Technical Yearbook 2024

FIGURE 3. Chemical oxygen demand (a) and pH (b) measurements from a pilot biosand reactor system during two consecutive crush periods. 3

Flow rates The flow of WWW through BSRs reduces after start-up because the functional microbial biofilm reduces the pore spaces between the sand particles. Microbial growth is a positive feature but requires consideration because of the knock-on effect on flow rates in BSRs. In an earlier pilot system that was operated in horizontal mode, the flow rates were retarded to the extent that the volume of WWW that could be treated was lower than design levels. Indeed, many stakeholders have queried whether BSRs become clogged with solids and whether they need backwashing. Studies have shown this is not the case and that the reduction in flow rate is positively correlated with COD loading and is naturally reversible after the crush period and/or lower loading as the organic solids degrade. Slow flow rates can be overcome by employing vertical operation, increasing the hydraulic head and regularly scarifying the surface of the sand. More recently, it was found in column experiments that if smaller sand particles (≤0.425 mm) are removed, the hydraulic conductivity of the sand can be increased a further 10-fold without having a negative impact on COD removal performance (Figure 4). Indeed, the achievable flow rates in systems containing fractionated sand are too high for effective WWW

remediation if non-controlled gravity feeding is employed. This is because a minimum contact time (hydraulic residence time) is needed between the incoming WWW and the microbial biofilm attached to the sand. In the proposed prototype, an optimal hydraulic residence time will be achieved by adjusting the BSR outlet to increase or decrease Based on our studies’ results and the projected WWW volume generated for wineries crushing from 10 to 5 000 tonnes of grapes per annum, the number of BSR modules with an internal sand capacity of 5.6 m 3 required to treat WWW was calculated (Table 1). The calculations accounted for two different peak flows: wineries that generate 80% or 50% of their effluent during the crush period. The data suggests that using the modular set-up, the systems are attractive for small to medium-sized wineries. The advantages of these systems over conventional systems are that: • after the initial start-up period, the systems are resilient to lengthy periods of inactivity without losing functionality; • they require minimal maintenance; the flow according to need, as shown in Figure 1. Biosand reactors for wineries of different sizes

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TECHNICAL YEARBOOK 2024