Evaluation of a Biochar Enhanced Constructed Treatment Wetland for the removal of Contaminants from Agricultural Wastewater

Nutrient-rich wastewater runoff from concentrated animal feeding operations (CAFO’s) is one of the largest sources of contamination of surface waters in the US and poses adverse ecological and human health effects. This study sought to evaluate a vegetated biochar system for passive treatment of CAFO wastewater. A wood biochar, pyrolyzed at 500°C was first characterized for its adsorption of ammonia (NH3), nitrate (NO3) and phosphate (PO43-) in batch sorption tests. It adsorbed up to 0.26 mg NH3 per g biochar (12% of the solution), but no NO3- or PO43-. A greenhouse experiment was set up with four 120 L simulated constructed wetland tanks as follows: T1 - 100% biochar planted with cattails, soft rush, parrots feather and knotweed; T2 - 50% biochar, 50% pea gravel and the same plants; T3 - 100% biochar and no plants; and T4 - pea gravel and plants.  Diluted swine waste was applied to each tank at 1 L per hour (residence time: 67 hours). The influent and effluent were analyzed for NH3-N, NO3--N, PO43-. In spring, using influent concentrations of ~100 mg/L NH3-N, T1 removed an average of 37% NH3-N, T2 25%, T3 24% and T4 only 14%. Influent NO3--N (~1.0 mg/L) was removed on average as follows: 9.2% (T1), 18% (T2), 17% (T3) and 5.4% (T4) and for PO43- (~20 mg/L): 78% (T1), 70% (T2), 28% (T3) and 60% (T4). Cattail growth was the greatest in T1 and the least in T4. The onset of winter and the introduction of concentrated influent killed the cattails in T4 whereas in T1 and T2 only ~25% and ~50%, respectively, died. These findings show that incorporation of biochar into the substrate of constructed wetlands media can significantly increase their treatment of agricultural wastewater. Additional studies using higher quality biochar could yield even better results.