Abstract: The invention relates to a system and method for treating wastewater onboard a marine vessel or other limited space application where wastewater treatment is required. In one embodiment of the invention, two polar opposite wastewater treatment processes are incorporated into the same treatment system. The system is a hybrid treatment unit that combines a conventional wastewater treatment using a moving bed biofilm reactor (MBBR) process-based biological treatment unit in combination with a novel electrochemical advanced oxidation process (EAOP) via an in-situ sodium hypochlorite recirculation. The clarified and disinfected effluent from this hybrid treatment unit is filtered to remove residual TSS. The filtered and disinfected effluent is discharged after dechlorination/neutralization. On-site maintenance disinfectant can be produced via the electrochemical/electrochlorination/electro-oxidation process and used for both treatment unit cleaning and disinfection.