Sewage testing played a crucial role in combating COVID-19 in Hong Kong, with a multidisciplinary team led by Professor Tong Zhang of the Department of Civil Engineering developing a COVID-19 sewage surveillance system. At the moment, the systematic routine sewage monitoring programme of the Environmental Protection Department (EPD) and the Drainage Services Department (DSD) covered 112 regular sampling sites, providing early warning signals for over five million people. The team also developed a method for identifying and quantifying different SARS-CoV-2 variants using allele-specific RT-qPCR technology. This method had a shorter turnaround time and served as a rapid screening tool for quick follow-up actions compared to genomic sequencing. Improvements to the sewage surveillance system included developing new sampling methods, simplifying sample pre-treatment procedures, increasing capacity, and integrating clinical and epidemiological data.