1. Research Background and Technical Necessity
Global water demand continues to grow at an average annual rate of about 1%. Combined with worsening water pollution and hydrological irregularities caused by climate change, droughts have become increasingly frequent in the Mediterranean region, putting rigid constraints on water supply. Taking Spain as an example, 2022 was recorded as the country’s third driest year in history, with annual precipitation about 26% lower than the multi‑year average, thus facing severe water shortage.
Against this background, water reuse and rainwater harvesting (RWH) have been listed by UNESCO as core sustainable supply strategies to cope with the global water crisis. Among them, rainwater reuse can replace up to 38% of freshwater consumption, providing an important pathway to relieve regional supply pressure .
High‑flow water treatment scenarios such as the continuous effluent of municipal wastewater treatment plants typically contain high concentrations of pathogenic microorganisms (e.g., Escherichia coli), which pose key risks of waterborne disease transmission. According to the EU Regulation (EU) 2020/741 and relevant Spanish regulations, reclaimed water used for agricultural irrigation or urban non‑potable purposes must keep E. coli and Enterococcus counts below 10 CFU/100 mL, while drinking‑water‑grade reuse must meet the strict requirement of 0 CFU/100 mL.
Although chlorine disinfection is widely used, it easily generates disinfection by‑products (DBPs) such as trihalomethanes, which pose environmental and health risks . In contrast, UV‑C (200–280 nm) physical disinfection — characterized by its absence of by‑products, broad‑spectrum high efficiency, and instantaneous inactivation — has become a preferred technology for high‑flow water treatment.
Compared with conventional mercury lamps, UV‑C LEDs — the new generation of ultraviolet light sources — offer higher energy utilization efficiency, flexible spectral control, and longer lifespan. They show remarkable potential under demanding conditions featuring high flow and short hydraulic retention times, making them a promising solution to overcome the bottlenecks of traditional disinfection technologies and to build safe, green, and efficient water disinfection systems.
2. Mechanism of UV‑C Disinfection
2.1 Microbial Inactivation Mechanism
UV‑C LED disinfection inactivates pathogenic microorganisms through wavelength‑specific photochemical reactions. Its mechanism features precise targeting and multi‑level synergy. Unlike traditional broad‑spectrum mercury lamps, LEDs allow narrow‑band emission precisely aligned with the target germicidal wavelengths, maximizing inactivation efficiency.
The 270 nm UV‑C band coincides with the absorption peak of microbial DNA and RNA bases. Photons in this wavelength penetrate cell walls and membranes, reaching nucleic acids and initiating photochemical reactions that form cyclobutane pyrimidine dimers and pyrimidine photohydrates between adjacent pyrimidine bases. This blocks microbial DNA replication and transcription, fundamentally suppressing microbial proliferation and achieving irreversible pathogen inactivation — the core mechanism of UV‑C disinfection.
Meanwhile, the 280 nm band exerts a synergistic effect by targeting structural proteins in cell walls and membranes as well as intracellular functional enzymes. It damages aromatic amino acid residues such as phenylalanine, tryptophan, and tyrosine, causing protein denaturation and structural collapse that disrupt cell integrity and metabolism. Furthermore, this wavelength inhibits synthesis and activation of photolyase and excision‑repair enzymes, thereby preventing photoreactivation and ensuring long‑term inactivation .
2.2 Microbial Inactivation Requirements for High‑Flow Conditions
Under high‑flow secondary effluent conditions, the first‑order kinetic inactivation rate constant of total coliforms and E. coli by UV‑C LEDs is about 1.4 times that of traditional low‑pressure mercury lamps. At the same UV dose, UV‑C LEDs achieve faster microbial inactivation, better meeting engineering requirements for short hydraulic retention times in large‑flow systems .
According to the Technical Code for Disinfection of Urban Wastewater Treatment Plants (CJJ 125‑2017) , typical design UV doses for different objectives are as follows:
Secondary effluent discharge standard: 20–25 mJ/cm²
Reclaimed water for urban non‑potable use: 40–60 mJ/cm²
High‑safety reuse applications: ≥ 80 mJ/cm²
3. Comparison of UV‑C Light Sources for High‑Flow Water Treatment
3.1 Traditional Mercury Lamp Systems
Low‑ and medium‑pressure mercury lamps remain the mainstream disinfection technologies for municipal high‑flow wastewater treatment. They feature high luminous efficiency and mature engineering applications but possess several inherent drawbacks:
Mercury toxicity and environmental risk: Mercury content poses disposal and leakage hazards that threaten ecosystems via air, water, and soil contamination, violating the Minamata Convention on Mercury.
Start‑up and lifespan limitations: Require preheating; frequent on/off cycles accelerate degradation. Operating life is only 8000–12000 hours, leading to higher maintenance costs and downtime risks.
Low spectral efficiency: Medium‑pressure lamps emit broad‑spectrum light, with only 250–265 nm being germicidal; energy at non‑germicidal wavelengths is wasted. Low‑pressure lamps emit mainly at 253.7 nm but still have energy loss in non‑target bands.
3.2 UV‑C LED Technology
Semiconductor‑based ultraviolet light‑emitting diodes (UV‑LEDs), primarily made from gallium nitride, represent a highly promising UV radiation source with unique engineering value in high‑flow systems [2, 5–7]:
Instant on/off without preheating; full output achieved immediately.
Precise wavelength control within the 265–280 nm germicidal range.
Frequent switching without shortening lifespan.
Modular integration compatible with open‑channel or pipeline reactors.
Environmentally friendly, mercury‑free, and lower lifecycle carbon footprint.
4. Conclusion
Water disinfection is a critical defense line for preventing the spread of waterborne pathogens and safeguarding water environment safety. In applications such as industrial water treatment, centralized drinking water supply, pressurized circulation systems, and large‑scale municipal wastewater treatment, increasing treatment capacity, complex operating conditions, and higher green‑low‑carbon demands are driving the need for new disinfection technologies that combine high‑flow adaptability, pressure stability, environmental safety, and energy efficiency.
Although traditional low‑ and medium‑pressure mercury lamp systems are technically mature, their limitations — mercury pollution risks, short service life, low spectral efficiency, and poor start‑stop characteristics — hinder long‑term sustainable operation. In contrast, UV‑C LED technology offers superior inactivation kinetics, flexible optical control, and mercury‑free environmental advantages. It shows great potential for high‑flow, short residence‑time conditions and represents an important upgrade direction for next‑generation municipal and industrial water disinfection systems, supporting the creation of safe, efficient, and sustainable water environments.
References:
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