Influence of the suspended and attached biomass on the nitrification in a two submerged biofilters in series system

Abstract

The performance and stability of a system formed by two submerged filters in series for removing organic matter, solids and ammonia nitrogen from urban wastewater were tested. The wastewater was pretreated through a fine screening (through a 1.5 mm sieve) and primary settlement, before entering the first filter. The hydraulic retention time (HRT) in both filters was adjusted to 2 hours. Superficial velocities of 1 m/h for the liquid and 8 m/h for the air were maintained throughout all the experiments. The averaged removal percentages of organic carbon and solids were the following: 84% of TSS, 85% of VSS, 88% of total COD, 86% of soluble COD and 93% of total BOD5. Filter backwash was conducted when the observed head loss through the filter raised up to 1.2 m of water column. Substratum particles were sampled from both filters for measuring the concentration of retained biomass in the two forms: attached (as Volatile Attached Solids, VAS) and suspended (as Volatile Suspended Solids, VSS) biomass. The activity of these solids was also determined by closed respirometry assays, which revealed that the biomass retained between the substratum particles as suspended biomass showed much higher activity that the attached biomass. The suspended biomass retained in the filter was responsible for removing most of the soluble COD and ammonia, while most of the nitrite oxidizers were inside the biofilm.