The article studies the occurrence of microplastics in the inlet and outlet wastewater streams at wastewater treatment plants in the Saigon-Dong Nai river basin, Vietnam and provide a suitable removal solution. The sampling method is suitable for the actual conditions of Vietnam combined with the application of Fourier-transform infrared spectroscopy to analyze the microplastic composition in the sample. The results show that microplastics exist in many different shapes and colors. Density of microplastics in the inlet stream is from 10.188-15.074 gL -1 . Density of microplastics in the outlet stream is from 0.684-2.107 gL -1 . In which, filaments with an average length of 524.68 μm and an average radius of 100.4 μm; slender form with an average length of 229.49 μm and an average width of 101.3-120.6 μm; granules with an average radius of 113.81μm. The removal efficiency of microplastics in the wastewater stream at the surveyed wastewater treatment plants ranges from 85.4% to 93.7% through the following main processes: pre-settlement, flotation, moving bed biofilm reactor, sedimentation, filtration. Solutions for the removal of microplastics from wastewater treatment plants in the Saigon -Dong Nai river basin were proposed and discussed.

The study was conducted to evaluate 5 main river basins: Long Song River, Luy River, Cai Phan Thiet River, Ca Ty River and Phan River in Binh Thuan province has a great impact on the province's socio-economic development. By method of survey, sampling and analysis of physico-chemical parameters and WQI method with hydrological regimes, typical minimum flow, the research has carried out the following contents: (1) An overview analysis of the water quality situation in the study area in the years 2018-2020;(2) Calculating the WQI index to determine the overall status of the water environment of 5 river basins; (3) Determine the environmental flow that needs to be maintained in the river to ensure the daily activities and production of people in the area. The research results have determined the current status of the water environment in rivers and canals and assessed the responsiveness of water sources to socio-economic activities in the locality. From there, propose solutions to effectively use surface water in river basins of Binh Thuan province in the direction of sustainable development.

In the karst regions of Vietnam, there are many different terrains. The most important one is karst polje. Because there are a lot of residential areas and important economic regions. Based on the study of Geomorphology, geological structure analysis, Geophysics document analysis, and karstification process research at Lang Son karst polje. The author gives conclusions: Lang Son karst polje was formed from the limestone of Bac Son Formation, 700 – 800 m thick. Lang Son karst polje was formed from anticline structure, the limestone of Bac Son Formation that has high karstification level. Lang Son karst polje has only one karst aquifer which is a water supply source for residential areas in Lang Son city. This karst aquifer distributes in limestone blocks of the Bac Son Formation. Geohazards on Lang Son karst polje include collapse, sinkhole, flood, and pollution of water. Among them, the most dangerous one is the collapse of the underground karst caves.