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.

 

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