Evaluation of sustainable biofilter for septic tank discharge: lessons from ancient biofilter techniques

ABSTRACT On-site wastewater treatment systems require reliable polishing processes to reduce organic matter, suspended solids, and nitrogen species in septic tank effluent before discharge or land application. This study evaluated alternative granular biofilter media, including 2A sand, crushed glass, biochar, and a crushed-glass/biochar mixture, using vertical PVC laboratory columns designed to simulate filtration through a discharge-control trench. Primary screened septic tank effluent from a residential onsite wastewater system in Wainui, Akaroa, New Zealand, was applied to columns containing 600 mm of media at a hydraulic loading rate of 25 Lm−2d−1. The investigation was conducted in three non-concurrent phases: Phase I (2019) evaluated 2A sand and crushed glass, Phase II (2023) introduced biochar, and Phase III (2025) assessed the crushed-glass/biochar mixture. Effluent quality was monitored using pH, turbidity, electrical conductivity, biochemical oxygen demand (BOD5), and nitrate-N. Because the media were not operated concurrently and independent replicated columns were unavailable across all phases, the results are interpreted as a phased descriptive evaluation rather than a statistically replicated comparison. All media improved effluent quality, achieving BOD5 removal rates of 92–98% and nitrate-N reductions of 60–89%. Crushed glass produced treatment outcomes comparable to 2A sand, while biochar achieved the lowest final nitrate-N concentrations.