Technical Yearbook 2024

FIGURE 1. Cross section of a biosand reactor module. 1

Equalization & settling tanks

Biosand reactors

Primary settling basin

FIGURE 2. Layout of a pilot biological sand reactor winery wastewater treatment system showing the flow of winery wastewater from settling basin. 1

Functionality of biosand reactors Removal of organics and neutralisation of acidic winery wastewater The main functions of BSRs are to reduce the chemical oxygen demand (COD) concentrations and increase the pH of acidic WWW. These parameters were measured in a pilot system over two crush seasons, shown in Figure 3. During the start-up period, which coincided with the first crush period, the functional microbial communities were slowly acclimatising to the BSR environment and the WWW. Once established, consistently high COD removal results were obtained during both crush periods (Figure 3a). The pH of WWW is highly variable, but the pH of acidic WWW is increased to near-neutral values in BSRs (Figure 3b). It was

found that the major alkalinisation mechanism is via the dissolution of calcite a (mineral form of calcium carbonate) and, to some extent, via microbial processes. Another valuable effect of calcite dissolution is the addition of calcium to the WWW, which decreases the sodium adsorption ratio. This is important when the WWW contains sodium. Addition of the divalent cation Ca 2+ mitigates against sodicity caused by excess monovalent Na + cations. Based on an average influent pH and results from extensive column and laboratory-based experiments, it was calculated that the calcite in the sand of BSRs treating WWW with a pH of 3 would be expended after 82 years, well beyond the projected ‘life’ of the sand. In reality, it is recommended that the sand is replaced every 10 to 15 years.

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TECHNICAL YEARBOOK 2024