Impact of the herbicide (RS)-MCPP on an anaerobic membrane bioreactor performance under different COD/nitrate ratios

The degradation of (RS  )-MCPP was investigated in an anaerobic membrane bioreactor (AnMBR) using nitrate as an available electron acceptor under different COD/NO3-–N ratios. Results showed high soluble COD removal efficiency (80–93%) when the reactor was operated at high COD/NO3-–N ratios. However, the COD removal started to decline (average 15%) at high nitrate concentrations coinciding with a drop in nitrate removal efficiency to 37%, suggesting that the denitrification activity dropped and affected the AnMBR performance when nitrate was the predominant electron acceptor. Additionally, the removal efficiency of (RS  )-MCPP increased from 2% to 47% with reducing COD/NO3-–N ratios, whilst the (RS  )-MCPP specific utilisation rate (SUR) was inversely proportional to the COD/NO3--N ratio, suggesting that a lower COD/NO3--N ratios had a positive influence on the (RS  )-MCPP SUR. Although nitrate had a major impact on methane production rates, the methane composition was stable (approximately 80%) for COD/NO3-–N ratios of 23 or more.

► At low COD/NO3-–N ratio, COD removal declined coinciding with a drop in nitrate removal efficiency.
► Reduced COD/nitrate ratio had a positive influence on the (RS)-MCPP specific utilization rate.
► (RS  )-MCPP degradation increased from 2% to 47% with reducing COD/NO3-–N ratios.