NOVELTY - A water treatment microplant comprises primary sedimentation compartment, anaerobic zone, aerobic zone, and secondary sedimentation zone in sequence. The primary compartment is divided by a partition to retain floating material and permit water flow to descend and ascend in the compartment. The anaerobic zone contains denitrifying biomass, and the aerobic zone contains nitrifying biomass.    USE - For treating wastewater from urban, industrial, and agricultural centers.    ADVANTAGE - The inventive treatment plant has dimension permitting the installation and operation at single family level and has margin of growth in case of increase in residual water unloading. It is economical in investment, and operation. Its treatment efficiently complies with the standard. It has minimal preventive and corrective maintenance conditions. It does not cause any inconvenience to the user e.g., bad smell, noise, and proliferation of insects and animals.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method of treating waste water comprising retaining water in a divided primary sedimentation compartment (1) to allow for sedimentation of suspended solids or for trapping fats, oils, and floating solids. The water is diversed from the sedimentation compartment into an anaerobic zone (7, 10) with biofilm of anaerobic and/or anoxic microorganisms to degrade organic matters and denitrify water. The water is diversed to an aerobic zone (11, 14, 15, 16, 17, 18, 19) containing heterophilic and autotrophic nitrifying aerobic bacteria to degrade organic matter and oxidize ammonia nitrogen in water. The water then is diversed to a second sedimentation compartment (24).    DESCRIPTION OF DRAWING(S) - The drawing shows the sequence of water treatment inside the inventive treatment plant.    Primary sedimentation compartment (1)    Anaerobic zone (7, 10)    Aerobic zone (11, 14, 15, 16, 17, 18, 19)    Air compressor (12)    Second sedimentation compartment (24)    Valves (29)