טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEpsztein Razi
SubjectGroundwater Hydrogenotrophic Denitrification in a
Pressurized Reactor
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Emeritus Michal Green
Full Thesis textFull thesis text - English Version


Abstract

Intensive use of fertilizers is the main source for groundwater contamination by nitrate (NO3-) observed worldwide. The common design approach for hydrogenotrophic denitrification is usually based on packed- or fluidized-bed reactors with H2 delivery scheme of continuous gas purging to the atmosphere in order to improve H2 transfer rates and enable discharge of N2 gas produced during denitrification. This operation results in a significant release of H2 gas to atmosphere with its related economic and safety concerns. The current research work proposes a novel pressurized high-rate hydrogenotrophic reactor for denitrification without gas purging, based on the hypothesis that during continuous operation a gas-liquid equilibrium is established in the reactor according to Henry’s law and excess N2 gas is carried out by the effluent in dissolved form. Therefore, no gas purging is required to discharge accumulating N2 gas and H2 loss is limited only to the dissolved H2 in the effluent. An unsaturated-flow pressurized reactor (UFPR) with water recirculation was chosen as the main topic of research.

 Reaching a gas-liquid equilibrium in the UFPR together with H2 utilization efficiencies above 92% and denitrification rates higher than most previously reported hydrogenotrophic denitrification rates were demonstrated for two effluent concentrations of 10 and 1 mg NO3--N/L. It was also shown that the residual dissolved H2 from the pressurized reactor can be further consumed by bacteria in a subsequent non pressurized up-flow submerged-bed polishing unit, thus increasing H2 utilization efficiency up to almost 100%.

 A mathematical model for predicting performance of the UFPR was developed and validated. A continuously stirred hydraulic regime could be assumed due to the relatively high recirculation flow rate required for efficient media wetting and the homogeneous gas phase in the closed reactor headspace, and simplified the model design for the UFPR. The reaction rate constant and the overall volumetric gas (H2)-liquid mass transfer coefficient (kLa) were determined for different recirculation flow rates at steady state.  A rate constant correction factor β was developed for different bulk concentrations of NO3--N and H2 to compensate for phenomena such as pH changes within the biofilm, deviation from intrinsic zero-order degradation kinetics and non-homogeneity of the biofilm. Final model results (with β) were based on single-step iteration and required a preliminary assessment of the bulk concentrations by the model (without β) in order to incorporate the adequate β value. Using the model, it was shown that high denitrification rates of up to 7.5 g NO3--N/(Lreactor•d) together with H2 utilization efficiencies above 90% can be achieved by the UFPR.

Comparison between the UFPR and a submerged-bed pressurized reactor (SBPR) with gas recirculation showed lower denitrification rates in the SBPR due to lower active biofilm surface area and kLa values in the SBPR. Moreover, effluent suspended solids concentration in the SBPR was substantially higher. On the other hand, in the SBPR the cleaning frequency was 2.5 times lower and gas recirculation is expected to reduce recirculation energy consumption by 0.35 kWh/m3 treated.