Two-decade study shows recycled greywater can be safe for landscaping—with caveats
A 23-year Norwegian experiment proves hybrid wetlands can purify household wastewater enough for reuse in landscaping, eliminating dangerous pathogens below infection thresholds. But the system's ability to remove nutrients degrades over time, raising questions about long-term maintenance costs and media replacement for facilities scaling this approach globally.
Originaltitel: Long-term evaluation of a hybrid treatment wetland for greywater treatment and microbial risks of reuse in landscaping
<p>This study evaluated a 23-year-old hybrid treatment wetland (HTW) treating greywater from a residential building in Norway using historic monitoring (2001–2014) and recent sampling campaigns (2023–2024), representing one of the longest documented systems operating in a cold-climate setting. The system consisted of an aerobic vertical flow filter with Filtralite®, and an anaerobic horizontal flow filter with Filtralite®P for enhanced phosphorus removal. Recent findings (2023–2024) show improved organic matter removal, with >98% BOD reduction and effluent BOD consistently <2 mg/L. However, nutrient removal declined over time, with effluent total nitrogen of 3.3–5.6 mg/L (59–74%). Total phosphorus increased from 0.02–0.08 mg/L (2001–2008) to 0.15–0.45 mg/L (2014–2024), indicating partial exhaustion of the Filtralite®P media. The HTW achieved effective log reductions of 1.6–3.4 for E. coli, 2.1–3.4 for enterococci, 1.6–3.1 for Clostridium perfringens, and 2.0 for Pseudomonas aeruginosa. Legionella spp. remained below detection limits, and Campylobacter was not detected. A quantitative microbial risk assessment (QMRA) showed the annual risk of infection was <10<sup>−4</sup> for E. coli and Clostridium perfringens. The HTW met Norwegian discharge standards of BOD <20 mg/L, phosphorus <1 mg/L, E. coli <100 MPN/100 mL, and complied with EU standards for agricultural water reuse.</p>