A novel sequential mechanical filter system was developed as an alternative primary treatment method for onsite wastewater treatment. The filter combines traditional screening with a novel type of counter-flow filter using wood-shavings as a biodegradable filter matrix. This study tested the system in a batch loading regime simulating high frequency toilet flushing using blackwater from a student dormitory. The filter removed 78–85% of suspended solids, 60–80% of chemical oxygen demand, and 42–57% of total-P in blackwater, giving a retentate with a dry matter content of 13–20%. Data analysis clearly indicated a tendency towards higher removal performance with high inlet concentrations, hence, the system seems to be most applicable to blackwater or other types of wastewater with a high content of suspended solids.