DEVELOPMENT OF PARTIAL NITRIFICATION AS FIRST STEP OF NITRITE SHUNT PROCESS IN A SEQUENTIAL BATCH REACTOR (SBR) USING AMMONIUM OXIDIZING BACTERIA (AOB) CONTROLLED BY MIXING REGIME

Shorcut biological nitrogen removal is a non conventional way of removing nitrogen from wastewater using two processes either nitrite shunt or deammonification. In the nitrite shunt process, the ammonia oxidation step stops at the nitrite stage, which is known as partial nitrification, then nitrite is directly reduced to nitrogen gas. Effective partial nitrification could be achieved  by accumulating  Ammonia Oxidizing Bacteria (AOB) and inhibiting Nitrite Oxidazing Bacteria (NOB).

The excessive nitrogenous compounds withdrawn in water streams from the effluent of wastewater treatment plants causes numerous problems for the aquatic system as it leads to eutrophication causing the excessive growth of algae and increase in the oxygen depletion and poisons in  the aquatic life. To avoid the aforementioned negatifve effects, reducing this compounds level has been a manner of great importance either by physical-chemical processes or biological processes. Due to its higher efficiency and lower cost over physical-chemical processes, Biological Nitrogen Removal (BNR) processes have been adopted widely. Conventional BNR processes comprise two main practices: nitrification and denitrification. Nitrification is the aerobic biological conversion of ammonia to nitrate with oxygen as electron acceptor via a group of autotrophic bacteria through two steps involving Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB), respectively. However, these two steps conventional BNR processes require 2mol of oxygen to oxidize the ammonia to nitrate. Afterthough, the nitrate is reduced via heterotrophic bacteria to nitrite and nitrogen gas, which also requires organic matter during the denitrification stage.

Hence, conventional BNR processes require high oxygen and external carbon sources along with a slow growth of the autotrophic and heterotrophic bacteria. To overcome the aforementioned challenges and reduce  the energy required for nitrogen removal of side stream high ammonia waste streams, Shortcut Biological Nitrogen Removal (SBNR) has ebeen developed. Based on the fact that nitrite is an intermediat compound in both nitrification and denitrification, SBNR relies on the direct conversion of nitrite produced in the first step of nitrification to atmospheric nitrogen instead of oxidizing it to nitrate then reducing the latter back to the former.Shortcut Biological Nitrogen Removal implies the reduction of oxygen consumption during the aerobic phase by 25% as aresult of skipping the nitrite oxidation to nitrate and consequently reduces the total energy  required by 60%. Additionally, SBNR eliminates the use of external electrone donor  by 40%;  resulting from skipping the nitrate reduction to nitrite; which makes it suitable for wastewaterwith low carbon to nitrogen ratio. Shotcut Biological Nitrogen Removal also results in a significant decrease of the sludge production in nitrification and denitrification processes by 35% and 55%, respectively. The SBNR process comprises both nitrite shunt and deammonification processes. In the deammonification process, 50% of the ammonia is oxidized to nitrite  subsequently the remaining ammonia is oxidized   anaerobically to nitrogen gas  using the nitrite produced as electron acceptor carried out by Anaerobic  Ammonium Oxidizing  (Annamox) bacteria.

 

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