Low Energy Biological Nitrogen Removal by Cation Exchange, Thin Film Oxygen Transfer, and Heterotrophic Nitrification in Sequencing-Batch, Packed-Bed Reactors

Sustainable solutions for biological nitrogen removal (BNtR) in wastewater require new treatment process concepts to lower energy requirements. A novel configuration of six packedbed, flood and drain (PFAD) reactors were used in a pilot study to remove nitrogen from a manufactured wastewater comprised of urea, cheese whey, and well water. The reactors operated in a sequencing-batch manner in alternating sets. Oxygen transfer in the PFAD system is mostly via nitrification in drained phases of ammonium ions that adsorb to manufactured aggregate in flooded phases. This method of oxygen transfer is substantially more energy efficient for nitrogen removal than activated sludge. The PFAD system is estimated to have approximately 20% of the energy requirement of a corresponding activated sludge treatment system using the modified Ludzack-Ettinger (MLE) process.Nitrogen removal in the PFAD system is dominated by bacterial mechanism that are inherently more energy efficient than conventional processes. Flood and drain cycles induce frequent large swings in dissolved oxygen concentrations (zero to saturation) and oxidation reduction potential (−200 to +600 mV) in PFAD biofilms. Bacteria in biofilms encounter organic carbon, ammonia oxidation products, ammonia, and molecular oxygen either all at once or in close temporal proximity. Under these conditions in a pilot study, heterotrophic nitrification and aerobic denitrification dominated nitrogen removal over autotrophic nitrification and facultative denitrification, as determined by quantitative fluorescence in-situ hybridization (FISH) probe analyses conducted 22 month apart. The population of Paracoccus denitrificans, a heterotrophic nitrifier, increased (50%) at the expense of all other nitrogen cycle bacteria. Autotrophic ammonia and nitrite oxidizing bacteria decreased by 68% and 63%, respectively. Divisions β- and γ-proteobacteria, in which all facultative denitrifiers had previously been found, decreased by 68 and 58%, respectively. The mean nitrification rate in the pilot was 95% at ammonia loadings up to 104 g TKN/ m3 aggregate·d. Total nitrogen removal averaged 67%. The maximum observed total nitrogen removal in the pilot was 92% at an influent TKN loading of 104 g TKN/ m3 aggregate·d. In a parallel column study, 99% of total nitrogen was removed used an organic carbon electron donor with nitrite as the apparent terminal electron acceptor. Nitrite oxidation to nitrate occurred only after organic carbon was consumed. This finding is consistent with heterotrophic nitrification, which also oxidizes ammonia directly to nitrous oxide under anoxic conditions. The novel operation and oxygen transfer mechanism of PFAD reactors are supported by unconventional process microbiology. Integrated knowledge of these mechanisms will be key to optimizing process control of the PFAD for total nitrogen removal.