A Pilot-Scale Hybrid Adsorption–Biological Treatment System for Nitrogen Removal in Onsite Wastewater Treatment

Abstract

Onsite wastewater treatment systems (OWTS) are significant nonpoint sources of nutrients to surface and groundwater worldwide. Advanced OWTS are challenged by highly transient nutrient loads, long idle periods (e.g., during vacations), and inadequate maintenance. This study investigated the nitrogen transformation mechanisms and performance of novel two-stage hybrid adsorption and biological treatment systems (HABiTS) that combine biological nitrogen removal and ion exchange to enhance OWTS under transient loading conditions. In the first stage, the natural zeolite mineral, clinoptilolite, which has a high capacity and selectivity for NH4+, was included in a passively aerated nitrifying biofilter. In the second stage, recycled tire mulch, which has a high adsorption capacity for NO3−, was combined with elemental sulfur pellets in a submerged anoxic biofilter for autotrophic sulfur-oxidizing denitrification. Two pilot-scale HABiTS were tested for 434 days, with and without Stage 1 effluent recirculation and predenitrification. Both pilot-scale HABiTS removed >50% of total nitrogen from septic tank effluent. Stage 1 recirculation significantly improved NH4+ removal at both a 1∶1 (84%) and a 3∶1 (87%) recirculation ratio compared to HABiTS without recirculation (∼50%). Recirculation and predenitrification reduced the organic load to the nitrifying biofilter, resulting in increased nitrification and reduced clogging and maintenance requirements. Consistently low effluent NO3− and NO2− concentrations were observed throughout the study in both pilot HABiTS under all conditions applied. The results show that HABiTS is a low-cost, low-energy, and robust process that can consistently achieve advanced secondary OWTS standards under transient loading conditions.