The numbers of people living legally in Bangkok City of Thailand has been estimated at 10 million, with about the same amount of people living there illegally. Thus, the total population living in the huge city of Bangkok is estimated at about 20 million population. This accounts for the daily community garbage of about 10,000 tons and wastewater of 4 MCM draining through 117 canals to Chao Phraya River and finally to the sea at the Gulf of Thailand. Due to the urban location on flatland there is one m MSL with the coverage area of 1200 km 2 . Despite a drainage system of 117 canals, the wastewater moves with difficulty from canals to the public Chao Phraya River. Then after that, the wastewater quality worsens with some evidences of black color and unpleasant smell. A key issue is that the water-borne diseases were found in every canal in the same degree as the urban drainage system. At the present time, the wastewater has been analyzed by many government agencies and they have found the same results as class 5 water quality, which indicated it could not be utilized for any purposes, except for transportation. In fact, the findings revealed that the wastewater elements could be identified as having high organic contents of BOD, COD, VOCs, total solids, suspended solids, very low dissolved oxygen, high concentration of heavy metals, etc. Such results traced the point sources as being the dense households and numerous numbers of active industrial factories. It could be seen that all those pollutants would contaminate Chao Phraya River and the Gulf of Thailand one way or another. Unfortunately, the lifestyle of Thai people depends upon the river water, meaning that the problems cannot be avoided. The above situation has been realized among Bangkok dwellers, especially H.M. the King Bhumibol, who introduced a means to treat such wastewater to better quality before draining it into public water sources such as the Chao Phraya River. In order to achieve the promising outcomes from an academic point of view, H.M. the King selected Bueng Makkasan Reservoir as the study area for wastewater inputs from the main canals and also from the repair factory belonging to the Royal Railways of Thailand. H.M. the King explained the study area of Bueng Makkasan pond as the kidney of Bangkok: It functions as a recycling system to treat wastewater so that they can be utilized for specific purposes such as crop cultivation, etc. In this recycling system, H.M. the King designated aquatic plants such as water hyacinth as withdrawing directly some toxic elements and decreasing indirectly organic substances. Fol- lowing treatment, the dead water hyacinth could be made into organic fertilizer. This paper was condensed from research reports in Thai original reports (printed during 1987–1990) as shown in the Appendix A. (1) to present the Royal knowledge of applying water hyacinth in terms of phytoremediation to extract the toxic elements in Bangkok wastewater. (2) to study the efficiency of wastewater treatment by water hyacinth. (3) to set aside water-hyacinth growing and harvesting in the wastewater reservoir for gaining the best benefits of biological treatment.