.425 mm) in self-compacting, high-performance concrete mixes.
• Digestion of PWWS via AD.
• Utilisation of biogas from AD to supplement the green energy mix in wineries.
• Utilisation of the digestate from AD as an agricultural fertiliser.

FIGURE 2. Schematic of zero-waste model for remediation and reuse of winery wastewater and primary winery wastewater sludge.

Funding

This work was supported by the Wine Industry Network of Expertise and Technology (Winetech) (CSUR 13091742538) and the Water Research Commission (Project (Project C2020/2021 – 00393). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and the funding entities do not accept any liability in this regard.

References

1. Welz, P.J. Kaira, W.M., Deepnarain, N., Holtman, G.A., Kimpiab, E., Ranjan, A., Oyekola, O.O. (2023). Valorization of primary winery wastewater sludge: a scoping study. WRC report TT 895/22 ISBN: 9780639202358.
2. Kaira, W.M., Kimpiab, E., Mpofu, A.B., Holtman, G.A., Ranjan, A., Welz, P.J. (2022). Anaerobic digestion of primary winery wastewater sludge and evaluation of the character of the digestate as a potential fertilizer. Biomass Conversion and Biorefinery (in press) https://doi.org/10.1007/s13399-022-03087-8
3. Holtman, G.A., Haldenwang, R., Welz, P.J. (2023). Biosand reactors for remediation of winery effluent in support of a circular economy and the positive effect of sand fractionation on hydraulic and operational performance. Journal of Water Process Engineering 53: 103849 https://doi.org/10.1016/j.jwpe.2023.103849

Contributing authors:

Walusungu Kaira, Ashton Mpofu, Elyse Kimpiab, Rainer Haldenwang & Pamela Welz.

For more information, contact Gareth Holtman at gareth@holtman.co.za.

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