NITRIFICATION AND DENITRIFICATION

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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The excessive nitrogenous compounds withdrawn in water streams from the effluent of wastewater treatment plants causes numerous problems for the aquatic system as it leads to eutrophication causing the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.

 

START-UP AND MICROBIAL COMMUNITIES OF A SIMULTANEOUS NITROGEN REMOVAL SYSTEM FOR HIGH SALINITY AND HIGH NITROGEN ORGANIC WASTEWATER VIA HETEROTROPHIC NITRIFICATION

With the rapid development of economy of the Three Gorges Resevoir area, a large amount of high salinity and high nitrogen organic wastewater about 3,5 million m3 per year is discharged by The Fuling Mustard Tuber Industry. It poses a serious threat to the water environtment of reservoir.

High salinity is a key problem that affects the processes of biological treatment  for Fuling Mustard Tuber waste water. In recent years, the effects of salinity on the activities of microorganism in bioreactors were studied. The studies have showed that high salinity in wastewater typically dampens the degrading enzymes and decreases cell activity, can even cauuse cell plasmolysis, and its also have negative effects on organic removal. Generally, high salinity wastewater needs to be diluted before treating , it means the long process flow and low operation load which demand gigantic investment and high operating cost. Consequently, building a microbial communityof salt-tolerant halophilic microorganisms that can improve the adaptability of system to high salinity is the key to treat the high salinity wastewater.

How to remove the nitrogen high salinity and high nitrogenorganic wastewater is another key problem. Traditionally, the nitrogen removal process were based on the autotrophic nitrification and heterotrophic denitrification. But high salinity wastewater can effect the conventional processes of ammonium oxidation, nitrification and denitrification. the traditional process of autotrophic nitrification was inhibited by organic matter.Therefore, the simultaneous removal efficiency of nitrogen and COD was low, Meanwhile, the low organic loading of outotrophic nitrification reactor can lead to large reactor volume, high investment and high operating cost. With the proposed concept of the heterotrophic nitrification and aerobic denitrification, heterotropic nitrification bacteria gradually was known by the public. Comparing with traditional nitrogen removal methods, neterotrophic nitrification has swveral advantages. Heterophic nitrification bacteria can remove COD and nitrogen simultaneouslyin one reactor via simultaneous nitrification and denitrification. Heterotrophic nitrification bacteria have the ability to utilize different kinds of materials. And it is conducive to coexist with other strains. Meanwhile, some species of heterotrophic nitrification bacteria have the charactheristic of high level ammonia resistant.These features of heterotrophic nitrification bacteria have great significance for treating the high salinity and high nitrogen organic wastewater. Recent studies of heterotrophic nitrification bacteria mainly focused on substrate removal, accumulation of intermediates, and the removal of COD and nitrogen via simultaneous nitrification and denitrification. But the applications of the heterotrophic nitrification -aerobic denitrificationhave been isolated, but they could not perform well in nitrogen removal with high salinity.

A simultaneous nitrogen removal system for high salinity and high nitrogen organic wastewater via heterotrophic nitrification was developed in a pressurized biofilm reactor. During the experiment, there is no anaerobic-aerobic conditionsand no excess sludge discharge in the single stage reactor. The perfomance of COD and nitrogen removal were examined. Meanwhile, PCR-DGGE was used to detectthe microbial diversity and community structure. This study can provide a more efficient and feasible solution to treat the high salinity and high nitrogen organic wastewater.

 

AMMONIA REMOVAL AND MICROBIAL CHARACTERISTICS OF PARTIAL NITRIFICATION IN BIOFILM AND ACTIVATED SLUDGE TREATING LOW STRENGTH SEWAGE AT LOW TEMPERATURE

Nowdays, many wastewaters do not contain sufficient amounts of biodegradable carbon, making them less suitable for nitrogen removal via nitrification-denitrification process. Moreover, with the development of anaerobic treatment process, most organic compounds in waste water are converted to biogas, which is feasible with the present state of the art. The autotrophic nitrogen removal process which based on partial nitrification and anaerobic ammonia oxidation (Anammox) could remove nitrogen  from wastewater with  no organic consumption, which has attracted increasing attention because of its ability to achine high nitrogen removal rate with less energy consumption. So it could be alternative technology for treating the sewage with low ratio of COD to nitrogen(C/N). The autotrophic nitrogen removal technology include two process type partial nitrification-anaerobic ammonia oxidation and completely autotrophic nitrogen removal over nitrite. since the occurence of anaerobic  ammonia oxidation requires a certainproportion of nitrite nitrogen of 1.32 in the feed solution, part of ammmonia in the wastewater should be oxidized to nitrite for the uptake by anaerob ammonia oxidazing bacteria. Thus, the partial nitrificationprocess which undertakes the oxidation of ammonia to nitrite is critical and indispensable for the autotrophicnitrogen removal from low C/N sewage.

In partial nitrification process, the oxidation of ammonia to nitrite by ammonia oxidizing bacteria should be enhanced while the oxidation of nitrite to nitrate by nitrite oxidizing bacteria should be inhibited since anaerob ammonia oxidizing bacteria consumes only ammonia and nitrite. in consequence, a prominent and stable partial nitrification requires enrichment of ammonia oxidizing bacteria and inhibition of nitrite oxidizing bacteria, to achieve high ammonia removal efficiency and nitrite accumulation rate. It didn’t seemquite so difficult to achieve high rate  and stable partial nitrification when treating sewage with high temperature or high ammonia in side-stream such as sludge digestion and landfill leachate with. However, there were still some challenges in partial nitrogen process for treating the main stream sewage with low ammonia or low temperature such as the influent of municipal plant. On one hand, the oxidation rate of ammoniaby ammonia oxidizing bacteriais severely affected by temperature, the lower temperature would lead to lower ammonia removal efficency. Moreover, the higher activation energy of ammonia oxidizing bacteria than nitrite oxidizing bacteria resulted in the difficulty of nitrite accumulation under low temperature, and many studies have reported failure during winter temperature.  On the other hand,the lower ammonia concentration would also impose restriction on partial nitrogen process, since the lower free ammoniacould not perform effectivesuppression on nitrogen oxidizing bacteria, In conclucion, the low temperature and ammonia concentrationwould not limit the efficiency, but also effect the stability of partial nitrogen, so the low ammonia sewage treated system could not reach the effuent demand in winter temperature. The operation of minicipal plant in winter was one problems that could not be ignored during the application of partial nitrogen, thus it was essential to gain more information about the performance and microbial characteristics of this process under low temperature.

Generally, there are two kinds of waste water treatment systems, including activating sludge system and biofilm system, in which organisms respectively survived in suspended and fixed condition. Activated sludge systemcoul achieve high removal loading since its better mass transfer, which could be flexiblyoperated and controlled. For biofilm system, it enables a larger biodiversity of the microorganism due to its long SRT, and has shown great potentials for partial nitrogen as aresult of sufficient oxygen usage and well stratified distribution of  ammonia oxidizing bacteria and nitrogen oxidizing bacteria within the biofilm. In previous studies, pertial nitrogen reactors have been conventionally operated as activated sludge system, whereas it has been confirmed that biofilmpartial nitrogen processes with attached biomass  also have advantages. Both the two systems could be effectively used for partial nitrogen process, but it was doubtless that the suitable operational condition could be different between each other. However, no study has been done simultaneously in both in two systems and   the specific suitable strategies for each system were still not clear, let alone for treating sewage in main stream.