Microbial Activities and Process Rates in Two-Step Vertical and Horizontal Subsurface Flow Gravel and Sand Filters

Abstract

Specific (nitrifying, denitrifying, and methanogenic) microbial activities were measured in simplified down-flow lab-scale columns simulating subsurface horizontal flow (HF) and vertical flow (VF) constructed wetlands (CWs). Extractable volatile solid (EVS) was 6.5-fold higher in VF system than in HF system. Potential nitrification rate was similar in both systems (2.6–3.2 g N/m2 day), while specific nitrifying activities were higher in the HF system (29–48 mg N/g EVS day) than in VF system (3.9–4.7 mg N/g EVS day), indicating a higher relative abundance of nitrifying bacteria in HF biofilm. Potential denitrification rates were 13–19 (HF) and 1–4 (VF) g N/m2 day and specific denitrifying activities were 178–195 (HF) and 2.3–5.6 (VF) mg N/g EVS day. However, VF units exploited better the potential nitrification and denitrification rates by removing significant amounts of nitrogen. Specific methanogenic activity was null in VF units but high (0.13–0.20 g COD-CH4/g EVS day) in HF units.