Acidic effluent such as winery wastewater is challenging to remediate. Biological sand reactors can simultaneously remove organics and neutralize winery wastewater via biotic and abiotic mechanisms. The systems have been shown to be suitable for treating the intermittent flow of wastewater at small wineries. It has been shown that dissolution of calcite is the most important abiotic mechanism for increasing the pH of the influent. In this study, sand column experiments were used to determine the effects of (i) sand particle size distribution on calcite dissolution kinetics, and (ii) the effects of calcite particle dissolution on the hydraulic conductivity. The results were then used to calculate the theoretical temporal abiotic neutralization capacity of biological sand reactors with differently sized sand fractions, including unfractionated (raw) sand. The results were compared with those determined from a pilot system treating winery wastewater over a period of 3 years. Sand fractions with larger particles contained lower amounts of calcite (using Ca as a proxy), but exhibited higher hydraulic conductivities (3.0 ± 0.05 %Ca and 2.57 to 2.75 mm·s⁻¹, respectively) than those containing smaller particles and/or raw sand (4.8 ± 0.04 to 6.8 ± 0.03 %Ca and 0.19 to 1.25 mm·s⁻¹, respectively). The theoretical abiotic neutralization capacity of biological sand reactors was compared with a pilot system with the same flow rates, and a temporal abiotic neutralization capacity of 37 years was calculated for biological sand reactors, which compared favorably with the theoretical results obtained for wastewater with pH values between 2 (8.2 years) and 3 (82 years). It was concluded that biological sand filters with around 10% calcite will be able to abiotically neutralize winery wastewater and other wastewaters with similar acidities for the projected life span of the system. Future work should focus on determining the effect of sand grain size on the bioremediation capacity, as well as the use of biological sand reactors for treating other acidic organic wastewaters such as fruit processing, food production and distillery wastewater.

Acidic wastewaters such as winery wastewater require treatment to increase the pH before discharge into the environment. Biosand filters have been shown to reduce the organic load while simultaneously providing a buffering function. Previous research has shown increases in pH which was assumed to mainly take place via dissolution of calcite from the sand particles. This study investigated the possible role of biotic mechanisms for pH adjustment in sand column experiments by comparing results obtained from irradiated (biotic) and non-irradiated (biotic and abiotic) sand columns extracted from biosand filters used to treat winery wastewater. The columns were fed with either synthetic winery wastewater or filtered water (control). It was shown that the specific hydroxide concentrations in the eluant from the non-irradiated columns was significantly (p < 0.05) higher than in the eluant from the irradiated columns (1.1 × 10⁻⁵ vs. 4.0 × 10⁻⁶ M/kgsand⁻¹), indicating the presence of both biotic (average 4.5 ± 0.13%) and abiotic (average 95.5 ± 0.16%) pH increases. Using multivariate statistical tools to analyze a combination of parameters linked with biotic and abiotic pH adjustment, significant differences (ANOVA, p < 0.05) were found between the four treatment groups (irradiated/non-irradiated SWW and control) and the groups showed good clustering in cluster plots (group average) linkages, and principal component analysis plots.

In 2020 there was approximately 260 million hectolitres of wine produced across the world. Many winemaking and cellar cleaning activities generate winery wastewater. In vine growing areas which are water stressed, this wastewater is often used for irrigation and if it is inadequately treated it can be detrimental to land and aquatic environments. It has been shown at pilot scale that horizontal flow biosand filters are suitable for treating winery effluent to comply with irrigation standards. In this study, it was envisaged that (i) the loading rates (organic and hydraulic) could be increased by changing the flow from horizontal to vertical flow, and that (ii) higher organic biodegradation rates could be achieved consequent to the increased redox potential from draw-down of atmospheric oxygen during system drainage in pulse mode in comparison to continuous mode. It was found that system performance was higher in continuous mode, attaining a hydraulic loading rate of 113 L.m -3 of sand a day -1 , organic loading rate of 279 gcod.m -3 of sand.day -1 and COD removal efficiency of 70% compared to pulse mode with 90 L.m -3 of sand a day -1 , 192 gCOD.m -3 of sand.day -1 and 70%, respectively. In comparison to other passive winery wastewater treatment systems (constructed/treatment wetlands), these biosand filters are able to treat winery wastewater at higher loading rates with smaller spatial footprints.