CHtank is an underground wastewater treatment system designed for decentralized domestic applications, with a treatment capacity of 0.6–5 m³/day. Featuring multi-stage A/O and simultaneous nitrification and denitrification (SND) processes, it delivers reliable nitrogen removal even under fluctuating influent conditions. Its compact, corrosion-resistant HDPE tank offers a service life of up to 30 years while preserving above-ground space. Integrated electrochemical phosphorus removal eliminates the need for additional chemical dosing, while flexible multi-tank configurations enable scalable treatment capacity. With sludge removal required only every 6–12 months, CHtank provides a durable, low-maintenance solution for decentralized wastewater treatment.