Comparing the effect of carbon media on nutrient removal and greenhouse gas production in laboratory-scale bioreactors

Abstract

The performance of locally available agricultural carbon media (barley straw and hemp straw) was compared to woodchips for removing nitrate (NO3-N) and orthophosphate (PO4-P) in up-flow laboratory bioreactors. These media were tested in three replicates to quantify variability. The production of greenhouse gases nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) were quantified. Influent water with NO3-N and PO4-P flowed continuously through bioreactors at a 4-h hydraulic retention time at 20 °C for 16 weeks. Nitrate removal reached up to 37% across all carbon media after the fifth week, with a removal rate of 64 g N m−3 d−1. Nitrate removal was affected by the type of carbon media in the order of barley straw > hemp straw > woodchips (P < 0.05). Most of the PO4-P rates declined rapidly after the first week for all carbon media meaning none were superior. Greenhouse gas production was dominated by CO2 with less CH4 and N2O produced. Production of N2O relative to nitrate removal for the three media remained low at 0.16 to 0.75%. The findings suggest that agricultural residues could perform better than woodchips for NO3-N removal although there was slightly higher N2O and CO2 production for these residues than woodchips. HIGHLIGHTS Barley straw was more effective in reducing nitrate compared to hemp straw and woodchips.; PO4-P removal efficiencies and rates declined rapidly for the three carbon media.; Greenhouse gas production was dominated by CO2 with less CH4 and N2O produced.; Production of N2O relative to nitrate removal for the three media remained low.;

Keywords

• agricultural residues
• bioreactors
• greenhouse gases
• nitrate
• orthophosphate
• woodchips