Ulva lactuca is a green alga that is popularly referred to as "sea lettuce." It is a member of the Chlorophyta division. It has been identified as a possible biofilter capable of assimilating nitrogen and phosphorus from seawater. This work established the viability of a flow system utilizing U. lactuca by developing a system performance model that met the ASEAN Marine Quality Criteria (AMQC) for nitrogen and phosphorus effluent concentrations. The performance model was validated using three treatments in a series of laboratory studies. The biofiltration system has been proven to be effective at removing nitrogen and phosphorus from seawater, with an average removal efficiency of 67 percent and 68 percent, respectively. The nitrogen and phosphorus concentrations in effluent were found to be significantly different from those in AMQC. However, biofiltration processes appeared to exhibit a decreasing decay ratio over time, implying that AMQC may be maintained when the process time is increased, allowing U. lactuca more time to recover from the stress induced by its new environment. A three-stage biofiltration system with enhanced acclimatization and residency time could significantly improve the system's effectiveness.