Photovoltaic Assisted Ultraviolet-C Treatment of Strawberry Drainage Solution for Reuse: Field Energy Balance, Optical Water Quality Constraints, and Microbial Indicator Reduction

초록

Drainage solution reuse in soilless strawberry production can reduce nutrient-rich discharge, but adoption requires microbial control, hydraulic reliability, and manageable energy demand. This field study evaluated a photovoltaic (PV) assisted ultraviolet-C (UV-C) treatment loop for substrate derived drainage solution in a 132 m2 three-tier natural light greenhouse producing ‘Solhyang’ strawberry in Sokcho-si, Republic of Korea. The system used an 11.25 kWp vertical windbreak-type PV facility and a 650 W treatment loop comprising a 250 W low-pressure mercury UV-C reactor, a 350 W pump, and a 50 W controller. The loop operated for 2.5 h day−1, processed 3.25 m3 day−1 as cumulative reactor throughput, and consumed 1.625 kWh day−1, equal to 4.22% of the measured daily PV alternating current (AC) output (38.5 kWh day−1). The drainage solution had low ultraviolet transmittance at 254 nm (UVT254; 25–50%) and moderate turbidity (5–30 NTU), conditions that can attenuate UV radiation and shield microorganisms. Across six post fruit set sampling events, the mean log10 reductions were 1.15 ± 0.09 for culturable molds/fungal propagules and 1.64 ± 0.09 for culturable aerobic bacteria; paired tests on log10 transformed counts were significant (p < 0.001). Total coliform bacteria were not detected after treatment, corresponding to a detection limit-based lower-bound reduction of ≥2.69 ± 0.17 log10. Apparent fluence values were treated as engineering estimates rather than validated delivered dose. The results support UV-C sanitation as a preliminary enabling step for drainage solution reuse, while biodosimetry, untreated circulation controls, multi-stage seasonal sampling, full-season recirculation, and crop response validation remain necessary.

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