With the rapid advancement of technology and the increasing environmental challenges, efficient waste management and energy harvesting solutions are crucial for sustainable development. This study presents a dual-mode triboelectric nanogenerator (TENG) powered by liquid droplets, developed for both energy harvesting and environmental monitoring. The dual-mode TENG leverages the charge transfer mechanism at the solid-liquid interface during contact. Through the integrated system comprising a solid-liquid triboelectric raindrop energy harvester (RD-TENG) and a contact-separation mode TENG, it enables the conversion of raindrop kinetic energy into electrical energy.  By optimizing the droplet impact frequency, height, and the tilt angle of the contact substrate, the energy conversion efficiency of the dual-mode TENG can be readily enhanced. The RD-TENG demonstrated an output variation according to pH changes, making it viable as both an energy harvester and a pH sensor. It can also effectively detect variations in rainfall intensity and frequency, providing a self-powered solution for continuous environmental monitoring. Notably, we have explored to use recycled milk cartons as substrates for TENG device, promoting sustainable (or waste) materials utilization in energy harvesting systems. Further optimization led to the development of an array of dual-mode TENG units, which significantly improved output power and capacitor charging rates. The system was tested under simulated rainfall conditions, demonstrating its ability to power portable devices such as calculators and hygrothermographs. Additionally, a machine learning model was employed to classify rainfall intensity based on triboelectric signals, realizing high classification accuracy. This research illustrates the potential of integrating TENG technology with environmental monitoring systems, providing a sustainable solution for both energy harvesting and waste recycling.