UV-LEDs for drinking water disinfection in the Netherlands as an alternative to mercury-based lamps

Abstract

The EU is progressively restricting the use of mercury to protect human health and the environment. Consequently, Dutch water utilities must find an alternative to their mercury-based UV disinfection systems before they are permanently banned. UV-LEDs are a promising replacement. However, their feasibility for full-scale water disinfection in the Netherlands remains unclear due to questions about their germicidal performance, required surface area, energy efficiency, and cost-effectiveness. To assess whether a simulated UV-LED reactor could match the performance of a real-world UV reactor with respect to these parameters, three mathematical models based on two reactor designs were optimized in Microsoft Excel. The reactor designs researched were the circular cylinder reactor (CCR) and the square prism reactor (SPR). The CCR consists of a series of meandering flow channels through which UV light propagates parallel to the flow direction. The SPR consists of a single square pipe in which UV light is emitted from the lateral walls of the flow channel, perpendicular to the flow direction. The three models are described below: • CCR: average fluence of 400 J/m2, assuming that fluence is not significantly affected by flow mixing. • SPR-1: average fluence of 400 J/m2, assuming that the flow is well mixed. • SPR-2: fluence of 400 J/m2 at the center of the cross section, and an average fluence greater than 400 J/m2. This model assumes that the flow is not well mixed. The optimized models were benchmarked against three full-scale Dutch or Flemish mercury-based UV reactors currently in use. The modeling results show that the SPR-1 is the most efficient of the reactor designs researched, as it delivers its target fluence with the overall best performance in terms of number of reactors, required surface area, energy efficiency, and cost of UV-LEDs. The optimal width for this kind of UV reactor ranges from 30 cm for UVT10 between 70%–85%, to 50 cm for UVT10 between 86%–92%. Although the SPR-1 does not match the energy efficiency of every real-world reactor considered for benchmarking, it would require the least technological development in wall-plug efficiency (above 15%) to do so. In conclusion, UV-LED technology is robust enough to at least be piloted in small full-scale (Q ≤ 1,000 L/s), high UV transmittance (UVT10 ≥ 85%) drinking water treatment plants (DWTPs) in the Netherlands. As the technology evolves, UV-LED reactors could be scaled up and installed in other Dutch DWTPs, until mercury-based UV lamps are completely replaced.