The Lhasa Najin Water Treatment Plant (WTP) was built in 2017 at total cost of 1.38 billion CNY (approximately 213 million USD) and has been designed to treat an average flow of 480,000 m3/day. Located in the Tibetan Plateau in the Tibet Autonomous Region of China, this plant is the world’s biggest water treatment plant treating surface water in the area, and it is also the world’s highest water treatment plant using UV disinfection at an altitude of 3,658m!

The plant can meet the daily water demand of 1 million people in the central area of the Plateau near the Himalayas. The plant provides water to many districts in Lhasa such as Chengguan District, Liuwu New District and the Lhasa Economic and Technological Development Zone, among others. The construction of the Najin WTP has solved many problems of the original water treatment plant, which provided insufficient water supply and only used conventional water treatment processes. The upgraded plant adopts a combined UV-chlorine disinfection technology which inactivates disease-causing microorganisms while also reducing the amount of various disinfection byproducts in the water quickly and efficiently, thus ensuring clean and safe water for the communities of Lhasa.

An in-pipe medium pressure UV system (Onyx-WH-12) with the input power of 72 Kw and the output power of 10.8 Kw is used throughout to the plant. The system has a photoelectric conversion efficiency of 15% and an effective dose of more than 40 mJ/cm2 to reach 4-log microbial inactivation rate. In addition, the service life of the medium pressure UV lamp selected for the UV system can last more than 9000 hours and the daily operation & maintenance cost of the UV system is less than 0.02 CNY/ton of water, making this approach extremely efficient and cost-effective.

After nearly 20 years of cooperative research and development efforts in UV disinfection technology, Tsinghua University, Beijing Onyx Environmental, and Research Institute for Environmental Innovation (Suzhou), Tsinghua have developed a number of domestic innovations which are also world-leading. For example, some of the technologies developed are:

3rd generation reactor using CFD simulation technology guaranteeing fast, efficient and thorough sterilization of systems;

ballast with double cooling mode which can be intelligently controlled based on the water quality and quantity to ensure the long-term stable operation of systems;

an online automatic cleaning system with a spiral cleaning structure which can fully guarantee cleaning and can maintain the casing transmission rate above 90% for a long period of time, and as a result, a stable UV dose output can be achieved;

Level-2 equipment protection system which prevents overheating of the surface temperature during the operation of medium voltage equipment;

Internet of Things (IoT) cloud control technology which allows the central control room and mobile phone apps to monitor the treatment processes in real-time and off-site.

The UV disinfection process adopted by Lhasa Najin WTP is an ideal example of cooperation between academia and industry, undertaking independent and innovative research & development to improve access to clean and safe water. In recognition of this, the project won the 2019-2020 honorary certificate of “Outstanding Solutions in Water Supply Industry – Quality Water Supply Providers” from the E20 Environmental Platform and www.lccrgm.cn.

Figure 1 Flow chart of water treatment process in Lhasa Najin Water Treatment plant

Figure 2 Photo of UV disinfection systems in Lhasa Najin Water Treatment plant

編輯：趙凡