Denitrification kinetics and stoichiometry of a moving bed biofilm reactor using methanol as an external carbon source

Wastewater treatment facilities discharging into Chesapeake Bay are facing Limit of Technology (LOT) standards to reduce their nitrogen discharge to more stringent levels (3-4 mg/L as Total Nitrogen (TN)). The Moving Bed Biofilm Reactor (MBBR) technology was determined to be a viable option for denitrification to achieve total nitrogen of between 3-4 mg N/L and nitrate concentration below 1 mg N/L. In an attempt to explore an effective, affordable and applicable technological alternative that can help meet the criteria, DC Water and Sewer Authority (DC WASA) operated a pilot scale post-denitrifying MBBR process from January 2008 through June 2008 as a dentirifcation polishing step downstream of the existing nitrification and denitrification activated sludge system.;Ex-situ Bench scale kinetic evaluation of MBBR system using methanol as an external carbon was conducted on the media grown in the pilot reactor throughout the pilot period. The primary objective was to better understand the denitrification kinetics and to determine kinetic and stoichiometric parameters for nitrogen removal of such system. Mainly, four parameters: specific denitrification rate (SDNR), half saturation constants (Ks) of limited NOx-N and Chemical Oxygen Demand (COD) and biomass density were measured over a range of temperatures of four individual reactors namely T1R1, T1R2, T2R1 and T2R2 that resembled the small scale pilot reactors. Stoichiometric Carbon to Nitrogen (C/N) ratio was also estimated for this system using COD and NO3-N values of SDNR test.;Average SDN rate of reactors T1R1, T1R2, T2R1 and T2R2 was estimated as 1.67 g NOx-N/m2/d, 1.47 g NOx-N/m2/d, 1.50 g NOx-N/m2/d and 1.28 g NOx-N/m2/d respectively expressed in terms of total biofilm carrier area. The SDNR expressed as g/m 2/d exhibited very little relationship with temperature suggesting a resilience of the overall process to temperature changes. SDN rate expressed in terms of g NOx-N/g biomass/d were estimated as 0.07-0.18 for T1R1 at a temperature range of 12°C-20°C; 0.10-0.20 for T1R2 and 0.07-0.13 for T2R2 at a temperature range of 13°C-18o°C, and 0.07-0.12 for T2R1 at a temperature range of 12.5°C-20°C. The SDNR expressed as g NOx-N/g biomass-d was observed to decrease with decreasing temperature suggesting an Arrhenius relationship and the Arrhenius constant (theta) was calculated as 1.08 for a temperature range of 12°C - 20°C.;Similarly, stoichiometric COD/N ratio was estimated in the range of 4.6 mg/mg to 5.3 mg/mg, 4.4 mg/mg to 6.1 mg/mg, 4.3mg/mg to 6.3 mg/mg and 4.3 mg/mg to 5.9 mg/mg for T1R1, T1R2, T2R1 and T2R2 respectively. This range is similar to the range observed for denitrification in activated sludge suspended growth process.;A kinetic model was developed based on the non-linear Monod model and operated on a Microsoft Excel Platform to estimate Ks. The model predicted Ks values for limited NOx-N between 0.61-2.55 mg N/L and 0.64-2.60 mg N/L, and for limited COD between 6.3-12.7 mg/L and 4.4-12.3 mg/L for T1R1 and T1R2 reactors respectively. These values seemed insensitive to change in temperature with a weak relationship with biomass density. Biomass Density values ranged widely from 6-21.6 g/m2, 4.37-16.52 g/m2, 7.46-20.48 g/m2 and 5.39-17.65 g/m2 for T1R1, T1R2, T2R1 and T2R2 reactors respectively for a temperature range of 11°C-23.5°C. The test indicated increase in biomass density with decrease in temperature.;The findings of this study are useful for the treatment plants that are considering to upgrade their facility to meet the stringent nutrient discharge limit. However, the testing in this research was limited to methanol only. Study of MBBR system on carbon sources other than methanol could complement the application of this technology in other existing as well as new treatment plants that use different carbon source.