The new Hong Kong International Airport is located at Chek Lap Kok. Annual passenger and cargo throughput reached 44.4 million and 3.6 million tones in 2006, respectively, which is over 250,000 aircraft movements. Aircraft maintenance produces large amount of wastewater. Wastewater treatment is necessary for removing contaminants from wastewater, which including chemical process to remove chemical contaminants. PLC was used for monitoring and control the treatment processes.